Effects of azadirachtin on levels of ecdysteroids and prothoracicotropic hormone-like activity in Heliothis virescens (Fabr.) larvae

Oral injections of 1 μg of the plant chemical azadirachtin into final (fifth)-instar larvae of the tobacco budworm, Heliothis virescens (Fabr.), reduced whole body and haemolymph titres of the moulting hormones, ecdysone and 20-hydroxyecdysone, below those of control larvae, thereby preventing normal development to the pupal stage. Likewise, oral injections of 1 μg of azadirachtin reduced brain levels of prothoracicotropic hormone (PTTH)-like activity, which is the stimulation of in vitro moulting hormone synthesis by the prothoracic glands. In addition, prothoracic glands from azadirachtin-treated larvae were less receptive to stimulation in vitro and in vivo by control brain extracts of PTTH compared to control glands. However, azadirachtin did not directly inhibit the in vitro synthesis of moulting hormone when the compound was included in the incubation media with prothoracic glands from control larvae. A timely haemocoel injection of 20-hydroxyecdysone into azadirachtin-treated larvae resulted in partial recovery of development to the pupal stage. Based upon these data, we suggest that in preventing normal development of final-instar larvae of H. virescens, azadirachtin reduces moulting hormone titres by reducing PTTH titres and the receptivity of prothoracic glands to produce ecdysone via stimulation by PTTH.