Adjuvants, organophosphorus insecticides

Formulation and initial placement influence the susceptibility of organophosphorus insecticides to transport in surface runoff, as well as their degradation by abiotic and microbial processes. Formulation affects the kinetics of insecticide release into soil water and overland flow, as well as sorption to soil solids and plant surfaces. Spray adjuvants affect initial placement by influencing the amount of insecticide depositing on foliar and soil surfaces. Initial placement determines the relative importance of such processes as volatilization, photolysis, biodegradation, and leaching out of the zone of interaction with overland flow. 

Prior to application of spray formulations, spray adjuvants are typically added to the insecticide mixture to enhance the efficacy of the active ingredient. Spray adjuvants include surfactants, compounds that impart adhesion and viscoelasticity to spray droplets (e.g., latex), compounds that provide protection from ultraviolet light and reduce volatilization, and activators. The coapplication of these compounds affects organophosphorus insecticide dissipation and transfer to surface runoff Some adjuvants and formulation ingredients are toxicologically significant themselves (e.g., nonylphenol ethoxylates). 

The effect of formulation and spray adjuvants on insecticide efficacy has received considerable attention from the pesticide industry. However, few detailed mechanistic studies on the role these additives play in environmental fate processes have appeared in the open literature. Application of laboratory-derived process information to field scenarios is hindered by the fact that most laboratory investigations have used technically pure (unformulated) organophosphorus insecticides. Including the effects of formulation ingredients on such processes as volatilization and sorption to soil solids would allow laboratory studies to better predict the environmental behavior of these compounds. 

Formulation strongly affects the amount of organophosphorus insecticide washed off from foliar surfaces. For example, azinphos-methyl and phosmet formulated as emulsifiable concentrates were highly susceptible to washoff, while wettable powder formulations were not (58). Thin-walled fenitrothion microcapsules were less prone to washoff than the emulsifiable concentrate (59). The composition of spray adjuvants also affected the degree of washoff (5<5). 

While the occurrence of most processes affecting organophosphorus insecticide concentrations in surface runoff are well documented, many are relatively poorly understood on a mechanistic level. Key processes requiring further study include sorption to plant and soil surfaces, and transfer from soil water into overland flow. Sorption to foliar surfaces determines the amount of applied insecticide available for washoff onto the soil surface or into surface runoff The kinetics of desorption from plant cuticular materials and the effect of adjuvants on foliar sorption processes need additional research. 

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