AZADIRACHTIN DEPRESSES THE RELEASE OF PROTHORACICOTROPIC HORMONE IN RHODNIUS-PROLIXUS LARVAE - EVIDENCE FROM HEAD TRANSPLANTATIONS

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. © 1990.