Interaction of treatment of both adult and immature Coleoptera with a chitin synthesis inhibitor affects mortality and development time of their progeny

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Abstract

Chitin synthesis inhibitors, like many other insect growth regulating insecticides, do not kill adult insects but cause mortality of the immature stages. Pre-exposure of adult stored grain Coleoptera, Rhyzopertha dominica (F.) (Bostrichidae) and Sitophilus oryzae (L.) (Curculionidae) and development of their progeny in grain treated with the chitin synthesis inhibitor, chlorfluazuron, influenced the mortality and development rate of the progeny. Hatch rate of eggs from R. dominica adults that had both developed and laid on wheat treated with 0.75 mg kg−1 chlorfluazuron was reduced by almost 50% compared with untreated eggs, with an LC50 of 0.84 mg kg−1. Eggs laid on treated wheat by R. dominica adults that had been exposed only to treated wheat for 2 weeks before oviposition showed greater reduction in hatch: 75% reduction of normal hatch rate at 0.25 mg kg−1 and almost 100% reduction at 2 mg kg−1 chlorfluazuron, with an LC50 of 0.19 mg kg−1 chlorfluazuron. X-rays were used to monitor the development and mortality of the immature stages of S. oryzae that developed within the wheat grains. Numbers of eggs laid were not affected by chlorfluazuron treatment. The combination of pre-exposure of adults and chlorfluazuron concentration had an additive effect on mortality of immature S. oryzae. Pre-exposure of adults caused most mortality in the first three weeks of development (eggs and larvae), whereas development in treated wheat caused mortality from weeks 3 to 8 (pupae and adults); higher concentrations of chlorfluazuron caused higher mortality. Development in wheat treated with 1 mg kg−1 chlorfluazuron caused 12% corrected overall mortality of progeny while pre-exposure to the same concentration and development in untreated wheat caused 29% corrected mortality. Pre-exposure combined with development in wheat treated with 1 mg kg−1 caused 30% corrected mortality. Thus, pre-exposure of adults appears to have a greater effect on mortality of S. oryzae progeny than development of immature stages in treated grain. Development on treated grain had no effect on development rate. Pre-exposure of adults did not appear to affect the rate of immature development, as assessed by X-rays, but did slow the emergence of adults, lengthening development time by about 2 days. This significant, additive effect of pre-exposure of adults on the mortality of their progeny will enhance the toxicity of chitin synthesis inhibitors such as chlorfluazuron, since most adults receive treatment when the immature stages are treated in crops either before they are harvested or in storage. Assessing the proportion of eggs that hatch from pre-exposed adults would be a simpler bioassay for CSIs.

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