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Deposition of insecticide on glass models of Lepidopterous larvae

Follett, P.A., Gould, F., Leidy, R., Bradley, J.R.
Journal of economic entomology 1994 v.87 no.6 pp. 1400-1406
permethrin, application methods, larvae, dosage, spatial distribution, canopy, leaves, Glycine max, insect control, chemical control
Glass models were used to simulate lepidopterous larvae for insecticide-deposition studies. Models were attached to foliage at various positions in the canopy of soybeans before treatment with permethrin (Pounce 3.2 EC [emulsifiable concentrate]). Experiments were done to determine differences in insecticide interception by models of three size classes, corresponding roughly to third-, fourth-, and fifth-instar Helicoverpa zea (Boddie), and to examine differences in deposits when insecticide was applied by aircraft compared with ground application by a tractor-mounted sprayer. Insecticide deposits on glass models were quantitated by high-performance liquid chromatography. Data from glass models indicated that larvae receive variable doses during insecticide application depending on their position in the canopy, their size, and the application technique used. Deposits on models generally decreased with increasing depth in the canopy; models attached to the upper surface of leaves intercepted more insecticide than those attached to the under surface. Models with smaller surface area intercepted less insecticide than larger models in the upper canopy, but size-dependent differences in deposits were less apparent in the middle and lower strata of the plant canopy. Aerial- and ground-applied insecticide showed similar patterns of deposition. Results from glass models are used to project the consequences of permethrin impingement on H. zea larvae. Based on levels of insecticide interception at the time of application, some larvae will receive a dose many times higher than a lethal dose, whereas other larvae initially will receive a sublethal dose. Means and variances in deposits are equally important for predicting the toxic effect of insecticide on a population.