Effects of methoprene on heat tolerance and progeny production in Tribolium castaneum (Coleoptera: Tenebrionidae)

Methoprene is a juvenile hormone analogue, and a reduced-risk insecticide. It disrupts development of insect immature stages. A few studies have shown that lower concentrations that permit adult emergence, can still cause sub-lethal effects. The objectives of this study were to examine the effects of methoprene on heat tolerance and progeny production in the stored-product insect pest, Tribolium castaneum (Coleoptera: Tenebrionidae, red flour beetle). Exposure to methoprene (Diacon II) at 3.33 ppm or higher reduced the heat tolerance of adults at 46°C. However, it did not affect heat tolerance of larvae at 0.07 ppm. The larvae (0.001 or 0.0165 ppm) or adults (1.67 to 66.6 ppm) exposed to methoprene were paired, held on flour for 1 week and progeny production measured. To determine the effect on sex, pairing of virgin male and female adults was performed as untreated x untreated, treated x untreated or treated x treated. There was 31% (0.001 ppm) and 66% (0.0165 ppm) reduction in emergence to adult stage when last instar larvae were treated with methoprene. The adults that survived methoprene treatment had normal movement and external morphology, but produced fewer offspring (both sexes treated, 24 progeny/pair/week) than untreated (88 progeny/pair/week). Males were more affected than females. There was no difference in progeny production between 0.001 and 0.0165 ppm. Adults treated with methoprene did not have lower progeny production. This work suggests that juvenile hormone is involved in heat tolerance and development of reproductive systems. The implications for using methoprene in stored-grain pest management are discussed.