Pestic. Sci. 1998, 52, 361È371 The Impact of Phenology , Exposure and Instar Susceptibilit y on Insecticide Effects on a Chr y somelid Beetle Population Christian Niels Elmegaard, A. Axelsen, Per N. Andersen & Kj~r,* JÔrgen Nanna Seidelin National Environmental Research Institute, Department of Terrestrial Ecology, PO Box 314, 25, VejlsÔvej DK-8600 Silkeborg, Denmark (Received 17 March 1997 ; revised version received 8 October 1997 ; accepted 19 November 1997) Abstract : 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 speciÐc exposure was evaluated. Two insecticides, cypermethrin and dimethoate, were investigated. In the laboratory, topical toxicity to eggs, second-instar larvae and adults was recorded in dose-response experiments. The spatial distribution of larvae and eggs were measured in the Ðeld. Deposition of insecticide onto eggs, second- instar larvae and adult specimens was measured at di†erent positions within the crop canopy by use of a dye tracer technique. A temperature-driven population model was constructed to simulate population development of all life stages in the Ðeld. The model was based on laboratory measures of growth and develop- ment at various temperatures. Mortality due to direct insecticide exposure was calculated as a function of population demography, spatial distribution of indi- viduals, spatial deposition of the insecticide, and stage-speciÐc susceptibility. Cypermethrin had the greatest impact, reducing population size by 19È32%. The life stages most sensitive to cypermethrin were the larval instars. As the popu- lation developed from eggs to larvae and imagines, the impact of one spraying Ðrst increased and then decreased according to the proportion of larvae in the population. Dimethoate had less e†ect on the population, i.e. 1É9È7É6% reduction. Dimethoate was most toxic to the egg stage, and consequently the e†ect on the population decreased as the proportion of eggs decreased due to hatching. The direct e†ect of insecticide spraying was signiÐcantly a†ected by all three factors investigated, i.e. phenology, life stage susceptibility and stage- speciÐc exposure. The latter factor is composed of both spray Ñux at various spatial positions in the canopy and the ability of di†erent life stages to retain spray droplets. 1998 SCI ( Pestic. Sci., 52, 361È371 (1998) Key words : insecticide side-e†ects ; exposure ; life stage susceptibility ; phenology 1 INTRODUCTION An important aspect of insecticide e†ect assessment is the estimation of e†ects on insect populations in the Ðeld for the purpose of protecting the crop against pests * To whom correspondence should be addressed. Contract/grant sponsor : Danish Environmental Protection Agency. Contract/grant no : MS-7041-0264. or assessing detrimental e†ects to non-target insects. Information on insecticide toxicity can be obtained from laboratory dose-response assays, but it is difficult to predict e†ects in the Ðeld from laboratory results for a number of reasons. First, the exposure of insects to insecticide spraying is composed of three routes of uptake : direct topical, residual and dietary. The relative importance of these three exposure routes di†ers among species of di†erent feeding guilds and trophic levels. There is, however, evidence that e†ects of direct spray 361 1998 SCI. Pestic. Sci. 0031-613X/98/$17.50. Printed in Great Britain (