The juvenile hormone analog pyriproxyfen affects ecdysteroid-dependent cuticle melanization and shifts the pupal ecdysteroid peak in the honey bee (Apis mellifera)

The control of the pupal melanization in the honey bee by ecdysteroids, and the modulation of these processes by a juvenile hormone analog were invest igated by a combination of in vivo and in vitro experiments. Injection of 1 -5 mug of 20-hydroxyecdysone (20E) into unpigmented pupae showed a dose- an d stage-dependent effect. The higher the dose and the later the injection w as performed, the more pronounced was the delay in cuticle pigmentation. Th is inhibition of cuticular melanization by artificially elevated ecdysteroi d titers was corroborated by in vitro experiments, culturing integument fro m unpigmented, dark-eyed pupae for 1-4 days in the presence of 20E (2 or 5 mug/ml culture medium). Topical application (1 mug) of pyriproxyfen to unpi gmented, white-eyed pupae had the opposite effect, leading to precocious an d enhanced melanization of the pupal cuticle. In vitro incubation of integu ments in the presence of this juvenile hormone analog (1 mug/ml) confirmed these results, showing that pyriproxyfen is apparently capable of triggerin g melanization. The in vivo mode of action of pyriproxyfen was further inve stigated by quantifying hemolymph ecdysteroids by radioimmunoassays. Topica l application leads to a delay of the pupal ecdysteroid peak by 4 days. The pyriproxyfen-induced low ecdysteroid titers during early pupal development could account for precocious pigmentation by removing an inhibition on pro phenoloxidase activation normally imposed by the elevated ecdysteroid titer during this phase. (C) 2000 Elsevier Science Ltd. All rights reserved.