Environmental fate of rice pesticides in California.

Abstract

Each of the pesticides reviewed is reported to dissipate from field water after application. Carbofuran is hydrolyzed rapidly under the alkaline conditions usually found in the rice field environment, and its hydrolysis products are also degraded rapidly. The longest half-life reported (18-26 d) was in water that overlaid soil treated with Furadan granular formulation. Generally, carbofuran dissipation ranged from 36 hr to 3 d. Under field application, bensulfuron methyl showed a half-life of 1-3 d, but others have recovered all of the "dissipated" herbicide in the soil compartment. MCPA applied to rice fields is reportedly degraded by the joint action of sunlight and microbial action with a half-life of 3-5 d. Methyl parathion showed a maximum half-life of 9-17 d in a model aquatic ecosystem, but other reports found more rapid dissipation. The half-life of molinate has been observed by numerous researchers to be less than 5 d, with volatility the major route of loss. A half-life as short as 5-7 d has been reported for thiobencarb applied to rice fields, but others report much longer periods; volatility again is expected to be a significant route of loss. Microbial degradation takes place with each of the subject pesticides. Numerous authors have reported enhanced degradation of carbofuran under conditions of repeated application, and this probably holds true for the others. A specialized segment of the microbial population (Pseudomonas spp.) is purported to carry out most of the degradation but is inefficient at degrading the hydrolysis product, carbofuran phenol. Biodegradation of bensulfuron methyl has been observed with actinomycetes, fungi, and bacteria, and takes place primarily by oxidation and hydrolysis. Methyl parathion is biodegraded primarily by nitro reduction to aminomethyl parathion. A fungus, an actinomycete, and a bacterium were shown to biodegrade molinate, primarily by oxidation at the sulfur atom and the azepine ring. Thiobencarb is biodegraded in anaerobic sediments at a slow rate; the dechlorinated thiobencarb was shown to cause dwarfing of rice in some fields. Otherwise, its aerobic biodegradation is rapid and follows the same routes as with molinate. Carbofuran is a systemic insecticide that is rapidly absorbed and translocated to aerial parts of the plant. Carbofuran is metabolized in rice plants to the corresponding phenol, which is irreversibly bound into the plant, as well as to 3-hydroxycarbofuran and other minor components; it is depurated through leaf exudate, from which it volatilizes. Rice plants were observed to take up more bensulfuron methyl through shoots than roots and to metabolize it to the 4-hydroxy analog. The half-life of methyl parathion in Hydrilla verticulla, an aquatic macrophyte, was 1 wk, but little has been reported on methyl parathion in plants. Barnyardgrass was found to absorb greater amounts of molinate than did rice, and it produced larger proportions of basic metabolites, which may form the basis for its selective toxicity. Thiobencarb has been shown to be rapidly absorbed, translocated, and metabolized in rice plants, barnyardgrass, and the broadleaved wild amaranth, smartweed, and lambsquarters. Translocation was more rapid and extensive in barnyardgrass than in rice, and most of the 14C radiolabel was recovered as metabolites. Its terminal metabolite, chlorobenzoic acid, was taken up into lignin-like plant constituents. It is apparent that information on plant uptake and biodegradation is limited--none exists for woody species--but the fact that some species appear resistant to the herbicides suggests that biodegradative ability is general. (ABSTRACT TRUNCATED)