Interactions Between Pesticides and Their Major Degradation Products

Chapter 12

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Interactions Between Pesticides and Their Major Degradation Products L. Somasundaram and Joel R. Coats Pesticide Toxicology Laboratory, Department of Entomology, Iowa State University, Ames, IA 50011

The i n t e r a c t i o n s between parent p e s t i c i d e compounds and t h e i r degradation product(s) can influence the f a t e o f both p e s t i c i d e s and degradation products. The r o l e o f p e s t i c i d e degradation products in i n f l u e n c i n g the persistence and degradation o f parent compounds, and the enhanced degradation o f degradation products are discussed. The s i g n i f i c a n c e o f these i n t e r a c t i o n s in crop protection is also addressed. P e s t i c i d e s applied t o a g r i c u l t u r a l lands are degraded through b i o l o g i c a l , chemical, and p h y s i c a l mechanisms (1). Some p e s t i c i d e s have very low persistence l e v e l s , r e s u l t i n g i n r a p i d transformation to degradation products. For example, 50% o f applied terbufos was oxidized to i t s sulfoxide w i t h i n three days of a p p l i c a t i o n (2). More than 90% o f applied metham-sodium was transformed to methyl isothiocyanate w i t h i n 3 hrs (3). I n the environmental matrices, parent compounds and t h e i r degradation products have been detected simultaneously. The i n t e r a c t i o n s between p e s t i c i d e s and t h e i r degradation products could be s y n e r g i s t i c , antagonistic, or a d d i t i v e . This paper focuses on the i n t e r a c t i o n s between some o f the commonly used p e s t i c i d e s and t h e i r major degradation products (Table I ) . Degradation Products as Inducers of P e s t i c i d e Degradation The f a i l u r e o f some s o i l - a p p l i e d p e s t i c i d e s to c o n t r o l t h e i r target pests (4) has resulted i n s i g n i f i c a n t research on m i c r o b i a l adaptation to p e s t i c i d e s (5). The enhanced biodégradation o f some p e s t i c i d e s has been a t t r i b u t e d to the substrate value of t h e i r degradation products (6-8). Degradation products o f p e s t i c i d e s belonging to d i f f e r e n t classes such as phenoxyacetic acids, carbamothioates, N-methyl carbamates, and organophosphates have been reported to condition s o i l s f o r r a p i d degradation o f t h e i r respective parent compounds (Table I I ) . 0097-6156/91/0459-0162$06.00A) © 1991 American Chemical Society Somasundaram and Coats; Pesticide Transformation Products ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Somasundaram and Coats; Pesticide Transformation Products ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

Butylate

EPTC

2,4-D

Pesticide

C-H.S-C-NC

X

2

7

C H - CI-KCHJ,

c^

/ ^ ^ T

C^S-C-NC

9

2

OCH COOH

Structure

s

Ο

Q Q n n

9 9

2

CH -

2

2

2

2

2

2

CHCHJ

/CH -CH(CHg)

N

2

CH -CH(CH3> C„H«S-C-N

Ο Ο Il II / C^S-C-N^

Structure

Continued on next page

Butylate sulfone

Butylate sulfoxide

EPTC sulfoxide

2,4-Dichlorophenol

Degradation product

Table I. Structures of Some Commonly Used Pesticides and their Degradation Products



Diazinon

Fonofos

Chlorpyrifos

Pesticide

2

s

s

C H O

2

/

/

3

CH(CH3>

CH

Cl

X=/

^ ° ) t o - / ~ \ c .

C H O

c

Cl

Structure

2

ο

S

ο

2-Isopropyl-6-methyl-4hyrdoxypyrimidine

C H

Methyl phenyl sulfone

Degradation product

χ

/

CH(CH3)

=

- | - Q

Structure


2

Pesticide-Degradation Product Interaction


12. SOMASUNDARAM & COATS



Metharo-sodium

Diphenamid

Pesticide

4

CH^H-CS-Na" "

S

Structure

Methyl isothiocynate

Diphen M-2

Degradation product

3

CH -N=C = S

Structure




Table I I . Degradation products as inducers of p e s t i c i d e degradation Inducer/ substrate

Susceptible p e s t i c i d e

Reference


Inducers of t h e i r respective parent compounds 2,4-dichlorophenol EPTC sulfoxide butylate sulfoxide carbofuran phenol 1-naphthol p-nitrophenol s a l i c y l i c acid 2 - aminobenz imidazole

2,4-D EPTC butylate carbofuran carbaryl parathion isofenphos carbendazim

(8,9) (10) (10) (7) (7) (6,8,11) (8) (12)

Inducers of degradation of other parent compounds butylate sulfoxide EPTC sulfoxide

EPTC butylate

(10) (10)

Hydrolysis Products. Hydrolytic reactions seem to play an important r o l e i n the i n i t i a t i o n o f p e s t i c i d e metabolism by adapted s o i l microorganisms (13,14). I n several instances, the presence of p e s t i c i d e h y d r o l y s i s products has r e s u l t e d i n induction of a p e s t i c i d e degrading population o f s o i l microorganisms (6,7). I n one of the f i r s t studies on the biodégradation of p e s t i c i d e s , Newman and Thomas (9) observed decreased persistence o f 2,4-D i n s o i l s pretreated with i t s hydrolysis metabolite, 2,4-dichlorophenol. In that study, pretreatment o f s o i l s with c l o s e l y r e l a t e d compounds such as phenoxyacetic a c i d and 2-chlorophenol, however, had no e f f e c t on the persistence of 2,4-D. Steenson and Walker (15) reported the a b i l i t y of an Achromobacter s t r a i n to use 2,4dichlorophenol as a substrate. I n a recent study i n our laboratory, the m i n e r a l i z a t i o n rate of 2,4-D increased with the number o f pretreatments with 2,4-dichlorophenol ( 8 ) . Fungicide carbendazim r a p i d l y degraded i n s o i l s previously treated with i t s metabolite 2-aminobenzimidazole. (12,16) 2Aminobenzimidazole and carbendazim were equally e f f e c t i v e i n conditioning the s o i l to carbendazim. Pre treatment of s o i l s with another p o t e n t i a l metabolite, benzimidazole, however, r e s u l t e d i n only a s l i g h t reduction i n the parent compound's persistence. Benz imidazole rings are generally l e s s susceptible to m i c r o b i a l degradation (17), and the benz imidazole-induced nonenhanced degradation of carbendazim (12) i s not c l e a r l y understood. Carbofuran phenol, a hydrolysis product of carbofuran, conditioned s o i l s f o r enhanced degradation of carbofuran under anaerobic environment (7) but d i d not have any e f f e c t i n aerobic conditions (8,18). Although the presence of carbofuran phenol i n anaerobic s o i l s accelerated carbofuran h y d r o l y s i s , the phenol was not used as an energy source and accumulated i n the s o i l . This f i n d i n g suggests that besides substrate value, other properties of


168

PESTICIDE TRANSFORMATION PRODUCTS


degradation products may be involved i n the induction process. The d i f f e r e n t r e s u l t s obtained from aerobic and anaerobic s o i l s r e v e a l the importance of environmental factors i n the i n t e r a c t i o n between p e s t i c i d e s and t h e i r degradation products. Oxidation Products. The sulfoxides of EPTC and b u t y l a t e induced enhanced degradation of t h e i r parent compounds, and the p o t e n t i a l o f the sulfoxides to promote r a p i d degradation of t h e i r parent compounds was s i m i l a r to that of the parent compounds themselves (10). Although butylate sulfoxide conditioned the s o i l s , p r e t r e a t i n g s o i l s with b u t y l a t e sulfone d i d not have any e f f e c t on the rate of degradation of the parent compound. Cross-enhancement. Degradation products can also enhance the degradation of p e s t i c i d e s other than t h e i r precursors. P r i o r exposure of s o i l s to butylate sulfoxide accelerated the degradation of EPTC (10). S i m i l a r l y , a higher b u t y l a t e degradation rate was observed i n s o i l s exposed to EPTC sulfoxide. This type of crossenhancement i s generally l i m i t e d to s t r u c t u r a l l y s i m i l a r p e s t i c i d e s . Location d i f f e r e n c e s were a l s o observed i n cross-enhancement t e s t s for EPTC and b u t y l a t e degradation i n s o i l s pretreated with sulfoxides of EPTC or butylate, suggesting the r o l e of s o i l properties i n i n t e n s i f y i n g the enhancement (10). Degradation Products as Promoters of P e s t i c i d e Persistence Most o f the a v a i l a b l e information on degradation products indicates t h e i r p o t e n t i a l to accelerate the degradation of subsequently applied p e s t i c i d e s . Contrary to t h i s , some degradation products could prolong the persistence of p e s t i c i d e s i n s o i l . Hydrolysis Products. P a r t i c u l a r l y when a p p l i e d to the s o i l repeatedly, 3,5,6-trichloro-2-pyridinol (TCP), a hydrolys i s metabolite of c h l o r p y r i f o s and t r i c h l o p y r , slowed the degradation o f c h l o r p y r i f o s (8). A t 100 ppm, TCP i n h i b i t e d the degradation of carbofuran and D0WC0 429X i n problem s o i l s but d i d not have any e f f e c t at 1 and 10 ppm (19). Recent studies have demonstrated the a n t i m i c r o b i a l a c t i v i t y of t h i s metabolite (20,21); the i n h i b i t o r y e f f e c t of TCP could be a r e s u l t of i t s m i c r o b i a l t o x i c i t y . The l e v e l s required to a f f e c t the microbes, however, are so high that the e f f e c t of a s i n g l e a p p l i c a t i o n of c h l o r p y r i f o s o r t r i c h l o p y r may not be s u f f i c i e n t to produce a s i g n i f i c a n t e f f e c t . I n contrast, under f i e l d conditions, p e s t i c i d e s are not homogeneously d i s t r i b u t e d through s o i l but are concentrated i n and around the p a r t i c l e s of a granular formulation. This could r e s u l t i n a high concentration of the p e s t i c i d e or metabolite i n a p a r t i c u l a r microenvironment. Cross-retardation. Fonofos, an organophosphorus i n s e c t i c i d e , increased the persistence of EPTC i n problem s o i l s (22). Dietholate, an extender used i n the new formulations of EPTC, i s s t r u c t u r a l l y very s i m i l a r to fonofos. I n laboratory t o x i c i t y studies, methyl phenyl sulfone (a metabolite of fonofos) was more t o x i c to Photobacterium phosphoreum a bioluminescent bacterium phylogenetically r e l a t e d to several important s o i l b a c t e r i a , than t




fonofos (20). The a n t i m i c r o b i a l a c t i v i t y of fonofos (22) could r e s u l t from the t o x i c i t y of i t s degradation product, methyl phenyl sulfone. The mechanism of the i n h i b i t o r y e f f e c t o f fonofos under f i e l d conditions has yet to be c l e a r l y elucidated, however.


Enhanced Degradation of Degradation Products Repeated applications of some p e s t i c i d e s can a l s o accelerate the degradation of the products formed (Table I I I ) . 2Aminobenzimidazole r a p i d l y d i s s i p a t e d i n s o i l s exposed to i t s parent, carbendazim, with only 6% remaining 4 days a f t e r a p p l i c a t i o n (16). I n these s o i l s , degradation o f 2-aminobenzimidazole was f a s t e r than that of the parent. The rate of degradation of benz imidazole, a s t r u c t u r a l l y s i m i l a r metabolite, however, was not a f f e c t e d by carbendazim h i s t o r y . Carbendazim, a l s o a f u n g i t o x i c h y d r o l y s i s product of benomyl, degraded more r a p i d l y i n s o i l with a benomyl h i s t o r y than i n a s o i l without one (23). The h a l f - l i f e o f carbendazim was reduced from 11 to 4 days i n benomyl-treated s o i l s . This reduction has been a t t r i b u t e d to the short lag-period observed i n these s o i l s . The degradation of desmethyldiphenamid (diphen M - l ) , a monodemethylated metabolite o f diphenamid, was much f a s t e r i n s o i l s pretreated with i t s parent compound than i n an untreated s o i l (24). There was no difference i n the degradation rate of Diphen M-2, a bidemethylated product o f diphenamid, i n s o i l s w i t h and without a diphenamid h i s t o r y . The rate of degradation of diphen M-2 was also much slower than f o r diphenamid or diphen M-l. Repeated applications of metham-sodium enhanced i t s transformation rate to methyl isothiocyanate and subsequent m i n e r a l i z a t i o n of the p e s t i c i d a l metabolite. The h a l f - l i f e of methyl isothiocyanate ranges from 0.5 to 50 days with the shorter h a l f - l i f e i n s o i l s previously treated with metham-sodium (3). Laboratory studies have investigated the fate of degradation products as influenced by t h e i r p r i o r treatment (8,12). The purpose of these studies was to confirm the substrate value of the degradation products. Self-enhancement of methyl isothiocyanate (3), 2-aminobenzimidazole (12), and 2,4-dichlorophenol (8) has been previously reported. The microbial adaptation to degradation products i s s i g n i f i c a n t , p a r t i c u l a r l y when the p e s t i c i d a l e f f e c t i s caused by the degradation product. Table I I I .

P e s t i c i d e degradation products susceptible to enhanced degradation

Susceptible degradation product

Reference

2-aminobenz imidazole carbendaz im methyl isothiocyanate de sme thy ldiphenamid

(16) (22) (3) ( 24 )


PESTICIDE TRANSFORMATION PRODUCTS

170


Conclusions A b e t t e r understanding o f the mechanisms o f pesticide-degradation product(s) i n t e r a c t i o n s i s important f o r studying the f a t e and the e f f e c t s o f p e s t i c i d e s i n the environment. Synergistic i n t e r a c t i o n s could r e s u l t i n inadequate host p r o t e c t i o n leading to r e s t r i c t e d use or withdrawal o f parent compound and thus influence the p e s t i c i d e c l a s s t o be used i n subsequent a p p l i c a t i o n s . The information generated from such studies w i l l help make e f f e c t i v e use o f e x i s t i n g biodegradable p e s t i c i d e s by promoting an understanding of the s p e c i f i c i t y and development o f enhanced biodégradation. Acknowledgements Journal Paper No. J-14292 o f the Iowa A g r i c u l t u r e and Home Economics Experiment S t a t i o n , Ames, Iowa, Project No. 2306. Literature Cited 1. Coats, J.R. chapter 2 i n this volume. 2. Lavelgia, J.; Dahm, P.A. J. Econ. Entomol. 1975, 4, 715-718. 3. Smelt, J.H.; Crum, S.J.; Teunissen, W. J. Environ. Sci. Health 1989, B24, 437-455. 4. Roeth, F.W. Rev. Weed Sci. 1986, 2, 45-65. 5. Racke, K.D. Coats, J.R. Enhanced Biodegradation of Pesticides in the Environment, Am. Chem. Soc.: Washington, D.C., 1990. 6. Ferris, I.G.; Lichtenstein, E.P. J. Agric. Food Chem. 1980, 28, 1011-1019. 7. Rajagopal, B.S.; Panda, S.; Sethunathan, N. Bull. Environ. Contam. Toxicol. 1986, 36, 827-832. 8. Somasundaram, L.; Coats, J.R.; Racke, K.D. J. Environ. Sci. Health 1989, B24, 457-478. 9. Newman, A.S.; Thomas, J.R. Proc. Soil Sci. Soc. Am. 1949, 14, 160-164. 10. Bean, B.W.; Roeth, F.W.; Martin, A.R.; Wilson, R.G. Weed Sci. 1988, 36, 70-77. 11. Sudhakar-Barik; Wahid, P.A.; Ramakrishna, C.; Sethunathan, N. J. Agric. Food Chem. 1979, 27, 1391-92. 12. Yarden, O.; Salomon, R.; Katan, J.; Aharonson, N. Can. J. Microbiol. 1990, 36, 15-23. 13. Sethunathan, N.; Pathak, M.D. J. Agric. Food Chem. 1972, 20, 586-589. 14. Racke, K.D.; Coats, J.R. J. Agric. Food Chem. 1987, 35, 94-99. 15. Steenson, T.I.; Walker, N. J. Gen. Microbiol. 1958, 18, 692697. 16. Aharonson, N.; Katan, J.; Avidov, E.; Yarden, O. In Enhanced Biodegradation of Pesticides in the Environment; Racke, K.D.; Coats, J.R.; Eds.; American Chemical Society, 1990; pp 113-127. 17. Sisler, H.D. In Biodegradation of Pesticides; Matsumara, F.; Krishnamurti, C.R. Eds.; Plenum Press: New York, NY, 1982; pp 131-151. 18. Harris, C.R.; Chapman, R.A.; Harris, C.; Tu, C.M. J. Environ. Sci. Health 1984, B19, 1-11. 19. Chapman, R.A.; Harris, C.R. In Enhanced Biodegradation of


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SOMASUNDARAM & COATS

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Pesticides in the Environment; Racke, K.D.; Coats, J.R.; Eds.; Am. Chem. Soc.: Washington, D.C. 1990; pp 82-96. Somasundaram, L.; Coats, J.R.; Racke, K.D.; Stahr. H.M. Bull. Environ. Contam. Toxicol. 1990, 44, 254-259. Felsot, Α.; Pedersen, W. 1991, chapter in this volume. Roeth, F.W.; Wilson, R.G.; Martin, A.R.; Shea, P.J. In Enhanced Biodegradation of Pesticides in the Environment; Racke, K.D.; Coats, J.R.; Eds.; Am. Chem. Soc.: Washington, D.C., 1990; pp 23-36. Yarden, O.; Katan, J.; Aharonson, N.; Ben-Yephet, Y. Phytopathology 1985, 75, 763-767. Avidov, E.; Aharonson, N.; Katan, J. Weed Sci. 1990, 38, 186193.

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Interactions Between Pesticides and Their Major Degradation Products

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