Pestic. Sci. 1986, 17, 183-193

Enhanced Degradation of Iprodione and Vinclozolin in Soil

Allan Walker, Pauline A. Brown and Andrew R. Entwistle

National Vegetable Research Staiion, Wellesbourne, Warwick CV35 9EF

(Revised manwcript received 23 April 1985)

Residues of iprodione and vinclozolin were measured following repeated application of the fungicides to a sandy loam soil in the laboratory. There was a progressive increase in rates of degradation with successive treatments. With iprodione, for example, the times for 50% loss of the first and second applications were about 23 and 5 days respectively. When treated for the third time, less than 10% of the applied dose remained in the soil after just 2 days. Similar results were obtained with vinclozolin in the same soil, and with both compounds in a second soil. In a third soil, which had relatively low pH, degradation of both compounds occurred only slowly and the rate of degradation of a second application was the same as that of the first. Degradation rates in this soil were increased by addition of 100 g kg-' of a soil in which degradation occurred more readily, and they were markedly increased by addition of 100 g kg-' of a soil in which enhanced degradation had been previously induced. Residues of both fungicides were also measured following repeated application in the field. When iprodione was applied to previously untreated plots, about 3% of the initial dose remained in the soil after 77 days. When applied to plots treated once before, less than 1% remained after 18 days, and when applied to plots treated twice previously less than 1% remained after 10 days. Similar results were obtained with vinclozolin. Enhanced degradation of subsequent soil treatments was also observed following a sequence of low-dosage sprays in the field.

1. Introduction The phenomenon of enhanced degradation of pesticides following repeated application to soil was first observed in the late 1940s for the phenoxy-alkanoic acid herbicides 2,4-D, 2,4,5-T and MCPA.'-2 Although similar behaviour has been observed with some other pesticides,394 9 5 it is only recently that consequences of direct practical importance have become apparent. For example, Rahman et aL6 and Obrigawitch et al.' reported reduced activity of the herbicide EF'TC in some soils receiving annual applications, and more rapid degradation of the herbicide in the previously-treated compared with previously-untreated soils was subsequently d e m ~ n s t r a t e d . ~ ~ ~ Similar observations were made with the insecticide carbofuran; there was evidence of reduced efficacy following repeated use at the same site,"." and enhanced microbial degradation was observed in these previously-treated soils."* l2

Another example with possible practical consequences is the use of the fungicide iprodione (3-(3,5-dichlorophenyl)-Nisopropyl-2,4-dioxoimidazolidine)-l-carboxamide) for the control of white rot disease (Sclerotium cepivorurn (Berk.)) of onions. Entwi~tle '~ reported failure to control the disease at a site where iprodione had been used repeatedly. The results of preliminary experiments indicated more rapid degradation in a number of previously-treated compared with similar untreated soils.14 These results were not conclusive, however, because of consistent differences in pH between the previously-treated and previously-untreated soil samples. The present experiments were therefore made to examine further the possibility of enhanced degradation of iprodione in soil. Samples of soil with no history of iprodione application were treated with the fungicide in both laboratory and field studies and changes in the rates of

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184 A. Walker et d.

fungicide degradation were monitored. Experiments were also made with the related compound vinclozolin ((RS)-3-(3,5-tlichlorophenyl)-5-methyl-5-vinyl-l,3-oxazolidine-2,4-dione) to investi- gate whether this might also degrade in soil more rapidly following repeated application.

2. Experimental methods and results

2.1. Soils and herbicides The soils were collected from the surface 10 cm of four sites with no known history of iprodione or vinclozolin application. Their properties are listed in Table 1. Organic carbon was determined by the Walkley-Black dichromate titration method, mechanical analysis by the pipette/ sedimentation procedure, and pH of soil suspensions in 0.01 M calcium chloride (1 g in 2.5 ml) by glass electrode. Water contents at an applied pressure of 33 kPa (0.33 bar) were determined using a pressure membrance apparatus similar to that described by Heining.15 The fungicides used were commercial wettable powder formulations of iprodione (‘Rovral’, 500 g kg-’ w.P.) and vinclozolin (‘Ronilan’, 500 g kg-’ w.P.), together with analytical grade samples of the two compounds.

Table 1. Soil properties

Mineral fraction (%) Organic Water content at

Site - ~ carbon applied pressure number Clay Sand Silt (%) pH of 33 kPa (%)

1 ;7.7 67.5 14.8 1.05 6.5 14.9 2 l9.1 68.9 12.0 0.68 6.3 11.7 3 8.6 75.7 15.7 1.13 5.0 15.4 4 15.9 70.0 14.1 0.88 6.1 12.4

2.2. Gas-liquid chromatography Residues of iprodione and vinclozolin were extracted from soil by shaking duplicate subsamples (30 g) with acetone (50 ml) for 1 h on a wrist-action shaker. The samples were centrifuged and clear supernatant (30 ml) was removed. This was evaporated to near dryness using a vacuum rotary evaporator; 50 g litre-’ aqueous sodium sulphate solution (50 ml) and ethyl acetate (20 ml) were then added and the mixture was shaken. The concentration of iprodione or vinclozolin in the ethyl acetate was determined by g.1.c. using a Pye-Unicam Series 104 gas-liquid chromatograph with a thermionic nitrogen detector. The column used for iprodione analysis was 5% SE30 on Chromosorb-WHP and flow rates were: carrier gas (nitrogen) 50 ml min-’, hydrogen 28 ml min-’, air 400 ml min-I. Temperatures of the injection port, column and detector were 240, 235 and 300°C respectively. Duplicate injections (3pl) of the unknown solutions were made and the peak heights were compared with those obtained from similar injections of solutions containing known concentrations of iprodione. The column used for vinclozolin analysis was 3% OV1 on Chromosorb-WHP and the chromatographic conditions were the same as those for iprodione, except that the column and injection port temperatures were 185 and 195°C respectively. Recovery from soil sample fortified in the range 0.4 to 4.0 mg kg-’ was 96(?3.1)% for iprodione and 95(?2.9)% for vinclozolin and the limit of detection was approximately 0.05 mg kg-’ for both compounds.

2.3. Repeat treatments in the laboratory 2.3.1. Degradation in a sandy loam soil Ten samples (500 g) of soil 1 (Table 1) were air-dried overnight while further samples (25 g) were dried in an oven at 110°C to determine soil moisture content. A suspension of the commercial formulation of iprodione in water was incorporated into six of the samples to give an

Degradation of iprodione and vinclozolin in soil 185

initial concentration of 4.0 mg kg-' soil with soil moisture at the appropriate 33 kPa percentage (Table 1). Water, but no fungicide, was mixed with the other four samples. This procedure was repeated with a further ten samples of the same soil and the fungicide vinclozolin. All samples were incubated in polypropylene containers at 25°C and moisture contents were maintained as described previ~usly. '~ At intervals during the subsequent 49 days, duplicate amounts of soil (30 g) were removed from two of the treatments containing iprodione and two containing vinclozolin. Residual concentrations of the fungicides were measured as described above.

Fifty days after the incubation began, all of the other fungicide-treated and untreated soil samples were removed from their containers and spread separately in plastic trays on the laboratory bench. They were weighed periodically, and when 10 g water had evaporated, this was replaced with water (10 ml) or with a suspension of the appropriate fungicide in water (10 ml) to give a fresh addition of 4.0 mg kg-' dry soil. The treatments prepared in this way were four replicate samples treated for the second time with iprodione or vinclozolin, two replicate samples treated for the first time with each fungicide and four samples containing no fungicide.

Days after initid treatment

Figure 1. Degradation of (a) iprodione and (b) vinclozolin in soil 1. 0 sequential treatments at time 0, 50 days and 100 days; 0 treated once only, after pre-incubation for 50 days; A treated once only, after pre-incubation for 100 days.

186 A. W.Urer et ol.

Two of the soil samples treated for the second time with the fungicides, and those treated for the first time, were sampled as before at intervals during the subsequent 49 days and the residual concentrations were measured.

After 50 days (i.e. 100 days from the start of the experiment), the soils that had not been subsampled were removed from their containers, dried until 10 g water had evaporated, and treated again with the appropriate fungicide as before. This gave duplicate samples treated for the third time with iprodione or vinclozolin and duplicate samples treated for the first time with each cornpound. The treated soils were sampled at intervals and the residual concentrations were measured. At the end of the experiment (150 days), the pH values of all 20 soil samples were measured to determine whether there were any differences between treatments. The average pH was 6.45 with standard deviation of 0.23, compared with the pH of 6.5 measured when the soil was collected (Table 1).

The results of the residue measurements are shown in Figure 1. In soil treated at time 0, the time for 50% loss of iprodione was about 23 days. Less than 2% of the applied dose was recovered after 49 days. In soil treated for the second time (day 50), iprodione degraded more rapidly, and the time for 50% loss was approximately 5 days. When treated for the third time (day 100), only 10% of the applied dose remained 2 days later, and none was recovered after 7 days. In the soil treated for the first time after pre-incubation without fungicide for 50 days, iprodione degraded more rapidly than when added to untreated soil at time 0, and the time for 50% loss was 12 to 15 days. It disappeared at a similar rate when added to soil pre-incubated without fungicide for 100 days. The results with vinclozolin (Figure 1) were similar to those for iprodione. When added to previously untreated soil at time 0, less than 10% of the applied dose remained after 40 days. Following re-treatment at 50 days, less than 10% remained 23 days later, and when treated for the third time, less than 10% remained after just 2 days. The rates of degradation in the samples treated for the first time on day 50 or day 100 were similar to the rate observed in the sample treated for the first time at the start of the experiment.

2.3.2. Degradation in a clay loam soil The above experiment was repeated with soil 2 (Table 1) in order to determine whether a similar enhancement of degradation rates could be achieved by repeated applications to a second soil. The procedure was identical to that described above except that no samples of untreated soil

Days

Flgme 2. Degradation of (a) iprodione and (b) vinclozolin in soil 2. 0 first treatment; 0 second treatment; A third treatment.

Degradation of ipradione and vinclozolin in soil 187

were incubated prior to fungicide application at 50 and 100 days. The results (Figure 2) demonstrate a progressive increase in the rate of degradation of both iprodione and vinclozolin with successive applications. With iprodione, for example, the time for 50% loss of the first treatment was approximately 7 days. With the second and third treatments it was approximately 5 and 2 days respectively. The results with vinclozolin were very similar; the times for 50% loss of the first, second and third applications were about 7, 4 and 2.5 days respectively.

2.3.3. Degradation in a soil of low pH In previous experiments with ip r~d ione , '~ degradation occurred very slowly in certain soils, particularly those with low pH ( 4 . 5 ) in which more than 50% of the initial dose remained after an incubation period of 77 days. A fresh sample of one of these soils was collected (soil 3, Table 1) and experiments were made to determine whether enhanced degradation could be induced either by the repeated application of the fungicides to this soil, or by the introduction of small quantities of other soils which were known to degrade the compounds more rapidly. Fourteen samples (800 g) of soil 3 (Table 1) were prepared and incubated as described previously (section 2.3.1.). Four of these contained iprodione and four contained vinclozolin at 4 mg kg-' dry soil. The other six samples were not treated with fungicide. One iprodione-treated and one vinclozolin-treated soil were sampled as before, at intervals during the subsequent 140 days. The results confirmed that iprodione and vinclozolin degraded slowly in this soil with respectively 64% and 47% of the initial doses remaining at 140 days.

After 140 days, the soils that had not been sampled were removed from their containers, dried until 10 g water had evaporated, and re-treated with the appropriate fungicide as before. This procedure gave triplicate soil samples treated with iprodione or vinclozolin for the first time and triplicate samples treated with each compound for the second time. The samples treated for the first time were incubated with an initial concentration of 4.0 mg iprodione or vinclozolin kg-' soil. Those treated for the second time were given an addition of 2 mg iprodione kg-' or 3 mg vinclozolin kg-' as appropriate, so that the overall concentration in each instance was approximately 4.5 mg kg-'. Further treatments were applied to individual soils within each group of three samples. One received an addition of the same untreated soil (soil 3, 80 g), one received an addition of untreated soil which had been shown to degrade the fungicides more rapidly (soil 1, 80 g), and one received an addition of soil 1 in which enhanced degradation of the fungicides had been induced previously (soil 1 pre-treated three times with the appropriate fungicide, 80 g). This additional soil was incorporated uniformly into the samples by passing several times through a 2mm mesh sieve. All samples were incubated at 25°C as before. They were subsampled at intervals during the subsequent 30 to 70 days and the residual concentrations of the fungicides were measured.

The results (Figure 3) once more confirmed the slow degradation of these fungicides in soil 3. When treated for the first time after pre-incubation without fungicide for 140 days, about 80% of the iprodione and 60% of the vinclozolin was recovered after 69 days. Treatment for the second time, following incubation with the appropriate fungicide for 140 days, had little effect on the rate of fungicide loss. Addition of 10% of soil 1 to the samples of soil 3 increased the rates of loss. There was a period of 14 to 21 days when both fungicides degraded at rates similar to those observed in the samples containing soil 3 only, but subsequent degradation occurred more rapidly in the soil 3 plus soil 1 mixtures. After 50 days, less than 10% of either fungicide remained in these soil samples. The increased rates of loss were more pronounced when soil 1, in which apparently enhanced rates of loss had been induced previously, was added to soil 3. Rapid degradation commenced immediately after addition of the 'conditioned' soil. With iprodione less than 20%, and with vinclozolin less than lo%, of the applied amount remained after 10 days. Additions of the untreated or previously-treated samples of soil 1 had similar effects on iprodione and vinclozolin degradation in soil 3, irrespective of whether the latter soil was treated with the fungicides for the first or second time.

Measurements were also made of soil pH in the various treatments. The initial pH of soil 3

A. Walker rt al.

100

80

60

40

'z 2c

a E

c

0

0

C

0

- .- c .- .- 'c

Days

Figwe 3. Degradation of (a) iprodione and (b) vinclozolin in soil 3. 0,O soil 3funtreated soil 3 (90+10 by weight); A, A soil 3+untreated soil 1 (W+10 by weight); D, 0 soil 3+pre-conditioned soil 1 (90+10 by weight); - - - - - soil pre-incubated with no fungicide for 140 days; __ soil pre-incubated with appropriate fungicide for 140 days.

Degradation of iprodione and vinclozolin in soil 189

was 5.0 and of soil 1 was 6.5 (Table 1). When measured 30 days after preparation of the soil 3/soil 1 mixtures (i.e. 170 days after the start of the experiment), the mean pH of the samples which comprised soil 3 alone was 4.69 (k0.02). The mean pH of the samples of soil 3 with 10% untreated soil 1 was 5.03 (k0.09), and with 10% 'conditioned' soil 1 was 5.10 (k0.05).

2.4. Repeat treatments in the field 2.4.1. Effects on persistence In order to determine whether enhanced degradation could be induced in soil in the field, an experiment was made at the site from which soil 1 (Table 1) was collected. The experiment comprised three replicate blocks of six plots with a plot size of 5 ~ 1 . 5 m. Two replicate plots in each block were sprayed with iprodione and two with vinclozolin at 5.0 kg a.i. ha-' on 28 September 1983. On the day of fungicide application and at intervals during the subsequent 140 days, 30 cores (2.5 cm diameter to a depth of 10 cm) were taken from each plot at random positions, bulked for each plot, and thoroughly mixed by passing several times through a 2 mm mesh sieve. Fungicide residues were determined as described previously (section 2.2.).

One plot sprayed with iprodione and one plot sprayed with vinclozolin in each block were resprayed with the appropriate fungicide (5.0 kg a.i. ha-') on 14 February 1Y84. These plots were sampled as before at intervals over the next 90 days, and fungicide residues were measurea by g.1.c. The whole experiment was sprayed again on 14 May 1984. All of the plots sprayed previously with iprodione plus one untreated plot per block were sprayed once more with iprodione (5.0 kg a.i. ha-') and appropriate plots were sprayed with vinclozolin. Residue measurements were made as before at intervals over the subsequent 77 days. Following application in September, residues of iprodione disappeared rapidly and less than 10% of the applied dose remained after 60 days. Vinclozolin was somewhat more persistent and about 25% of the initial dose remained after 100 days.

At the time of re-treatment, after 140 days, less than 5% of the initial dose of either compound was recovered from the soil. When applied in February, iprodione degraded more rapidly than before, and less than 5% of the initial dose was recovered after 34 days. Vinclozolin was again somewhat more persistent than iprodione with about 14% of the initial dose remaining after 34 days and 3% remaining after 55 days. The patterns of residue decline following application in May 1984 are shown in Figure 4. When applied for the first time, the time for 50% loss of iprodione was about 22 days and approximately 3% of the initial dose was recovered after 77 days. When applied for the second time, the time for 50% loss was only 5 days and less than 1% of the applied dose remained after 18 days. On plots treated for the third time, less than 1% of the applied dose remained after 10 days. There were similar marked differences in rates of degradation of vinclozolin (Figure 4(b)).

2.4.2. Changes in degradation rates following application of low doses Although iprodione and vinclozolin are applied as seed and soil treatments for the control of white rot disease of onions, their main use is for control of foliar diseases in several crops. This can involve the application of a sequence of sprays at a relatively low dose during the growing season. Although intended for retention by the foliage, some of the applied chemical will reach the soil and might lead to enhanced degradation of subsequent soil treatments. An experiment was made to examine this possibility. An area of autumn-sown onions grown at the National Vegetable Research Station, Wellesbourne (soil properties at the site: soil 4, Table l), was sprayed routinely as a precaution against Botrytis infection, with 0.5 kg a.i. iprodione ha-' on 9 September, 10 October, 17 November, 16 December 1983 and 18 January 1984. After onion harvest in July, and before any subsequent soil cultivation, the soil of this area was sampled (100 cores taken at random positions; 2.5. cm diameter to a depth of 5 cm) and an adjacent area which had remained fallow throughout the growth of the onion crop was sampled in a similar way. The two soil samples were sieved separately (2 mm mesh) and iprodione was incorporated into duplicate subsamples (500 g) of each at a concentration of 5 mg kg-' dry soil. These soils

190 A. Walker d d.

I

60 -

40 -

20 -

\

0 20 Days after 14 May 1984

Figure 4. Persistence of (a) iprodione and (b) vinclozolin in soil in the field following application in May 1984. 0 previously untreated plots; 0 plots sprayed once previously with the appropriate fungicide; A plots sprayed twice previously with the appropriate fungicide.

were incubated at 25°C and the soil's standard soil water content (Table 1) and sampled at intervals during the subsequent 50 days. Measurements of residual concentrations (Figure 5) demonstrated more rapid degradation in the soil collected from the sprayed area than in that from the non-sprayed area.

3. Discussion One approach to the study of enhanced degradation of pesticides in soil following repeated application, is to examine their persistence in paired samples of 'problem' and 'non-problem'

Degradation of iprodione and vinclozolin in soil 191

Days

Figure 5. Degradation of iprodione in soil 4 in the laboratory. 0 soil from field plots sprayed with 5 ~ 0 . 5 kg a.i. iprodione ha-’ at monthly intervals; 0 unsprayed soil from the same site.

soils. Problem soils are defined as those in which the chemical has failed to control the target pest in the field and non-problem soils as those which have identical properties but with no known history of use of the particular chemical.16 An alternative approach is to study degradation in soils having no history of treatment with the pesticide of interest and to measure in the laboratory the effects of pre-treatments on disappearance rates of subsequent applications.” One disadvantage of the first approach is the difficulty of obtaining paired soil samples with identical properties; a disadvantage of the second is that behaviour in the laboratory may not always give a true indication of the significance of the effects in the field.

In previous studies with iprodione using paired samples of soil from several fields, degradation was more rapid in the previously-treated than in the previously-untreated samples. l4 However, the untreated samples were collected mainly from headland areas and field boundaries and they were of consistently lower pH than the corresponding treated soils. The results were therefore inconclusive. In the present experiments, soil with n o history of iprodione or vinclozolin application was treated with the appropriate fungicide in both the laboratory and the field. Differences in pH between treatments were generally very small and there were increased rates of loss of subsequent applications, irrespective of whether pre-treatments were made in the laboratory (Figures 1 and 2) or in the field (Figures 4 and 5) . These results therefore provide more conclusive evidence that enhanced degradation of these compounds does occur following repeated use.

The results in Figures 1 and 2 are similar to those reported previously for degradation of EPTC, butylate’ and carbofuran,” all of which show increased rates of loss with repeated application. There appear to be differences between compounds, however, in the relative changes in degradation rate and in the number of treatments required to achieve maximum rates of loss. Obrigawitch et d9 for example, demonstrated that the time for 50% loss of butylate from a silt loam soil treated with the herbicide for the first time, was approximately 25 days. The rate of loss progressively increased following treatment on three subsequent occasions (after 50, 100 and 150 days). The time for 50% loss following the fourth application was about 5 days. In the

192 A. Walker el d.

same experiments, the time for 50% loss of the initial application of EPTC was 13 days; for the second application it was ,just 3 days. The degradation curves for the third and fourth applications were similar to the curve observed for the second, indicating that the full potential of the soil to degrade EPTC was achieved after a single application. Even more striking results were observed with carbofuran. Harris ef ~ 1 . ' ~ reported that the time for 50% loss of carbofuran added at 10 mg kg-' to a previously untreated sandy loam was about 12 days, with approximately 10% remaining after 21 days. When a further 10 mg carbofuran kg-' was added at 28 days, less than 10% remained 1 day later. The present results with iprodione and vinclozolin (Figures 1 and 2) show changes in degradation rates similar to those reported for butylate-there was a progressive increase in rates of loss with successive treatments.

With iprodione in particular, there was also an apparent increase in degradation rate in soil incubated without fungicide for 50 or 100 days, although the changes were relatively small compared with those observed in the samples treated with the fungicides at time 0. This effect was also observed previously by Harris et al.17 with carbofuran and attributed to either contamination of the previously untreated soil with 'active' soil during mixing or to increases in the total numbers of microorganisms in the soil during the first 3 weeks' incubation.

There were differences between soils in the rates of iprodione and vinclozolin degradation. The relative order of degradation in soils treated for the first time was soil 2=soil 4>soil l>soil 3 for iprodione, and soil 2>soil l>soil 3 for vinclozolin. Degradation of both compounds was considerably slower in soil 3 than in the other soils. This again suggests that soil pH has an effect on degradation since the major difference between soil 3 and the others was its relatively low pH (Table 1). There was no evidence that the degradation of a second application of iprodione or vinclozolin in this soil proceeded more rapidly than that of the first. On the assumption that degradation is mi~robia l , '~ this suggests that either the organisms responsible for degradation were present in very low numbers or that their growth was not stimulated by the presence of the compounds in this soil, possibly because of the low pH. Degradation of the fungicides was stimulated by incorporating 80 g soil 1 into 800 g soil 3 (Figure 3), even though the pH of the mixture was not very different from that of the original slow-degrading soil. This implies that the organisms can survive in soil at pH 5.0, at least for periods of up to 40 days. Kaufman et al." suggested that the inability of some soils to induce rapid degradation may be due to strong adsorption of the pesticide which renders it unavailable for degradation. In the present experiments, the total fungicide content of the samples of soil 3 treated for the second time comprised approximately equal amounts of residual and freshly added iprodione or vinclozolin. This mixture degraded rapidly in soil 3+pre-treated soil 1 (Figure 3) which strongly suggests that the fungicides were not so tightly bound as to prevent degradation in soil 3.

Accelerated degradation of iprodione and vinclozolin was induced following application in the field (Figure 4). A single application of 5 kg ha-' of either compound appeared to be sufficient to cause more rapid loss from subsequent treatments, and the effects were observed eight months after the first treatment. Relatively low doses of iprodione applied repeatedly ( 5 ~ 0 . 5 kg ha-') also caused more rapid degradation of this fungicide seven months after the final treatment (Figure 5), even though not all of the chemical applied initially would have reached the soil. It therefore appears that accelerated degradation of iprodione and vinclozolin can be induced quite readily in certain soils and that the effects may last for some time following application.

The practical significance of these results is not clear. Failure of iprodione to control white rot has been confirmed at one site'3 and recent evidence suggests that vinclozolin has also lost effectiveness at this site.'' There is, at present, no indication that the effects are more widespread. An alternative reason for loss of control could be development of resistance to the fungicides in the pathogen. S. cepivorum has the capacity to develop resistance to dicarboxamide fungicides in vitro,20 but in the field resistant isolates did not appear to contribute to loss of control.'' There is a need to monitor the performance of these fungicides in commercial practice and if a decline in effectiveness against white rot becomes apparent, to determine whether this is a consequence of development of resistance in the organism or of enhanced degradation in the soil.

Degradation of iprodione and vinclozolin in soil 193

Acknowledgements The authors are grateful to May & Baker Ltd and BASF United Kingdom Ltd for their interest in this work and for providing samples of analytical grade iprodione and vinclozolin respectively.

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10. Felsot, A. S.; Wilson, J. G. ; Kuhlman, D. E.; Steffey, K.L. 1. Econ. Entomol. 1982, 75, 1098-1103. 11. Felsot, A. S.; Maddox, J. V.; Bruce, W. Bull. Environ. Contam. Toxicol. 1981, 26, 781-788. 12. Read, D. C. Agric. Canada Res. Sta., Charlottetown, Research Report 1983, 68-69. 13. Entwistle, A. R. Phytopathol. 1983, 73, 800. 14. Walker, A,; Entwistle, A. R.; Dearnaley, N. J . British Crop Protection Council Monograph No. 27, Soils and Crop

Protection Chemicals, 1984, 117-123. 15. Heining, B. J . Agric. Eng. Res. 1963, 4-49, 16. Kaufman, D. D.; Edwards, D. F. Proc. 5th lnt. Congr. Pestic. Chem. 1983, 4, 117-123. 17. Hams, C. R.; Chapman, R. A.; Harris, C.; Tu, C. M. 1. Environ. Sci. Health Part B 1984, 19, 1-1 1. 18. Kaufman, D. D.; Katan, Y.; Edwards, D. F. ; Jordan, E. G. BeltsviIle Agric. Res. Cent. Agricultural Chemicals of the

Future. Symp. No. 8, (J. L. Hilton, Ed.), Rowman and Attenheld, Washington D. C. 1984. (In press.) 19. Entwistle, A. R.; Hawling, S . L. Rep. Natn. Veg. Res. Stn. (Wellesbourne, Engl.) 1983 1984, 67. 20. Entwistle, A. R. Proc. 10th lnt. Cong. Plant Prot. 1983, 629.