Pestic. Sci. 1979, 10, 254-270

Time Related Factors in Heliothis Control on Cottona

Neil Morton

ICI Ltd, Plant Protection Division, Fernhrirst, Haslemere, Surrey

(Manuscript received 8 August 1978)

Most factors in Heliothis control on cotton are crucially related to time but all are interrelated one with another and form two complex interactions hinging on the pest life cycle. One is the crop-pest interaction and the other the control strategy. This paper reviews most of the factors and reports work carried out in Swaziland to elucidate some of them. In the crop-pest interaction the timing of the initial attack and a cue for spraying can be indicated by development of an alternate host and, in Swaziland, by reference to the average date of maize tassellation. Later infestations are measured by scouting and a comparison is made of the two main systems, the first based on a small part of many plants and used in the USA, and the second based on the whole of a few plants in Africa. Their respective purposes are discussed and economic spray thresh- olds based on them are presented. The compensatory ability of the plant diminishes as the season progresses but in Swaziland manual bud/flower removal experiments over 3 years on raingrown and irrigated cotton showed that prior to 12 January (9-12 weeks from planting) removal increased the yield in half the cases whilst later removal could cause a decrease. The natural Heliothis infestation was allowed to remove fruits in an experiment by leaving spray gaps and the results confirmed those of the previous manual experiments except that damage continued for about 2 weeks after spraying restarted. This advanced the critical date to 3 1 December but up to three sprays could be saved without loss of yield and possibly with a yield bonus. In the control strategy the different stages of the life cycle are considered as targets but the most obvious is normally the first instar larvae. Rapid plant growth can dictate frequent spray applications regardless of deposit persistence which may need to be longer when using aircraft in spite of Heliothis oviposition occurring predominantly on exposed surfaces. Temperature-independent or fumigant insecticides or those affecting the adult may best be sprayed in the evening when cooler and calmer post-treatment conditions occur and adults are starting to fly.

I . Introduction

About 40% by value of current insecticide usage in the western world is to control cotton pests, the majority of this in the USA. The reasons for such high usage are well known but it will be helpful to reiterate them: (a) the need to protect the fruiting forms from nearly continuous pest attack for about 2-3 months; (b) the rapid growth rate of the plant making frequent applications necessary; (c) the wide variety of pests, ranging through two or three insect orders and the consequent difficulty of controlling all of them with either a single compound, or an inadequate range of specific com- pounds; (d) the difficulty of effecting spray coverage good enough to give complete control; (e) the presence of key pests at the very beginning of the fruiting period requiring insecticidal control which can reduce beneficial arthropods and promote pest attack later on ; (f) the existence since the early 1960s of insecticide resistance in bollworms which has required ever increasing quantities of active ingredient to give control, and the introduction of new higher cost insecticides.

organised by the Physicochemical and Biophysical Panel (Pesticides Group), Society of Chemical Industry. Presented at the symposium on Insecticiclc> kinetics and time reluted effects in pesticide action on 7 February 1978,

0031-613X/79/0600-0254 $02.00 0 1979 Society of Chemical Industry

2.54

Heliothis control on cotton 255

Consequently, there is a long established desire amongst government officials and a more recent wish amongst farmers to reduce their dependence on insecticides. Most factors in promoting more efficient insecticide use relate to timing and this paper reviews these briefly and includes original work carried out in Swaziland.

2. The pest life cycle

The single most important consideration in Heliofhis control strategy is the stage of the life cycle against which control measures should be aimed. Adult female Heliothis lay eggs on a wide range of preflowering or flowering plants and larvae emerge 3 4 days later and attack a nearby bud or flower. In the absence of these it eats leaves or the growing point. Later instars are capable of eating more mature fruit but after about 3 weeks or six instars it drops or crawls down to the soil to pupate. Pupation lasts about 3 weeks in summer and up to 7 weeks in cooler conditions. As cooler weather approaches an increasing proportion of pupae enter diapause.

The first infestation of Heliofhis armigera on cotton traditionally originates from maize1.2 and still does throughout most of Africa, but there are now more crops to provide pre-cotton hosts. The idea of controlling Heliofhis on its alternate hosts has frequently been suggested but nothing practical has been achieved. However, strip cropping an alternative host with cotton can have practical value either by providing a nucleus for biological control agents or by acting as a trap crop to be sprayed off; the literature abounds with examples.3 The poisoned baiting of moths on the crop is discussed later.

In. these cases the strategy is to make use of or modify the oviposition behaviour of the moth. The moth itself has been the target of an ambitious air-to-air spray programme in the Sudan (Joyce, V.; Uk, S. unpublished) which used radar to locate swarms of moths in flight at night. Swarms are found to aggregate at the confluence of air streams.

If a pesticide is particularly active upon direct contact of wet spray with larvae or in some way affects the adult, more frequent applications of reduced quantities of active ingredient may improve control. Splitting the normal dose and making two applications 3 days apart has been tested and found successful in the Sudan. The logistics of this exercise is particularly suitable for ultra low volume (ULV) spraying using especially small droplets (60 pm volume median diameter) and wide swaths (60 m) with aircraft.

The Heliofhis egg is laid on fairly exposed surfaces of cotton such as bud bracts or the upper surface of young leaves and if spray coverage is adequate an ovicide can be effective. Spray coverage is not reliable enough however and in any case rapid plant growth and continuous oviposition rule out the use of ovicides by themselves. Damage is caused by larvae penetrating buds, flowers or bolls. They hatch from the egg and wander about until a fruiting form is located. They may then eat their way inside or merely eat a small hole before transferring to another fruit. The average number of transfers during 26-day average larval life was nine in Rhodesia4 but it is the first and second which are most important in the control strategy because the amount of insecticide necessary is dramatically more for old than young larvae. For DDT, Matthew@ reported double the median lethal concentration was necessary after the first 11 days and by 18 days, 56 times as much was necessary. Gaste reported a loo0 fold increase in LD50 (the dose required to kill 50% of the test species) of DDT applied topically to Heliofhis zea over a five-fold weight increase. It is thus most desirable to time sprays to coincide with the hatching of eggs.

The full grown larva leaves the plant and pupates in a tunnel just below the soil surface. This is the most inaccessible stage of the life cycle but it is not of course immune to damage from agronomic practices such as inter-row cultivation or irrigation (particularly furrow), nor from natural enemies such as beetles and ants. When a pest, such as the red bollworm Diparopsis castanea Hmps or the boll weevil Anthonomus grandis Boh, is restricted or nearly restricted in habit to one host, there is logic in closely examining the stage which passes the critical period between one season and the next, but this is not the case with Heliothis.

256 N. Morton

3. The timing of Hefiothis attack and economic thresholds The Heliothis species attacking cotton are known as bollworms: H. armigera Hb in the Old World, Europe, Africa, India, Asia and Australia; H. punctigera Wllgr in Australia; H. peltigeru (Schiff) in Europe and Near East; H. zea (Boddie) in the New World, the Americas, or budworm (tobacco) H . virescens (F) in the New World, and their common name tells that part of the cotton plant they attack. The timing of the first attack and the severity of the infestation vary from season to season and from one locality to another, but work as early as the 1930s’ showed how ‘the annual march of bollworm incidence’7 could be traced by reference to those plants in the bud and flowering stage. Thus the sequence of crops attacked in the Barberton Valley of South Africa after moth emergence from diapause pupae in August-September was: first gene- ration, winter crops and citrus; second generation, wild hosts and early maize (November- December); third generation, late maize and cotton (January); and two further generations on cotton. This pattern may have changed slightly in recent years because only 80 km away in neigh- bouring Swaziland, oviposition of second generation eggs now sometimes occurs on cotyledonary stage cotton and the larvae cause severe defoliation. Nevertheless, H. urmigera in the Old World and H. virescens and H. zeu in the New World characteristically oviposit on cotton from the time the first flower buds are formed.

The time to start the spray programme can be determined on two different bases. One is to investigate the seasonal occurrence of attack and determine whether any generalisations about its timing, or predictions based on its prior occurrence in other crops or weeds can be made; the second is to make regular assessments of actual infestation levels in the crop, commonly referred to as ‘scouting’.

3.1. Predicting the time of the initial infestation; maize tassellation Maize plants at flowering are known to be at least as attractive if not more so to ovipositing female Heliothis than flowering cotton. To H. armigera it is the male flower, the tassel, which is theattractive stage whilst to H. zea it is the female flowers, the silks, formed 2 weeks after tasselling. In southern Africa H. armigera oviposition starts immediately tassels appear and continues for as long as they are present, 15-20 days on the varieties observed by Parsons and Ullyett.1 Maize is the staple diet and is the most widespread crop in the region. It is planted in preference to cotton as soon as sufficient spring rain allows adequate land preparation. When cotton and maize are sown on the same date under rainfed conditions peak maize tasselling occurs when the cotton has 9-1 1 nodes and squaring (bud formation) is just beginning.

Thus H. armigera oviposition on cotton follows that on maize and because of the short period that maize tassels in each locality the first infestation on cotton usually occurs suddenly, providing a synchronous infestation. Maize tassellation may thus provide a sensitive local key for the commencement of vigilance in protecting cotton from H . armigera. It could also help by preventing the occasional spray from being carried out too early before the main H. armigeru attack occurs during which period little fruit is on the plant; early damage can in any case be beneficial and is mentioned later. It is also believed that natural enemies would benefit.

To test the hypothesis several trials were carried out on both irrigated and raingrown cotton in Swaziland.8-10

3. I. I. Irrigated cotton

One trial was carried out each season from 1969-70 to 1971-72 at Big Bend in the lowveld. The date of peak tassellation for local raingrown maize was surprisingly constant at about 24 December in each season. Maize tasselling was clearly a good alert and in two of the three seasons, early sprays were saved compared with using either the officially recommended or an ‘on sight’ spray threshold (Table 1) [Figure I(a)]. No increase in H. armigera larvae or loss of yield resulted from omitting the early sprays.

3.1.2. Raingrown cotton

Trials were carried out in the hot lowveld at Big Bend and Vuvulane and in the more mild middle-

Heliothis control on cotton

P

z 5 1.0

0.5

0 ’

251

f-7 “ i’”i

I 1 . 5 - ( b ) Maize tossets

- p---‘4 I

I I -

I 9 - 2

/ /

J ,

Table 1. Sprays applied before maize tasselling for Heliorhis armigera control under three spray programmes with irrigated

cotton at Big Bend, Swaziland

Spray programme

Aftcr maize 0 . 5 eggs ‘On sight’ Season tasselling per plant of eggs

1969-70 0 1 4 1970-71 0 0 0 197 1-72 0 1 1

~-

Figure 1. The liming of egg and larvae infestations on cotton in relation to maize tasselling: (a) irrigated cotton; (b) raingrown cotton. ( 0 ) 1969-70; (0) 1970-71; ( A ) 1971-72. (-)Eggs; (----)larvae.

Maize 1 o ssel s - ( a )

\ - 1 I

I I I I 1

Maize 1 o ssel s - ( a )

\ - 1 I

I I I I 1

December January

veld at Luve (dry) and Nhlangano (wet) where tasselling was about 2 weeks later. At Big Bend in 1969-70 and 197&71 tasselling was prior to any larvae record but in 1971-72 larvae were damaging cotton buds by that time [Figure I(b)]. This damage may have stimulated production because first pick and total yields were numerically higher than other treatments, though not statistically different. Adherence to the tasselling ‘cue’ saved one spray compared with the recommended spray threshold of 0.5 eggs per plant. At the other lowveld site, Vuvulane, tasselling also occurred at the correct time to provide a spraying cue. In the middleveld the cue was satisfactory except in one year at Luve when maize tasselling was extremely late due to a drought. At Nhlangano in one season a single spray was saved with no adverse effects on larval numbers or yield. Thus on all raingrown sites maize tasselling was a satisfactory cue for detecting H. urmigeru oviposition on cotton and led to a saving of sprays in 3 out of 12 trials. Exceptionally, tasselling may not occur before cotton flowering and the solution may be to use the average date of tasselling over a period of years.

258 N. Morton

3.2. Scouting

The assessment of pest infestation levels is a task farmers instinctively do but in too cursory a manner to provide accurate enough information for use when spraying to economic thresholds. Accurate techniques are based on a sound knowledge of pest identification and of natural distri- bution about the plant of the relevant pest stage.

Two different scouting systems have been developed; the first, in the USA,ll is to scout a small part of many plants [see (a) below], and the other, in central and southern Africa,12 is to scout the whole of a relatively few plants [see (b) below]. Both approaches are born of the need to scout for another major pest, in North America the boll weevil Anthoriomits grandis, and in Africa the red bollworm Diparopsis castanea.

(a) The boll weevil punctures buds from the beginning of the season so that the top 15 cm of the mainstem (the terminal) where many new buds are present is a logical sampling unit. Of these terminals, 100 provide a statistically satisfactory sample. Heliothis eggs are laid on young leaves and fruiting points and so terminal scouting can also be used for Heliothis. However, it has important weaknesses. Only about 25 % of the eggs laid on the plant are laid on these terminals and as this proportion varies, it gives no indication of the presence of larvae elsewhere on the plant. Because of this certain states in the USA have introduced an alternative system for Heliothis scouting based on an assessment of damage to squares (buds). A hundred successive squares are examined and the proportion damaged is checked against the locally recommended spray threshold level. The danger is that it is an assessment of larval damage and if used for deciding whether to spray the most susceptible target for insecticidal control, the period from egg hatch to first penetration of a bud may have passed. There is a counter argument which maintains that the delay in control allows biological control of eggs and young larvae more expression. Lincoln et a1.13 assessed whether in fact biological control had been effective on farms and found it had been in only 28% of the cases. There is also the danger of a sampling bias when the scout comes across a ‘flared’ square (when a bud is damaged the bracts flare, that is, they fold back away from the bud), because this indicates the presence of a larva, if not in the flared square, probably in the next fruit lower down the branch. If strictly successive squares are counted this bias can be avoided.

A refinement devised in Arkansas for boll weevil is the point sampling system.’-‘ A point is randomly selected and marked in a representative area of cotton and the next 50 squares (both flared and normal for bollworms) are examined and the number damaged recorded. The length of row over which the 50 were found is then roughly measured. The whole process is repeated nearby to provide a total of 100 squares. The system thus provides an indication of the rate of square production per unit area and of percentage damage to squares. Economic thresholds have been calculated for each successive week of square production and with reference to these the need to spray is determined. A problem is that plant and cotton variety are factors affecting the rate of squaring.

(b) In Africa the red bollworm lays its eggs throughout the height of the plant often on stalks and petioles low down; to scout for it the whole plant has to be searched. In Malawi and Rhodesia where the first African recommendations were devised12 the red bollworm was at least as important as Heliothis if not more so; scouting for Heliothis was therefore done on the whole plant at the same time as for the red bollworm. Thus it is recommended that at least 24 plants in Malawi and Rhodesia, or 20 plants in Swaziland,l5 are randomly selected and examined in their entirety. It has been confirmed that this system determines the level of infestation accurately and provides the basis for effective spray thresholds.

It has been thought in Rhodesia that the red bollworm was less of a problem than it used to be and a smaller standardised sampling unit, comprising the top ten sympodial branches including the upper part of the mainstem, was compared with the whole plant.16 The ‘top ten’ sampling unit detected 97 % of the whole plant total of H . armigera eggs and is now the recommended procedure. Although only 55 % of red bollworm eggs were actually located in the top ten unit, it was found by comparing scouting records of the two systems, that records using the top ten unit gave 60-70X of the whole plant total. The difference between the proportion of eggs actually laid on the top ten

Heliothis control on cotton 259

unit and the proportion located by scouts is probably the result of more efficient scouting in the smaller sampling unit. The red bollworm spray threshold has been altered from six eggs per 24 plants to four eggs per 24 top ten units.16

Sequential sampling of cotton for H . armigeru has been investigated as a means of simplifying and, when infestation levels are high, shortening the job of scouting.17 On the basis of the scouting records stored away in most cotton research stations it is possible to calculate the necessary mathe- matical constants suitable for the locality; where there is an intensive agricultural extension effort, sequential sampling appears worth examining.

3.3. Sugar-line baits and light traps as survey tools Insecticidal sugar baits were used for bollworm ( H . zea) control in the 1930s in the USA18 and molasses-insecticide sprays have since been tested widely in the USAlg and Africa.10~20-29 Mixtures of carbaryl and molasses are now recommended in southern Africa as a safe alternative to DDT for H . armigera control whilst still giving red bollworm control. Both adult and larval kill of H . armigera occurs.

However, it is as a survey tool that sugar baits are of more concern here. In 1963-64 Lincoln et a/.30 sprayed a 19% sugar bait containing an insecticide in long single row lines on a variety of crops. The spray was applied in large droplets late each afternoon and in the late evening between 20.30 hours and midnight the rows were checked for H . zea and H . virescens moths. Apparently the sugar line was an effective trap for moths of both species in each crop and gave a realistic picture of the pattern of moth activity through successive crops throughout the year. The sugar line is a research station survey tool of under-used potential, whilst on the farm it might find ready accept- ance as a n adjunct to egg counting if it could be developed for use on alternate nights or three times a week.

Light traps have been used as sampling tools for decades and there are notes from 1928-29 describing how, in the Swaziland lowveld, acetylene flares were placed at regular spacings around cotton fields in an attempt to attract red bollworm moths away from the cotton.31 However, light traps are notoriously difficult tools for population assessments because the catch is affected by factors such as phase of the moon, degree of cloud cover, wind speed and age of adult. H . virescens moths were rarely caught at a black-light trap whilst H. zea were.3o

Pheromones are recently of interest as baits or mating disruptants and field work has been conducted with H . z m 3 2 , 3 3 and H . virescens.34 It seems most likely that pheromones will become of greatest value as survey tools aiding the correct timing of sprays.

4. Economic thresholds

To devise economic spray threshold levels, research must first relate numbers at the stage being counted to a probable extent of damage, based on one or other scouting system as already discussed. This is complicated by the ability of the plant to compensate for lost fruit when the season is long enough and depends on the stage of plant growth when the fruit was lost. Consequently, the economic threshold will alter as plant growth progresses and some work in Swaziland on defining the most susceptible period is reported in the next section. In Arkansas economic thresholds based on stage of plant growth have already been determined and Table 2 shows how these have e v ~ l v e d . ~ ~ ~ ~ ~ Generally speaking most states in the USA advise Heliothis economic thresholds of four to ten larvae plus eggs per 100 terminals but some recommend 5 or 10% square damage.

Scouting is widely recommended in the southern and central parts of Africa (Table 3). Since the original recommendations were made in central and southern Africa12 investigations have been aimed at simplifying scouting, for example, by using the top ten sampling unit and the maize tasselling cue mentioned earlier. To confirm the thresholds for local use a series of raingrown and irrigated trials were carried out in Swaziland.8-10.36 On irrigated cotton the recommended threshold of 0.5 eggs per plant was not the most profitable when averaged over four seasons; a threshold of 0.25 eggs per plant doubled the number and cost of sprays (using DDT) for H . armigera control but resulted in neither significantly better pest control nor profits. However with the lowest threshold, ‘on sight’,

260 N. Morton

Table 2. Economic spray thresholds for bollworm (and budworm) in Arkansas showing how they have evolved from 1962 (scouting based on point sampling)

1962

1963

1966

1970

1971

4-5 worms plus eggs per 100 terminals 20 worms plus eggs per 100 terminals if it is the first spray and biological control is possible (only in first 2 weeks of squaring) 25 % damaged squares Ten worms per 100 terminals plus 2-3 % damaged squares 4-10 worms per 100 terminals plus 4 6 % damaged squares (only in first 2 weeks of squaring) 25 % damaged squares (Terminal counts dropped) First 2 weeks of squaring 2nd to 5th week of squaring after 5th week of squaring 1st week of squaring 2nd week of squaring 3rd week of squaring 4th week of squaring 5th week of squaring after 5th week of squaring (Started to use squaring rates per unit area) weeks 1 to 5 , 15 OOO damaged squares acre-' (37 065 ha-1) after 5th week 10 000 damaged squares acre-' (24 710 ha-1) 4 OOO damaged bolls acre-' (9 884 ha-1)

or or

or or

25 % damage 8 % damage 5 % damage

20 % damage 16 % damage 12 % damage 10 % damage 8 % damage 5 % damage

or

Table 3. Economic spray thresholds for Heliothis armigeru in Africa

Scouting method Recommended threshold

Entire plant 0 . 5 eggs per plant all season or two consecutive counts of 0.25 or more. (In Swaziland this now applies only to raingrown cotton) As above (Rhodesia only). Top ten unit

2.7 times as many sprays were applied (13.5 compared with 5 ) and significantly higher yields occurred in one season and numerically higher in two other seasons. The increased cost of these extra sprays was assessed in detail at E20 ha-1 (including labour, material, machinery, etc at 1974 prices) which was theoretically covered by a 100 kg ha-l increase in seed cotton yield. As long as the cost of spraying does not more than double compared with the value of the resultant increase in yield, it was concluded that the 'on sight' threshold, after using the early season maize tasselling cue, was most profitable.

5. Plant response to damage

Most cotton varieties have an indeterminate fruiting habit; they continue to initiate new fruit for as long as growing conditions are suitable. The final fruit load or fruit set is governed by a combination of agronomic and climatic factors not discussed here. If insect attack causes loss of buds, flowers or bolls the plant compensates by initiating and setting more buds and this makes economic spray thresholds for cotton pests difficult to determine. Furthermore the amount of compensation may be limited intentionally, for example, by close-season legislation forcing early uprooting, or demand for a second crop from the same land, or unintentionally as a result of, for example, a late attack of pests or a cool snap of weather. If detailed and accurate thresholds are to be used, the amount of compensation possible at different times during the season in any particular region has therefore to be determined.

Heliothis control on cotton 261

It has been suggested that the ability to compensate might be used to manipulate harvest timing so as to avoid periods of unfavourable ~ e a t h e r 3 ~ and this has been achieved in Australia,38 where plant response to delayed measures for H . punctigera and H. armigera control was investigated in detail. In the trial environment (Ord River Valley, Western Australia), climate did not limit the length of season and it was found that cotton compensated for fruit lost to Heliothis if control measures were delayed for up to 10 weeks after planting. Thus the crop was set after the cool wet period from December to March rather than during it, thus preventing excessive fruit shedding well before the onset of storms in October.

The likelihood of compensatory fruiting contributing to final yield varies as the season progresses. A single threshold used throughout the season has some value but a variable threshold which altered with plant development or the time of season should be more effective. That devised in Arkansas14 has been described above and the following section describes recent work in Swaziland based on fruit removal experiments and the possibility of beneficially using the early H. armigera attack.

5.1. Bud and flower removal experiments in Swaziland In the low, hotter parts of Swaziland there is ample length of season for compensatory production and it was decided to find out which periods of fruit production were most important and how much compensation was generated by the loss of fruit in each period. Apart from aiding in the setting of more accurate economic thresholds it was hoped it would indicate whether compensation could allow the characteristically sudden early attack of H. armigera to be left uncontrolled so encouraging biological control agents to become established.

Two irrigated and two raingrown experiments were sown each year for 3 years (2 years rep0rted2~9~~ interim), one of each pair sown early and the other later but both within the normal period of sowing on commercial farms (Table 4). Each treatment comprised the manual removal of all buds and white flowers twice a week for a period of 2 weeks. Each experiment was made up of 5 or 6 treatments dated successively from the first appearance of floral buds and except in

Table 4. Total seed cotton yields after bud and flower removal

Seed cotton yield (kg ha-1)

Bud and flower removal (weeks after first bud) ~~

~~ ~~~ Date of Water Planting first Control 0-2 2-4 4-6 6-8 8-10 10-12 12-14

Season regime" time bud (BRO) (BRI) (BR2) (BR3) (BR4) (BR5) (BR6) (BR7) s.e.b

1972-73= I 31 Oct 10 Dec 3821 3399 3792 3575 3857 3766 f 188NS I 14 Dec 23 Jan 2778 3106 3118 3033 3073 2867 f 199NS R 17 Nov 2 Jan 959 1034 827 970 861 833 * 94NS R 30 Nov 9 Feb 888 913 768 930 1067 706 k 114NS

1973-74 I 1 Nov 24 Dec 3692 3736 3738 2772 3073 3279 3277 k 9 9 P=0.005 I 12 Dec 21 Jan 2648 2480 2072 2520 2540 2599 * 153NS R 17 Oct 20 Dec 2807 2258 2543 2269 1210 1798 1987 + I 1 7 P = O 005 R 21 Nov 5 Jan 1724 1239 1566 1342 1836 1679 +81 P=0.005

1974-75" I 10 Oct 18N0v 3083 3443 3594 3837 3404 3528 2754 f-I76 P=0 .005 I I 1 Nov 18 Dec 2676 2636 2292 2718 2572 2479 + 9 7 P=D.05

R 27 Nov 23Jan 2486 2380 2186 1960 2098 2545 177NS R 5 Nov 27 Dec 3375 3489 3299 3361 3412 2931 3442 f201NS

a I =irrigated ; R = raingrown. * NS = not significant.

The results in 1972-73 experiments were affected by erroneous bud removal from all treatments on one occasion

The 0-2 week period missed in 1974-75 was due to exceptionally early fruiting. at the onset of fruiting.

BRO-BR7 are bud removal periods and relate to Figures 2-6.

262 N. Morton

1972-73 all had an untouched control. Throughout the season continuous protection was given against bollworm attack (H. armigera, D. castanea, Earias biplaga and E. insulana) by spraying carbaryl(O.5 %) and molasses (15 by volume) mixture at 56 to 280 litres ha-1, depending on crop height, through knapsack sprayer and an 8-nozzle tailboom. The treatments were randomised in six replicate blocks in each experiment and the cotton variety was an Albar Acala cross. Harvests of seed cotton were made weekly from the time the earliest plots were ready.

Total yields of seed cotton obtained from these experiments are shown in Table 4. In seven of the 12 experiments bud and flower removal for 2-week periods at any time during the first 12 weeks of bud production had no significant effect on total yield. In the other five experiments there were significantly increased or decreased yields and these are shown graphically in Figures 2-6. When accumulative yield curves were plotted it was possible to discern in all 12 experiments the effect of successive bud removal periods; these are arrowed on the figures.

There appear to be three periods of different crop response and they vary in length depending on the time of planting and on the degree of water stress. The early period was when compensation and sometimes an increase in yield occurred; the middle period caused yield losses whilst the later period had no effect, fruit set having already occurred. These are illustrated in Figures 2-6. In the

4000

3000

- , 0 L

D f 2000 U

> - Y

I000

5 9 I7 2 4 I a 16 21 2 8 5 I 2 I 8 2 5

A c r i I rbi o Y June

Figure 2. Accumulative harvest curves, 1973-74, for irrigated cotton (first planting). Arrows show times o f loss due to bud removals; see Table 4 for key to bud removal (BR) periods.

Heliothis control on cotton 263

300C

200c

I OO(

Figure 3. due to bud

BR2

/ /

/ BR3

BRI ERO

ER6

ER5

0 R 4

22 27 5 9 17 24 I 8 16 21 5

March April MOY June

Accumulative harvest curves, 1973-74, for raingrown cotton (first planting). Arrows show times of loss removal; see Table 4 for key to bud removal (BR) periods.

2ooo r

8 17 24 I 8 16 21 5

Apri I Moy June

Figure 4. Accumulative harvest curves, 1973-74, for raingrown cotton (second planting). Arrows show times of loss due to bud removal; see Table 4 for key to bud removal (BR) periods.

264 N. Morton

4 10 17 24 I 7 I 5 21 28 5 12 19 26 2 I0

March April MOY June

Figure 5. Accumulative harvest curves, 1974-75, for irrigated cotton (first planting). Arrows show times of loss due to bud removal; see Table 4 for key to bud removal (BR) periods.

earliest sowings in 1973-74 and 197475, raingrown and irrigated respectively, yields of 22 and 24 % more than the control resulted from fruit removal carried out in the first 10 days of January (Figures 3 and 5). Water stress prevented compensation for early bud removal in the late planted raingrown trial in 1973-74 and lower yields occurred (Figure 4).

In combining the results of the five trials which showed significant differences it was not possible to correlate the ability to compensate with the time after planting or the time after first bud appear- ance, but a correlation was evident with calendar date (Figure 7). Figure 7 shows that of seven fruit removal treatments prior to 12 January, four were not significantly different to the control indicating good compensation, whilst three were significantly higher.

After the 12 January, 12 treatments were not significantly different to the control whilst nine were significantly lower. It thus seems possible to create a simple strategy of minimising early sprays and at the same time utilising the early H. armigera outbreak to provide nearly a 50% chance of increased yields. As nine out of 21 treatments after 12 January gave lower yields it is clear that a low economic spray threshold should operate from 12 January onwards.

To develop the strategy further it was necessary to confirm that manual removal of buds had the same effect as attacks of H. armigera. This was found to be the case in Australia3* where Heliothis attack gave similar effects to manual bud removal reported from Australia39 and the USA.40v41 It was thought necessary to confirm this in Swaziland and a trial was laid out in the 1975-76 season.

5.2. Testing the strategy in Swaziland The trial was laid out on sprinkler-irrigated cotton as follows: plot size, 17.4 m x 19 rows (17.4 m),

Hcliothis control on cotton 265

3000

2000

- - I ,a .c

(r 1 - 2 .- >

IOOC

BRO

24 1 7 15 21 28 5 12 19 26 2 10

Apri l May June

Figure 6. Accumulative harvest curves, 1974-75, for irrigated cotton (second planting). Arrows show the times of loss due to bud removal; see Table 4 for key to bud removal (BR) periods.

Figure 7. Effect of bud removal on final yield of seed cotton compared with control. Significant differences are circled. I =irrigated; R=raingrown; 1 =first planting; 2=second planting.

18

266 N. Morton

sampling-from 12 m x 13 rows; design, six treatments replicated six times in randomised blocks; pIanting date, 20 October 1975; variety, Albacala 72. Pest control, H. armigera: weekly sprays of DDT (0.5%); D. custaneu red bollworms and Eurias spp. spiny bollworm; weekly sprays of carbaryl (0.5 %). Aphids: two sprays dimethoate, five sprays pirimicarb. Spider mites: eight sprays dicofol. Application method : knapsack sprayer with eight-nozzle vertical tailboom applying from 56 to 224 litres ha-1 in accordance with crop height. Treatments: details are in Tables 5 and 6.

Heliothis larvae infested the unprotected crop at a moderate level (0.25-0.75 larvae per plant) throughout the first half of the season. The timing and level of the red bollworm infestation was very similar. The effect of withholding sprays in treatments 1 4 , compared with continuously sprayed cotton (treatment 5) , was reflected in first pick yields; the amount of subsequent com- pensation could be estimated by comparing these yields with final yields (Table 6). Only when the first spray was delayed until 3 February (15 weeks after planting) was there a significant effect on final yield (treatment 4). The difference between treatment 5 (continuous spraying) and treatments 1, 2 and 3 was also possibly real though not proven statistically. The spray gap leading to most initial yield loss (288 kg ha-l) was treatment 1 , 23 December to 13 January (9-12 weeks after planting) but compensatory production almost exactly halved the loss so that the final loss was only 145 kg ha-1 which was not significant. The gaps, either earlier or later, led to less initial yield loss. The amount of compensation was poorer the later the spray gap or the start of spraying (Figure 8).

Table 5. Spray treatments and corresponding seed cotton yields in Swaziland trial

Treatment Seed cotton yield

(kg ha-')

1 . 2. Start spraying 1 3 January 3. 4. Start spraying 3 February 5. Continuous spraying 6 . No spray

Spray gap 23 December to 13 January

Spray gap 1 3 January to 3 February

2718" 2665" 2104a 23306 2863" 1221"

Treatment yields with a common letter suffix are not significantly different at P = 5 %.

Table 6. Effect of spray gaps on yield, and the consequent compensation

Spray gap Initial yield (weeks after planting) loss compared

-~ to treatment 5 5 9 12 15 (A)

Treatment 25 Nov 23 Dec 13 Jan 3 Feb (kg ha-') (%)

3 gap 238 23

1 288 27

I YS 2 delay

4 delay

2 delay _ _ _ _ ~ _ _ _ _ _ _ _ ~ ~~

196 19

701 65

484 46

Total yield loss compared to treatment 5

(B)

(kg ha-') (%) -

Amount of compensation

(A - B)

(kg ha-') (%)

159NS 5 . 6

145NS 5 .1

53NS 2

533s 19

19gNS 7

19 33

143 50

143 73

168 24

286 59

S = significant; NS = not significantly different to treatment 5 (P= 5 %).

Heliothis control on cotton 267

Figure 8. Early yield loss (-) from bollworm damage during three periods when the crop was un- protected, and the consequent degree of compensation (----) for irrigated cotton, 1975-76, at Big Bend, Swaziland.

Weeks after plant inq

5.3. Discussion: the state of the strategy

Shed collections showed that the damage by bollworm persisted for 2-3 weeks after the start of spraying, as found in Australia.38 This was presumably because larvae in the crop when spraying starts are of a large enough size to be unaffected by the spray, especially as the residual deposit from earlier applications was small as in treatments 1 and 3, or nil as in treatments 2 and 4.

The bud removal experiments showed that, before the 12 January, removal did not significantly decrease yield and in fact could cause an increase, and also that the 2 weeks prior to 10 January elicited most compensation. In the bollworm trial, because of the delayed effect, this period corre- sponded roughly to the difference between treatments 2 and 1 , that is, to a spray gap from 14 to 31 December, and this period (treatment 1 vs 2) gave the highest degree of compensation, 73 %. The basic indications from the bud removal experiments were thus confirmed except that no increase in yield could be demonstrated, possibly because in the treatment covering the above period of maximum compensation (treatment 2), spraying did not start until about 3 weeks after the end of that period.

The strategy should be therefore that bollworm sprays need not be applied before 31 December and that a bollworm attack prior to this could generate a higher yield if uncontrolled than if control measures were taken. Protection of the crop from 31 December onwards however must be good and reliable. In the early planted crop ;his could mean a saving of three sprays and in later plantings, two, one or none. If maize tassellation were used as a cue, only one spray would be saved.

Two other implications of fruit removal are changes in plant structure and alterations in the timing of fertiliser requirements. Two varieties used in these experiments were locally bred strains of Albacala which are recommended for all areas in Swaziland. It is a potentially tall growing bush prone to rankness if over-fertilised but its response to disbudding was to increase its height and its number of central monopodial nodes. There was no increase in production of lateral monopodia, which is fortunate if labour shortages generate more interest in mechanical harvesting.

Wilson et al.38 found that, with their longest delay before crop protection (16 weeks from sowing), that yields were reduced unless a second application of nitrogen was made shortly before protection was due. In Swaziland the application of nitrogen is already split, just less than a third being applied prior to sowing and the remainder topspread 6 weeks later. It is thus unlikely that any alteration to current practice in Swaziland would benefit the 3 weeks delay in crop protection proposed here.

18*

268 N. Morton

6. Insecticide persistence, plant growth and spray application method

Cotton plants grow very rapidly during their first 3 months. An average 2 cm per day increase in height was recorded over 2 weeks in Swaziland42 and plants 25 cm high kept in a well-lit room increased in leaf area 2.5 times over 14 days in M a l a ~ i . ~ Growth dilution of an insecticide deposit therefore can be rapid. In the latter experiment, first instar larvae of the red bollworm D. castanea infesting on a 14-day-old deposit, compared with those on a 1-day-old deposit, had a five times greater ‘wandering time’ (length of wandering time being inversely related to % mortality) and whilst 100% mortality occurred on a 1-day-old deposit, 75% of the larvae successfully penetrated the growing point on the 14-day-old deposit.

Because of growth dilution, and no matter how persistent the insecticide, if it relies for its effect on a residual deposit, frequent applications are necessary during the main growth period, usually at intervals of 5-10 days. In spite of this, residual insecticides of short persistence cannot be introduced for cotton pest control because of the following interaction with the spray application method.

When sprays are applied at low to medium volume (100-200 litres ha-]) through multi-nozzle drop arms or blasted downward with an airblast sprayer, the lower part of the plant at each spray receives at least some insecticide. If it persists from the previous sprays the net insecticide deposit is composed incrementally over a period and may compensate for growth dilution. The concentration of spray has to be sufficient for a new deposit to kill larvae on new growth and it is clear that, depending on the persistence of the active ingredient, a large deposit could accumulate on leaves as they become older. It may be for these reasons that in Swaziland, although endosulfan is intrinsically poor in controlling the red bollworm D. castanea which occurs characteristically low down the plant, a weekly spray programme as applied for control of Heliothis, gave, in fact, good control of D . castanea.

However, the most widely used system, aerial application, provides relatively poor penetration to the lower parts of the plant. Fortunately Heliothis eggs are laid at the top of the plant, 95 % in the top half,13 and 97% on the top ten branches including mainstem,16 so that aerial control of Heliothis can be effective with residual compounds. It is less easy to control escapes from previous applications, however, without using a compound having fumigant or plant systemic properties. To control Heliothis purely by a residual contact effect, a more persistent insecticide deposit may therefore be needed for aerial application than is necessary for ground use.

7. Time of day for application

The time of day for spraying is an aspect certainly not overlooked by entomologists but any restric- tions suggested in pest control recommendations are unlikely for practical reasons to be acceptable to most farmers. The correct time of day is a compromise between the dictates of pest behaviour, the application method, and the technical properties of the insecticide. The last factor is probably the best as a basis for discussion.

Insecticidal activity may be temperature dependent and there could be a 10-15°C difference between the few hours following a morning spray and those following an evening spray. DDT and other insecticides are known to be temperature independent whilst the activity of organophosphorus insecticides is not.43 A larvicidal compound, active particularly through direct spray impingement but also as residual deposit, should be applied when first instar larvae hatch or when larvae go from one fruit to another. Both H . armigera4 and H . zea30 show no preferred time of day for egg-hatch or inter-fruit transfer. Thus timing is not dictated by this aspect unless the insecticide has a pro- nounced fumigant activity, in which case the still conditions of dusk suggest late afternoon spraying.

Insecticides and pheromones with pronounced ovicidal, adulticidal, or adult attractant/repellant properties would also best be sprayed in the late afternoon and early night time, particularly from sunset until dark. Lincoln et aL30 observed a higher proportion of male moths earlier in the evening and an equal ratio of male to female moths later. They also recorded oviposition starting at about 19.00 hours at a light intensity of 2.6 lux continuing until 21.30 hours. It thus appears that the time of day for spraying is important when the insecticide has to affect the adult or egg.

Heliothis control on cotton 269

Aerial spraying of less than 20 litres ha-1 requires a reduced droplet size in order to retain an adequate number density of droplets on the target surface. In these circumstances, high tempera- tures and low humidity cause severe evaporation of water-based sprays and restrictions occur on the time available for spraying. Those suggested in and Rhodesia28s45 when applied to conditions in the Swaziland lowveld in the peak spraying months of January and February would mean that flying time was severely curtailed (Table 7). Any refinement of aerial spraying practice which increases the droplet transit time between nozzle and target, such as spraying only in definite crosswinds (at up to 37 km h-1)46 will result in increased evaporation of water-based spray drop- lets. It is possible, though not proven, that in frictional turbulence as occurs in any wind, pest control will be satisfactory because the resultant smaller droplets will be well distributed about the plant, despite the proportion of emitted active ingredient impinging on the crop probably being less than if spraying had occurred in calmer conditions. One advantage to be gained by spraying in crosswinds is the elimination of one of the factors otherwise restricting spraying time.

Table 7. Effect of climatic restrictions on the aerial application of water-based low-volume sprays (20 litres ha-') at Big Bend, Swaziland. Restriction occurs when the difference between wet and

dry bulb thermometers exceeds 4.5 "C (mean of 5 years data)

Number of days Stopping Re-starting when limits are time time

Month effective (hours) (hours)

January 30 09.20 19.28 February 27 09. I9 19.00

The deposition of non-volatile droplets however as used in ULV spraying can be carried out in greater turbulence than with water-based spraying. High temperatures alone, greater than 30°C, can lead to excessive volatilisation of solvent ULV formulations and Johnstone and Johnstone47 proposed that suitable formulations should lose not more than 30% of their weight in the first 20 min when 0.5 ml is exposed on a suspended filter paper at 30°C.

8. Conclusion

There is an element of time in most aspects of Heliothis control and a view from this dimension is both illuminating and unifying. All aspects hinge on the pest life cycle and join at that point but also separate from there into two distinct bodies of relationships. Firstly, the crop-pest interaction and secondly the control strategy. These have been considered in their composite fragments in this paper but the conclusion is that the fragments are all inextricably linked in a single interaction.

References 1. 2.

3. 4.

5. 6. 7. 8. 9.

10.

Parsons, F. S.; Ullyett, G. C. Bull. Entomol. Res. 1934, 25, 349. Pearson, E. 0. In The Insect Pest of Cotton in Tropical Africa Empire Cotton Growing Corporation, Common- wealth Institute of Entomology, London, 1958. Johnson, E. K.; Young, J. H. ; Mulnar, D. R . ; Morrison, R. D. Environ. Entomol. 1976, 5, 508. Tunstall, J. P.; Matthews, G. A. Annu. Rep. Cotton Pest Res. Scheme 1960-61 Dept. of Agric., Nyasaland, 1962. Matthews, G. A. Bull. Entomol. Res. 1965, 57, 77. Gast, R. T. J . Econ. Entomol. 1959, 52, 11 15. Parsons, F. S. Bull. Entomol. Res. 1939, 30, 321. Morton, N. Annu. Rep. Res. Division 1969-70 Ministry of Agric., Swaziland, 1971. Morton, N. Annu. Rep. Agric. Res. Division 1970-71 University of Botswana, Lesotho and Swaziland, Swaziland, 1972. Morton, N . Cotton Res. Rep. Swaziland 1971-72 1972.

270 N. Morton

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Boyer, W. P.; Warren, L. 0.; Lincoln, C. Arkansas Agric. Exp. Stn. Bull. 656University of Arkansas, 1962. Matthews, G. A.; Tunstall, J. P. Cotton Grow. Rev. 1968,45, 115. Lincoln, C.; Boyer, W. P.; Dowell, G. C.; Barnes, G.; Dean, G. Arkansas Agric. Exp. Stn. Bull. 7.74 University of Arkansas, 1970. Lincoln, C.; Dowell, G. C.; Boyer, W. P.; Hunter, R. C. Arkansas Agric. Exp. Stn. Bull. 666 University o f Arkansas, 1963. Morton, N. In The Swaziland Cotton Pest Control Handbook, Advisory Bull. 4 Division Agric. Res., University of Botswana, Lesotho and Swaziland, Swaziland, 1974. Gledhill, J. A. Gatooma Res. Stn. Exp. Rep. 15, Rhodesia, 1973. Ingram, W. R.; Green, S. M. In Proc. Cotton Insect Control Conf., Blantyre, Malawi, 1971 Agric. Res. Council, Malawi, 1971. Burdette, R. C. J. Econ. Entomol. 1934, 27, 213. Lincoln, C.; Dean, G.; Phillips, J. R.; Matthews, E. J.; Nelson, C. S. Arkansas Farm Res. 1966,15. Tunstall, J . P. Cotton Grow. Rev. 1968, 45, 198. Matthews, G. A. Occasional Rep. I Agric. Res. Council, Malawi, 1972. Mowlam, M. D. Cotton Res. Rep. (Malawi) 1969-70, 1972. Hayward, J. A.; Beeden, P. Cotton Res. Rep. (Northern States Nigeria) 1973-74, 1975. Kirkby, R. A. Cotton Res. Rep. (Tanzania) 1973-74, 1975. Percy, H. C. Cotton Res. Rep. (Tanzania) 1973-74, 1975. Robertson, 1. A. D. Cotton Res. Rep. (Kenya) 1973-74, 1975. Morton, N. Cotton Res. Rep. (Swaziland) 1973-74, 1975. Rhodesian Farmer 1970, 30 October. Annu. Rep. Cotton Pest Res. Team, Annu. Rep. Gatooma Res. Stn. 1972-73, Rhodesia, 1974. Lincoln, C. and Staff, Graves, J. B. and Staff. Arkansas Agric. Exp. Stn. Bull. 720, University of Arkansas, 1967. MacDonald, D. Rep. Exp. Stn. Swaziland 1928-29, 1930. Kaae, R. S.; McLaughlin, J. R.; Shorey, H. H.; Gaston, L. K. Environ. Entomol. 1972, 1 , 651. Mitchell, R. E.; Jacobson, M.; Baumhover, A. H. Environ. Entomol. 1975, 4, 577. Roelofs, W. L.; Hill, A. S.; Carde, R. T. L f e Sci. 1974, 14, 1555. 1971 Corton Insect Recommendations University o f Arkansas Agric. Extension Service and U.S. Department of Agriculture, 1971. Morton, N. Cotton Res. Rep. (Swaziland) 1972-73, 1975. Singh, B. N.; Chaudri, R. S . Empire Cotton Grow. Rev. 1937, 14, 126. Wilson, A. G. L.; Basinski, J. J.; Thornson, N. J. Cotton Grow. Rev. 1972, 49, 308. Evenson, J. P. Cotton Grow. Rev. 1969, 46, 37. Eaton, F. M. J. Agric. Res. 1931, 42, 447. Dale, J. E. Ann. Bot. 1959, 23, 636. Lea, J. D.; Catling, D.; Jackson, P.; Cornish-Bowden, M. E.; Gibbs, J. D. Misc. Rep. 46 Research Division, Dept. of Agric., Swaziland, 1966. Metcalf, R. L. In Organic Insecticides Wiley (Interscience), New York, 1955. Lomas, J.; Frankel, H.; Hirsch, 1. Agric. Meteorol. 1964, 1, 225. Erasmus, M. N. J. Chiredzi Res. Stn InJ Bull. 3/71 Dept. Res. Specialist Services, Rhodesia, 1971. In D r f t from Aerial Spraying Bayer Australia Ltd, 1977. Johnstone, D. R.; Johnstone, K. A. PANS 1977, 23, 13.