Pestic. Sci. 1984, 15,61&623

The Effect of Simulated Rain on Deposits of Some Cotton Pesticides"

Frans E. Pick, Louis P. van Dyk and Pieter R. de Beer

Plant Protection Research Institute, Private Bag X134, Pretoria, South Africa OOO1

(Revised manuscript received 28 June 1984)

Commercially grown cotton and potted cotton plants were sprayed with five pesticides, and simulated rain was applied 1-72 h later. Leaf samples were analysed to determine the effect of the rain on the original deposits. It was found that 2 to 5 mm of simulated rain applied 1 h after spraying, washed off 50% or more of the original deposit. An increase in rainfastness of the pesticides occurred over a period of time after spraying. The type of formulation seemed to affect rainfastness, but the origin of a formulation, the addition of wetting agents and the intensity of the simulated rain did not.

1. Introduction If rain removes insecticide that has just been applied to cotton plants, pest mortality will be reduced to a greater or lesser extent, depending on how much insecticide is washed off. Little work seems to have been done on this problem. However, despite inconclusive tests with parathion-methyl' and malathion,' there is good evidence that rain washes off deposits of DDT, monochrotophos and malathion from cotton leaves.*' In South Africa, if rain falls shortly after an application of insecticide to cotton, an arbitrary rule is used by farmers to decide whether to respray. The rule, for which there is no experimental support, is that they should respray if more than 9 mm rain falls within 4 h of spraying.

The work reported here was undertaken to determine which kinds of rainfall and which properties of a pesticide formulation affect rainfastness. Two types of experiments were carried out. First, a field of commercially grown cotton was sprayed with various pesticides, and an overhead sprinkler was used to simulate rainfall. As the overhead sprinkler system could not be finely adjusted, a second set of experiments was carried out with a more controllable garden sprinkler and potted cotton plants.

2. Materials and methods 2.1. Experiments in a field of commercially grown cotton

with simulated rain from an overhead sprinkler Insecticides used to control the American bollworm, Heliothis arrniger Hiibner, were sprayed in successive experiments. The insecticides were endosulfan 50% emulsifiable concentrate (e.c.) (Hoechst) at 650ml (325ga.i.) in 200 litres water ha-'; carbaryl 85% wettable powder (w.P.) (Union Carbide) at 1.5kg (1275ga.i.) in 200 litres water ha-'; carbaryl 30% in molasses suspension concentrate (s.c.) (Union Carbide) at 1.75 litres (525ga.i.) in 200 litres water ha-'; and cypermethrin 20% (e.c.) (Shell) at 350ml (70ga.i.) in 200 litres water ha-'. A previously calibrated rucksack sprayer with ten nozzles was used. Plants were sprayed between 0800 and 1000 hours, when they were free from dew. The plants were between 3 and 5 months old and were grown in rows 1 m apart.

'Part of this paper was presented at the Third Entomological Congress of the Entomological Society of Southern Africa, held on 16-18 September 1980 in Pretoria.

616

EfTect of simulated rain on pesticide deposits 617

An overhead irrigation pipeline was laid across an experimental plot (130~22m) marked out in a large cotton field that had been sprayed with one of the above-mentioned insecticides. Five sprinklers were mounted on the pipeline at 18m intervals. The areas covered by the sprinklers did not overlap. One sampling plot was selected in each area. In these sampling plots, the quantity of water applied was measured by a grid of rain gauges placed at the comers of a 4 ~ 4 m square, with a further gauge in the middle of the square. They were mounted on poles 130cm high (about as high as the top of the plants). The mean quantity measured by the five gauges was taken as the amount of simulated rainfall that fell on a sampling plot. The intensity of applied rain was constant within the range 15-18mm h-'.

The effect of the quantity of simulated rain on rainfastness was determined on one plot by taking samples before and then immediately after 1, 2, 5 and 10mm of simulated rain had been applied. After spraying, 1 h was allowed to elapse before the application of rain. To determine the effect of a constant quantity of simulated rain on an aged deposit, 5 mm of simulated rain was applied to separate plots 1, 4, 7, 24 and 72h after spraying.

Leaf samples were collected from the top sections of cotton plants in the square formed by the rain gauges. Twenty leaf discs were taken per sample, and four replicate samples per square. The discs (2.54cm diam.) were punched with a leaf punch;' they were collected in 500ml glass bottles to which organic solvent (100 ml) was added, and the bottles were stored in a freezer as soon as possible.

The weather during the field trials was sunny and warm to hot; the mean minimum temperature ranged from 15.5 to 16.5"C and the mean maximum from 26 to 28°C. The average sunshine was 6.5 hday-'. Rain fell on the second day of the endosulfan experiment (11 mm) and on the night after the start of the carbaryl-molasses experiment (27 mm). This upset the planned experiments, and the effect of rainfall on an aged deposit could not be determined.

2.2 Experiments with potted cotton plants and simulated rain from a garden sprinkler The experiments with potted cotton plants were designed to test five hypotheses: (a ) that an increase in the amount of rainfall increases the removal of a pesticide deposit; (b) that the time of onset of rain after pesticide application affects the amount of pesticide removed; (c) that the rain intensity (volume per unit time) affects the amount removed; ( d ) that products formulated by different agrochemical companies behave differently as far as rainfastness is concerned; and (e) that wetting agents used as adjuvants affect the rainfastness of a pesticide deposit.

Only one insecticide, parathion, was used in these experiments. Although it has a limited use on cotton, it is produced in various formulations by different agrochemical companies, and deposits can be determined by gas-liquid chromatography (g.1.c.). It was sprayed with a rucksack with ten nozzles. Parathion 50% e.c. (Bayer and Agro-Serve) was sprayed at 650ml (325g active ingredient) in 100 litres water ha-', and parathion 25% w.p. (Bayer) was sprayed at 500g (125ga.i.) per 100 litres water ha-'. The two wetting agents used were Effekto G49 Wetter (Agncura) at 0.2ml litre-' of spray mixture, and Agrogard Wetting Agent (Agro-Serve) at 0.5 ml litre-' of spray mixture.

A simple circular garden sprinkler, the Procast Mist Spray, was selected for use as a rainfall simulator. This type of sprinkler gives an even distribution within the irrigated area. Water is forced upwards through a 6-mm orifice of the sprinkler, and the droplets formed then descend vertically. The sprinkler was mounted on a 170-cm pole and connected by ordinary garden hosepipe to a tap. The quantity of water delivered was measured by eight rain gauges mounted on 130-cm poles arranged in a circle of 180-cm radius round the sprinkler. The sprinkler delivered simulated rain at 24mm h-'. In experiments where a higher intensity was required, a Procast adjustable spray nozzle was used which could deliver a higher intensity; however, this did not deliver such an even distribution of water as the mist sprinkler. The experiments were carried out between 0800 and 1000 hours when there was little or no wind, and the temperature ranged from 20 to 24°C.

Cotton plants of the cultivar Reba were grown in large plastic bags under a hail screen. When the plants were between 3 and 5 months old they were used for the experiments. In each

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Efleet of simulated rain on pesticide deposits 619

experiment, 30 bags containing cotton plants were placed 50cm apart in a row. The plants were then sprayed by an operator moving first along one side and then the other at a predetermined speed to deliver the required dose of parathion. The leaves were allowed to dry (approximately 30xhin) and the plants were then moved to the rainfall plot and placed within the circle formed by the eight rain gauges. To determine the initial parathion deposit before the application of simulated rainfall, leaf disc samples were collected 1 h after spraying. A specific quantity of simulated rain as measured in the rain gauges was then applied. Plants were removed at intervals as required and allowed to dry; leaf disc samples were then collected with the leaf punch to determine the parathion deposit; deposits left after rainfall were calculated as a percentage of the initial deposit.

Each sample, taken before and after the simulated rainfall, consisted of 20 leaf discs and was placed in a glass bottle; three replicates were taken. To the discs in each bottle, 100ml of hexane+chloroform (3+ 1 by volume) was added, and the samples were refrigerated until extraction and analysis.

To test the first hypothesis, plants were sprayed with both an e.c. and a w.p. formulation, and then exposed to 1, 3 and 5mm of simulated rain. To test the influence of ageing on the rainfastness of a deposit, 3mm of simulated rain was applied 0.5, 1.5 and 2.5h after spraying. This was done with both the e.c. and w.p. formulations. To determine the influence of rain intensity, the effects of applying simulated rain at 56 and 20mmh-' were compared using plants sprayed with both an e.c. and a w.p. The e.c. formulations of the two commercial companies were compared by applying 3 mrn of simulated rain to sprayed plants, and the effects of the two wetting agents were compared by spraying the w.p. formulation plus each agent, and testing the rainfastness with 3 mm of simulated rain. It was decided to use 3 mm of simulated rain in many of these experiments because the results of the first experiment (Table 1) indicated that 5 mm of simulated rain would remove too much of the deposit, thus making comparisons difficult.

2.3. Extraction and analysis The concentrations of the pesticide deposits on cotton leaves were determined by extraction and analysis by g.1.c. The results were expressed as ng cm-' of leaf.

For endosulfan extra~t ion,~ each sample was blended in a mixer with lOOml of hexane+acetone (1 +1 by volume); the extract was filtered through anhydrous sodium sulphate and concentrated on a rotary evaporator to 10ml. For analysis by g.l.c., a column (180cm long, 3mm i.d.) packed with 3% SE 30 on Chromosorb 750 (100-120 mesh) was used at 200°C; the inlet temperature was at 225°C. The nitrogen carrier gas flow rate was 77mlmin-'. A flame-photometric detector was used in the sulphur selective mode at 185°C. External standards were used, but only a-endosulfan was determined because it is the more toxic isomer and it simplified the analyses.

Table 2. Effect of the age of deposits on the rainfastness of three pesticides on commercially grown cotton ~~ ~

a Endosulfan e.c. Carbaryl w.p. Cypermethrin e.c.

Time after Mean Deposit Mean Deposit Mean Deposit spraying Rain deposit left deposit left deposit left

(h) (mm) (ng L X - ~ ) S.d. (%) (ng cm-2) S.d. (%) (ng cm-*) S.d. (%)

1 0 5

4 0 5

7 0 5

24 0 5

72 0 5

317 102 128 19 183 37 70 22 93 17 84 11 88 19 73 22

Washed out Washed out

100 40

100 38

100 90

100 83

135 37 52 8

135 44 70 6 97 10 57 4 89 6 50 2 49 6 42 6

100 39

100 52 100 58

100 56

100 86

138 17

200 148 198 139 182 164 167 168

21 17 14 15 19 17 13 5

26 23

100 56

100 74

100 70

100 90

100 101

620 F. E. Pick er d.

Table 3. Rainfastness of e. .c. and w.p. formulations of parathion on potted cotton leaves, as affected by the quantity of simulated rain (tested 1 h after application of

the pesticide)

Parathion e.c. Parathion w.p. Quantity of simulated Mean Deposit Mean Deposit

rain deposit left deposit left (mm) (ngcm-') S.d . (%) (ngcm-') S.d. (%)

0 328 8 100 139 8 100 1 189 10 58 44 8 32 3 98 7 30 35 3 25 5 88 4 21 28 9 20

Carbaryl was extracted and analysed according to the method of Tilden and Van Middelem." Cypermethrin was extracted with 200 ml of light petroleum+acetone (1+ 1 by volume), according to a method by Shell Chemical Company." The acetone was washed from the filtrate with distilled water, and the light petroleum extract was dried over anhydrous sodium sulphate. Clean-up was effected on a Florisil column using diethyl ether+light petroleum (1+9 by volume) as eluting solvent. The purified solution was concentrated on a rotary evaporator to 101111. For analysis by g.l.c., a column (142cm long, 3mm i.d.) packed with 3% SP 2100 on Gas Chrom Q (80-100 mesh) was used with an electron-capture detector. The temperatures of the inlet, column and detector were 240, 225 and 275"C, respectively. The nitrogen carrier gas flow rate was 50mlmin-', and external standards were used.

For the extraction of parathion, a sample was homogenised with 100ml of hexane-tchloroform (3+ 1 by volume); the extract was filtered through anhydrous sodium sulphate and concentrated on a rotary evaporator to 10 ml. This method is similar to the method used by Van Dyk" and has been applied in this laboratory for many years. For analysis by g.l.c., a column (144cm long, 3mm i.d.) packed with 10% DC 200 on Chromosorb WHP (80-100 mesh) was used with a flame-photometric detector in the phosphorus mode. The temperatures were 225,210 and 200°C for the inlet, column and detector, respectively. The nitrogen carrier flow rate was 88mlmin-', and external standards were used.

3. Results

The results of the rainfastness experiments with commercially grown cotton plants are given in Tables 1 and 2. The standard deviation from the mean is relatively large for most of the results; this can probably be attributed to the inherently large variation normally encountered in this kind of experiment. The collection of a large sample of 20 leaf discs should reduce the variance, but it is the author's experience" that this large variance is to be expected in determining deposits on leaves-it is impossible to place an equal deposit on all the leaves. It may be reduced by taking

Table 4. Rainfastness of e . c . and w.p. formulations of parathion on potted cotton leaves, as affected by ageing of the deposit

Emulsion concentrate Wettable powder

Time after spraying Rain

(h) (mm)

0.5 0 3

1.5 0 3

2.5 0 3

Mean deposit

(ng cn-')

356 55 353 103 230 115

S.d.

15 9 20 17 8 16

___

Deposit left (%)

I00 15 100 29

100 50

Mean deposit

(ng cm-') S.d.

102 8 36 19 66 9 22 13 53 13 18 11

Deposit left (%)

100 35 100 33 100 34

EtYect of simulated rain on pesticide deposits 62 1

Table 5. Rainfastness of e x . and w.p. formulations of parathion on potted cotton leaves, as affected by the intensity of simulated rain (tested 1 h after application of the pesticide)

Emulsion concentrate Wettable powder

Rain Rain intensity quantity (mm h-’) (mm)

0 56 3

0 20 3

Mean Deposit Mean Deposit deposit left deposit left

(ngctW2) S.d. (%) (ng cm-’) S.d. (%)

524 15 100 82 33 100 203 6 39 26 18 32 458 2.5 100 89 7 100 158 18 34 28 14 31

very large samples or a large number of samples, both of which greatly increase the cost. A very rapid loss of a-endosulfan was found in the absence of rain; however, this loss cannot be explained (Table 2).

Except for cypermethrin, increasing the amount of rainfall removed progressively more deposit; 10mm removed 44-72% of the original amount (Table 1). The difference found in the amounts deposited between the carbaryl w.p. and the carbaryl S.C. in Table 1, may be due to a better wetting of leaves by the latter formulation. However, the w.p. was applied at about twice the dose of the S.C. (1275ga.i. versus 525ga.i.). The rainfastness of deposits seemed to increase with the time after spraying (Table 2).

Table 6. Rainfastness of parathion on potted cotton leaves, as affected by the origin of the e.c. formulation (tested 1 h after application of the pesticide)

Bayer parathion e.c. Agro-Serve parathion e.c. Quantity of simulated Mean Deposit Mean Deposit

rain deposit left deposit left (mm) (ng cm-’) S.d. (76) (ng cm-’) S.d. (%)

0 348 21 100 272 25 100 3 72 14 21 63 6 23

The results of experiments with parathion on potted cotton plants are presented in Tables 3 to 7. Because of the relatively small number of plants available for these experiments, only three replicates could be made. This may explain the few high standard deviations found. Small quantities of simulated rain removed a large part of the deposit of parathion e.c. and w.p. (Table 3). The rainfastness of the ex . , but not of the w.P., increased noticeably over a few hours after spraying; however the e.c. appeared to be less fast to rain falling within 0.5 h of treatment (Table 4). Little difference was found between the effects of two intensities of rain on deposits of the e.c. and w.p. (Table 5). The two e.c. formulations tested seemed to be equally affected by simulated rain (Table 6), and the two different manufactures of wetting agents had about the same effect on rainfastness of the w.p. (Table 7).

Table 7. Effect of wetting agents on the rainfastness of parathion w.p. on potted cotton plants (tested 1 h after application of the pesticide)”

Effekto wetting agent Agrogard wetting agent Quantity of simulated Mean Deposit Mean Deposit

rain deposit left deposit left (mm) (ng cm-’) S.d. (%) (ng cm-’) S.d. (%)

0 89 16 100 116 23 100 3 26 7 29 39 27 34

“For comparison with Tables 3-5.

622 F. E. Pick el al.

4. Discussion

The effect of simulated rain on pesticide deposits can be severe. Even l m m of simulated rain, within 1 h after application was sufficient to remove an appreciable part of the deposit of some pesticides. About 70% of a parathion w.p. was removed (Tables 3-5, and 7). Other formulations were more rainfast, for example, 1 mm of simulated rain removed only 4% of the original deposit of a carbaryl-molasses S.C. (Table 1). This difference was most probably due to the inherent characteristics of each formulation, such as the type of wetting agents used. The fact that deposits of some of the pesticides seem to become resistant to simulated rain faster than others is probably due to the inherent qualities of the formulations concerned, which may promote the rainfastness of a deposit and may also speed up penetration of the pesticide into the plant surface. No generalisation is possible, and thus each pesticide formulation has to be evaluated separately for rainfastness. Nevertheless, it is clear from the results with the five pesticide formulations tested, that between 2 and 5mm of raiii, 1 h after spraying, will remove about 50% of the original deposit.

Deposits that have aged generally become more rainfast. The speed at which a pesticide penetrates the leaf surface probably determines its increased resistance to wash off. The pesticide formulations tested varied in the speed at which they became resistant to simulated rain. The cypermethrin e.c. formulation rapidly became rainfast (Table 2), whereas the parathion w.p. formulation showed no signs of increased rainfastness over 2.5 h (Table 4), unlike the parathion e.c. formulation. The speed of penetration of a formulation may be influenced by the nature of the active ingredient, which may be lipophilic. The formulating agents added to the mixture may also increase the transport of the active ingredient into the plant tissue; the two formulations of parathion behaved differently, the e x . formulation showed a rapid increase in rainfastness, whereas the w.p. formulation did not show any increase in rainfastness over 2.5 h (Table 4). The relatively low value of the e.c. remaining after 3mm of rain, at 0.5h after treatment, suggests that the e.c. was least fast to rain falling within 0.5 h of treatment (Table 4).

Surprisingly, the intensity of the rain (defined as volume per time unit) over the range tested did not affect the rainfastness of the parathion e.c. and w.p. (Table 5). It was expected that rain of higher intensity would remove more of the deposits because the droplets would have greater impact. Perhaps rain of very low intensity (less than 20mmh-') would remove less of a deposit, but this type of rain seldom falls in the cotton-growing areas of South Africa.

Two products of parathion ex., formulated by different companies, did not behave differently with respect to rainfastness (Table 6) and wetting agents added to a parathion w.p. formulation did not seem to change its rainfastness from that of the original formulation (Table 7 compared with Tables 3-5).

These results illustrate the problems inherent in this type of experiment. Nobody outside an agrochemical company knows the exact composition of its formulation, and this lack of knowledge makes results of experiments on rainfastness applicable only to the specific formulations tested. If the manufacturer decides to change the formulation at any time, conclusions drawn from experiments with the previous formulation may be invalidated. This problem should not prevent the recognition of the tremendous effect rainfall may have on the eventual control of pests. While it may sometimes be desirable to have a pesticide removed from a crop by rainfall, especially once the pest has been killed, in most instances it is desirable to have the pesticide on the leaves for at least a couple of days. Anything that may increase rainfastness would thus improve control of the pest and reduce the cost of pest control.

Acknowledgements The assistance of N. C. J. Basson, P. Schutte, D. A'Bear, J. L. Neen, S. Prinsloo, T. von Reiche, M. Lynch and R. Lombard in field and laboratory work is gratefully acknowledged. Bayer (SA) and Agro-Serve are thanked for donating pesticide formulations. Special thanks are also due to Mr E. F. Whiteside for his valuable comments on the paper.

Effect of simulated rain on pesticide deposits 623

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