Chapter VII-15

Application and evaluation of entomopathogens for controlof pest insects in mint

Ralph E. BerryEmeritus Professor of EntomologyOregon State University18160 Cottonwood Road, PMB #798Sunriver, OR 97707, USA

1 Introduction

Peppermint (Mentha piperita), native spearmint(M. spicata), and Scotch spearmint (M. cardiaca)are grown commercially for their essential oils.Essential oils, which are produced in leaf glandsprimarily on the lower leaf surface, containvolatile terpenes and sesquiterpenes that producethe unique flavoring for candy, chewing gum,toothpaste, medicines and other products. Theoil, initially recovered by on-farm steam distil-lation, is further processed and blended toproduce oils to meet specifications requested byproduct manufacturers.

Commercial peppermint production in theUSA started in Ashfield, Massachusetts in1812, and subsequently its cultivation movedrapidly to New York and the Midwest. Culti-vation of peppermint reached the northwesternUnited States in 1909 and was grown inwestern Oregon and central Washington andthen in Idaho and central Oregon (Sievers,1948; Green, 1963; Landing, 1969; Matzat,1981). Presently, peppermint and spearmintis produced commercially in the Midwest inIndiana, Michigan and Wisconsin (Lacy et al.,1981) and in the West in Oregon, Washington,Idaho and Montana. Smaller production areasinclude California, Nevada, and North andSouth Dakota. The principal peppermint cultivarsare black Mitcham, Todd’s Mitcham, Murray’sMitcham, redefined Murray’s Mitcham, andRobert’s Mitcham. Over 49,776 ha of peppermint

were in production in the USA in 1998. In 1997,4,004,310 kg of oil, valued at about $124 million,were produced. Mint is cultivated commerciallyin fields ranging from 8 to 50 ha producing agross value of approximately $2,818/ha.

Peppermint is a perennial crop that spreadsby underground rhizomes or stolons. Propagationis through digging and replanting rhizomes orpropagating tip cuttings in greenhouses. Newfields are planted in rows either in the fall orspring and the rhizomes spread in all direc-tions sending up new growth at the nodes orjoints. Peppermint grows best on rich, well-drained, mineral sandy loam or clay loam soilsor soils high in organic matter, such as themuck soils found in some areas of the Midwest(Martin et al., 1976; Jackson et al., 1983).Optimum production occurs in areas with amidsummer day length of at least 15 h andcool nighttime temperatures, such as those foundjust north and south of the 45th parallel. Inmost production areas, irrigation is requiredfor maximum mint production because of itsshallow root system. Irrigation creates a cool,humid environment that enhances the habitat forsurvival of entomopathogens.

In many production areas, mint is not tilled orcultivated after it is established because of thelikelihood of spreading verticillium wilt (Verti-cillium dahliae). This practice allows popula-tions of soil arthropods to increase and reducesthe opportunity to incorporate soil pesticides.Therefore, the increase in soil arthropods and

599L.A. Lacey and H.K. Kaya (eds.), Field Manual of Techniques in Invertebrate Pathology, 599–607.© 2007 Springer.

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other pests such as weeds, nematodes, and verti-cillium wilt may limit the production life of mintstands to 5 or 6 years.

2 Insect pests

The major insect pests in mint are all subter-ranean and either feed on the roots and crowns orbore into the underground rhizomes. In the West,larvae of mint root borer, Fumibotys fumalis,strawberry root weevil, Otiorhynchus ovatus, andmint flea beetle, Longitarsus ferrugineus, arethe most damaging pests. The mint stem borer,Pseudobaris nigrina, only occurs in Idaho. Mintroot borer and strawberry root weevil are notpests in the Midwest, but the mint flea beetlefrequently causes serious damage. Subterraneancutworms, Euxoa, Agrotis and Mamestra spp.,occur in the West and cause sporadic injury tomint in the spring (Berry, 1975; Berry and Fisher,1993). Other species of cutworms, such as varie-gated cutworm, Peridroma saucia, and loopers(Autographa californica and Trichoplusia ni) areimportant pests in most production areas.

3 Entomopathogens

Table 1 summarizes the entompathogens thathave been used against key insect pestsof mint. The primary entomopathogens havebeen entomopathogenic nematodes and Bacillusthuringiensis subsp. kurstaki (Btk).

A Entomopathogenic nematodes

1 Mint root borer

Steinernema carpocapsae was evaluated in largefield plots against mint root borer (Takeyasu,1994) and was used commercially by growerson approximately 1,000 ha in 1993 and 1994.Post-harvest applications in mid-August throughsprinkler irrigation at a rate of 2.5 or 4�9 × 109

infective juveniles (IJs)/ha controlled 75.2% and85.5% of the mint root borer larvae, respec-tively (Takeyasu, 1994). The low rate was recom-mended for fields with a root borer density lessthan 1.0 larva/929 cm2, and the high rate wasrecommended for fields with densities greaterthan 1.0 larva/929cm2. Split applications ofnematodes (applied pre-harvest in late July andpost-harvest in mid-August) also have beenevaluated at rates of 1.2 and 1�8×109 IJs/ha. Pre-harvest applications of 1.2 and 1�8 × 109 IJs/hacontrolled 28.9% and 60.9% of the mint rootborer larvae, respectively. A second post-harvestapplication of 1.2 and 1�8×109 IJs/ha increasedcontrol of mint root borer larvae to 88.9% and94.3%, respectively (Takeyasu, 1994).

S. riobrave injected into the top 2.5 cm ofsoil in furrow irrigated mint reduced the mintroot borer population by 54% (Gillespie et al.,1996; Research Reports, Mint Industry ResearchCouncil, unpublished). Application of nematodesthrough sprinkler irrigation allows for a moreeven distribution than injection into the soil on

Table 1. Insect pests in mint and their control with entomopathogenic nematodes in the field

Key target pests Nematode species Application rates�109 IJs/ha�

References

Mint root borer Steinernema carpocapsae 3�7–4�9 Takeyasu, 1994S. oregonense 4�9S. riobrave 4�9Heterorhabditis marelatus 4�9H. bacteriophora 4�9

Strawberry root weevil S. carpocapsae 7�4H. marelatus 4�9H. bacteriophora 7�4

Mint flea beetle S. carpocapsae 4�9–9�8 Morris, 1990S. feltiae 4�9–9�8S. riobrave 3�7H. bacteriophora 7�4 Morris, 1990

Variegated cutworm S. carpocapsae 4�0

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the furrow ridge and may help explain the reasonfor the low level of control.

Large quantities of S. carpocapsae were notavailable after 1995 and growers discontinuedusing entomopathogenic nematodes to controlthis pest. However, growers are likely to useentomopathogenic nematodes if large quantitiesbecome available at costs competitive with insec-ticides. Additional nematode species, such asHeterorhabditis marelatus, H. bacteriophora,S. riobrave and S. oregonense, have beenevaluated in the laboratory for control of mintroot borer. In these experiments, all nematodespecies were applied at rates equivalent to 4�9×109 IJs/ha and control ranged from 90 to 100%.

2 Strawberry root weevil

Entomopathogenic nematodes, S. carpocapsae,H. marelatus and H. bacteriophora, providecontrol of strawberry root weevil larvae andpupae in mint in late April and early May.Higher rates of S. carpocapsae are required tocontrol strawberry root weevil and pupae thanH. marelatus and H. bacteriophora. For example,at a rate of 7�4 × 109 IJs/ha, S. carpocapsaeonly reduced the weevil population 68.1% whenapplied through sprinkler irrigation after harvest(Takeyasu, 1994). However, 4�9 × 109 IJs ofH. marelatus/ha reduced the weevil populationby 96%. Applications of entomopathogenicnematodes in late April or early May will controlweevil larvae, pupae, and teneral adults. Post-harvest applications in August will control bothstrawberry root weevil larvae and mint root borerlarvae, but application rates may need to beadjusted depending on the insect pest and thespecies of nematodes being applied. Applicationsto control strawberry root weevil in the springhave been less successful largely because soiltemperatures are often below 16 �C and irrigationmay not be available.

3 Mint flea beetle

Control of mint flea beetle larvae has beenachieved with S. carpocapsae and H. bacte-riophora (Morris, 1990). Spring applicationsof S. carpocapsae and H. bacteriophora ata rate of 7�4 × 109 IJs/ha in late May orearly June may provide control of mint flea

beetle larvae, prepupae, pupae, and teneraladults. For example, Morris (1990) showedthat S. carpocapsae reduced the mint fleabeetle population by 68.6% and H. bacterio-phora reduced the population 93.9%. Mahr andWyman (1996 Research Reports, Mint IndustryResearch Council, unpublished) demonstratedthat S. feltiae, S. carpocapsae and S. riobravecould disperse in muck soils and infect Galleriamellonella mellonella larvae buried in the soil.They suggested that these nematode speciesmight be good candidates for mint flea beetlecontrol in the Midwest. Morris (1990) reportedthat applications should be made in the eveningthrough sprinkler irrigation after soil tempera-tures exceed 16 �C in the spring and after about400 day-degrees have been accumulated. Earlyspring applications may not be effective becauseof cold soil temperature, lack of adequateirrigation water, and small size of larvae and theirconcealment in rhizomes.

4 Variegated cutworm

S. carpocapsae applied at 7�4 × 109 IJs/hareduced populations of variegated cutworm by94.5% in experiments conducted in 532 cm2

plots. However, when applied to 1 ha plotsthrough sprinkler irrigation at 4�9 × 109 IJs/ha,the population was only reduced 45.8%. Earlyinstars of variegated cutworm feed on the foliageand may avoid infection by S. carpocapsae.Later instars remain on the soil surface or underdebris and may be more susceptible to infection,but additional research is needed to determinewhether S. carpocapsae will provide adequatecontrol of this pest.

B Bacillus thuringiensis subsp. kurstaki (Btk)

Btk controls populations of small cutworms andloopers feeding on mint leaves. However, Btkfailed to control subterranean cutworms, suchas the redbacked cutworm, in mint becausethis species feeds below ground and does notconsume above ground plant parts treated withBtk until nearly mature. The major disadvantageof Btk is that the dense mint foliage preventspenetration and adequate coverage of the lowerleaves. Further studies are needed to determine if

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new formulations of Btk may be effective againstfoliage feeding cutworms and loopers.

4 Protocols for applicationof entomopathogens

A Crop characteristics

Destructive sampling of mint rhizomes is themost effective method to evaluate pest densitiesand determine efficacy of entomopathogens. Inmost situations, a 464 cm2 or 929 cm2 soil sampletaken 10 cm deep is sufficient to estimate thepopulation. Soil samples may be either sorted orsieved directly in the field or transported to thelaboratory and extracted using Berlese funnels.A core sampler also may be used for root weevilsand mint flea beetles, but the core size shouldbe no less than about 10 to 15 cm diameter andsamples should be taken 10 cm deep. When thesamples are processed in the field, the rhizomesmay be replaced, which reduces the impact onthe plant stand.

B Plot characteristics

Evaluation of entomopathogens in mint may beconducted either in small plots ranging from 1 m2

to 15 m2 or in larger plots ranging from 0.5 to3.0 ha sized plots, depending on the insect pestand its distribution and the methods of appli-cation. Buckets with the bottoms removed (ca.532 cm2) or PVC pipe cylinders 16 cm insidediameter and 32 cm long pushed 10 cm intothe soil also may be used to evaluate entom-pathogens in mint. A known number of larvaecollected from the field site may be placed inthese micro-plots or the micro-plots may beplaced within a known area of high insect infes-tation. Insect larvae are the most damaging stageson mint rhizomes and have limited powers ofdispersal. Because of the host-seeking behaviorof entomopathogenic nematodes, a 1 to 2 mbuffer should be left between adjacent plots.Except for cutworms, adults of the major insectpests on mint have limited powers of dispersal,but if left unchecked their populations can reacheconomic levels in a few years. Also, since mintrhizomes are dug and used to establish newmint plantings, several of the serious insect pests

may be transported with the rhizomes and soil,resulting in infestation of new plantings. Mintrhizomes or tip cuttings, certified free of insectpests, should be used to plant new mint fields.

C Application methods

Entomopathogenic nematodes are packageddifferently depending on the supplier. Forexample, some are packaged in inert carryingmaterial such as a clay-based formulation orin finely ground vermiculate. Both formulationsare suspensible in water, but since the particlesize may vary, all filters should be removedfrom sprayers. Spray nozzle opening should beat least 500 �m. Others supply nematodes ina water soluble gel formulation or on smallpieces of sponge. The nematodes are releasedfrom the gel or sponge when agitated in water.Entomopathogenic nematodes also are shippedas “nematode wool” that can be added directlyto water. It is important to keep nematodes cooland protected from sunlight until application.Nematodes may be stored up to 3 months at 2to 6 �C in the dark, but the length of storageand nematode viability after storage will dependon the formulation. To check for viability,follow directions given by Koppenhöfer (seeChapter IV-5).

Entomopathogenic nematodes may be appliedto small plots with a backpack pressure sprayeror a small garden watering can, but for largerplots, tractor drawn pressure tank sprayers with0.38 to 1�0 m3 capacity may be used.1. If needed, pre-moisten the soil with a minimum of

6 mm of water prior to application of nematodes,2. Estimate the actual nematode concentration

applied in the plots, place five or more smallcups (3.6 cm inside diameter, 4.0 cm deep) in eachtreatment plot before applying the nematodes.

3. Collect the cups immediately after application,and measure the quantity of water and count thenumber of nematodes in each cup to estimate theactual concentration reaching the soil surface. Thistechnique should be used in small and large plots.

4. For large plots, directly inject the nematodes intothe irrigation system over a 30–60 min interval toensure an even distribution (Takeyasu, 1994).

5. After application, irrigate the plots weekly withapproximately 25 mm of water.

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Btk applied before harvest in July at a rate of5.6 to 11.7 billion international units (BIUs)/haprovides control of variegated cutworm andloopers feeding on mint leaves. Btk is commonlyapplied with a ground sprayer or airplane in aminimum of 93.5 liters of water/ha. The additionof a spreader sticker or wetting agent improvesefficacy. Btk is more effective if applied atnight, when larvae are actively feeding on thefoliage. Btk incorporated into baits also has beeneffective against cutworms and loopers, but theyare not widely used because bait formulationsare generally unavailable and are difficult toapply uniformly with ground equipment duringthe growing season.

D Pattern and type of infestation

The major soil insect pests on mint (except forcutworms) have an aggregated or clumped distri-bution and five or more replications are requiredfor field efficacy testing. The number of replica-tions should be increased for small plot exper-iments. Pretreatment soil samples may providesome indication of the relative infestation levelin a particular area. To do this, small repli-cated plots can be randomly established withinthe field. Artificially infesting small plots witha known number of insect larvae collected fromthe field is frequently the best method to evaluateentompathogens in mint. This method is partic-ularly useful if bottom less buckets or plasticcylinders are used as the plots. Burying smallscreen cages such as screen mesh tea strainers(4 cm diameter) artificially infested with a knownnumber of larvae (except large cutworms) alsois a good method to evaluate efficacy ofentomopathogenic nematodes. Burying screencages containing insect or G. mellonella larvaeat different depths in the soil may be used todetermine nematode vertical movement.

To determine persistence of nematodesin the soil:1. Bury G. mellonella larvae in screen tea strainers

in the soil or collect treated soil and bait it with 5to 10 G. mellonella.

2. After 2 days, remove tea strainers containingG. mellonella larvae or G. mellonella that wereplaced in treated soil and count the number ofinfected larvae.

3. Rinse all G. mellonella larvae in distilled water,and store them at room temperature in petri disheswith moistened filter paper for 3 days.

4. Dissect G. mellonella cadavers before thenematodes reproduce (within 3 days in studieswith Steinernema spp. or within 4 days for studieswith Heterorhabditis spp.) to validate that theywere killed by nematodes and count the number ofentomopathogenic nematodes (Koppenhöfer et al.,1998). The percentage of nematodes recoveredfrom the cadavers in the soil samples can becalculated relative to the nematode concentrationapplied.

E Application dates

1 Mint root borer

Application of entomopathogens against insectpests in mint depends on the pest phenology. Formint root borer, S. carpocapsae provides controlof larvae when applied pre-harvest in late Julyor early August or post-harvest from mid-Augustto mid-September. Studies in the laboratoryhave shown that H. marelatus also controlsroot borer larvae. Since pre-harvest applicationstarget earlier instars, crop damage is minimized.However, the nematodes may be applied tooearly. The mint root borer has a prolongedadult emergence over a 2-month period (Berry,1974). This, in combination with the short persis-tence of S. carpocapsae in the soil, makestiming an important factor in achieving goodcontrol. To benefit from a pre-harvest appli-cation, nematodes should be applied as earlyas possible to minimize crop damage, yet lateenough to ensure control of larvae resulting fromadults that emerge later in the summer.

Just as pre-harvest applications may be appliedtoo early, a post-harvest application may beapplied too late. Prior to entering the prepupalstage, late instar mint root borers construct asilk-lined earthen cell, a hibernaculum, in whichto overwinter (Berry, 1974; Pike et al., 1988;Berry and Fisher, 1993). Once it enters theprepupal stage, it is no longer susceptible toS. carpocapsae (Takeyasu, 1994). Therefore,fields must be treated before hibernacula areformed. Since nematode applications must beaccompanied by irrigation to move nematodesinto the soil and to ensure nematode survival,

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proper timing of a post-harvest application maybe hampered by the lack of irrigation immedi-ately after harvest. Also interfering with post-harvest timing is the time-consuming nature ofdiagnosing and treating fields. Processing soilsamples either by hand or in Berlese funnelsis a slow, labor-intensive process. If an infes-tation is found, growers typically need up to aweek to treat a field. In most situations, applica-tions of entomopathogenic nematodes at 3.7 to4�9×109 IJs/ha should be timed to coincide withearly instar mint root borers. Preharvest treat-ments have generally been less effective thana single post-harvest application. Split applica-tions, such as one application before harvest at1.8 to 2�4×109 IJs/ha and a second application at1.8 to 2�4×109 IJs/ha after harvest, are effective,but the labor required may outweigh the advan-tages of a single application after harvest.Perhaps, a higher pre-harvest rate would providecontrol and minimize crop damage earlier inthe season, followed by a lower post-harvestrate, if necessary. This combination may givebetter control than a split application using thesame rate. In mint, entomopathogenic nematodespersist in the soil for about 2 to 3 weeks so asingle application, if properly timed, will effec-tively reduce the root borer population.

2 Strawberry root weevil

To control strawberry root weevil larvae andpupae, entomopathogenic nematodes may beapplied in mint in late April or early May.Applications after harvest in mid-August or earlySeptember will control root weevil larvae. Inthe spring, cool soil temperature and lack ofirrigation may limit efficacy of S. carpocapsae.H. marelatus is a cool-temperature adaptednematode that has been effective against rootweevil larvae and pupae at temperatures as lowas 10 �C (Berry et al., 1997). If properly timed,a single application of 4�9 × 109 IJs/ha afterharvest may provide control of both root weevilsand mint root borer larvae.

3 Mint flea beetle

Mint flea beetle eggs hatch in April and earlyMay and larvae begin feeding on small mintroots and later tunnel into rhizomes. Larvae

complete development during late May and Juneand the pupal stage is completed in about 3 to 4weeks. Spring applications of entomopathogenicnematodes at a rate of 7�4×109 IJs/ha should bemade in late May or early June to control larvae,prepupae, pupae, and teneral adults. Applicationsof S. carpocapsae should be made in the eveningthrough sprinkler irrigation after soil tempera-tures exceed 16 �C. In areas East of the CascadeMountains, cool spring temperatures and the lackof irrigation water may limit the effectiveness ofearly spring applications.

4 Variegated cutworm

Variegated cutworm larvae are susceptible to S.carpocapsae, but only when applied preharvestin late June and July. Takeyasu (1994) demon-strated that there was a 94.5% reduction of varie-gated cutworms in small plots, but only about a46% reduction in the population in large plots.Early instars feed on foliage and later instarsfeed near the soil surface or beneath crop debris.However, because of the overlap in larval stages,a single application may not provide adequatecontrol because larvae feeding on above groundfoliage may escape infection by S. carpocapsae.

F Sampling and monitoring

1 Mint root borer

Mint root borer larvae cause damage by feedinginside peppermint and spearmint rhizomes fromlate July through mid-September (Berry, 1974).Damage resulting from feeding injury weakensmint stands, which overwinter poorly and regrowslowly in the spring.

Inspection of fields during July and Augustshould be performed when adults are active andcan be detected by observing moth flight as youwalk through the field. When disturbed, adults flya few meters and land on the underside of a leaf.Adults may be collected by sweeping foliage,but in most instances the wing scales have beenlost and adults cannot be easily distinguishedfrom other small moths. A sex pheromone isavailable commercially to sample for mint rootborer males (Davis et al., 1984, 1991). Thesuccessful development of this sex pheromone in

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a trapping system could provide valuable early-season information to growers concerning theneed to control mint root borer larvae. However,Takeyasu (1994) was unable to correlate adultdensity in pheromone trap catches with larvalinfestation. Therefore, the use of pheromonetraps for mint root borer may be limited torevealing the presence or absence of mint rootborer adults in a field or concentration of adultactivity in a certain area.

Presence of adults in late June and July servesas a signal to take postharvest soil samplesfor larvae in August or early September. LateSeptember and early October soil samples willusually only detect mature larvae and hibernaculaafter damage is done and control with nematodeswill be ineffective.1. Soil samples �929 cm2� should be taken from

several locations in the field. It is recommendedthat one sample be taken/ha with a minimum of 25samples per field. Samples should include soil andrhizomes to a depth of 5 to 10 cm. These samplesalso may be used to estimate the population ofroot weevil larvae in the fall, although the depthof samples may be too shallow to collect a repre-sentative sample of weevils.

2. Samples may be sorted by hand in the field orwith the use of a Berlese funnel. If using Berlesefunnels, separate the soil and rhizomes to enhancerecovery of larvae. If samples are processed inthe field, examine rhizomes for damage and larvaethat may be within the rhizome.

3. Depending on soil texture and moisture, the useof screens to sieve soil may speed up the processof locating larvae in the samples that have leftrhizomes. Treatment is justified if an average of 2to 3 larvae is found/929 cm2.

2 Strawberry root weevil

Strawberry root weevil larvae are best estimatedby taking 929 cm2 soil samples after harvestin late August and September or the followingspring in April or early May (Emmenegger,1976; Emmenegger and Berry, 1978). Samplesfor root weevils may be combined with samplesfor mint root borer in the fall or combinedwith samples for cutworms and flea beetles inApril and May. Take at least 25 soil samplesfrom different areas of the field (approxi-mately one sample/ha), screen the soil, count

the number of larvae and calculate the averagenumber per sample. Treatment is warranted if theaverage number of root weevil larvae exceeds2�0/929 cm2.

A sequential sampling program has beendeveloped for strawberry root weevil in centralOregon (Cacka, 1982).1. This sampling plan is based on taking a minimum

of 25, 929 cm2 soil samples and accumulating thenumber of adults, pupae, and larvae found in eachsample.

2. Using this method, treatment is recommended ifthe total accumulated number of adults, pupae,and larvae exceed 5�5/929 cm2. Treatment is notrecommended if the total weevils/sample is lessthan 2�7/929 cm2.

3. These sequential sampling plans are based ontreatment thresholds, which were calculated usingan oil value of $22.00/kg and a cost of treatmentof $47.35/ha. However, as the value of the oiland the cost of treatment change, the recom-mended treatment thresholds also change (Berryand Fisher, 1993).

4. In practice, mint fields suspected of being infestedwith root weevils should be sampled for adultswith a sweep net in the evening, a couple of hoursafter sunset. Still, warm and dry evenings are best.Windy, cool and/or rainy periods produce fewerweevils/sweep, giving the impression of a smallerfield population than is actually present.

5. Take 10 straight-line sweep samples in at leastfive different sites in fields up to 12 ha. Add oneadditional sample site for each additional 4 ha.

3 Mint flea beetle

Root damage caused by mint flea beetle larvaecan be seen in the spring. Regrowth of mintinjured by this pest is slow and characterized byspotty stands and reddish plants that are stuntedand appear stressed. However, these symptomsalso may be typical of other factors, such as waterstress, inadequate fertility or other root feedingpests, such as plant-parasitic nematodes and rootweevils.

Recognition of adult flea beetle feedingdamage on mint leaves is the easiest clue todetermine if this pest is present. Feeding is inter-veinal and appears as many small “shot holes”through both leaf surfaces.

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1. Use a sweep net in late June and July to monitorfor adult mint flea beetles. The small, yellowish-brown beetles are most efficiently sampled earlyin the morning on dry foliage.

2. Sample at least five sites for every 8 ha taking 10to 20 straight-line sweep net samples through thefoliage at each site. No treatment threshold hasbeen developed for adult flea beetles, but yieldreductions have been observed for an average of 5to 10 adults/sweep sample.

3. To sample for larvae, take soil samples in late Mayand early June or after 300 to 400 day-degreeshave been accumulated (Morris, 1990).

4. Take a 12.8 by 18 cm sample of soil and roots to adepth of 10 cm (350 cm3 core) every 0.4 to 0.8 ha.Take a minimum of 25 samples/field.

5. Screen the soil samples in the field or use Berlesefunnels to collect larvae or pupae (Morris, 1990;Shields et al., 1981). If Berlese funnels are used,it is recommended that the soil and rhizomes beseparated to enhance recovery. Carefully inspectthe soil and roots for small cream-colored larvae(1 to 2 mm in length with brown heads). Theyalso can be found in black or brown “tracks”within tissue just below the surface of the root.Occasionally they will be in the soil near the rootor protruding from the root itself.

6. At an oil price of $33.07/kg, the economic injurylevel ranges from 0.61 to 1.12 larvae/350 cm3

depending on cost of control (Morris, 1990).

4 Variegated cutworm

The decision to control variegated cutworm isusually based on the average number of larvaefound in 929 cm2 samples taken on the soilsurface. To estimate populations of fourth, fifth,and sixth instars:1. Inspect the soil surface by first vigorously shaking

mint foliage and closely observing and recordingthe number of larvae/929 cm2 randomly throughthe field.

2. Take a ground search sample every 2 ha for fieldsup to 12 ha. Add an additional site for every 4 hain fields that exceed 12 ha. Look very closely forsmall and curled-up larvae under and in foldedleaves on the ground. When leaf chewing is quiteevident and cutworm counts from ground searchesare low, return after dark and sample with a sweepnet to collect larvae actively feeding on the foliage.

a. Sequential sampling plans have been developedfor variegated cutworm using sweep netsamples to estimate larvae (instars 2 to 4)and for ground search samples �929 cm2� toestimate instars 4 to 6 (Coop, 1987; Coopet al., 1995). These sequential sampling plansare based on treatment thresholds which werecalculated using an oil value of $26.46/kgand a cost of treatment of $47.35/ha; as thevalue of the oil and cost of treatment change,the recommended treatment thresholds alsochange (Berry and Shields, 1980; Berry andFisher, 1993).

b. Using these plans, treatment of instars 2 to 4,sampled with a sweep net, is recommendedif 60 larvae are collected from a minimumof 11 different field sites (a minimum of 10sweep net samples should be taken at each site).Treatment is not recommended if fewer than 44larvae are collected in sweep net samples.

c. For ground search sampling, treatment ofinstars 4 to 6 is recommended if 24 larvaeare collected in 929 cm2 samples taken froma minimum of 18 different sites. Treatment isnot recommended if fewer than 17 larvae arecollected in the samples.

A sex pheromone is commercially availableand can be used to detect and monitor adultmales of the variegated cutworm in the spring.Trapping males could provide valuable earlyseason information to growers concerning thepotential need to control cutworm larvae duringJune and July (Coop, 1987). Sticky traps baitedwith this pheromone can be set in fields in lateApril and monitored weekly or biweekly throughJune. Although action levels for treatment havenot been developed relative to moth catches, it islikely that large and continuous catches greaterthan 25 adults/week will result in similarly largepopulations of larvae being observed approxi-mately 2 weeks following peak trap counts. Thereal value of pheromone traps lies in the factthat they signal when to begin inspecting fieldsfor larvae, thereby greatly improving timing ofcontrol. However, large trap catches will notalways result in large larval populations. This isparticularly true when the mint field does nothave a resident population of variegated cutwormand the trap catches are a result of males beinglured into traps from other crops. Conversely, a

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small trap catch does not necessarily mean thatthe larvae will not exceed an action level.

5 References

Berry, R. E. 1974. Biology of Pyrausta fumalis onpeppermint in Oregon. Ann. Entomol. Soc. Am.67,580–582.

Berry, R. E. 1975. Redbacked cutworm: flight periodand egg development under field conditions. Environ.Entomol. 4, 603–605.

Berry, R. E. and Fisher, G. 1993. A guide to peppermintinsect and mite identification and management. PNW182. Oregon State Univ. Ext. Ser. Corvallis, OR.

Berry, R. E., Liu, J., and Groth, E. 1997. Efficacy andpersistence of Heterorhabditis marelatus (Rhabditida:Heterorhabditidae) against root weevils (Coleoptera:Curculionidae) in strawberry. Environ. Entomol. 26,465–470.

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