Toxicity of selected acaricides on Tetranychus urticae ... · Janet L. Ashley Thesis submitted to...

Toxicity of selected acaricides on Tetranychus urticae Koch (Tetranychidae: Acari) and Orius insidiosus Say (Hemiptera: Anthocoridae) life stages and

predation studies with Orius insidiosus

Janet L. Ashley

Thesis submitted to the Faculty of the Virginia Polytechnic Institute and State University in partial fulfillment of the requirements for the degree of

Master of Science

in Entomology

D. Ames Herbert, Chair

Edwin Lewis, Co-Chair

Carlyle Brewster

Randy Huckaba

December 3, 2003 Blacksburg, Virginia

Keywords: Tetranychus urticae, Orius insidiosus, predator, prey, acaricides

Copyright 2003, Janet L. Ashley

Toxicity of selected acaricides on Tetranychus urticae Koch (Tetranychidae: Acari) and Orius insidiosus Say (Hemiptera: Anthocoridae) life stages and predation studies with O.

insidiosus

Janet L. Ashley

(ABSTRACT)

Most management tactics for Tetranychus urticae (TSSM) rely upon applying acaricides.

Multiple applications are required, which impact natural enemies. Growers will benefit from a

more complete understanding of acaricide toxicity. My objectives were to determine: 1.) stage-

specific direct and residual efficacy of three acaricides to TSSM; 2) direct and residual toxicity

of these acaricides to O. insidiosus; 3) the functional response of O. insidiosus to mobile and

egg stages of TSSM, in laboratory and greenhouse studies; 4) the abundance of O. insidiosus

relative to TSSM densities in peanut.

Direct toxicity of three acaricides to TSSM was measured on peanut cuttings. All acaricides

caused significant mortality, however; mortality did not differ among the acaricides. Residual

toxicities against TSSM were not found to be toxic compared with untreated controls 24 and 72

hours after treatment. When acaricide toxicity to eggs was tested, the hatch rate for all

treatments was significantly lower than the control hatch rate.

Direct toxicity of the acaricides was tested against O. insidiosus. Fenpropathrin and

propargite caused 100% mortality and etoxazole resulted in mortality > 50%. Residual toxicity

of acaricides to O. insidiosus adults varied. Fourteen days after treatment, fenpropathrin left

residues highly toxic to O. insidiosus.

In laboratory studies, the functional response of O. insidiosus to TSSM eggs resulted in a

Type III response whereas the functional response to adults was Type II. The data suggest either

a Type II or linear response in greenhouse studies. A definitive conclusion cannot be drawn

because of the sample size.

iii

Acknowledgements First, I would like to thank Dr. Ames Herbert, my major advisor. If not for him, I would

not have been here in the first place. He believed in me and gave me a chance, when others may

not have done so. His guidance and support over the last three years has helped me to grow, not

only as a scientist but as a person, as well. I will forever be grateful. I would also like to thank

my co-advisor, Dr. Edwin Lewis. There were times during my graduate studies I did not believe

I would ever complete my research. It seemed I was going to Dr. Lewis every other day with

some problem I was positive could not be fixed. He would answer my questions and allay my

fears, but never in a way to make me feel insignificant, and for that alone, I am most

appreciative. I would also like to thank my advisory committee members, Dr. Carlyle Brewster

and Dr. Randy Huckaba. Whenever I had questions, they always made time for me, no matter

how busy they were. I want to send a special thank you to Dr. Brewster for all of his help with

my statistics. Without his help, two of my chapters would never have been finished. I am very

grateful to the Entomology department at Virginia Tech, for the support from the faculty, staff,

and fellow graduate students. A very special thanks to the Tidewater Agricultural Research and

Extension Center, where I performed my summer research. I want to thank Mike Arrington,

Mike Ellis, and Ken Bradshaw. Without their assistance in the field, and for making work fun,

the summers would have been unbearable. I also want to thank Erin Holden for her help with

my summer research and for sticking it out in the peanut fields when the heat was so bad. I also

want to thank Sean Malone for answering many questions. I�m sure he is tired of hearing �Sean,

can I ask you a question?�

Last, but not least, I want to thank my family, Mom, Dad, Jimmy, Andy, and Alisha. I

could not have made it through graduate school were it not for their love and support. I am very

lucky to have such a warm and caring family. I hope they know how much I love them and how

much I appreciate all their help and support over the past few years.

I also want to acknowledge my funding sources for this project, the Virginia Peanut

Board and the National Peanut Board. Without their support, this project would not have been

possible.

iv

Table of Contents Abstract .................................................................................................................................... ii

Acknowledgements.................................................................................................................. iii

Table of Contents .................................................................................................................... iv

Tables and Figures .................................................................................................................. vi

Introduction...............................................................................................................................1

Chapter 1: Review of literature ...............................................................................................4

Tetranychus urticae history .....................................................................................................4 Tetranychus urticae biology ....................................................................................................4 Orius insidiosus biology..........................................................................................................7 Tetranychus urticae and peanut ...............................................................................................8

Chapter 2: The Toxicity of Three Acaricides on Tetranychus urticae..................................10

Introduction...........................................................................................................................10 Materials and methods...........................................................................................................10 Results ..................................................................................................................................13 Discussion.............................................................................................................................14 Conclusions...........................................................................................................................16

Chapter 3: The toxicity of three acarcides to Orius insidiosus. ............................................22


Chapter 4: Functional response of Orius insidiosus with Tetranychus urticae as prey........32


v

Chapter 5: A preliminary survey of Orius associated with different densities of ................42

Tetranychus urticae..................................................................................................................42


Summary .................................................................................................................................46

Literature cited........................................................................................................................49

Vita ..........................................................................................................................................54

vi

Tables and Figures Table 2.1. Treatments��...17 Figure 2.1. Modified Huffaker cell��...��17 Figure 2.2. Direct toxicity to T. urticae��..��.18 2.2a. 24 hours after treatment��.��...18 2.2b. 72 hours after treatment��.��...18 2.2c. 7 days after treatment��...19 Figure 2.3. Residual toxicity to T. urticae��...��20 2.3a. 24 hours after treatment��...20 2.3b. 72 hours after treatment��...20 Figure 2.4. Toxicity to T. urticae eggs��...��..21 Table 3.1. Treatments��...28 Figure 3.1. Modified Huffaker cell��...��...28 Figure 3.2. Direct toxicity to O. insidiosus��...��...29 Figure 3.3. Residual toxicity to O. insidiosus��...��...30 3.3a. 24 hours after treatment��...30 3.3b. 72 hours after treatment��...30 3.3c. 7 days after treatment��.��..�31 3.3d. 14 days after treatment��.��31 Figure 4.1. Functional response experimental arena��38 Figure 4.2. Functional response of O. insidiosus to T. urticae eggs��...�.39 Figure 4.3. Functional response of O. insidiosus to T. urticae adults��..40 Figure 4.4. Functional response in greenhouse studies��41 Figure 5.1. Mean number of Orius found per TSSM population level.��...45

1

Introduction Tetranychus urticae (TSSM), the twospotted spider mite, is a major pest on many plant

species (Cagle 1949). Peanut is one of its preferred host plants. In the early 1980�s, mite

infestations in peanut changed from occasional and sporadic, into a major economically

important factor in peanut production (Smith & Mozingo 1983). In 1995, Georgia reported their

combined cost of damage and control for the TSSM to be $2,445,000 (Brown, Jones & Todd

1995).

There are many factors that allow the TSSM to achieve pest status in peanut. The TSSM

exhibits arrhenotokous parthenogenesis, where the female does not have to copulate to produce

offspring (Brandenburg & Kennedy 1987). Unfertilized eggs will result in all male offspring,

but fertilized eggs can result in either male or female offspring. The average oviposition period

for a female is 2.4 to 2.5 days (Cagle 1949; Laing 1969). During this time, she will lay an

average of 38 eggs or more (Laing 1969). Peanuts are typically grown in hot, dry areas, and are

often planted in well-drained sandy soils. These areas are highly susceptible to drought

throughout the growing season, creating conditions that can lead to an increase in TSSM

fecundity (Brandenburg & Kennedy 1987). High levels of nitrogen in host plants can result in

higher female weight and a high oviposition rate (Brandenburg & Kennedy 1987). In times of

high mite numbers, the quality of the host plant can decline very rapidly. Direct and indirect

effects of mite feeding can occur. Defoliation, leaf burning, and even plant death are examples

of direct effects. Indirect effects, which may lead to other problems in the plant, include

decreases in photosynthesis and transpiration. This combination of effects on the host plant

often reduces the amount of yield for that crop (Huffaker & McMurty 1969).

Because of their high numbers and high reproductive rates, management of the TSSM

population can be difficult. Growers most often rely on acaricide applications to manage mite

infestations. However, these are not always effective. When mites begin to feed on a plant, they

produce webbing. This webbing can protect both motile and egg stages from the acaricide

(Brandenburg & Kennedy 1987). Other reasons for the lack of success with acaricide

applications would include reduced efficacy of the product in hot weather, poor plant coverage

by grower application, and poor timing of the initial application. Often growers wait until they

see plant damage before making the first application. By this time the density of all mite stages

(egg, immatures, and adults) is high. Because the initial sprays never fully eradicate the

2

population, the females that are left on the plant continue to produce offspring, causing a

resurgence in the pest population. Another important factor in TSSM population resurgence is

lack of toxicity to eggs. Many of the acaricides that are applied to the crop do not have ovidical

properties.

Resistance to pesticides is a problem with many pest species. Repeated applications of

the same chemical or chemicals with the same mode of action can increase the chances of a pest

population developing resistance (Helle & Sabelis 1985). Twospotted spider mite populations

have developed resistance to hexyathiazox in Australia (Hernon et al. 1993). Because of poor

management with acaricides and the possibility of resistance, preservation of natural enemies is

important.

Diverse natural enemies have an important role in the ecology of the TSSM

(Brandenburg & Kennedy 1987). Orius insidiosus, is a common predator in peanut and other

cropping systems in the eastern and Midwestern United States (Isenhour, Wiseman & Layton

1989). Orius insidiosus exhibits facultative phytophagy, which is important in biological control

as it allows for maintenance of predator populations during periods of prey scarcity

(Wiedenmann & O�Neil 1991; Naranjo & Gibson 1996; Coll 1997). Both immature and adult

Orius spp. can consume 30 or more spider mites per day (Wright 2001). They often only

partially consume their prey, and in some instances they do not feed on prey they have killed

(Rajasekhara & Chatterji 1970). Orius insidiosus has similar developmental times to the TSSM.

Both are highly dependent on temperature (Richards & Schmidt 1996; Laing 1969). Similar

developmental temperatures to T. urticae, and the fact that it can feed on other prey and plants in

times of low prey numbers, make O. insidiosus a good candidate for augmentative biological

control. However, to combine O. insidiosus with acaricide applications, chemical residues must

be non-toxic to the predators and they must possess a level of tolerance to direct contact.

Economically damaging levels of spider mites can be closely associated with insecticide

applications (Wilson et al. 1991; Bartlett 1968; Iftner & Hall 1984; van de Vrie et al. 1972). One

explanation for this is that insecticides reduce natural enemies, causing a reduction in predation

pressure, which may allow mite numbers to increase. Foliar applications to crops are known to

greatly reduce Orius spp. numbers (Wright 2001); however systemic applications can also

reduce numbers because of its omnivorous feeding behavior (Brown & Shanks 1976).

3

Acaricide applications may be necessary to suppress a TSSM population, however

selective use of acaricides that are compatible with natural enemies may preserve predator

populations and enhance control (Trumble & Morse 1993). To combine O. insidiosus with

acaricide applications, the chemical must be non-toxic to the predators Trumble and Morse

(1993) conducted studies on the harvest value of strawberries and they found that a combination

of predator releases followed by applications of abamectin after mite thresholds were reached

provided a higher value crop than abamectin alone. The same could be true for other crops, but

studies need to be conducted.

The objectives of this study were to 1.) determine the stage-specific efficacy of three

acaricides on TSSM; 2.) determine the residual toxicity of the acaricides to adult TSSM; 3.)

measure the ovicidal efficacy of the acaricides; 4.) measure the direct and residual toxicity of

these same three acaricides on O. insidiosus; 5.) measure the predation functional response of O.

insidiosus to both mobile and egg stages of the TSSM, in both laboratory and greenhouse

studies; and 6.) determine the abundance of O. insidiosus in a naturally occurring infestation of

TSSM in peanut and determine the relationship between mite density and O. insidiosus density.

4

Chapter 1: Review of literature

Tetranychus urticae history

The twospotted spider mite (TSSM), Tetranychus urticae (Koch), belongs to the group of

acarines known as Acarifórmes, in the suborder Prostigmàta, and the family Tetranychidae

(Borror et al. 1989). The TSSM is about 0.5mm long with an oval shaped body which varies in

color from greenish-yellow, to virtually transparent, brown, and red-orange (Fasulo & Denmark

2000). The TSSM was first described by Koch in 1836 (Pritchard & Baker 1955). It is thought

to originate from temperate climates (Fasulo & Denmark 2000).

Tetranychus urticae biology

The TSSM passes through five developmental stages during its life cycle: egg, larva,

protonymph, deutonymph, and adult (Huffaker et al. 1969). Each active immature stage is

followed by a quiescent period. One TSSM generation is completed in about 19 days when the

temperature is between 21° C and 23° C (Mitchell 1973). However, when temperatures are

higher (30° C), development time from egg to adulthood can be reduced to seven days (Thomas

2001).

The TSSM exhibits arrhenotokous parthenogenesis (Brandenburg & Kennedy 1987).

Fertilized eggs will result in female offspring, whereas unfertilized eggs will produce male

offspring. Males typically complete the last quiescent stage before adulthood earlier than

females (Mitchell 1973). Instead of feeding, the males actively search out female deutonymphs

and await their emergence from the quiescent period into adulthood. Just before the female

emerges, the male stays in close contact with the female, often touching her (Laing 1969). When

the exoskeleton splits open, the male will often assist the female in freeing herself from the

exuvium. Mating will sometimes take place as soon as the anterior portion of the exoskeleton is

released. Copulation can last from a few seconds to several minutes (Cagle 1949). When a

female mates with more than one male, sperm precedence is given to the first male (Potter et al.

1976). The average oviposition period per female is about 2.4 to 2.5 days (Cagle 1949; Laing

1969). Females generally lay an average of 38 eggs in total, but it is possible for a single female

to lay well over one hundred eggs during the oviposition period (Laing 1969). Higher numbers

of eggs generally occur when relative humidity is low (25-30%).

5

While on uninjured plants, TSSM are uniformly distributed over the leaf surfaces. When

the plant begins to decline, resulting in a reduced food supply, the mites will enter a dispersal

phase and aggregate in the uppermost parts of the plants (Suski & Naegele 1963). Mites in the

dispersal phase show a greater directional response to light than in the sedentary phase. The

declining condition of the plant partially triggers the change from the sedentary phase to the

dispersal phase (Suski & Naegele 1963).

Dispersal is movement away from the colony in which the TSSM developed

Brandenburg & Kennedy 1987). It includes both intraplant and interplant movement.

Crawling is a common means of dispersal through the host plant, however, it can also be

an effective means of interplant dispersal (Brandenburg & Kennedy 1987). TSSM will often

climb over intertwined foliage of adjacent plants or simply crawl over the ground to reach new

plants, which they colonize. Aerial dispersal begins with the mites aggregating on the uppermost

portions of the plants (Suski & Naegele 1963). The mites face the opposite direction of the light

source with their forelegs raised upward above their bodies (Brandenburg & Kennedy 1987).

The mites produce a thread of silk, which they use to �balloon� into the wind, sometimes

carrying them great distances. Another method of dispersal, phoresy, is common when mites

move by �hitchhiking� on other organisms (Weeks et al. 2000).

The tetranychid mites maximize fitness in several ways (Mitchell 1970). When the host

plant begins to decline, reproductive rates of females are greatly increased. Since mating of

females usually occurs just after emergence of the quiescent deutonymph stage, most are mated

before they disperse, which increases their probability of founding new colonies. When a

dispersing female reaches a new resource, she immediately begins to feed close to a leaf vein and

produce webbing (Brandenburg & Kennedy 1987). Eggs are deposited beneath the webbing and

larvae and nymphs develop within it. The webbing basically defines the colony boundaries, and

as the colony grows, the webbing also expands (Brandenburg & Kennedy 1987). In addition to

providing the boundaries of the colony, the webbing also serves as a means of protection from

rain, wind, and predators. It is thought that the webbing and deposition of fecal pellets within the

webbing, is a mechanism to regulate humidity (Hazan et al. 1974). When a heavy infestation

occurs, the plants often become matted with webbing (Cagle 1949). If the webbing is dense

enough, protection may also be provided from acaricide sprays (Brandenburg & Kennedy 1987).

TSSM feeding on the underside of leaves (Cagle 1949) generally results in the typical

6

�stippling� damage, which is white or grayish colored spots due to the punctures made by

feeding (Brandenburg & Kennedy 1987). Mites insert their stylets into the plant cells and suck

out the cell contents. Feeding can damage protective leaf surfaces, stomata, and the palisade

layer (Huffaker et al. 1969). They may also damage the lowest parenchymal layer

(Brandenburg & Kennedy 1987). Defoliation, leaf burning, and even plant death can occur due

to direct feeding damage. Indirect effects of feeding may include decreases in photosynthesis

and transpiration (Brandenburg & Kennedy 1987). However, moderately damaged leaves may

have increased transpiration. This combination of direct and indirect effects often reduces the

amount of harvestable material (Huffaker et al. 1969).

Diapausing females or eggs are the most common overwintering stage for tetranychids

(Mitchell 1970). In warm areas during the winter, some mites may continue to reproduce and

diapause is facultative (Brandenburg & Kennedy 1987; Mitchell 1970). Diapause is often in

response to short daylengths and cooling temperatures (Mitchell 1970). During diapause, TSSM

do not feed or oviposit, and they generally seek shelter in crevices in tree bark and shrubs, clods

of dirt, and in leaf litter (Brandenburg & Kennedy 1987). Longer daylengths and warming

temperatures terminate diapause.

The twospotted spider mite is an important economic pest of many host plants including,

but not limited to, peanut, cotton, corn, soybean, and many orchard crops and ornamentals

(Cagle 1949). Host plant species, cultivar, or phenological stage can affect TSSM developmental

rate, survival, reproduction and longevity (Brandenburg & Kennedy 1987). There is also

evidence that the nitrogen-phosphorus-potassium ratio can influence TSSM female weight,

preoviposition period and oviposition rate (Brandenburg & Kennedy 1987). High levels of

nitrogen improve host quality, which results in higher female weight, shorter preoviposition

period, and a high oviposition rate. Water stress can enhance plant susceptibility to TSSM

because it causes an accumulation of soluble leaf carbohydrates, which can increase fecundity.

Diverse natural enemies have an important role in the ecology of TSSM (Brandenburg &

Kennedy 1970). The orders of arthropods that prey on TSSM include Thysanoptera,

Coleoptera, Hemiptera-Heteroptera, Neuroptera, Diptera, Acarina, and Araneida. TSSM is also

attacked by emtomopathogenic fungi (Brandenburg & Kennedy 1987), including Neozygites

spp., Verticillium lecanii (Zimmerman), Entomophthora spp., and Paecilomyces terricola

(Miller, Giddens, and Foster) (Helle & Sabelis 1985). Most research has been focused on

7

phytoseiid mite predators because they possess a number of characteristics that allow them to

control TSSM at low densities (Brandenburg & Kennedy 1987).

Predators can effectively suppressing spider mite populations (Brandenburg & Kennedy

1987), however, when pesticides are applied to crops, outbreaks of high numbers of TSSM can

occur. In crops such as peanut, where fungicides are applied throughout the growing season,

serious outbreaks can occur. Predators are not capable of suppressing the high densities of

TSSM, which results in yet another application of pesticide. Several acaricides listed for the

control of TSSM include propargite, aldicarb, lambda-cyhalothrin, and fenpropathrin (Herbert

1999). While these acaricides are effective against mobile forms of TSSM, little is known about

their ovicidal properties. Campbell et al. (1974) reported that in laboratory tests, several

pesticides have ovicidal properties, including triphenyltin hydroxide, chlordimeform, and

carbophenothion. Studies by Smith and Mozingo (1983) have shown that some fungicides can

increase mite populations. However, more timely studies are needed to test acarcides and

pesticides that are currently on the market.

Orius insidiosus biology

The insidious flower bug, Orius insidiosus (Say), is considered an important natural

enemy of numerous insect pests (Ruberson, Bush & Kring 1991). Their prey includes thrips,

mites, aphids, whiteflies, lepidopteran eggs and small larvae (Zhang & Shipp 1998). Both adults

and nymphs feed on prey, when available (Coll 1996). Orius insidiosus is omnivorous, meaning

they will feed at more than one trophic level (Coll 1996). Omnivory is important because it

allows for maintenance of predator populations during times when prey are not readily available

(Armer, Wiedenmann & Bush 1997). Orius insidiosus finds prey by touch, then grabs it with its

front legs, inserts the rostrum, and drains the prey of its body fluids (Koppert Biological Systems

2003). Orius insidiosus can be found on a wide variety of vegetation, including trees, shrubs,

wild plants, and numerous field crops (McCaffrey & Horsburgh 1986). Orius spp. can consume

30 or more mites per day (Wright 2001). They often only partially consume the prey they have

killed (Rajasekhara & Chatterji 1970), thus killing more than necessary for their own feeding.

Adult female O. insidiosus insert eggs into plant tissue, usually in stems or near leaf veins

(Lindquist 1999). Adults usually prefer to lay eggs on wrinkled or bumpy surfaces rather than

on a smooth surface (Richards & Schmidt 1996). At 17°C, eggs will hatch in about 12 days,

however, as temperature increases, egg development time decreases (McCaffrey & Horsburgh

8

1986). A relative humidity of at least 70% is required for eggs to hatch (Richards & Schmidt

1996). At 35° C, the eggs can complete development in as little as 3 to 4 days. The eggs hatch

into nymphs, which then go through five instars before becoming adults (McCaffrey &

Horsburgh 1986; Lindquist 1999). Development from egg to adult requires from 9 to 25 days,

depending on temperature (Lindquist 1999). At 25° C, O. insidiosus can go from egg to adult in

about 18 days (Coll 1996). Females usually live for about 15 days, and have a 4-day

preoviposition period.

In temperate zones many Orius species are known to hibernate as adults in dry and

protected places (van den Meiracker 1994) including leaf litter and other organic material

(Ruberson, Bush & Kring 1991). Ruberson et al. (1991) reported that the critical photoperiod to

induce diapause, at 20 ºC, was between 12 and 13 hours. Orius insidiosus usually terminates

diapause within two weeks after temperatures return to 25° C and photophase is at least 16 hours

(van den Meiracker 1994). When O. insidiosus emerge from hibernation, they are mostly mated

females, but males also occasionally occur (van den Meiracker 1994).

Pesticides can have a detrimental effect on naturally occurring populations of O.

insidiosus. Effects of pesticides on O. insidiosus may not come just from foliar sprays, but also

from the use of systemic pesticides, because of their omnivorous feeding behavior (Brown &

Shanks 1976). Several of these systemics include aldicarb, disulfoton, demeton,

oxydemetonmethyl, and imdiacloprid. In strawberry and lima bean, carbofuran has also been

shown to kill Orius spp. These pesticides can reduce the O. insidiosus populations, while having

very little effect on TSSM populations.

Tetranychus urticae and peanut

Peanut, Arachis hypogaea L., constitutes over four billion dollars of the U.S. economy

per year (Owens 1996). The Virginia-Carolina area produces about 20% of the nation�s peanuts

annually (Herbert 1998; Owens 1996). In this area, the TSSM has become difficult to manage.

Peanuts are generally planted around the end of April to the first of May (Owens 1996).

During July and August, the plants are putting their energy into pod development. This is a very

critical time for peanut. Because they are planted in sandy well-drained soils, adequate rainfall is

important. However, in the Virginia-Carolina area, the months of July and August are typically

hot and dry. These conditions favor TSSM development. High densities can quickly

9

develop. Because TSSM sucks chlorophyll from the leaves, photosynthesis is decreased. If

TSSM feeding is severe, yield can be reduced.

Chemical control is the most common form of management for TSSM in peanut. Two of

the more common acaricides used are fenpropathrin and propargite. Fenpropathrin is a synthetic

pyrethroid, which acts as an axonic poison that affects both the peripheral and central nervous

system (Ware 1994). Propargite is an organosulfur (Ware 1994), which causes inhibition of

stimulated ATP-ase (IRAC 2003).

Even though acaricides can usually offer adequate reduction of TSSM populations,

eradication is not possible with TSSM. Also, economically damaging levels of spider mites are

closely associated with insecticide applications (Wilson et al. 1991; Bartlett 1968; Iftner & Hall

1984; van de Vrie et al. 1972). One explanation is insecticides decimate large numbers of

natural enemies, causing a reduction in the natural enemy population. This can lead to a

resurgence in the TSSM population.

10

Chapter 2: The Toxicity of Three Acaricides on Tetranychus urticae Introduction

The twospotted spider mite (TSSM), Tetranychus urticae Koch, is a major economic pest

of many host plant species including, but not limited to, peanut, corn, cotton, soybeans, many

orchard crops, and ornamentals. Twospotted spider mite damage can be classified as either

direct or indirect (Brandenburg & Kennedy 1987). Direct effects include stippling, webbing,

defoliation, leaf burning, and in extreme outbreaks, plant death. Indirect effects of mite feeding

may include decreased photosynthesis and transpiration. A female TSSM can develop from egg

to adult in approximately 6.5 days at 30º C (Sabelis 1981) and lay an average of 38 eggs per

individual in five days (Laing 1969). In optimal conditions, a single female has been reported to

lay well over one hundred eggs (Laing 1969). Most management tactics in the U.S. rely upon

application of chemical pesticides (Brandenburg & Kennedy 1987; Smith & Mozingo 1983).

Coincidentally, outbreaks of TSSM are closely associated with the use of various pesticides

(Brandenburg & Kennedy 1987). The development of synthetic pyrethroids further increased the

frequency and severity of outbreaks because of their toxicity to natural enemies.

Because the numbers of each life stage of the TSSM vary in a natural population, it is

important to know how a chemical treatment applied to manage the TSSM infestation will affect

each life stage. I determined the stage-specific efficacy of three acaricides on TSSM. I had three

main goals: 1.) determine the direct toxicity of three acaricides against a mixed-stage population

of TSSM; 2.) determine the residual toxicity of the acaricides to adult TSSM, and 3.) measure

the ovicidal efficacy of the acaricides. Two acaricides commonly used by growers to control

TSSM, fenpropathrin and propargite, and one experimental acaricide, etoxazole, were tested.

Etoxazole has since been registered for use in cotton and some fruit crops.

Materials and methods

Rearing conditions for all experiments

Twospotted spider mite colonies were maintained on lima beans (Phaseolus lunatus) in

rearing cages at 28° C and 16:8 hr (L:D) photoperiod. Lima beans were grown in a peat based

soil-less medium in plastic pots. No pesticides were applied to stock plants. The individuals

used to initiate the colony were collected from infested weeds. Each rearing cage was a 20 x 40

x 30-cm Plexiglass box with an open top, fitted with No-Thrips screening. Double-sided tape

11

was placed along the outside rim and petroleum jelly along the inside rim to prevent mite escape

and contamination of the colonies.

Direct toxicity to a mixed-staged population

Peanuts were grown in plastic containers in a mixed medium that contained 70% sand

and 30% peat based soil-less medium. No pesticides were applied to stock plants during

cultivation. Cuttings with eight leaflets were used for all treatments. Each cutting was installed

into a 473.18-ml canning jar that contained a piece of copper screening near the bottom with a

small hole for leaflet stems to prevent the peanut leaves from falling into the water. Three to six

centimeters of water were added to the jars to prevent desiccation of the cuttings. The canning

jar with leaflets was placed inside a glass battery jar that contained five centimeters of water in

the bottom and petroleum jelly applied to the top of the battery jar to prevent mite escape. The

entire battery-canning jar system was placed into a large plastic tray. Double-sided tape was

applied to the outside perimeter of the tray. The entire unit was placed under HID (high intensity

discharge) mercury vapor lighting programmed for a 14:10 hr (L:D) photoperiod.

Ten adult female TSSM were released onto each of the leaflet cuttings. A mixed-stage

population that contained eggs, nymphs, and adults was allowed to develop on the cuttings for

one week. After one week, the total numbers of eggs, nymphs, and adults combined were

counted on all leaflets for each cutting. After pretreatment counts were taken, the cuttings were

sprayed with one of the treatments listed in Table 2.1 with an aerosol hand-held sprayer under a

fume hood until run-off. The rates for the sprays are the manufacturer�s labeled rates for that

acaricide, for use against TSSM in peanut. The amount of product per 100 ml of water was

determined. The treated cuttings were allowed to dry under the fume hood for approximately 30

minutes. At 1, 3 and 7 days following application, the total numbers of eggs, nymphs, and adults

were counted. Temperatures during the experiment averaged 29° C and ranged from 27-30° C.

Two trials of five cuttings each, which served as replicates, were conducted for each

treatment. These data were blocked according to trial, analyzed with ANOVA (SAS Institute),

and the means were compared with Tukey�s test.

Residual toxicity

All treatments (Table 2.1) were applied to whole lima bean plants using a hand-held

sprayer until run-off. One, 3, 7 and 14 days after treatment, 10-cm2 leaf disks were cut from the

treated plants and transferred to a modified Huffaker cell system (Munger 1942; Huffaker 1948).

12

The cells were made from three 7.6 x 7.6 x 0.6-cm thick Plexiglass pieces bolted together at the

corners (Figure 2.1). A 4.5-cm diameter hole was cut in the middle layer of Plexiglass to create

a chamber to house leaf disks for the assay.

One adult female TSSM was placed onto each of the treated bean leaf disks, after which

the cells were sealed by bolting together the three layers of Plexiglass. The cells were placed

under mercury vapor lighting programmed for a 14:10 hr (L:D) photoperiod. After 24 hours of

exposure to the treated surface, each mite was scored as dead or alive. A mite was scored as

dead if no movement was detected after prodding. Seven- and 14-day old residues were not

tested if mortality from 3-day old residues was not significantly greater than control mortality.

Two trials of twenty disks each, which served as replicates, were conducted for each treatment.

Temperatures during the experiment averaged 25.4° C and ranged from 25-29.8° C. Data were

analyzed using χ2 analysis.

Toxicity to eggs

Peanut plants were grown in a greenhouse, in one-gallon plastic containers with a peat

based soil-less medium. No pesticides were applied to stock plants.

To provide eggs to test, five 15-mm Petri dishes were lined with peanut leaflets. Ten

adult female TSSM were placed onto the leaflets and the dish was sealed with Parafilm. After 48

hours, the active stages of the TSSM were removed from the leaflets and the number of eggs per

leaflet was counted. The eggs were then subjected to a dip-test (Shearer 2000) with each of the

treatments listed in Table 1. To perform the dip-test, each leaflet was dipped into the treatment

for 5 seconds. The leaflets were dried under a fume hood for at least 30 minutes (Shearer 2000).

After drying, the leaflets were placed into a modified Huffaker cell as described in the residual

toxicity studies. A small piece of slightly moistened cotton was placed into the chamber with the

leaflet to prevent desiccation of the leaf and the eggs.

All leaflets were examined daily for seven days. Egg hatch rate was determined each day

by counting the numbers of hatched eggs and larvae on the leaflets. If any TSSM were found,

they were removed from the leaflets. For each treatment, two trials were conducted with 10

replicates per trial. Temperatures averaged 28.3° C and ranged from 27.5-29° C during the test

period. These data were transformed by √arcsin and were analyzed by ANOVA (SAS Institute).

The means were compared with Tukey�s test.

13

Results

Direct toxicity to a mixed-stage population

Twenty-four hours after treatment, mortality of TSSM eggs, nymphs, and adults varied

significantly among the compounds tested (Figure 2.2a) (F = 5.56, df1 = 6, df2 = 53, P<0.0002).

Mortality caused by the low and high rates of fenpropathrin (41 and 16%) were not significantly

different from the control (9%). The mortality rates for the other compounds tested were

significantly different from controls, but were not significantly different from one another. The

low and high rates of etoxazole caused 53 and 49% mortality, respectively and propargite

resulted in 63% mortality.

Seventy-two hours following treatment, mortality rates for all compounds and rates tested

were significantly different from the control (Figure 2.2b) (F = 13.74, df1 = 6, df2 = 53,

P<0.0001). The control caused 18% compared to propargite, which caused 83%. Mortality at

the high rate of fenpropathrin was not significantly different from the low rate of fenpropathrin,

but was significantly different from the mortality rates for the remaining treatments. The low

rate of fenpropathrin caused 60% mortality, compared to 54% mortality with the high rate. The

low and high rates of etoxazole resulted in 81 and 85% mortality, respectively.

Seven days after treatment, the mortality rates for all the compounds tested were

significantly greater than the control, but did not vary among the compounds tested (Figure 2.2c)

(F = 15.02, df1 = 6, df2 = 53, P<0.0001). The low and high rates of fenpropathrin caused 72 and

82% mortality, respectively, while propargite caused 96% mortality. The low and high rates of

etoxazole provided 85 and 87% mortality, respectively.

Residual toxicity

The duration of acaricide residue toxicity to TSSM did not vary significantly among the

acaricides or rates tested (Figure 2.3). At 24 hours after treatment, there were no significant

differences among any of the acaricides or rates tested compared with the untreated controls

(Figure 2.3a). All of the compounds caused less than 20% mortality.

The results at 72 hours after treatment were consistent with the results at 24 hours after

treatment. Mortality was not significantly among any of the acaricides or rates tested compared

with the untreated controls (Figure 2.3b). All of the compounds caused less than 30% mortality.

14

Toxicity to eggs

The hatch rates of eggs treated with different acaricides varied significantly (Figure 2.4)

(F = 48.82, df1 = 6, df2 = 112, P<0.0001). Untreated eggs hatched at a rate of 82%. The low

(41%) and high (17%) rates of fenpropathrin resulted in hatch rates that were significantly

different from one another, significantly lower compared with the untreated control, but

significantly higher than other treatments. The hatch rates for both rates of etoxazole and

propargite were 0% and significantly different from the control and both rates of fenpropathrin.

Discussion

The objective of this study was to determine the effects of three acaricides on various life

stages of TSSM. Tests included efficacy trials, residual toxicity trials, and ovicidal trials.

Sublethal effects, which can occur from direct and residual contact with the acaricides, were not

measured.

Direct application of all the treatments that we tested caused some level of mortality.

Blair (1989) found that propargite is an effective adulticide of Tetranychus evansi. Efficacy of a

specific acaricide may vary among different species of spider mite, however our findings agree

with those of Blair (1989). Three days after treatment, propargite caused 83% mortality of

TSSM. A similar study found 80% mortality when Panonychus ulmi, the European red mite, was

treated with propargite at 1000 ppm (A.I.) (Marshall & Pree 1991). The pyrethroid

fenpropathrin is highly toxic to nymphs and adults of Panonychus ulmi (Koch), the European red

mite (Marshall & Pree 1991). At 48 hours with the field rate of fenpropathrin (25ppm), 98 and

100% of the P. ulmi nymphs and adults were killed, respectively. In my study, 72 hours

following treatment with fenpropathrin, mortality rates remained significantly different from the

controls. The low rate of fenpropathrin caused 61% mortality, while the high rate resulted in

54% mortality. Tests were conducted by the Valent USA Corp. (Longwood, FL) company on

TSSM with etoxazole on cotton. Seven days after treatment, the number of TSSM was reduced

from 80 mites per leaf to 7 mites per leaf. Fourteen days after treatment, the number decreased

even further to 4.1 mites per leaf. In our studies, similar results were found. Seven days after

treatment, mortality rates were at 86 and 87% for the low and high rates of etoxazole,

respectively. Our test lasted seven days after treatment because the peanut cuttings desiccated to

the point where mites died. Results from Valent Corp. USA (Longwood, FL) (2001) show that

15

the number of mites per leaf 21 and 28 days after treatment began to increase, indicating a

resurgence in the TSSM population.

Residual toxicity tests were run on TSSM adults 24 and 72 hours after treatment. None

of the compounds tested resulted in mortality greater than the untreated controls. A similar study

was conducted on the TSSM with propargite by Schiffhauer & Mizell (1988) found different

results. They treated leaf surfaces by dipping leaves for five seconds into different

concentrations of propargite. Mortality assessments were made at 72 hours after treatment. The

highest concentration in the experiment, 1,760 mg [A.I.]/liter (176 mg [A.I.]/100 ml water)

resulted in 48.8% mortality of adult TSSM. This is much higher than the mortality rate I

observed. Etoxazole is more effective against juvenile and egg stages of TSSM versus efficacy

against adults (Valent USA Corp. 2001). Our residual toxicity results support this statement. At

72 hours after treatment, the highest rate of etoxazole only caused 25% mortality, compared with

15% mortality in the control.

The numbers of hatched eggs were counted daily in the egg toxicity tests and the total

hatch rate was determined at seven days after treatment. The low and high rates of fenpropathrin

caused 41 and 17% hatch rates, respectively. A similar study conducted on the European red

mite, P. ulmi (Koch), reported similar results (14% mortality) (Marshall & Pree 1991). Mortality

differences could have occurred because of the method of treatment used on the mite eggs, as

well as differences in mite species. Blair (1989) performed a similar dip-test study on T. evansi

eggs and reported 100% mortality at a dosage between 10 and 50 mg [A.I]/liter (1 and 5 mg

[A.I]/100 ml water). Tetranychus evansi could be more susceptible to fenpropathrin than the

TSSM. It is common for different species of mites to have different reactions to the same

acaricide. An acaricide may be toxic to one species, but not to another.

I found 100% mortality of the eggs treated with propargite, as did Blair (1989). A similar

study that used the Potter spray tower technique, found 80% mortality when P. ulmi eggs were

treated with the field rate of propargite (Marshall & Pree 1991).

The low and high rates of etoxazole caused 100% mortality of the treated eggs in our

study. Because etoxazole has just recently been registered with the EPA, very little data could

be found. However, unpublished information from Valent USA Corp. (Longwood, FL) states

that etoxazole is most effective against juvenile stages of TSSM, including eggs (Valent USA

Corp. USA 2001).

16

Conclusions

When choosing an acaricide to treat a TSSM infestation, it is important to know how it

will affect different life stages. It would be beneficial to have one acaricide that is effective

against all life stages: eggs, larvae, nymphs, and adults. Growers often rely on multiple sprays

because many acaricides have limited stage-specific activity. If an acaricide is active against

adult TSSM only, a second spray will be needed in a few days to control the newly hatched

TSSM larvae. If an ovicide is used, the adult TSSM will lay more eggs, and thus a second spray

will be needed. A management strategy that uses two acaricides, which are toxic to different life

stages, may extend the duration of TSSM control. For example, if etoxazole and fenpropathrin

were used together, etoxazole would decrease the hatch rate of eggs that are already on the plants

and fenpropathrin would control the motile stages. However, this would not kill all of the eggs

or adults in the field. Better coverage of the acaricides is possible in the laboratory versus the

field, causing much higher mortality than would be found in a field situation.

17

Table 2.1. Treatments

Common Name Trade Name + Formulation

Labeled Field Rates mg A.I./100mL H2O

0.2 lb a.i. / acre 120.95 Fenpropathrin Danitol® 2.4 EC

(Valent USA Corp., Longwood FL)

0.3 lb a.i. / acre 183.05

Propargite Comite® 6.55 (Crompton, Uniroyal

Chemical, Middlebury, CT)

1.64 lb a.i. / acre 976.10

0.09 lb a.i. / acre 54.00 Etoxazole Zeal� (Valent USA Corp.,

Longwood, FL) 0.135 lb a.i. / acre 80.93

Control (Water)

Bottom Layer Middle Layer Top Layer

7.6 cm

7.6 cm

4.5 cm

(0.6 cm thick) Bolt holes for assembling the three layers. Chamber for assay material

Figure 2.1. Modified Huffaker cell. Three Plexiglass layers are bolted together to create the housing unit for assays.

18

24 H

ours

Aft

er

Tre

atm

ent

72 H

ours

Aft

er

Tre

atm

ent

0

0.2

0.4

0.6

0.8

1

Contr

olFe

npro

path

rin

(Low

)Fe

npro

path

rin

(High

)

Etox

azol

e (L

ow)

Etoxa

zole

(High

)Pr

opar

gite

Mea

n M

orta

lity

Rat

e +/

-SE

M

A

ABC

AB

C

C

C

2.2a

0

0.2

0.4

0.6

0.8

1

Contro

lFe

npro

path

rin

(Low

)Fe

npro

path

rin

(High

)Eto

xazo

le (L

ow)

Etoxa

zole

(High

)Pr

opar

gite

A

BC B

CCC

Mea

n M

orta

lity

Rat

e +/

-SE

M

2.2b

19

7 D

ays A

fter

T

reat

men

t

Figure 2.2a-c. Mortality of total egg, nymph, and adult TSSM. Treatments with the same letter are not significantly different (ANOVA, Tukey�s test, α=0.05).

0

0.2

0.4

0.6

0.8

1

Contro

lFe

npro

path

rin

(Low

)Fe

npro

path

rin

(High

)Et

oxaz

ole

(Low

)

Etox

azole

(H

igh)

Prop

argit

e

A

BB

B BB

Mea

n M

orta

lity

Rat

e +/

-SE

M

2.2c

20

Figure 2.3a-b. Mortality of individual adult TSSM when exposed to acaricide residues 24 and 72 hours after treatment (χ2, α=0.05).

24 h

ours

aft

er

trea

tmen

t 72

hou

rs a

fter

tr

eatm

ent 3b

0

0.2

0.4

0.6

0.8

1

No Significance

Fenpr

opath

rin

(Low

)

Fenp

ropa

thrin

(High

)

Etoxaz

ole

(Low

)

Etoxaz

ole

(High

)

Prop

argit

e

Mea

n M

orta

lity

Rat

e

Control

Treatment

2.3a

0

0.2

0.4

0.6

0.8

1

No Significance

Mea

n M

orta

lity

Rat

e

Fenpr

opath

rin

(Low

)

Fenp

ropa

thrin

(High

)

Etoxaz

ole

(Low

)

Etoxaz

ole

(High

)

Prop

argit

e

Control

Treatment

2.3b

21

Figure 2.4. Mean hatch rate of TSSM eggs 7 days after treatment with three acaricides. Treatments with the same letter are not significantly different (ANOVA, Tukey�s test, α=0.05).

0

0.2

0.4

0.6

0.8

1

Cont

rol

Fenp

ropa

thrin

(Low

)

Fenp

ropa

thrin

(High

)

Etox

azol

e(L

ow)

Etox

azol

e(H

igh)

Prop

argi

te

A

B

C

D D D

Mea

n H

atch

Rat

e +/

-SE

M

22

Chapter 3: The toxicity of three acarcides to Orius insidiosus. Introduction

Natural enemies are believed to suppress spider mite populations and in many cases are

effective in delaying population buildup (Schoenig & Wilson 1992). They exhibit the capacity

to lower early- to middle- season spider mite abundance. Orius insidiosus (Say) is a common

predator in various cropping systems in the eastern and Midwestern United States (Isenhour,

Wiseman, & Layton 1989). Prey is known to include thrips, mites, whiteflies, leafhoppers, and

lepidopteran eggs and early instars (Barber 1936). A study in Virginia indicated that O.

insidiosus is a potentially important predator that responds numerically and temporally to

Panonychus ulmi (McCaffrey & Horsbugh 1986), a spider mite biologically similar to the

twospotted spider mite (TSSM), Tetranychus urticae. Orius insidiosus exhibits facultative

phytophagy, which is important in biological control as it allows for maintenance of predator

populations during periods of prey scarcity (Wiedenmann & O�Neil 1991; Naranjo & Gibson

1996; Coll 1997). As with the TSSM, temperature can greatly affect the development time of O.

insidiosus. At 35º C, O. insidiosus completes development in as little as three to four days

(Richards & Schmidt 1996). This is comparable TSSM development at approximately the same

temperature. Similar developmental time and the fact that it can feed on other prey and plants in

times of low prey densities, makes O. insidiosus a good candidate for augmentative biological

control against this pest. To combine O. insidiosus with acaricide applications, chemical

residues must be non-toxic to the predators and there must be an acceptable level of tolerance to

direct contact. My objectives were to determine the lethality of three acaricides to adult O.

insidiosus. My specific goals were to determine: 1.) direct toxicity; and 2.) residual toxicity.

Two acaricides commonly used by growers to manage the TSSM, fenpropathrin and propargite,

and one experimental acaricide, etoxazole, were tested. Etoxazole has since been registered for

use in cotton and some fruit crops.


Predator handling

Orius insidiosus were obtained as a mixture of nymphs and adults from Koppert

Biological (Romulus, MI). Upon arrival, they were placed into 20 x 40 x 30-cm Plexiglass

cages. The top of the cage was open and fitted with No-Thrips screening. The cages contained

arthropod-free lima bean plants (Phaseolus lunatus). Cages were kept under mercury vapor light

23

emitting 250 fc, 16:8 hr (L:D) photoperiod. Prey were not given to the O. insidiosus for 24 hours

prior to the experiment.

Direct toxicity

Cuttings were excised from arthropod-free lima bean plants to fit in the bottom of a 15-

mm Petri dish. Ten adult O. insidiosus were placed onto the cuttings inside the Petri dishes. The

dishes were then placed into a freezer for approximately three minutes to immobilize the O.

insidiosus. The O. insidiosus and the bean cuttings were then sprayed with a hand-held aerosol

sprayer under a fume for five seconds with the treatments listed in Table 3.1. Materials were

applied at the manufacturer�s labeled rates for use against TSSM in peanut. For the purposes of

my sprays, the amount of product per 100 ml of water was determined. The Petri dish was then

sealed and kept under the fume hood for 24 hours. After 24 hours, the numbers of dead O.

insidiosus were counted. Two trials were conducted with five replicates for each treatment.

Temperatures during the experiment averaged 27û C, and ranged from 21-34û C. These data

were blocked by trial, transformed by √arcsin and analyzed with ANOVA (SAS Institute). The

means were compared by Tukey�s test.

Residual toxicity

The treatments in Table 3.1 were applied to whole lima bean plants with a hand-held

sprayer under a fume hood until run-off. The plants were left under the fume hood for 30-45

minutes, or until the leaf surface had dried. They were then placed under mercury vapor light,

emitting 250 fc, 16:8 hr (L:D) photoperiod. The plants did not receive overhead watering after

treatment. Twenty10-cm2 diameter leaf disks were excised from the treated plants 24 and 72

hours after treatment. If significant mortality was recorded, then cuttings were also taken at 7

and 14 days after treatment.

Treated leaf disks were put into modified Huffaker cells (Munger 1942; Huffaker 1948).

These cells were made from three 7.6 x 7.6 x 0.6-cm Plexiglass pieces bolted together (Figure

3.1). A 4.5 cm diameter hole cut in the middle layer of Plexiglass created a chamber to house the

leaf disk for the assay.

A single adult O. insidiosus was placed into each cell containing a treated leaf disk. The

housing unit was then sealed and the O. insidiosus was exposed to the acaricide residue for 24

hours. After 24 hours, the O. insidiosus was scored as dead or alive. Dead was assigned to the

insect if no movement was detected after prodding.

24

Two trials of 20 cuttings each were conducted for each acaricide. Temperatures during

the experiment averaged 24û C, and ranged from 22-25û C. These data were analyzed by χ2

analysis and ANOVA (SAS Institute). The means among acaricides were compared with

Tukey�s test.

Results

Direct toxicity

Twenty-four hours after treatment, O. insidiosus mortality varied significantly among the

acaricides tested (Figure 3.2) (F = 68.61, df1 = 6, df2 = 53, P<0.0001). The mortality rates for the

low (54%) and high rates (65%) of etoxazole were not significantly different from one another,

but were significantly higher than the untreated control, which had a mortality rate of 17%, but

significantly lower than the other treatment. Both rates of fenpropathrin and propargite resulted

in 100% mortality 24 hours after treatment and were also significantly higher than the untreated

control.

Residual toxicity

The duration of acaricide residue toxicity varied with the number of days after treatment.

Twenty-four hours after treatment, all of the acaricides tested caused significantly higher

mortality when compared to the control (F = 29.15, df1 = 6, df2 = 5, P<0.0010) (Figure 3.3a).

The low and high rates of fenpropathrin caused 95 and 100% mortality, respectively, while the

control had 2.5% mortality. The two rates of fenpropathrin did not cause significantly different

mortalities when compared to one another. The mortality caused by the high rate of

fenpropathrin was significantly higher than all other compounds tested. The lowest mortality

rate 24 hours after treatment occurred with the low rate of etoxazole, which was 50%. The

mortality rate for the high rate of etoxazole was 60%. Propargite caused a mortality rate of 63%.

Seventy-two hours after treatment, all acaricides, except the high rate of etoxazole,

resulted in significantly higher mortality of O. insidiosus than the untreated control (F=9.94,

df1=6, df2=5, p<0.0117) (Figure 3.3b). Again, the high rate of fenpropathrin caused a mortality

rate of 100%. The mortality rate for the low rate of fenpropathrin was 93%, as was the mortality

rate for propargite. Mortality caused by both the low (60%) and high (30%) rates of etoxazole

were not significantly different from one another.

At 7 days after treatment, the mortality caused by the low and high rates of fenpropathrin

were identical (95%), significantly different from the mortality in the untreated controls, and

25

significantly higher than the other treatments (F=40.96, df1=6, df2=5, p<0.0004) (Figure 3.3c).

Propargite (55%) also caused a significantly different mortality rate compared to the untreated

control. Neither rate of etoxazole caused a significantly different mortality rate when compared

to the untreated control, nor were their mortality rates significantly different from one another.

Fourteen days after treatment, mortality rates caused by all acaricides tested remained

approximately the same (F=68.93, df1=6, df2=5, p<0.0001) (Figure 3.3d). The mortality rates of

the low and high rates of fenpropathrin (95 and 98%, respectively) continued to have

significantly different mortality rates than the untreated control (7.5%). Mortality caused by

propargite and the high rate of etoxazole, 28% for each, was not significantly different than the

untreated controls. The low rate of etoxazole resulted in 38% mortality, which was significantly

different than the control and both rates of fenpropathrin.

Discussion

The objective of this study was to determine the direct and residual toxicity of three

acaricides to O. insidiosus. I wanted to look at how these acaricides would affect O. insidiosus

mortality if it came into direct contact with the acaricide during a spray application and the

mortality of O. insidiosus when exposed to acaricide residues at various days after treatment.

For the purposes of this study, I only considered mortality of O. insidiosus, not sublethal affects.

Little information could be found on the direct toxicity of acaricides against O. insidiosus

because of their high motility. However, if the residual toxicity to O. insidiosus was high, one

can assume that the direct toxicity would follow the same trend. In my study, I found the low

and high rates of fenpropathrin to cause significant mortality to O. insidiosus in both the direct

and residual toxicity tests. A similar study also found fenpropathrin residues to cause significant

mortality to O. insidiosus (Michaud & Grant 2003). The field rate of fenpropathrin (309 ppm)

was used and a survival index of less than ten was found for O. insidiosus. The survival index

ranged from 0-100, where 0 equals no survival and 100 equals 100% survival.

I found that propargite caused significant mortality when O. insidiosus was directly

exposed to the acaricide. Mortality after residual contact was significant when compared to the

control 72 hours after treatment. A similar study found that propargite caused 30-40% mortality

for the first 13 days after treatment when applied at a rate of 5.04 kg [A.I.]/ha (11.11 lb

[A.I.]/acre) (Morse et al. 1987). These results are similar to mine, with the exception of 72 hours

after treatment, where I found 93% mortality. Morse et al. (1987) found less than 20% mortality

26

19 days after treatment, but mortality never dropped below 10% until 90 days after treatment. If

my study ran for longer than 14 days after treatment, my results may have shown similar results.

The mortality at 14 days after treatment had already begun to show a decrease in mortality

compared with 7 days after treatment.

Because etoxazole is a relatively new acaricide, studies on direct or residual toxicity

could not be found. In the direct toxicity tests, I found significantly higher mortality 24 hours

after treatment, when compared to the controls. In the residual toxicity study etoxazole caused

variable mortality of O. insidiosus.

Conclusions

Natural enemies are an important component in a cropping system. They are believed to

suppress spider mite populations and delay population build-up (Schoenig & Wilson 1992).

However, acaricide applications are still often needed to manage mite populations. Because of

their importance, we need to know how various acaricides will affect natural enemy populations.

My study has shown that O. insidiosus, a common predator of the TSSM, is highly

sensitive to certain acaricide applications. In this study, I only studied O. insidiosus mortality.

Some of our results were quite dramatic, but I want to point out that these results were obtained

in a laboratory setting and may vary from those observed under field conditions. The O.

insidiosus were forced to remain on the treated surfaces, whereas in the field, they would most

likely leave the treated area. Other factors in the field could cause a reduction in mortality with

these treatments. Weather is a major issue with application of any type of pesticide in the field.

Rain or excessive heat could reduce the efficacy/toxicity of a product. Another issue is

coverage. In the laboratory, we achieved complete coverage of the lima bean plants; however, in

the field, a reduction in coverage is to be expected. Because of the duration of the acaricide

residues indicated in this study, recolonization of O. insidiosus may be slow, which could lead to

a resurgence of the pest population.

When one is considering an acaricide, they should look at its effectiveness against mites,

but also look at how it will affect the beneficial species. A balance should be met between these

two issues, where mite populations will be effectively managed and natural enemies not be

eradicated from the field. From this study on O. insidiosus, it appears that etoxazole would be a

practical choice. Although mortality was higher than 50% from direct contact, residues do not

seem to persist as long as the other treatments. If residues do not persist for more than a few

27

days, it is likely that the O. insidiosus will return to the field. If this is the case, then they may be

able to suppress the mites that are also returning, thus preventing pest resurgence. However, I

want to point out, that etoxazole has only been registered in cotton and some fruit crops, not in

peanut. This being the case, then propargite would be my second choice as a management

solution. Direct exposure to O. insidiosus caused high mortality; however, the residues did not

persist for more than a few days. Again, this could allow O. insidiosus to return to the field and

help prevent pest resurgence.

28

Table 3.1. Treatments

Common Name Trade Name + Formulation

Labeled Field Rates mg A.I./100mL H2O

0.2 lb a.i. / acre 120.95 Fenpropathrin Danitol® 2.3 EC

(Valent USA Corp., Longwood FL)

0.3 lb a.i. / acre 183.05

Propargite Comite® 6.55

(Crompton, Uniroyal Chemical, Middlebury,

CT)

1.64 lb a.i. / acre 976.10

0.09 lb a.i. / acre 54.00 Etoxazole Zeal�

(Valent USA Corp., Longwood, FL)

0.135 lb a.i. / acre 80.93

Control (Water)

Figure 3.1. Modified Huffaker cell. Three Plexiglass layers are bolted together to create the

housing unit for assays.

7.6 cm

7.6 cm

(0.6 cm thick)

4.5 cm

Bottom Layer Middle Layer Top Layer

Bolt holes for assembling the three layers. Chamber for assay material

29

Figure 3.2. Adult O. insidiosus mortality rate after 24 hours exposure to three acaricides (ANOVA, Tukey�s test, α=0.05).

0

0.2

0.4

0.6

0.8

1

Contro

lFen

propath

rin

(Low

)Fen

prop

athrin

(High

)

Etoxaz

ole(L

ow)

Etoxaz

ole(H

igh)

Proparg

ite

Mea

n M

orta

lity

Ra t

e+ /

-SE

M

C

AA

BB

A

30

24 H

ours

Afte

r T

reat

men

t 72

Hou

rs A

fter

Tre

atm

ent

3.3a

3.3b

0

0.2

0.4

0.6

0.8

1

Control

Treatment

Fenp

ropa

thrin

(Low

)

Fenp

ropa

thrin

(High

)

Etoxa

zole

(Low

)

Etox

azole

(H

igh)

Prop

argit

e

Mea

n M

ort a

lity

Rat

e

****

**

** **

AB A

CBC BC

0

0.2

0.4

0.6

0.8

1

Control

Treatment

Mea

n M

o rta

lity

Rat

e

Fenp

ropa

thrin

(Low

)

Fenp

ropa

thrin

(High

)

Etoxa

zole

(Low

)

Etoxa

zole

(High

)

Prop

argit

e

****

**

**AB A AB

ABC

BC

31

7 D

ays A

fter

Tre

atm

ent

14 D

ays A

fter

Tre

atm

ent

Figure 3.3a-d. Adult mean O. insidiosus mortality 24 hr, 72 hr, 7 and 14 days after treatment when exposed to acaricide residues. Treatments with the same letter are not significantly different (ANOVA, Tukey�s test, α=0.05). ** Indicates significant difference compared to untreated controls

(χ2analysis, α=0.05).

3.3c

3.3d

0

0.2

0.4

0.6

0.8

1

Control

Treatment

Mea

n M

orta

lity

Rat

e

Fenp

ropa

thrin

(Low

)

Fenp

ropa

thrin

(High

)

Etoxa

zole

(Low

)

Etox

azole

(H

igh)

Prop

argit

e

** **

**

A A

BB

B

0

0.2

0.4

0.6

0.8

1

Mea

n M

ort a

lity

Rat

e

Fenp

ropa

thrin

(Low

)

Fenp

ropa

thrin

(High

)

Etoxa

zole

(Low

)

Etoxa

zole

(High

)

Prop

argit

e

Control

Treatment

** **

**

A A

BCBC

B

32

Chapter 4: Functional response of Orius insidiosus with Tetranychus urticae as prey Introduction

Orius insidiosus (Say) is an important natural enemy of numerous insect pests (Ruberson,

Bush & Kring 1991). Some prey includes thrips, spider mites, and aphids (Zhang & Shipp

1998). Orius insidiosus finds prey by touch, then grabs it with their front legs, inserts their

rostrum and drain the prey of its body fluids (Koppert Biological Systems 2003). They

sometimes kill more insects than necessary for their own feeding. To quantify the impact of

these anthocorid predators, it is important to assess their functional and numerical response to

prey density (van den Meiracker & Sabelis 1999). Three types of functional responses are

described by Holling (1959a,b). Type I functional responses occur when there is a linear

increase in the maximum number of prey eaten per predator as prey density increases. Type II

occurs when the predation rate increases to a maximum at a decreasing rate. This has long been

considered as typical for insect predators. A Type III response is sigmoid and approaches an

upper asymptote at satiation. All of the three types of functional responses are characterized by

the presence of a plateau at high prey densities (Holling 1959a,b). The type II functional

response is described by the Hollings disk equation,

Na = a�TN (1)

1+ a�ThN

where Na is the prey attacked per predator per time period T, at a given prey density N; Th is

handling time; and a� is the attack rate.

When the predator is limited by time, the plateau is determined by the ratio of the total

time and the time spent handling a prey (van den Meiracker & Sabelis 1999). In many

predatory arthropods handling time comprises only a small proportion of the total time available.

Handling times in Orius spp. are relatively short (Isenhour & Yeargan 1981a). Given this,

Orius could have a maximum predation rate on adult thrips of 76-160 per day (Isenhour &

Yeargan 1981a). Orius predatory bugs often consume their prey only partially, and sometimes

do not even feed on prey they have killed (Rajasekhara & Chatterji 1970; Askari & Stern 1972;

Isenhour & Yeargan 1981b), thus resulting in a reduction in handling time.

33

My objectives were to determine the predation functional response of O. insidiosus to the

TSSM. In the laboratory studies, various numbers of mites were exposed to a single O.

insidiosus. Cage greenhouse studies were used to test predictions from the laboratory tests.


Functional response: Laboratory studies

Twospotted spider mite colonies were maintained on lima beans (Phaseolus lunatus) in

rearing cages at 28û C and 16:8 hr (L:D) photoperiod. The individuals used to initiate the

colonies were collected from infested weeds. Each rearing cage was a 20 x 40 x 30-cm

Plexiglass box with an open top, fitted with No-Thrips screening. Double sided tape was placed

along the outside rim and petroleum jelly along the inside rim to prevent mite escape and

contamination of the colonies.

Experimental arenas were constructed by placing a 10-cm2-bean leaf disk excised from

uninfested bean plants on top of a small piece of clay in the bottom of a 15-mm Petri dish. The

dish was filled with enough water to float the disk (Figure 4.1). Orius insidiosus were obtained

as a mixture of adults and nymphs from Koppert Biological (Romulus, MI). Upon arrival, they

were placed into cages similar to the rearing cages for the TSSM. Prey were not given to the O.

insidiosus for 24 hours prior to the experiment

Twospotted spider mite eggs or adults were placed onto leaf disks at densities of 5, 10,

20, 40, and 60 per disk (Figure 4.1). Adult TSSM were transferred directly to the arenas from

the colony on the day of the experiment. Eggs were obtained by placing 25 adult, female TSSM

on each of five pest free bean leaves inside a 15-mm Petri dish for 48 hours at 30û C prior to the

experiment. On the day of the experiment, eggs were transferred from the bean leaves inside the

Petri dish to the arenas. Five replicates were conducted for each prey density. The arenas were

left undisturbed for at least one hour. They were then examined to determine if any eggs had

hatched or adults had laid eggs. One O. insidiosus was placed into the center of each of the leaf

disk and allowed to feed for eight hours. Eggs were removed from adult arenas every hour for

the duration of the experiment. Egg arenas were inspected hourly for the presence of newly

hatched TSSM. After eight hours, the numbers of adult TSSM cadavers were counted in the

adult arenas and the numbers of viable eggs were counted in the egg arenas to determine how

much prey had been consumed/killed. Temperature and relative humidity were recorded hourly.

34

Data were analyzed using least squares regression by fitting a line through the data points as

predicted by the descriptive functional response equation:

)]exp(/[1 cXXaY

tT

h

t

′+= (2)

described by Fujii et al. (1986). In equation (2), Y is the number of attacks per predator Tt, the

total time prey is exposed, X is the prey density, th is the handling time per prey, a� is the rate of

successful search, and c is the facilitation coefficient, which determines the form of the

functional response. When c = a� th the functional response is Pseudo Type I, c = 0 gives a Type

II response, for c > a� th the response is Type III, and c<0, the response is Type IV (Fujii et al.

1986). The data that were collected were fitted to equation (2) using the curve fitting software

TableCurve 2D version 5 for Windows (Systat Software Inc., Richmond, CA.).

The results of the lab study were used to determine the number of TSSM and O.

insidiosus to use in the functional response cage studies described below.

Functional response: Greenhouse studies

Peanuts were grown in a peat based soil-less medium in 3.785-liter plastic pots. No

pesticides were applied to stock plants. On the day of the experiment, peanut leaves were

removed from the plants so that only eight leaflets remained on each plant. Twospotted spider

mite colonies were maintained as previously described. Three densities (5, 20, or 60) of adult

TSSM were then placed onto the peanut plants. Once the TSSM were transferred to the peanuts,

a single adult O. insidiosus was placed onto the plant and nylon screening was placed over the

top of the plant. The screening was 3.785-liter paint strainers with elastic around the bottom

edge. The elastic edge was placed around the top of the pot. This prevented the escape of the

TSSM and O. insidiosus and also kept out unwanted arthropods, which could serve as additional

prey. The O. insidiosus were allowed to feed on the TSSM for 24 hours. After 24 hours, the

screening was removed and the plant was cut at the soil line. The peanut plant was then placed

into a plastic bag and placed into a freezer for 24 hours. This arrested further development and

prevented further movement of the TSSM so as to facilitate counting. Freezing as a technique to

arrest development did not appear to cause any of the symptoms associated with predation. After

24 hours, the numbers of �viable� TSSM were counted for each cut peanut. The TSSM were

considered viable if they did not show signs of O. insidiosus predation. These signs would

35

include leaking of fluids, drawn up bodies, or desiccated corpses. Four replicates for each TSSM

density were tested and two trials were conducted. Data were analyzed as described above.

Results

Laboratory studies

The functional response of adult O. insidiosus to eggs of the TSSM was found to be a

Type III curve and the functional response to TSSM adults was a Type II curve. The type of

functional response curve was determined using the facilitation coefficient (c) from the

descriptive functional response equation (2) described by Fujii et al. (1986). The calculations

were performed by fitting Holling�s disk equation to the data points to derive the search rate and

handling time. Then, these terms were substituted into the descriptive equations to derive a c-

value, which was used to interpret the type of functional response curve. At a density of 60 prey

per arena, O. insidiosus attacked an average of 32 eggs and 43 adults over an eight-hour period.

Total calculated handling times were 13.9 minutes for eggs and 4.8 minutes for adults (Figures

4.2 and 4.3).

Greenhouse studies

The functional response of O. insidiosus on adult TSSM in the greenhouse studies was

also assessed. At a density of 60 TSSM per cage, O. insidiosus attacked 53 TSSM over a 24-

hour period (Figure 4.4). The calculated handling time of TSSM adults was 5.4 min/adult.

Although the result (i.e., the c-value) suggests that this was a Type III functional response, the

data set was too small to make a definitive conclusion. One could look at the graph and easily

assume that the functional response was linear. Predation by Orius on higher densities of TSSM

that were used in this study (i.e., >60 mites/cage) needs to be assessed.

Discussion

In the laboratory studies with TSSM eggs, we found a Type III functional response. It is

generally assumed that sigmoid functional responses are more characteristic of vertebrate than

invertebrate predators or parasitoids (Hassel et al. 1977). However, there are several examples

showing sigmoid responses with invertebrates. Single-prey systems may be a function of

aggregative behavior (McCaffrey & Horsburgh 1986). Examples of this behavior can be seen

with O. tristicolor (White) (Sheilds & Watson 1980) and Anthocoris confuses (Reuter) (Evans

1976). These hemipteran predators show increases in the frequency of turning movement

following feeding and confine their search in a limited area until a threshold time. If they do not

36

find anymore prey, they will straighten out their search path. Once an area of high prey density

is found, they will exploit it. This may explain the sigmoid functional response we found with

O. insidiosus with TSSM eggs as prey. The eggs were placed randomly around the leaf disk in

groups of five or ten. Once the O. insidiosus found a group of eggs, they may have fed there

until those eggs were mostly consumed, then they would move on to find other prey.

In the laboratory studies with adult TSSM as prey, we found a Type II functional

response, which is what one generally assumes will occur with invertebrate predators. In another

study with O. insidiosus and western flower thrips, results implied that O. insidiosus had a

maximum predation rate of 76-160 thrips per day (Isenhour & Yeargan 1981). In my study, at a

density of 60 TSSM per leaf disk, a single O. insidiosus killed an average of 43 TSSM adults in

eight hours. It is thought that the high rate of predation for O. insidiosus may be due to the fact

that they do not completely consume their prey and therefore the gut capacity would not be

reached and satiation would not occur (van den Meiracker & Sabelis 1999). This would reduce

the handling time per prey, thereby increasing the number of prey they can kill in one day.

Our data for the greenhouse studies suggest a Type II functional response; however, one

can argue that it was more of a linear response. The data set is too small to make a definitive

conclusion. More TSSM densities need to be added to accurately determine the functional

response in a cage study. However, we did find results with the greenhouse studies that are

consistent with O. insidiosus predation in the adult studies conducted in the laboratory. At a

density of 60 TSSM per peanut plant, 43 TSSM were killed in the laboratory, and an average of

52.5 TSSM were killed in the greenhouse studies. This number may seem large, but in this case,

the O. insidiosus were given 24 hours to find prey, instead of eight, because they in were a large

experimental �arena�. Again, the explanation offered above concerning gut capacity and

satiation could work in this situation as well. Because the O. insidiosus do not fully consume

their prey, they do not reach satiation, and therefore they can attack more prey.

Conclusions

It would appear from the results that O. insidiosus is an excellent predator of TSSM. It is

capable of killing large numbers of mites per day. However, in a field infestation of TSSM, O.

insidiosus may not show the same feeding behavior as in the laboratory. There are several

reasons for this. The O. insidiosus used in these studies were obtained commercially. They were

not given prey for 24 hours prior to the experiment, so they were very hungry when exposed to

37

the mites. Field-dwelling O. insidiosus would have more choices of prey and would most likely

not be starved. Orius insidiosus exhibits generalist feeding behavior, so they do not feed on just

one species of prey. In an agroecosystem, prey from several families are available to the Orius.

The TSSM may not be the preferred prey of O. insidiosus. Even though the TSSM may be in

abundance, if the Orius can feed on something else that it prefers, the TSSM will not be preyed

upon at the same rate as it would if it were the only prey available.

38

Arena Design Egg Placement

.

15-mm Petri dish with water

10-cm2-leaf disk

5 or 10

20

40

60

Figure 4.1. Functional response assay arenas. Eggs or adults: 5, 10, 20, 40, and 60 per leaf disk (10 disks per treatment).

Groups of 10 eggs

Leaf disk

39

Figure 4.2. The functional response of commercially available adult O. insidiosus on TSSM eggs in laboratory studies. Search rate (a�) = 0.058, handling time (Th) = 0.232 hr/egg, and c = 0.049. (R2 = 0.9914).

0

5

10

15

20

25

30

35

0 20 40 60 80

TSSM Eggs: Laboratory studies

Num

ber

Att

acke

d

Observed

Predicted

TSSM Density

40

Figure 4.3. The functional response of commercially available adult O. insidiosus on TSSM adults in laboratory studies. Search rate (a�) = 0.15, handling time (Th) = 0.08 hr/adult, and c =1e -09. (R2 = 0.9792).

0

10

20

30

40

50

0 20 40 60 80

Observed

Predicted

TSSM Adults: Laboratory studies

Num

ber

Att

acke

d

TSSM Density

41

0

20

40

60

10 30 50 70

Because the predicted values are so close to the observed values, they are directly behind the observed line

Observed

Predicted

Num

ber

Atta

cked

Adult TSSM: Greenhouse studies

Figure 4.4. The functional response of commercially available O. insidiosus adults on TSSM adults in greenhouse studies. Search rate (a�) = 0.083, handling time (Th) = 0.09 hr/adult, and c=0.02. (R2 = 0.9999).

TSSM Density

42

Chapter 5: A preliminary survey of Orius associated with different densities of Tetranychus urticae.

Introduction

Natural enemies are believed to suppress spider mite populations and in many cases are

effective in delaying population buildup (Schoenig & Wilson 1992). They exhibit the capacity

to lower early to middle season spider mite abundance. Orius insidiosus (Say) is a common

predator in various cropping systems in the eastern and Midwestern United States (Isenhour,

Wiseman, & Layton 1989). Prey are known to include thrips, mites, whiteflies, leafhoppers, and

lepidopteran eggs and early instars (Barber 1936). A study in Virginia indicated that O.

insidiosus is a potentially important predator that responds numerically and temporally to

Panonychus ulmi (McCaffrey & Horsbugh 1986), a spider mite biologically similar to the

twospotted spider mite (TSSM), Tetranychus urticae Koch. Orius insidiosus exhibits facultative

phytophagy, which is important in biological control as it allows for maintenance of predator

populations during periods of prey scarcity (Wiedenmann & O�Neil 1991; Naranjo & Gibson,

1996; Coll 1997). As with the TSSM, temperature can greatly affect the development time of O.

insidiosus. At 35º C, O. insidiosus completes development in as little as three to four days

(Richards & Schmidt 1996). This is comparable to TSSM development at approximately the

same temperature. Similar developmental temperatures and the fact that it can feed on other prey

and plants in times of low prey densities, make O. insidiosus a candidate for augmentative

biological control against this pest.

My objective was to determine the abundance of O. insidiosus in a naturally occurring

infestation of TSSM in peanut and to determine the relationship between mite density and O.

insidiosus density.


Peanut fields in southeastern Virginia were surveyed for the presence of the TSSM. Field

surveys to determine the presence of the TSSM began around the first of July, which is when the

TSSM is commonly be found in peanut. Once the TSSM populations were found in the peanut

fields, the level of infestation was determined. For the purposes of this study, TSSM populations

were classified into three levels: low, medium, and high. A low level infestation was considered

to be an average of five adult TSSM or fewer per leaf, a medium level infestation was 10 to 20

adult TSSM per leaf, and a high level infestation was more than 20 adult TSSM per leaf.

43

Sampling began on July 12, 2002 and concluded on August 14, 2002. Thirteen fields

were sampled throughout this experiment. In each of the fields, the areas were located where a

low, medium, or high levels of TSSM existed. Then, in each area, two sampling procedures

were initiated. Beat sheet samples were taken from 0.91 meters of row length. Beat sheet

samples were taken by placing a 0.91-meter long beat sheet on the ground between two rows of

peanut, then reaching halfway across the plants bordering the sheet on one side and shaking the

foliage vigorously towards the middle. This was repeated on the opposite row. Two beat sheet

samples were taken in each of the three levels of mite infestation. Any predatory arthropods

found on the beat sheets were placed into alcohol for later identification. Because the plants in

some fields were small due to drought conditions, beat sheet samples were not possible in all of

the fields. In each infestation level, two soapy water samples were taken. For soapy water

samples, 10 leaves were taken from 10 different randomly selected plants and placed into a 473-

ml jar containing mildly soapy water. The soapy water samples were examined under a

microscope in the laboratory and all predatory arthropods were placed into alcohol. The alcohol

vials were later examined and the numbers of O. insidiosus and other predatory arthropods were

recorded.

Results

In this study, the densities of O. insidiosus were compared against TSSM densities

(Figure 5.1). When these data were collected, drought conditions were evident in all locations of

sampling. This, in combination with other factors, led to an explosion of TSSM numbers. In

most cases, only the high level population of TSSM could be found. This caused unequal

sampling among the TSSM densities. Because of this, only the means and standard errors were

used to describe these data.

In the high level populations of TSSM, the number of Orius ranged from 0 to 11 per

sample location, with a mean of 1.12. In the medium level population, no Orius were found. In

the low level population of TSSM, the number of Orius found ranged from 0 to 14 per sample

location, with a mean of 3.22.

Discussion

A similar study was conducted by Wilson et al. (1991). The objective of their study was

to assess the relationship between spider mites and natural enemies during the growing season of

cotton. The natural enemies they sampled for were all generalist and included Frankliniella

44

occidentalis, Orius tristicolor, and Geocoris spp. They found that when these predators were

found during the early part of the growing season, the greater the reduction in TSSM population

explosions. The abundance of mid-to late-season spider mites also decreased. In early season

populations of spider mites, the numbers are generally low. In our study, we found that the

highest numbers of Orius occurred in the low level populations of TSSM. This would indicate

that Orius is capable of suppressing low numbers of TSSM. Because higher numbers of Orius

were found in the low level population, there would be a higher ratio of Orius to the TSSM. In

the high level population, there would be a smaller ratio of Orius to the TSSM.

Conclusions

One Orius is capable of consuming 30 or more spider mites per day, however, they seem

to be more effective at managing TSSM populations when the mite numbers are low. There are

several possible explanations for this. The TSSM could reproduce at a higher rate than the

Orius. This would allow the mite numbers to explode rapidly, while the Orius cannot reproduce

fast enough. Also, Orius is an omnivorous predator. This is important when prey numbers are

low because it allow for predator maintenance. However, this could also explain why they do

not seem to be able to manage TSSM effectively. There could be some other prey that the Orius

prefers over the TSSM. If the mite numbers are low, the Orius will feed on other prey, allowing

the few TSSM that are on the peanut to reproduce rapidly.

More studies need to be undertaken before the true nature of the Orius-TSSM complex is

fully understood. New studies could determine if prey preference or reproduction rates are the

cause behind why Orius do not seem to be able to manage high TSSM numbers.

45

0

1

2

3

4

5

6

High

Med

ium Low

Mea

n N

umbe

r O

rius

±SE

M

Figure 5.1. Mean number of Orius found per TSSM population level.

46

Summary I found significant differences in mortality caused by all acarcides compared to the

untreated controls when the acaricides were tested on a mixed-stage population of TSSM. At 72

hours and 7 days after treatment, the mortality with all of the compounds was significantly

higher than the untreated controls. Differences among the treatments occurred at 24 and 72

hours after treatment. At each day after treatment (24 and 72 hours), mortality rates for both

rates of fenpropathrin were significantly different from propargite and both rates of etoxazole.

However, at 7 days after treatment, there were no significant differences in mortality among the

treatments. Mortality for all treatments increased with each day after treatment. There are a

couple of reasons for this. Egg toxicity could have occurred with the compounds that held the

populations in check after treatment. Also, nymphs would be more susceptible to acaricide

residues than adults. The residues from the sprays could have been toxic to emerging nymphs,

resulting in increasing mortality over the course of 7 days.

The duration of acaricide residue toxicity to adult TSSM did not vary significantly among

the acaricides or rates tested at either 24 or 72 hours after treatment. At both days, all

compounds resulted in less than 30% mortality.

The egg toxicity tests produced some interesting results. Seven days after treatment, the

untreated control had a hatch rate of 82%. This is significantly higher than the acaricides I

tested. The low and high rates of fenpropathrin were not significantly different from each other,

but their mortality rates were significantly lower than the control. Propargite and both rates of

etoxazole resulted in a 0% hatch rate. This supports the explanation made for the direct toxicity

tests. If they prevented egg hatch for up to 7 days after treatment, this would have caused the

mortality rates to continue to increase in the direct toxicity tests.

In the direct toxicity tests against O. insidiosus, significant mortality was found with all

acaricides tested. The low and high rates of etoxazole caused the lowest mortality rates, but they

were still significantly higher than the untreated controls. Both rates of fenpropathrin and

propargite were found to be highly toxic to O. insidiosus, causing 100% mortality at 24 hours

after treatment.

The duration of acaricide residue toxicity to O. insidiosus varied with the number of days

after treatment. At 24 hours after treatment, all treatments caused significantly higher mortality

compared to the untreated controls. Both rates of fenpropathrin caused the highest mortality

47

(higher than 80%). At 72 hours after treatment, all acaricides caused significantly higher

mortality than the control, with the exception of the high rate etoxazole. Propargite and both

rates of fenpropathrin did not have significant differences in their mortality. At seven days after

treatment, only propargite and both rates of fenpropathrin resulted in significant mortality when

compared to the untreated controls. Mortality for both rates of etoxazole was significantly lower

than all other acaricides tested. At 14 days after treatment, mortality rates for both rates of

etoxazole and propargite were significantly different from both rates of fenpropathrin. The low

and high rates of fenpropathrin continued to cause significantly higher mortality compared to the

untreated controls, as well as the low rate of etoxazole.

The functional response studies showed that O. insidiosus is an effective predator of the

TSSM. The data suggests the functional response of O. insidiosus to TSSM eggs is a type III

response. The laboratory data on the functional response to TSSM adults suggests a type II

response. The same response was suggested from the data in the greenhouse studies, however I

could not draw a definitive conclusion from the data, which could also be interpreted as a linear

response. The data set is too small to form a set conclusion.

In the comparative studies of Orius and TSSM densities, higher numbers of Orius were

found in the low-level populations of TSSM. While this indicates that Orius are more abundant

in low levels, other factors could be involved. More studies need to be conducted to determine

how and why Orius may be more capable of subduing low numbers of TSSM versus high

numbers.

The information gained from this study could be used to improve recommendations made

to growers in the management of the TSSM. Having an understanding of not only how toxic

acaricides are to different life stages of TSSM, but also how toxic these acaricides are to O.

insidiosus could help improve management practices. For instance, crop advisors cold

recommend a product that would provide maximum management of different life stages of

TSSM, as well as offering low toxicity to O. insidiosus. Etoxazole would be a good choice. It is

effective at managing most life stages of TSSM and its residual toxicity does not last as long as

other products. Etoxazole would manage the TSSM population, as well as allowing O.

insidiosus populations to return to the crop. Understanding how effective O. insidiosus alone is

at managing TSSM numbers could also improve management recommendations. If we know O.

insidiosus is most effective in low-level populations of TSSM, we could then allow the O.

48

insidiosus to feed until the TSSM population begins to increase, then an acaricide application

could be made.

Even though this study has provided many useful insights into chemical and biological

control of the TSSM, the information gained here can be expanded into new research projects.

Field studies to research the toxicity of acaricides on O. insidiosus, or other beneficial species, in

naturally occurring populations of TSSM could be done. Also, studies could be done to identify

other beneficial species that feed on the TSSM. Once these are found, pertinent questions could

be answered, such as 1.) Which beneficial species are most prevalent?; 2.) which ones are most

effective at managing TSSM numbers?; and 3.) how toxic are acaricides on these other beneficial

species? The more research done and the more knowledge we have on interactions between

beneficial species, the TSSM, and acaricide applications the better our management

recommendations will be.

49

Literature cited Askari, A. & V.M. Stern. 1972. Biology and feeding habits of Orius tristicolor

(Hemiptera: Anthocoridae). Ann. Entomol Soc. Am. 65: 96-100. Armer, C.A., R.N. Wiedenmann & D.R. Bush. 1997. Plant feeding site selection on

soybean by the facultatively phytophagous predator Orius insidiosus. Entomologia Experimentalis et Applicata 86: 109-118.

Barber, G.W. 1936. Orius insidiosus (Say), an important natural enemy of the corn earworm. USDA Technical Bulletin 504. Bartlett, B.R. 1968. Outbreaks of two-spotted spider mites and cotton aphids following

pesticide treatment. I. Pest stimulation vs. natural enemy destruction as the cause of outbreaks. J. Econ. Entomol. 61: 297-303.

Blair, B.W. 1989. Laboratory screening of acaricides against Tetranychus evansi Baker & Pritchard. Crop Protection 8: 212-216. Borror, D.J., C.A. Triplehorn & N.F. Johnson. 1989. An Introduction to the Study of

Insects. Saunders College Publishing. 875pp. Brandenburg, R.L. & G.G. Kennedy. 1987. Ecological and agricultural considerations in the

management of twospotted spider mite (Tetranychus urticae Koch). Agricultural Zoology Reviews 2:185-236.

Brown, G.C. & C.H. Shanks, Jr. 1976. Mortality of two spotted spider mite predators caused

by the systemic insecticide, carbofuran. Environ. Entomol. 5: 1155-1159. Brown, S.L., D. C. Jones & J.W. Todd. 1995. Peanut insects. <www.bugwood.caes.uga/s195/peanut.html > Accessed 2002 Feb. 12. Cagle, L.R. 1949. Life history of the two-spotted spider mite. Virginia Agric. Exp. Sta. Tech.

Bull. 113: 31pp. Campbell, W.V., R. W. Batts, R.L. Robertson & D.A. Emery. 1974. Suppression of the

twospotted spider mite on peanuts. Peanut Science 1: 30-34. Coll, M. 1997. Feeding and living on plant in predatory Heteroptera. In: M. Coll & J.R.

Ruberson, (eds.), Predatory Heteroptera in agroecosystems: Their ecology and use in biological control. Thomas Say Publications in Entomology, Lanham, MD.

Coll, M. 1996. Feeding and ovipositing by an omnivorous insect predator. Oecologia 105:

214-220. Evans, H.F. 1976. The searching behavior of Anthocoris confuses (Reuter) in relation to prey

50

density and plant surface topography. Ecol. Entomol. 1: 163-169. Fasulo, T.R. & H.A. Denmark. (August 2000). Twospotted spider mite, Tetranychus

urticae Koch. UF/IFAS Featured Creatures EENY-150. Fujii, K., C.S. Holling, & P.M. Mace. 1986. A simple generalized model of attack by

predators and parasites. Ecol. Res. 1: 141-156. Hassell, M.P., J.H. Lawton & J.R. Beddington. 1977. Sigmoid functional responses by

invertebrate predators and parasitoids. J. Anim. Ecol. 46: 249-262. Hazan, A., U. Gerson & A.S. Tahori. 1974. Spider mite webbing. I. The production of

webbing under various environmental conditions [Tetranychus cinnabarinus]. Acarologia 16(1): 68-84.

Helle, W. & M.W. Sabelis. 1985. Spider mites: their biology, natural enemies and control.

Volume 1B. Elsevier Amsterdam. 458 pp. Herbert, D.A., Jr. 1999. Peanut production guide: Insect control in peanuts. Tidewater

Agricultural Research and Extension Center. http://vaes.vt.edu/tidewater/peanut. Herbert, D.A., Jr. 1998. Insect pest management in Virginia peanuts, soybean, and cotton.

Tidewater Agric. Res. and Ext. Ctr. Info. Ser. No. 410. TAREC, Suffolk, VA. Hernon, G., V. Edge & J. Rophalia. 1993. Clofentezine and hexythiazox resistance in

Tetranychus urticae Koch in Australia. Exp. Appl. Acarol. 17: 433-440. Holling, C.S. 1959a. The components of predation as revealed by a study of small

mammal predation of the European pine sawfly. Can. Entomol. 91: 293-320.

Holling, C.S. 1959b. Some characteristics of simple types of predation and parasitism.

Can. Entomol. 91: 385-398. Huffaker, C.B. 1948. An improved cage for work with small insects. J. Econ. Entomol. 41: 648-649. Huffaker, C.B., M. van de Vrie & J.A. McMurty. 1969. The ecology of tetranychid mites

and their natural control. Ann. Rev. Entomol. 14: 125-174. Iftner, D.C. & F.R Hall. 1984. The effects of fenvalerate and permethrin residues on

Tetranychus urticae Koch fecundity and rate of development. J. Agric. Entomol. 1: 191 200.

Insecticide Resistance Action Committee. 2003. IRAC Mode of Action Classification v. 33. http://www.irac-online.org/documents/moa/moa.pdf.

51

Isenhour, D.J., B.R. Wiseman & R.C. Layton. 1989. Enhanced predation by Orius insidiosus

(Hemiptera: Anthocoridae) on larvae of Heliothis zea and Spodoptera frugiperda (Lepidoptera: Noctuidae) caused by feeding on resistant corn genotypes. Environ. Entomol. 18(3): 418-422.

Isenhour, D.J. & K.V. Yeargan. 1981a. Interactive behavior of Orius insidiosus [Hem.: Anthocoridae] and Sericothrips variabilis [Thys: Thripidae]: predator searching strategies and prey escape tactics. Entomophaga 26: 213-220.

Isenhour, D.J. & K.V. Yeargan. 1981b. Predation by Orius insidiosus on the soybean

thrips, Sericothrips variabilis: Effect of prey stage and density. Environ. Entomol. 10: 496-500.

Koppert Biological Systems. 2003. Pirate bugs. http://www.koppert.com Accessed 2003 Jan 1. Laing, J.E. 1969. Life history and life table of Tetranychus urticae Koch. Acarologia 11:32

42. Lindquist, R.K. 1999. Non-pests (beneficials) of the month: Other Important Predators.

http://floriculture.osu.edu/archive/dec99/benefit.html. Marshall, D.B. & D.J. Pree. 1991. Effects of miticides on the life stages of the European red mite, Panonychus ulmi (Koch) (Acari: Tetranychidae). Can. Entomol. 123: 77-87. McCaffrey, J.P. & R.L. Horsburgh. 1986. Biology of Orius insidiosus (Heteroptera:

Anthocoridae): A predator in Virginia apple orchards. Environ. Entomol. 15: 984-988. Michaud, J.P. & A.K. Grant. 2003. IPM-compatibility of foliar insecticides for citrus:

Indices derived from toxicity to beneficial insects from four orders. J. Insect Sci. 3:18, Available online: insectscience.org/3.18.

Mitchell, R. 1973. Growth and population dynamics of a spider mite (Tetranychus urticae K.,

Acarina: Tetranychidae). Ecology 54: 1349-1355. Mitchell, R. 1970. An analysis of dispersal in mites. The American Naturalist 104: 425-431. Morse, J.G., T.S. Bellows, Jr., L.K. Gaston & Y. Iwata. 1987. Residual toxicity of acaricides to three beneficial species on California citrus. J. Econ. Entomol. 80: 953-960. Munger, F. 1942. A method for rearing citrus thrips in the laboratory. J. Econ. Entomol. 35: 373-375. Naranjo, S.E. & R.L. Gibson. 1996. Phytogaphy in predaceous Heteroptera: Effects on life

history and population dynamics. In: O. Alomar & R.N. Wiedenmann (eds.),

52

Zoophytophagous Heteroptera: Implications for life history and integrated pest management. Thomas Say Publications in Entomology, Lanham, MD, pp. 57-93.

Owens, B. 1996. Information. http://aboutpeanuts.com/infova.html. Potter, D.A., D.L. Wrensch & D.E. Johnston. 1976. Guarding, aggressive behavior, and

mating success in male twospotted spider mites. Ann. Entomol. Soc. Am. 69: 707-711. Pritchard, A.E. & E.W. Baker. 1955. A revision of the spider mite family Tetranychidae.

Vol. 2 The Pacific Coast Entomol. Soc. (436). Rajasekhara, K. & S. Chatterji. 1970. Biology of Orius insidiosus (Hemiptera:

Anthocoridae), a predator of Taeniothrips nigricornis (Thysanoptera). Ann. Entomol. Soc. of Am. 63: 364-367.

Richards, P.C. & J.M. Schmidt. 1996. The suitability of some natural and artificial substrates

as oviposition sites for the insidiosus flower bug, Orius insidiosus. Entomologia Experimentalis et Applicata 80: 325-333.

Ruberson, J.R., L. Bush & T.J. Kring. 1991. Photoperiodic effect on diapause induction an

development in the predator Orius insidiosus. (Heteroptera: Anthocoridae). Environ. Entomol. 20: 786-789.

Sabelis, M.W. 1981. Biological control of two-spotted spider mites using phytoseiid predators. Part 1. Modeling the predator-prey interaction at the individual level. Agricultural Research Reports No. 910. Wageningen. SAS Institute. 1992. SAS/STAT user�s guide. SAS Institute, Cary, N.C. Schiffhauer, D.E. & R.F. Mizell. 1988. Behavorial response and mortality of nursery populations of twospotted spider mite (Acari: Tetranychidae) to residues of six

acaricides. J. Econ. Entomol. 81(4): 1155-1162. Schoenig, S.E. & L. T. Wilson. 1992. Patterns of spatial association between spider mites (Acari: Tetranychidae) and their natural enemies on cotton. Environ. Entomol. 21(3): 471-477. Shearer, P.W. 2000. Pyridaben baseline assays for adult female European red mite (Acari:

Tetranychidae) and eggs. J. Agric. Urban Entomol. 17(2): 65-69. Shields, E.J. & T.F. Watson. 1980. Searching behavior of female Orius tristicolor. Ann.

Entomol. Soc. Am. 73: 533-535. Smith, J.C. & R.W. Mozingo. 1983. Effect of twospotted spider mites (Acari: Tetranychidae)

on large-seeded, Virginia-type peanuts. J. Econ. Entomol. 76: 1315-1319.

53

Suski, Z.W. & J.A. Naegele. 1963. Light response in the two-spotted spider mite. II. Behavior of the �sedentary� and �dispersal� phases. Advances in Acarology 1: 193 198.

Thomas, C. 2001. Biological control of twospotted spider mite. Integrated Pest Management Program, Pennsylvania Department of Agriculture. http://paipm.cas.psu.edu/BugAgBug/BAB-July2001.html.

Trumble, J.T. & J.P. Morse. 1993. Economics of integrating the predaceous mite Phytoseiulus persimilis (Acari: Phytoseiidae) with pesticides in strawberries. J. Econ. Entomol. 86: 879-885.

Trumper, E.V. & J. Holt. 1998. Modeling pest population resurgence due to recolonization of fields following an insecticide application. J. of App. Ecology 35: 273-285. Valent USA Corp. 2001. What is etoxazole? Unpublished data. Longwood, FL. van den Meiracker, R.A.F. 1994. Induction and termination of diapause in Orius predatory

bugs. Entomologia Experimentalis et Applicata 73: 127-137. van de Vrie, M., J.A. McMurty & C.B. Huffaker. 1972. Ecology of tetranychid mites and

their natural enemies: a review. III. Biology, ecology, and pest status and host-plant relations of tetranychids. Hilgardia 41: 343-432.

Ware, G.W. 1994. The Pesticide Book. Fresno, CA: Thomson Publications, 384 pp. Weeks, A.R., M. Turelli & A.A. Hoffman. 2000. Dispersal patterns of pest earth mites

(Acari: Penthaleidae) in pastures and crops. J. Econ. Entomol. 93: 1415-1423. Wiedenmann, R.N. & R. J. O�Neil. 1991. Searching behavior and time budgets of the

predator Podisus maculiventris. Entomologia Experimentalis et Applicata 60: 83-93. Wilson, L.T., P.J. Trichilo & D. Gonzalez. 1991. Natural enemies of spider mites (Acari:

Tetranychidae) on cotton: Density regulation or casual association? Environ. Entomol. 20(3): 849-856.

Wright, B. 2001. Know Your Friends: Minute Pirate Bugs.

http://www.entomology.wisc.edu/mbcn/kyf101.html Zhang, Y. & J.L. Shipp. 1998. Effect of temperature and vapor pressure deficit on the flight

activity of Orius insidiosus (Hemiptera: Anthocoridae). Environ. Entomol. 27(3): 736 742.

54

Vita

Janet L. Ashley

I was raised on a family farm in Suffolk, VA. Over the years, our area of production has

been row crops, such as peanut, cotton, soybeans, wheat, and corn. In December of 2000, I

received a B.S. degree in Horticulture. During my studies and work experiences, I was constantly

exposed to entomology. I decided to pursue a Master�s degree in Entomology in the fall of 2001

at Virginia Tech. I would like to pursue a career that would allow me to bring my knowledge of

plants and insects back to the growers.

Toxicity of selected acaricides on Tetranychus urticae ... · Janet L. Ashley Thesis submitted to...

Documents

Transcript of Toxicity of selected acaricides on Tetranychus urticae ... · Janet L. Ashley Thesis submitted to...