Enhanced Degradation of Insecticides in Soil - ACS Symposium

Chapter 7

Enhanced Degradation of Insecticides in Soil Factors Influencing the Development and Effects of Enhanced Microbial Activity

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R. A. Chapman and C. R. Harris Agriculture Canada Research Centre, 1400 Western Road, London, Ontario N6G 2V4, Canada Treatment of soils with pesticides at 10 ppm generated enhanced microbial activity within 6 weeks for 14 of 18 materials tested. Increases in degradation rates ranged from 2 - 100x depending on the material and soil type. The aryl methylcarbamates and isofenphos showed the largest increases. Generally, only activity against the sulfoxide and sulfone metabolites resulted from treatment with thioether containing materials (aldicarb, phorate, terbufos, etc.). Enhanced activity to carbofuran was not generated at low temperatures (3°C), low soil moistures or with low insecticide concentrations (164 wk for carbofuran and isofenphos. The effectiveness of enhanced microbial populations was dependent on the form of the insecticide encountered. Granular formulations were less affected than more dispersed forms. For Furadan 15G there was a strong soil moisture dependence. The activity of enhanced microbial populations was significantly reduced by thermal or chemical biocide treatment. Cross enhancement occurred generally among the aryl methylcarbamates and among a few structurally similar organophosphorus insecticides.

Felsot et a l . (1) demonstrated that the disappearance rate of carbofuran from s o i l was affected by previous treatments with the i n s e c t i c i d e i n 1981. Since then, the phenomenon now generally described as enhanced microbial degradation has been intensively examined, p a r t i c u l a r l y for i n s e c t i c i d e s used to control corn 0097-6156/90/0426-0082S06.00/0 Published 1990 American Chemical Society

Racke and Coats; Enhanced Biodegradation of Pesticides in the Environment ACS Symposium Series; American Chemical Society: Washington, DC, 1990.

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rootworms, Diabrotica spp., i n North America (2). The existence of enhanced degradation of herbicides i n s o i l has been known f o r 40 years (3). The e f f e c t was f i r s t reported f o r a s o i l - a p p l i e d carbamothioate herbicide i n 1980 (4,5) and t h i s prompted renewed i n t e r e s t i n the f i e l d f o r herbicides (6). Several fungicides are now known t o be affected by t h i s phenomenon (7-9).


Laboratory Generated Enhanced S o i l Microbial A c t i v i t y In 1982, we decided that the conditions required t o produce a "history s o i l " capable of causing enhanced carbofuran degradation should be better defined. A laboratory experiment was designed that incorporated the following features: 1) a s o i l that had not been previously treated with any pesticide, 2) a method of treatment that introduced only the pure t e s t chemical to the s o i l (10), 3) frequent mixing of the s o i l t o maintain a homogeneous d i s t r i b u t i o n of the microbial populations and 4) an extensive series of samples incubated under the same conditions that could be treated at i n t e r v a l s and s t i l l permit comparison of disappearance rates with a previously untreated c o n t r o l . Under these conditions, enhanced microbial a c t i v i t y developed f o r carbofuran during the 28 days required f o r the i n i t i a l 10 ppm treatment t o decline t o ca. 0.5 ppm (11). C l e a r l y a long "history" of carbofuran treatment was not a requirement. Using t h i s technique, we have examined a v a r i e t y of i n s e c t i c i d e s and herbicides for t h e i r a b i l i t y to r a p i d l y generate enhanced microbial a c t i v i t y i n soils.The r e s u l t s of these studies are summarized i n Table I. During the course of the work i t became c l e a r that a quantitative measure of the degree of enhancement was required

Table I. Ε-Factors for Laboratory Treated

Material Sand Butylate Carbofuran 35 Furadan 4.8F Furadan 15G Chlorpropham Cloethocarb Diazinon DOWCO 429X EPTC Fensulfothion(p) 14 (t) Isofenphos Isofenphos(th) Oftanol 15G Phorate(p) (t) Tefluthrin

Soils

Ε-Factor f o r S o i l Type Indicated Sandy Loam Clay Loam Muck 1.3 44 28 100 47 16* 13 36 4.8 3.5 3.6 0.9 8.8 4.6 6.9 24 1.0 42 49 0.5 1.0 1.2

ρ = parent only; t = parent + metabolites (sulfoxide/sulfone) *based on t e s t i n g with carbarvl; t h = technical


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to keep r e s u l t s i n t h e i r proper perspective. We have found the r a t i o of the pseudo f i r s t order disappearance rate constants to be useful for comparison. (The rate constant for previously treated / the rate constant for not previously treated = the Ε(nhancement)-factor. For example, the f i r s t entry for carbofuran i n Table I i s derived as follows: 2.46 day" / 0.07 day" =35). For s o i l s i n which degradation i s very rapid, the factors are based on only a few d a i l y observations and therefore may vary somewhat among experiments. Tests for enhanced degradation were made 28 days a f t e r the i n i t i a l 10 ppm application. A n a l y t i c a l grade materials were used for a l l t e s t i n g and for the i n i t i a l applications except for the carbofuran and isofenphos formulations and technical isofenphos as noted. Enhanced microbial a c t i v i t y developed quickly for s o i l s treated with a n a l y t i c a l carbofuran, chlorpropham, cloethocarb, diazinon, DOWCO 429X and fensulfothion with Ε-factors ranging from 4 to 100 depending on the chemical and s o i l type. Commercial flowable and granular formulations of carbofuran also rapidly generated enhanced a c t i v i t y . The behavior of the granular formulation was p a r t i c u l a r l y i n t e r e s t i n g . The carbofuran concentration after 28 days was s t i l l above 5 ppm and too high to r e l i a b l y t e s t for a c t i v i t y against the usual 10 ppm t e s t treatment. This d i f f i c u l t y was circumvented by taking advantage of the known c r o s s - a c t i v i t y of carbofuran-enhanced microbes to carbaryl (12) and by using 10 ppm of carbaryl for the enhanced degradation t e s t . The s o i l was indeed active, but the disappearance of the residual carbofuran from the o r i g i n a l granular treatment was apparently unaffected. Enhanced a c t i v i t y also developed quickly for s o i l s treated with technical and formulated isofenphos, but those treated with a n a l y t i c a l isofenphos only developed a c t i v i t y occasionally. S o i l s treated with a n a l y t i c a l grade EPTC, butylate, t e f l u t h r i n and phorate did not develop a c t i v i t y within 28 days. This was somewhat surprising behavior for the carbamothioates which apparently can develop a c t i v i t y within 6 days of a f i r s t s o i l treatment (13). Their behavior and that of isofenphos implies that there are s t i l l unidentified factors involved i n the generation of enhanced a c t i v i t y and suggests that pure chemicals, formulated products and metabolites need to be examined to f u l l y understand the phenomenon.


1

1

Field-Generated Enhanced S o i l Microbial A c t i v i t y 2

Samples from f i e l d experiments c a r r i e d out i n microplots (ca. 2 m ) (14, Chapman,R.A., Harris,C.R. unpublished data) have proven valuable i n determining the rate of development of enhanced dégradâtive a c t i v i t y . These small plots were developed o r i g i n a l l y to allow economical evaluation of the e f f i c a c y and persistence of i n s e c t i c i d e s i n various, previously untreated s o i l types at one location (15). Their size permits the simultaneous study of a large number of materials, treatments and retreatments under natural conditions i n a small area. They also make i t possible to prevent the transfer of s o i l and associated microbial populations among plots by c a r e f u l cleaning of the simple equipment required for treatment, sampling and plot maintenance. Table II l i s t s the E-factors determined i n the laboratory for s o i l samples removed from the microplots 6 weeks after the 1st and 3rd annual subsurface band


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applications. The method used was that described previously f o r the laboratory-treated s o i l (11) ( i e . f i e l d sample vs. not previously treated c o n t r o l ) . Also included i n Table II i s the corresponding factor obtained from the r a t i o of the 0 - 8 week disappearance rates for the 3rd and 1st applications of the commercial formulations from the f i e l d microplots. Enhanced microbial a c t i v i t y was present f o r carbofuran, chlorethoxyphos, DOWCO 429X, isofenphos, fensulfothion and trimethacarb and the sulfoxide/suIfone metabolites of aldicarb, disulfoton, fensulfothion and terbufos within 6 weeks of the i n i t i a l application. E-factors ranged from 2 t o 31. Chlorpyrifos and

Table I I . Ε-Factors f o r Microplot Treated Clay Loam Ε-Factor Laboratory Test Material (Formulation) l s t / C o n t r o l 3rd/Control 1.0 Aldicarb (Temik 15G)(p) 5.1 (t) Y 31 Carbofuran (Furadan 15G) 2.1 Chlorethoxyphos (Fortress 15G) 2.0 0.8 Chlorpyrifos (Lorsban 15G) 0.9 0.8 Disulfoton (Di-Syston 15G)(p) 7.9 7.6 (t) 5.4 5.4 DOWCO 429X (5G) 6.8 Fensulfothion (Dasanit 15G)(p) 6.0 14 (t) 12 5.7 0.8 Fonofos (Dyfonate 20G) 6.8 27 Isofenphos (Amaze 20G) 0.8 T e f l u t h r i n (Force 1.5G) 1.3 1.0 Terbufos (Counter 15G)(p) 5.0 4.4 (t) Y Y Trimethacarb (Broot 15G) 1.1 Phorate (Thimet 15G)(p) 5.3 (t) -

Field 3rd/lst

1.6 1.3 1.1 1.1 4.3 1.6 1.1 1.4 1.6 1.6 1.4 1.4 2.0 1.7 1.0 2.1

ρ = parent only; t = parent + metabolites (suifoxide/sulfone) Y = active i n aqueous incubation t e s t

t e f l u t h r i n treated s o i l s were not tested f o r year 1 because of the negative r e s u l t s observed f o r the 3 year samples. A factor could not be calculated f o r trimethacarb because the test f o r enhanced degradation was done by an alternate aqueous incubation procedure (16). Ε-factors f o r the s o i l s receiving the 3rd annual treatment were s i m i l a r to those receiving the 1st except f o r those treated with fonofos and isofenphos indicating that repeated annual treatments with most materials d i d not increase the enhanced degradative a c t i v i t y of the s o i l i n the f i e l d . Of a l l the materials tested which generated enhanced a c t i v i t y , fonofos was the only one requiring more than 6 weeks after the 1st treatment to generate i t . The 3-year sample from phorate-treated s o i l rapidly degraded the sulfoxide/



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sulfone metabolites at rates s i m i l a r to those observed f o r the corresponding metabolites i n the aldicarb and terbufos-treated s o i l s . The "enhancement factors" calculated from the 0 - 8 week disappearance rates of the commercial formulations from the o r i g i n a l microplot treatments (14, Chapman,R.A., Harris,C.R. unpublished data) (year 3 vs. year 1) were much smaller than those derived from the shorter term laboratory tests of the same s o i l s using pure chemicals and controlled temperature and s o i l moisture. This demonstrates the d i f f i c u l t y one faces i n establishing the presence of enhanced degradative a c t i v i t y based on studies of the persistence of f i e l d applications even when a previously untreated plot i s available f o r comparison. There are at least two reasons f o r these factors being smaller. F i r s t , as we have demonstrated, enhanced a c t i v i t y develops r a p i d l y (within 2-3 weeks i n some cases (14))· This r e s u l t s i n increased 0 - 8 week disappearance rates even i n the 1st year treatment plots and thus reduces the enhancement factor. Second, s o i l treated with a granular formulation of i n s e c t i c i d e i s a heterogeneous system with respect t o the location of the i n s e c t i c i d e . The concentration of i n s e c t i c i d e i n " s o i l " measured on samples taken from t h i s system represents a combination of the i n s e c t i c i d e remaining on the o r i g i n a l granules and that which has become dispersed through the s o i l . I f the i n s e c t i c i d e remaining on the granule i s not as susceptible t o enhanced degradation as that which i s dispersed through the s o i l , the t o t a l concentration i n " s o i l " w i l l decrease less rapidly than expected f o r an active s o i l . This i s r e f l e c t e d i n a reduced enhancement factor. Factors A f f e c t i n g the Generation of Enhanced A c t i v i t y by Carbofuran in Soil The laboratory and microplot experiments have shown that enhanced degradative c a p a b i l i t y i s generated quickly following a single treatment i n a v a r i e t y of s o i l types by at least 14 i n s e c t i c i d e s or t h e i r metabolites. They also demonstrate that various factors can influence the a c t i v i t y of the enhanced microbial population following i t s development. To better understand these two aspects of the phenomenon, we have studied carbofuran intensively both with regard to the generation of enhanced degradative a c t i v i t y and the subsequent behavior of the active microbial population generated i n a sandy loam s o i l (17). For these tests 28°C, 11.3% moisture, 10 ppm carbofuran treatments and 10 ppm carbofuran retreatments a f t e r 28 days were considered "standard". S o i l s maintained at 3°C following an i n i t i a l treatment demonstrated no enhancement, but those at 15 and 28°C showed Ε-factors of 16 and 41, respectively (Table I I I ) .

Table I I I . E f f e c t of Temperature on E-Factors Generated by Carbofuran 8

Temperature ( C) 3 15 28

E-Factor 1.0 16 41



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The e f f e c t of d i f f e r e n t s o i l moistures on the a c t i v i t y developed i s shown i n Table IV. There was a c r i t i c a l lower moisture l e v e l between 9 and 4.5% f o r t h i s s o i l below which enhanced a c t i v i t y f a i l e d to develop. The enhanced a c t i v i t y developed at 36% (9x) was s i g n i f i c a n t l y lower than that developed at intermediate moistures. To examine the e f f e c t of i n i t i a l concentration on the development of a c t i v i t y , four s o i l types were i n i t i a l l y treated with f i v e l e v e l s of carbofuran ranging from 0.01 to 100 ppm. The r e s u l t s of the subsequent a c t i v i t y tests are summarized i n Table V. Concentrations of 1 ppm were s u f f i c i e n t to cause a c t i v i t y to develop i n the three mineral s o i l s . Concentrations i n excess of 1 ppm were required f o r the organic s o i l . A c t i v i t y appears to develop quickly over a broad range of conditions that one would encounter n a t u r a l l y during use of the i n s e c t i c i d e .

Table IV.

E f f e c t of S o i l Moisture on E-Factors Generated by Carbofuran E-Factor 0.2 0.4 46 41 50 54 9

Moisture f%) 1.3 4.5 9 11.3 18 27 36

Table V.

E f f e c t of Insecticide Concentration on E-Factors Generated by Carbofuran

Concentrât ion 0.01 0.1 1.0 10 100

( ppm )

Sand 1.1 3.1 35 35 =

Ε-Factor Sandy Loam Clay Loam 0.8 0.7 14 13 =

1.3 1.0 22 28 -

Muck

1.8 1.0 100 110

Factors A f f e c t i n g the A c t i v i t y of S o i l Microbial Populations Enhanced by Carbofuran Treatment Enhanced microbial populations, once developed, were found to be r e l a t i v e l y i n s e n s i t i v e to changes i n temperature and moisture. Carbofuran continued to disappear from an active s o i l held at 3 C with an E-factor of 12 (Table VI). The E-factors declined with lower s o i l moistures, but remained s i g n i f i c a n t even i n the 4.5-2.3% range (Table V I I ) . The e f f e c t of additional 10 ppm carbofuran treatments made at monthly i n t e r v a l s on the Ε-factors observed f o r a sandy loam s o i l c o l l e c t e d from the same location i n three consecutive years are shown e


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in Table VIII. Because the second and subsequent applications disappeared r a p i d l y from the s o i l , i t was possible to c a l c u l a t e an enhancement factor 1 week following these applications. Tests were

Table VI. E f f e c t of Temperature on Enhanced A c t i v i t y for Carbofuran 8


Temperature ( C) 3 15 28

E-Factor 12 40 44

Table VII. E f f e c t of Moisture on Enhanced A c t i v i t y for Carbofuran Moisture 1.3 2.3 4.5 6.8 9 11.3

m

E-Factor 8 days, not active; < 5 days, a c t i v e ) . Enhanced a c t i v i t y was generated i n the uncovered p l o t s

Table XII.

Treatment Natural Solarized Solarized Solarized

E f f e c t of Solarization on Enhanced A c t i v i t y

Period(wkl 0 - 10 0 - 10 3 - 10 1) 3 - 10 2)

DT/50idavs) for • Timefwk) Indicated 2 5 4 10 12 15 36 7 4.8 4.5 4.4 4. 7 5.1 4.9 5.1 5.4 >14 >14 >14 11. 6 >14 4.5 4.9 9.6 4.1 4.8 5.1 7.4 5.7 5.5 5.0 5.5 4.6 4.9 5.2 7.2 8.9 5.4 8.1 12.7

within 2 weeks, but not i n the solarized p l o t s . At 3 weeks, more p l o t s were covered and the sampling was continued. The p l a s t i c covers were removed from a l l plots after 10 weeks (August 13) and the plots were further sampled at 12, 15 and 36 weeks. The non-solarized plots became active by 2 weeks and the a c t i v i t y remained the following spring as expected from previous work. The plots s o l a r i z e d f o r 10



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weeks immediately following carbofuran treatment d i d not develop enhanced a c t i v i t y during the s o l a r i z a t i o n period, but d i d i n the subsequent 2 weeks. The a c t i v i t y that developed was unusual i n that, 1) the phenolic hydrolysis product of carbofuran, which usually accumulated i n the t e s t solution, was only present i n trace amounts, and 2) enhanced a c t i v i t y d i d not p e r s i s t t o the following spring. S o l a r i z a t i o n of p l o t s i n which enhanced a c t i v i t y was allowed t o develop naturally for 3 weeks d i d not cause a rapid decrease i n t h i s a c t i v i t y with the s o i l s being only s l i g h t l y less active at 7 weeks. Following t h i s , d i f f e r e n t and somewhat e r r a t i c r e s u l t s were observed between the two plots with one remaining active and the other becoming i n a c t i v e . The differences may be due to the depth t o which the enhanced microbial population moved into the s o i l p r i o r t o s o l a r i z a t i o n . The farther an enhanced population e x i s t s below the surface the less l i k e l y i t would be affected by the s o l a r i z a t i o n because of the i n s u l a t i n g e f f e c t of the intervening s o i l . This aspect of the experiment i s currently being investigated more f u l l y . As pointed out by Yarden et a l . ( 8 ) , i t i s u n l i k e l y that s o l a r i z a t i o n would be economical i f used s o l e l y t o a f f e c t microbial populations enhanced t o degrade i n s e c t i c i d e s . In those cases where the cumulative benefitβ merit i t s use, i t would appear best, based on our l i m i t e d r e s u l t s with carbofuran, t o design the treatment t o coincide with i n s e c t i c i d e application. Chemical Biocide Treatment. Chlorpyrifos i s one i n s e c t i c i d e that does not appear t o undergo enhanced microbial degradation i n s o i l . Since t o x i c i t y to microbes has been reported for both c h l o r p y r i f o s (Somasundaram,L., Coats,J.R., Racke,K.D. unpublished r e s u l t s ) and i t s degradation product, 3,5,6-trichloro-2-pyridinol, which can accumulate i n soil(20,21), t h i s i s not surprising. The r o l e of the t r i c h l o r o p y r i d i n o l i n the behavior of chlorpyrifos i n s o i l i s d i f f i c u l t t o e s t a b l i s h , but Racke et al.(20) have shown that concentrations of 100 ppm of the p y r i d i n o l severely i n h i b i t the microbial degradation of both isofenphos and carbofuran i n s o i l . We have examined the e f f e c t s of treatment l e v e l s of 1, 10, 100 and 1000 ppm of the t r i c h l o r o p y r i d i n o l on the a c t i v i t y of s o i l microbial populations enhanced to degrade carbofuran, DOWCO 429X and

Table XIII. E f f e c t of T r i c h l o r o p y r i d i n o l on Enhanced A c t i v i t y Soil Enhanced bv Carbofuran DOWC0429X Isofenphos

E-Factor• for Level(ppm) Indicated _ 1 10 100 1000 27 29 31 0.8 0.6 2.5 2.8 1.9 0.5 0.3 91 46 73 80 0.3

isofenphos. The r e s u l t s are shown i n Table XIII. A c t i v i t y was tested 28 days a f t e r the t r i c h l o r o p y r i d i n o l treatment. Treatment l e v e l s of 1 and 10 ppm had no e f f e c t . At 100 ppm, the degradation of carbofuran and DOWCO 429X was i n h i b i t e d , but t h i s concentration had no e f f e c t on isofenphos degradation. The l e v e l s required to a f f e c t the microbes


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are such that i t would not be f e a s i b l e to treat large quantities of s o i l . The treatment of formulated granules with biocides to provide a protected sphere around the granule appears to be possible, but the l i k e l i h o o d of eventual development of enhanced degradation toward the biocide should not be overlooked.


Cross Enhancement Cross enhancement i s the enhanced degradation of a chemical i n a s o i l i n which enhanced microbial a c t i v i t y was i n i t i a l l y generated by a d i f f e r e n t chemical. Numerous examples for pesticides are found i n the reviews by Felsot (2) and Roeth (6). This f i r s t appeared to be an important area of investigation i n t h i s f i e l d because i t provided information on materials that might be used with some degree of assurance of performance, even i n s o i l s enhanced to degrade other chemicals, and allow t h e i r use i n a chemical rotation scheme to control the pest of i n t e r e s t . The discovery that the generation

Table XIV.

Cross Enhancement Factors for Carbamate Pesticides

Aldicarb Test Material Aldicarb (p) (t) Bendiocarb Bufencarb Butylate Carbaryl Carbofuran Cloethocarb Chlorpropham EPTC Furathiocarb Methomyl Oxamy1 Phorate Propoxur Trimethacarb

1.0 5.1

3.8

2.4

E- or CE-Factor for S o i l Enhanced to Carbofuran Cloethocarb Chlorpropham 1 3 2 1.2 2.3 4.7 1.1 10 6.4 Y 3.4 5.2 1.0 0.9 0.6 0.8 7.0 16 7.2 Y 10 1.0 22 12 71 6.6 36 8.4 17 0.9 13 0.5 0.7 0.9 0.5 1.0 0.5 1.4 1.2 1.1 0.6 3.4 3.4 Y 1.5 1.0 2.3 0.7 0.7 9.2 14 13 27

of enhanced a c t i v i t y can be a rapid process and that i t can p e r s i s t for extended periods indicates that chemical r o t a t i o n w i l l have l i m i t e d usefulness for s o i l - a p p l i e d i n s e c t i c i d e s where e f f i c a c y 8-10 wk a f t e r application i s often required. Studies of cross enhancement may s t i l l provide information on the nature of the basic processes involved i n t h i s phenomenon. Our observations on cross enhancement are summarized i n Table XIV for carbamate enhanced s o i l s towards various carbamates and phorate and i n Table XV for carbofuran and various organophosphorus i n s e c t i c i d e enhanced s o i l s towards organophosphorus i n s e c t i c i d e s and t e f l u t h r i n . Only a small f r a c t i o n of the possible combinations have been examined. E n t r i e s f o r three d i f f e r e n t carbofuran enhanced s o i l s are included to provide a



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sampling to the v a r i a t i o n that can be observed. Entries 1 and 2 are for sandy loam c o l l e c t e d i n 1983. They received 8 treatments at 10 ppm and 5 treatments at 100 ppm, respectively. Entry 3 i s for sandy loam c o l l e c t e d i n 1984 which received 2 treatments at 10 ppm. Cross enhancement factors (CE-Factors) were calculated i n a manner analogous to the enhancement factors. In four cases, the q u a l i t y of the data d i d not allow quantitative treatment and the presence or absence of cross enhancement i s indicated by Y or N, respectively. Cross enhancement was observed between carbofuran and a l l the a r y l methylcarbamates examined except furathiocarb. The behaviour of the oximino methylcarbamates (aldicarb, methomyl, oxamyl) was more variable. Both aldicarb and cloethocarb enhanced s o i l s showed cross enhancement to carbofuran and oxamyl. The chlorpropham enhanced s o i l was not cross enhanced to carbofuran. The carbamate enhanced s o i l s were not cross enhanced to butylate, EPTC or phorate. No cross enhancement was observed between carbofuran enhanced s o i l and t e f l u t h r i n or any of the 11 organophosphorus i n s e c t i c i d e s l i s t e d i n Table XV. Diazinon and DOWCO 429X showed mutual cross

Table XV. Cross Enhancement Factors for Organophosphorus Insecticides and Carbofuran E- and CE-Factors for S o i l Enhanced to Carbofuran Diaz. D i s u l f . DOWCO Fensulf. Phorate Test Material 1 2 3 Chlorfenvinphos 0.8 0.5 1.2 Chlorpyrifos 0.8 0.3 1.0 Diazinon 0.8 0.5 0.9 3.5 2.4 Disulfoton(p) 1.1 1.4 0.9 0.9 0.9 (t) 7.9 DOWCO 429X 2.0 3.6 Fensulfothion(ρ) 0.8 0.3 1.0 8.8 (t) 0.5 0.9 24 (sulfide) 2.0 Fenthion 0.7 Fonofos 0.8 0.9 Isofenphos Ν Ν Tefluthrin 0.9 1.3 Terbufos(p) 0.4 1.2 0.9 2.1 0.9 1.9 (t) 0.6 0.9 3.9 0.9 3.4 Phorate(p) 1.5 2.3 1.0 0.9 1.1 it) 1.2 0.9 1.5 5.3

enhancement. S o i l s enhanced to the sulfoxide and sulfone metabolites of d i s u l f o t o n and phorate were cross enhanced to the corresponding metabolites of terbufos. Fensulfothion (a sulfoxide) enhanced s o i l did not show cross enhancement to the sulfoxide/sulfone metabolites of disulfoton, terbufos or phorate. It d i d show s l i g h t enhancement t o i t s parent s u l f i d e , but not to the s t r u c t u r a l l y s i m i l a r material fenthion. The r e s u l t s of a study of carbofuran treatment i n t e n s i t y on the cross enhancement factors f o r carbaryl, cloethocarb and oxamyl are


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shown i n Table XVI f o r the sandy loam s o i l c o l l e c t e d i n 1982 and 1984. L i t t l e change i n the CE-factors for carbaryl was observed f o r 1, 2 and 4 treatments of the 1982 s o i l . The response of carbaryl was s i m i l a r to that of carbofuran (see Table VIII). The factors increased s i g n i f i c a n t l y f o r cloethocarb and oxamyl between the 2nd and 4th treatments. Higher CE-factors for cloethocarb and oxamyl and a lower


Table XVI. E f f e c t of Treatment Intensity on Cross Enhancement to Carbofuran

Treatment No. 1

Pretest Interval(wk) 4

2

1

4

1

Year 1982 1984 1982 1984 1982 1984

CE-Factors Carbaryl Cloethocarb Oxamyl 0.7 1.1 13 2.2 13 7.5 0.8 1.2 9.0 3.6 17 5.7 1.9 7.9 7.4 145 2.7 18

factor for carbaryl were observed for the 1984 s o i l after the 1st treatment. After the 4th treatment, the factor for oxamyl had changed l i t t l e , but those for carbaryl and cloethocarb had increased markedly r e f l e c t i n g a s i m i l a r increase i n the E-factor f o r carbofuran that occurred with t h i s s o i l (see Table VIII). I t i s c l e a r that the e f f e c t of treatment i n t e n s i t y on cross enhancement i s not simple. Summary Our observations show: 1) some enhanced microbial a c t i v i t y i s generated quickly (6 weeks or less) for most of the s o i l applied i n s e c t i c i d e s (or t h e i r i n s e c t i c i d a l metabolites) currently i n use, 2) the s o i l temperatures, s o i l moistures, s o i l types and chemical concentrations required, both to generate and maintain the enhanced a c t i v i t y of the s o i l microbes, are those normally encountered i n agriculture, 3) enhanced degradative a c t i v i t y , once generated, can p e r s i s t for variable periods exceeding 164 weeks for some materials and 4) i n s e c t i c i d e s applied as granular formulations are often much less susceptible to the e f f e c t of enhanced microbial populations than more f i n e l y dispersed forms. Clearly, agrochemical s c i e n t i s t s face a d i f f i c u l t problem i f they wish to exert some control over s o i l microbial populations and t h e i r adaptability to degrade i n s e c t i c i d e s . It i s important to remember that, for i n s e c t i c i d e s , enhanced degradation i n s o i l i s not a new phenomenon, but only a newly observed one. I t i s unreasonable to think that the e f f e c t of the f i r s t encounter between an i n s e c t i c i d e and a s o i l microbial population i n 1983 or 1989 would d i f f e r from that same encounter occurring i n 1965 or 1970. I t follows that, some s o i l applied i n s e c t i c i d e s were subjected to "enhanced degradation" within a few weeks of t h e i r introduction. Therefore, i t i s u n s c i e n t i f i c to describe a p a r t i c u l a r chemical as "once having been a good material but now i s of questionable value because enhanced degradative a c t i v i t y developed after a history of use". A more s c i e n t i f i c


96


approach would be to ask, which variables involved have changed to permit the natural and rapid adaptation of the s o i l microbial population to the degradation of t h i s material to become a factor controlling i t s efficacy?


Acknowledgment The authors wish to thank our laboratory s t a f f , Colleen Cole, Carol Harris, Karin Henning and Pat Moy, and our f i e l d crew, Glen McPadden and Murray Cates, under the supervision of Dr. J e f f Tolman, f o r t h e i r u n t i r i n g e f f o r t s and innumerable suggestions during the course of the work described.

Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21.

Felsot, Α.; Maddox, J. V.; Bruce, W. Bull. Environ. Contam. Toxicol. 1981, 26, 781-8. Felsot, A. S. Ann. Rev. Entomol. 1989, 34, 453-76. Audus, L. J. Plant Soil 1949, 2, 31-6. Obrigawitch, T.; Martin, A. R.; Roeth, F. W. Proc. North Cent. Weed Control Conf. 1980, 35, 20. Schuman, D. B.; Harvey, R. G. Proc. North Cent. Weed Control Conf. 1980, 35, 19-20. Roeth, F. W. Rev. Weed Sci. 1986, 2, 45-65. Bailey, A. M.; Coffey, M. D. Phytopathology 1985, 75, 135-7. Yarden, O.; Katan, J.; Aharonson, N.; Ben-Yephet, Y. Phytopathology 1985, 75, 763-7. Walker, Α.; Brown, P. Α.; Entwistle, A. R. Pestic. Sci. 1986, 17, 183-93. Miles, J. R. W.; Tu, C. M.; Harris, C. R. Bull. Environ. Contam. Toxicol. 1979, 22, 312-8. Harris, C. R.; Chapman, R. Α.; Harris, C.; Tu, C. M. J. Environ. Sci. Health 1984, B19, 1-11. Chapman, R. Α.; Harris, C. R.; Harris, C. J. Environ. Sci. Health 1986, B21, 125-41. McCusker, V. W.; Skipper, H. D.; Zublena, J. P.; Dewitt, T. G. Weed Sci. 1988, 36, 818-23. Harris, C. R.; Chapman, R. Α.; Tolman, J. H.; Moy, P.; Henning, K.; Harris, C. J. Environ. Sci. Health 1988,B23, 1-32. Harris, C. R.; Svec, H. J.; Sans, W. W. J. Econ. Entomol. 1971, 64, 493-6. Chapman, R. Α.; Moy, P.; Henning, K. J. Environ. Sci. Health 1985, B20, 313-9. Chapman, R. Α.; Harris, C. R.; Harris, C. J. Environ. Sci. Health 1986, B21, 57-66. Chapman, R. Α.; Harris, C. R.; Harris, C. J. Environ. Sci. Health 1986, B21, 125-41. Katan, J. Annual Review of Phytopathology 1981, 19, 211-36. Racke, K. D.; Coats, J. R.; Titus, K. R. J. Environ. Sci. Health 1988, B23, 527-39. Chapman, R. Α.; Harris, C. R. J. Environ. Sci. Health 1980, B15, 39-46.

RECEIVED February 21,

1990


Enhanced Degradation of Insecticides in Soil - ACS Symposium

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