Azadirachtin depresses the release of prothoracicotropic hormone in Rhodnius prolixus larvae: Evidence from head transplantations

Abstract

Azadirachtin A (1.0 μg/ml) if fed to last-instar larvae of Rhodnius prolixus through a blood meal, affects PTTH production as shown by a reduction or absence of stimulation of ecdysteroid titres in decapitation experiments. The host larvae, when decapitated 5 days after feeding, showed steadily declining haemolymph ecdysteroid titres whereas untreated larvae at this stage maintained their moulting hormone titre. Head transplantations from untreated donors 4–5 days after feeding onto headless larvae sustained hormone production for about 18 h. Heads from azadirachtin-treated donors were unable to sustain a constant ecdysteroid level which declined immediately after transplantation. In converse experiments, heads transplantation from untreated donors 5 days feeding stimulated the production of ecdysteroids in azadirachtin-treated recipients (40 days after azadirachtin A treatment) for about 18 h. It is suggested that the synthesis and release of PTTH was deficient in the azadirachtin-treated animals. The significance of these results is discussed in relation to a general mode of azadirachtin action in insects.