Pesticide persistence on foliage

Abstract

A pesticide spray droplet may evaporate before reaching the intended target surface, drift out of the target zone, or be intercepted by a plant or soil surface in the target area. The amount of applied pesticide intercepted by target plants varies widely depending on meteorological conditions: spray droplet size, carrier and fall time; and the extent and density of ground cover presented by the target plants. Representative literature values reveal that the percent of applied pesticide intercepted by various targets was 62+-27 and 45+-20% for ground and aerial application, respectively. A need for improved pesticide delivery methods and systems is indicated. Plant-intercepted pesticides may be adsorbed, absorbed, altered, volatilized or removed by water. The integrated result of these processes is usually expressed as an initial rapid decline in surface residues followed by a slower, asymptotic decrease. Generally (ignoring formulation, extraction method, etc.), field half-lives of organochlorine (5.0+-4.6 d) and pyrethroid (5.3+-3.6 d) insecticides on foliage were longer than those of organophosphorus (3.0+-2.7 d) and carbamate (2.4+-2.0 d) insecticides. The absence of data that verifies the amount of pesticide intercepted by plants and that characterizes the rapid loss of pesticides from foliage immediately after application can result in over-estimated half-lives for foliar-intercepted pesticides. Many of the studies cited in this review typically suffer from one or both of these shortcomings. Thus, many of the half-livespresented in this review may be overestimates. Future studies concerning pesticide interception/persistence on foliageshould include sampling protocols that ensure acquisition of reliable data necessary for calculating half-life values. Plant and pesticide characteristics such as the form and wettability of the leaf cuticule and the pesticide's polarity/formulation regulate pesticide penetration of the leaf surface and/or the degree to which the pesticide is retained on the leaf surface. Several meteorological variables including wind, rain, sunlight, temperature, relative humidity, and dew are reported to affect pesticide persistence on plants. However, only in a few cases has pesticide disappearance from plants been related quantitatively to weather variables. The data presented in this review suggest that, in general, insecticide persistence on foliage appears to be shorter in the southern U.S. than in the northern U.S. and Canad…