THE IMPACT OF PHENOLOGY, EXPOSURE AND INSTAR SUSCEPTIBILITY ON INSECTICIDE EFFECTS ON A CHRYSOMELID BEETLE POPULATION

Direct topical impact of an insecticide spray on a population of a non target leaf-eating beetle, Gastrophysa polygoni, was studied, and the relative importance of phenology, instar susceptibility and instar spe cific exposure was evaluated. Two insecticides, cypermethrin and dimet hoate, were investigated. In the laboratory, topical toxicity to eggs, second-instar larvae and adults was recorded in dose-response experim ents. The spatial distribution of larvae and eggs were measured in the field. Deposition of insecticide onto eggs, second-instar larvae and adult specimens was measured at different positions within the crop ca nopy by use of a dye tracer technique. A temperature-driven population model was constructed to simulate population development of all life stages in the held. The model was based on laboratory measures of grow th and development at various temperatures. Mortality due to direct in secticide exposure was calculated as a function of population demograp hy, spatial distribution of individuals, spatial deposition of the ins ecticide, and stage-specific susceptibility. Cypermethrin had the grea test impact, reducing population size by 19-32%. The life stages most sensitive to cypermethrin were the larval instars. As the population d eveloped from eggs to larvae and imagines, the impact of one spraying first increased and then decreased according to the proportion of larv ae in the population. Dimethoate had less effect on the population, i. e. 1.9-7.6% reduction. Dimethoate was most toxic to the egg stage, and consequently the effect on the population decreased as the proportion of eggs decreased due to hatching. The direct effect of insecticide s praying was significantly affected by all three factors investigated, i.e. phenology, life stage susceptibility and stage-specific exposure. The latter factor is composed of both spray flux at various spatial p ositions in the canopy and the ability of different life stages to ret ain spray droplets. (C) 1998 SCI.