Carbaryl volatilization

Phenylcarbamates, or carbanilates, generally exhibit low water solubilities, and thus they are almost immobile in soil systems. Chlorpropham and Propham are readily volatilized from soil systems, but Terbutol and Carbaryl (Fig. 10, Table 3) are not. Ester- and amide-hydrolysis, N-dealkylation and hydroxylation are among the chemical reactions that carbamates undergo. The N-methylcar-bamate insecticides (Fig. 10, Table 3) commonly used in soils are Carbaryl, Methiocarb,Aldicarb,and Carbofuran [74,173]. 

The amounts volatilized in 10 days were extremely low in most cases. Dicamba was the only herbicide exhibiting significant volatilization and that only in the presence of high water evaporation. Apparently the insecticide carbaryl, has the potential to volatilize significantly from the soil with or without water evaporation, particularly when it is present near the soil surface. The reference insecticide, lindane, will appreciably volatilize when it is present near the soil surface, but its volatility decreases markedly when present within the entire 0 - 10 cm depth. Volatilization of the other insecticides will be essentially insignificant due either to their rapid degradation rate or low Kh-... 

Their volatilization from litter on the forest floor will also be appreciable. With the possible exception of carbaryl, their volatilization after being washed into the soil will be relatively low or insignificant because of their low volatility, low Henry s constants, Kh> and/or their high rates of degradation in the soil environment. The rapid disappearance of the phenoxy herbicides (2, 31) and the insecticide, fenitrothion (28) from vegetation and the forest floor is supporting evidence that volatilization is an important pathway for loss of applied pesticides from the forest canopy and litter on the forest floor. 

Evidence was obtained recently that pesticide vapors may enter the air by still another mechanism, involving plant circulation and water loss (57). Rice plants were found to efficiently transport root-zone applied systemic carbamate insecticides via xylem flow to the leaves, eventually to the leaf surface by the processes of guttation and/or stomatal transpiration, and finally to the air by surface volatilization. Results from a model chamber showed that 4.2, 5.8, and 5.7% of the residues of carbaryl, carbofuran, and aldicarb, respectively, present in rice plants after root soaking vaporized within 10 days after treatment. The major process was evaporation of surface residues deposited by guttation fluid. 

The most important reaction that occurs in all compounds of this class is hydrolysis. For example, carbaryl is hydrolyzed to 1-naphthol (Figure 4.9). The carbamates are slightly to moderately soluble in water (10-500 ppm), moderately volatile (v.p. 10 4-10-5 mm Hg), and readily biodegradable. 

Unlike chlorpropham and propham, terbutol and carbaryl were not volatilized from soil systems (282, 344). 

In 1980, five applications of a carbaryl/captan mixture were used, two by ground and three by air, as indicated in Table I. Analyses of off-target depositions showed that carbaryl consistently drifted further than captan when applied by air (Figure 1). One would expect that the drifting particulates would have both pesticides present, and that levels of each would be detected wherever drift occurred. This unexpected result cannot be readily attributed to sensitivity of the analyses since all samples were well above the sensitivity limits (10 ng for captan and 0.01 ng for carbaryl). The vapor pressure of captan is appreciably lower than that of carbaryl and if volatilization from drifting minute particulates was a factor, captan should have been found in greater quantities than carbaryl. 

Hydrothermal processes lead to a large reduction of residue levels, which is associated not only with volatile substances that vaporise with water vapour, but also with elevated temperatures that accelerate the hydrolysis of pesticides. For example, the loss of malathione can reach almost 100% in cooked spinach and 92% in cooked rice, while cooking tomatoes contaminated with carbaryl can decrease the pesticide concentration to 31%, and cooking meat contaminated with organophosphate insecticides similarly has a decontaminating effect. 

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