Effects of methoprene on Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)
Wijayaratne, Leanage Kanaka Wolly
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Due to concerns with synthetic neurotoxic insecticides used in insect pest management, alternative control methods are sought. Methoprene is a juvenile hormone analogue, which is well-known for its lethal effects on immature insects, and is registered as a stored-grain insecticide in the U.S.A. and Australia. However, less is known about its sub-lethal effects. Experiments were conducted to investigate the effects of methoprene (Diacon II) on heat tolerance, cold tolerance and progeny production of Tribolium castaneum (Herbst) when late-instar larvae or adults were exposed to wheat treated with methoprene. Methoprene at 3.33 ppm or higher, reduced heat tolerance of adults at 46ºC. In contrast, methoprene did not affect the heat tolerance of larvae. Cold tolerance and cold acclimation of both adults and larvae was not affected by methoprene. Exposure of larvae to 0.001 or 0.0165 ppm of methoprene on wheat had 37 and 72% reduction in adult emergence, respectively. The surviving adults had normal movement, but their progeny production was reduced by 71%. Males were affected more than females. Exposure of adults to methoprene (66.6 ppm) did not reduce progeny production. Methoprene is used as a surface treatment to control insects in empty grain bins, processing facilities and warehouses. Experiments were conducted to assess the effect of surface material, temperature and cleaning practices on methoprene residual efficacy. A bioassay with late-instar larvae at 30ºC until adult emergence was used to measure methoprene residual efficacy. Methoprene applied at the label rate (0.0003 mg/cm2) on varnished wood, held at 20, 30 or 35ºC for 24 weeks, prevented all development of larvae into adults. Conversely, concrete surfaces allowed 22% adult emergence after 8 weeks and 69% after 24 weeks. Temperatures, 20, 30 or 35ºC, that the concrete surfaces were held before the bioassay, did not affect this decline in efficacy. Presence of flour or repeated removal of flour (cleaning) reduced the efficacy of methoprene on concrete surfaces, but not on varnished wood. Maintaining concrete surfaces at 65ºC for 48 hours did not reduce efficacy. The implications of these results are discussed in the context of controlling stored-product insects, as well as insect physiology.