D. D. K A U F M A N
PERSISTENCE AND BIODEGRADATION OF PESTICIDE COMBINATIONS
Pesticide Degradation Laboratory
Agricultural Environmental Quality Institute, ARS
United States Department of Agriculture, Beltsville, Maryland, USA
INTRODUCTION
The combined residues of herbicides, fungicides, insecticides, and nematicides may occur in soil during a single growing season. Such combinations may result from either the successive application of in dividual pesticides, with the concomitant cumulation of their residues, or the intentional application of pesticide combinations. Problems in volving the degradation, persistence, or toxicity of pesticides may thus arise when several pesticides or their residues are present in the soil together.
Numerous types of pesticide interactions have been observed. Inter actions in terms of plant response have been noted for herbicide mixtures [11], herbicide-insecticide mixtures [1, 9, 17, 35, 38, 40], fungicide-herbi- cide mixtures [34, 36, 45], and for fungicide-insecticide mixtures [2, 13, 37]. While most of these interactions have resulted in increased phytotoxicity [1, 2, 9, 11, 13, 17, 34, 35, 38, 40], a few have involved reduced phytotoxicity [36, 45]. Chemical interactions among pesticides are known to occur [32], but these are generally recognized during the development stages of pesticides and appropriate precautions are ob served. Only a few interactions in terms of soil persistence have been reported [3-6, 10, 12, 18, 22-26]. For more detailed review of these inter actions the reader is referred to cited literature. The purpose of this presentation is to briefly discuss some of the possible interactions one might observe during the degradation of pesticide combinations. Common and chemical names of pesticides mentioned in text are listed in Table 1.
56 D. D. K au fm an
T a b l e 1
Common and chemical namee, and use of pestioldes diacuaaed in text
Comm on name Chemical name Use
Ajnitrole 3-aminо-в-triazole herbicide
С apt an B-/trichloromethylthio/-4— cyclohexene-l9 2-dicarboximide fungicide
Carbaryl l-naphthyl methylcarbamate insecticide
Chloramben 3-amino-2,5-dichiorobenzoic acid herbicide Chlarobenzilate ethyl-4f4*-dichlorobenzilate acaricide Chlaropropylate iaopropyl-4t4*-dichloroben2ilate acaricide Chlorpropham isopropyl m-chlorocarbanilate herbicide 2,4—D /2 ,4— dichlorophenoxy/ acetic acid herbicide
Dalapon 2,2-dichlaropropionic acid herbicide
D-D 1,3-dichloropropene nematicide
DDT 1,1,1-trichlar0-2 , 2-bis/E-chlorophenyl/ ethane insecticide Dia zinon 0,0-diethyl 0-/2-is oprqpy 1-4—methyl-6-pjpimidiny 1/
phosphorothioate insecticide
Dlcamba 3,6-dichloro-o-anisic acid herbicide
Dinoseb 2-sec-butyl-4,6-dinitrcphenol herbicide
EDB ethy le no dibromide nematicide
Heptachlor 1,4,5 * 6,718,8-hept achl oro-^a,4,7 ,7.a-te trahy dr
0-4,7--methanoindane insecticide
MGPA [/4— chloro-o-toiyiyoxyj acetic acid herbicide Nenagon l,2-dibromo-3-chl or opr opene nematicide Parathion 0,0-diethyl 0-£-nitrophenyl thiophosphate insecticide PCMC D-chlorophanyl methylcarbrmate
РС1Ш pcntachloronitrobenzene fungicide
Propanil 3»^ ’-dichloroprppionanllide herbicide
SMDC sodium U-methyldithiоcarbamate fumigant
2,4,5-T /2,4,5-trichlorophenoxy/ acetic acid horbicide
DECREASED PESTICIDE PERSISTENCE
Pesticide interactions leading to decreased persistence may result from chemical reactions or enhanced biodégradation of the combined pesticides. The deactivation of SMDC by halogenated hydrocarbon nema- ticides is an example of premature chemical inactivation. SMDC was detoxified when combined with D-D, nemagon, EDB, or related poly- halogenated alkenes [32], thus shortening its residual life in soil. It was suggested that SMDC inactivation occurred through esterification of the carbamic acid by the halogenated hydrocarbon nematicides. SMDC is a volatile soil fumigant and interactions o f this type which further limit its residual life in soil are, therefore, deleterious.
The ability of soil microorganisms to at least partially metabolize a homologous series of molecules, once adapted to metabolizing a single member of the series, is well documented in pesticide research and in accordance with the principles of adaptation outlined by S t a n i e r [38]. A u d u s [3-6] reported that soil microbial populations pre-induced with 2,4-D could rapidly degree MCPA and vice versa (Table 2). He also observed that an MCPA-induced microbial population could decompose 2,4,5-T, which a 2,4-D-enriched population could not. B r o w n b r i d g e [10] observed that MCPA-induced microbial populations could rapidly degrade all phenoxyacetates supplied to it. Similar observations have been reported by others [31, 39].
T a b l e 2
Effect of inducer chemicals on the degradation and persistence of 2,4-D, 2,4,5-T, and MCPA in a soil perfusion system /5, 20/
Inducer chemical p
Days required for degradation of
2,4-D MCPA
None 16.0 47;5 8 5.7
2,4-D 2.4 > 35 11.5
MCPA 2 7.3 8.3
Practical application of such phenomena may be possible. Incorpo ration of small amounts of MCPA or a similar inducer into 2,4,5-T for mulations may facilitate a more rapid dissipation of the more persistent 2,4,5-T. Some caution should be expressed, however, since the microbial degradation of most phenoxyacetates proceeds by cleavage of the ether linkage to acetate and the corresponding phenol. The accumulation of the toxic 2,4,5-trichlorophenol may present an undesireable problem un less the enhanced degradation of 2,4,5-T is accompanied by further me tabolism of the phenol. Consideration of the use of inducer molecules to enhance pesticide degradation must, therefore, be accompanied by an understanding of both the mechanisms involved and the ecological ac ceptability of the residues formed.
In an analagous experiment we examined the degradation of the herbicide dicamba in perfused soils treated with dicamba, or previously enriched with 2,4-D. Degradation was measured by liberation of chloride ion. Dicamba applied to previously untreated soil was quite persistent and no Cl~ liberation was observed. In soils previously perfused with 2,4-D, however, rapid liberation of Cl” from dicamba was observed following initial treatment with dicamba. Subsequent dicamba treatments were degraded more slowly, however, presumeably due to a loss of ’'effectiveness” by the soil population.
58 D. D. K au fm an
Cooxidation, or cometabolism, is a similar process which mght ac count for the decreased persistence of chemicals applies in combination. Cooxidation is the phenomenon whereby a microorganism may oxidize a compound without being able to utilize the energy derived from the oxidation to sustain growth [14]. M i y a z a k i et al. [33] observed that the rate of chloropropylate and chlorobenzilate degradation was greatly increased when citrate was incorporated into the microbial growth media as a carbon supplement. A basic similarity exists between the reductive decarboxylation process of citric acid and 4,4'-dichlorobenzilic acid (Fig. 1) a degradation product of chloropropylate and chlorobenzilate.
C'DOH C %
DBA DBP
0H NAD+ NADH о
HOOCH*
c - с-
CH2 COO H — — HOOCC- C % - CH2 COO Hс*оон
с*о
2Citric acid a* К G
Fig. 1. Parallelism observed in the decarboxyla tion of citric acid and 4,4'-dichlorobenzilic acid; DBP = 4,4,-dichlorobenzophonone; a-K G = a-ke-
toglurate
The reductive decarboxylation process of the pesticides was apparantely stimulated by citrate exploitation of such phenomena could prove bene ficial in controlling or reducing residual accumulation of pesticides.
IN C R EASED PESTICIDE PERSISTENCE
Increased persistence of one or more pesticides applied in combination has been observed [18, 21, 23-26]. All of the interactions presently known involve the parent pesticide molecules, i.e., the interaction of one intact pesticide molecule with another. The diversity and multiplicity of pesti cide residues and degradation products increases the probability that similar interactions may also occur at various other stages of pesticide degradation.
interactions : (a) chemical and physical interactions of the pesticides occurring in combination may preclude their normal degradation in soil and thus increase their persistence in soil; (b) the biocidal properties of pesticides to soil microorganisms may preclude their biodégradation; (с) direct inhibition of the adaptive enzymes of effective soil micro organisms; and (d) inhibition of the proliferation processes of effective microorganisms. Increased pesticide persistence resulting from these types of interactions could be accompanied by prolonged periods of chemical control of the pest. The results of periodic soil chemical ana lyses would be expected to reflect the increased persistence of the pesticide in question.
•An apparent increased pesticide persistence may also be explained by reactions of either target or nontarget organisms to the pesticide combinations. The sensitivity of the treated organisms to a given pesti cide may be greater in the presence of a second pesticide. Thus, the organism may be affected for a longer period of time due to the enhanced activity of lower pesticide concentrations during the time when the soil pesticide concentration is actually decreasing. In this case, how ever, soil chemical analyses would not necessarily reflect an increased persistence of the pesticide.
Examples of several of the aforementioned reactions have been re ported in the literature. Wheat seedling bioassays and chemical analyses of soils treated with various dalapon-amitrole combinations substantiated the fact that dalapon persisted longer in soils when applied in com bination with amitrole than when dalapon was applied alone [18]. While the precise mechanism by which amitrole inhibits dalapon degradation is not known, the results of enrichment investigations indicated amitrole did not interfere with the lag period or adaptation of effective micro organisms, but did inhibit their proliferation [18]. Dalapon had very little effect on amitrole behavior in soil.
Carbaryl (Fig. 2) heptachlor, and PCNB increased the persistence of chlorpropham in soil when applied either alone or in combination with other pesticides [20]. W a l k e r [44] also observed an increased soil persistence of chlorpropham in the presence o f PCNB. Combinations of DDT and captan also increased soil persistence of chlorpropham, although neither pesticide alone affected chlorpropham persistence [20]. DDT has been reported to inhibit certain esterases of soil microor ganisms [15]. The microbial enzymes responsible for chlorproham hydro lysis are esteratic or amidase type enzymes [27, 28]. Apparently, how ever, the combined effect of DDT and captan was necessary to more effectively inhibit chlorpropham degradation.
60 D. D. K au fm an
with phenylamide herbicides has been extensively investigated [22, 23, 25]. Soil microorganisms are active in the biodégradation of phenyl amide herbicides in soil. The degradation pathway of phenylcarbamate and acylanilide herbicides involves an initial hydrolysis to the corres ponding phenyl and acid or alcohol moieties. Esterase and amidase type enzymes catalyzing these hydrolyses have been isolated and characterized [8, 15, 27]. Detailed enzyme investigations have demonstrated that me- thylcarbamates lacking steric hindrance of the carbamate linkage are competitive inhibitors of many of these enzymes [23]. Certain of these enzymes are also sensitive to inhibition by organophosphate pesticides such as parathion and diazinon [21, 25].
Fig. 2. Effect of carbaryl on micro bial degradation of chlorpropham
(CIPC) in soil perfusion [23]
The practical significance of these observations is currently under extensive investigation. The deliberate combination of methylcarbamates with acetamides, phenylcarbamates, and acylanilides for purposes of controlled persistence of biodegradable pesticides shows considerable promise. Application of this phenomenon has been made to dodder control with soil applied herbicides [12]. Surface or incorporated appli cation of chlorpropham are used to control dodder. The addition of PCMC to chlorpropham applications doubles, or more than doubles the period of dodder control by chlorpropham.
Simultaneous application of PCMC with propanil to soil retarded propanil degradation in soil and greatly reduced the formation of 3,3',4,4'- tetrachloroazobenzene (TCAB) [22] (Fig. 3). Propanil persistes in soil for only a few hours [22] to a few days [46]. Thus, incorporation of a me- thylcarbamate into propanil formulations could improve the residual
activity of propanil in soil as well as reduce the formation of TCAB. Since some propanil-methylcarbamate combinations are phytotoxic to some crop plants the selection of a methylcarbamate which could function as an inhibitor of propanil degradation in soil but not interact undesi rably within the crop plant would be essential.
% propanil converted to
Fig. 3. Effect of PCMC on the conver sion of propanil to 3,3’,4,4’-tetrachlo-
roazobenzene (TCAB)
PESTICIDE M O B IL IT Y IN SOIL
Interactions affecting biodégradation of a pesticide may ultimately affect the mobility of the pesticide or its residues in soil. T a l b e r t , R u n y a n , and B a k e r [41] observed an interesting interaction be tween the herbicides chloramben and dinoseb. Chloramben leaches in soil to 15-25 cm, when aplied as the ammonium salt. In autoclaved soil, however, the methyl ester remains near the soil surface and does not leach. In nonsterile soil the methyl ester is readily hydrolyzed micro- biologically to the free acid which then leaches readily to the 15-26 cm depth. In the presence of dinoseb, however, hydrolysis of the chlor amben methyl ester is inhibited and no leaching occurs. Thus, inhibition of microbial degradation may ultimately affect mobility in soil.
CO M P L E X RESIDUE F O RM AT IO N
The formation of pesticide conjugates and condensation products of single pesticides has been well documented. The significance or fate of these complexes in our environment is unknown. The occurrence of multiple pesticide residues in soil provides opportunity for the for mation of complex or hybrid residues. Such residues may result from
62 D. D. K au fm an
the biochemical interaction of the residues о-f a single pesticide, or the multiple interactions of residues from combinations of pesticides. Our knowledge of such interactions occurring in soil is very limited at present.
The esterification of SMDC by halogenated nematicides results in the formation of a hybrid residue [32]. Interactions among methylamine, CS2, H2S, and methylisocyanate, the initial degradation products of SMDC [19, 42, 43], results in the formation of methyl thiourea, N,I\T~ dimethylthiourea, iV,N'-dimethylthiuram monoeulfide, and N,2V'-dimethyl- thiuram disulfide. The presence of N,N'~dimethylthiourea has been as sociated with phytotoxicity symptoms following SMDC treatments [16]. Symmetrical azobenzene formation occurs through the condensation of two identical molecules of aniline which may result from the degra dation of a single aniline-based pesticide. K e a r n e y et al. [29] and B a r t h a [7] have recently demonstrated that simultaneous or successive applications of tw o different aniline-based pesticides may result in the formation of asymmetric azobenzenes. The probability of asymetrical azobenzene formation would depend upon several factors including : (a) application concentration; (b) degradation rate of the individual and/or combined chemicals; or (c) their application timing. The re cognition and identification of potential residue problems accompanied by appropriate adjustments in chemical control programs will reduce or eliminate the occurrence of such pesticide residues.
CONCLUSIONS
Interactions among the various components of pesticide combinations or their residues in soil may ultimately affect their fate and behavior in soil. While no interactions have been observed with most pesticide combinations, decreased persistence, increased persistence, altered mo bility, and complex residue formation have resulted from the appli cation and degradation of some pesticide combinations. Ideally a pesticide should not persist in the environment any longer than necessary to provide the pest control desired. Careful selection of pesticide combi nations or formulations may facilitate controlling the residual toxicity and products of some of our present pesticides.
REFERENCES
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D. D. K A U F M A N
TRW AŁOŚĆ I RO ZKŁAD PESTYCYD ÓW M IESZAN YCH
Pracownia Rozkładu Pestycydów,
Instytut Środowiska Rolniczego, ARS, Beltsville, Maryland, USA
S t r e s z c z e n i e
Równoczesne lub następcze stosowanie różnych pestycydów może prowadzić do powstawania różnych kombinacji ich pozostałości w glebie. Łączenie się pozo stałości może wywoływać zjawiska niepożądane, jeżeli spowodują przedwczesną inaktywację pestycydów lub uszkodzenie plonów. Pozytywnym zjawiskiem będzie, jeśli zwiększy się spectrum działania pestycydów lub zmieni się w pożądanym stopniu szybkość działania pestycydów. Kombinowanie mieszanek pestycydów w celu zwiększenia ich efektywności lub regulowanie szybkości rozkładu jest obiecujące i stanowi przedmiot szeroko zakrojonych badań.
Д. Д. К А У Ф М А Н СТАБИЛЬНОСТЬ И РАЗЛОЖЕНИЕ СМЕШ АННЫ Х ПЕСТИЦИДОВ Лаборатория по деградации пестицидов Института исследования сельскохозяйственной среды Департамент сельского хозяйства Соединенных Штатов Сев. Америки, Бельствиль, Мэрилэнд, 20705, США Р е з ю м е Одновременное или последовательное применение разных пестицидов мо жет вызывать разные комбинации их остатков в почве. Соединение остатков может вызвать нежелательные явления, в случае если произойдет прежде временная инактивация пестицидов или повреждение урожаев. Положительным явлением будет факт, если повысится спектр действия пестицидов или изме нится в желательной степени быстрота действия пестицидов. Комбинирование смесей пестицидов с целью повышения их эффективности или регулирования быстроты разложения подает надежды и является объектом широко запла нированных исследований.
