Reduced titres of ecdysone following juvenile hormone treatment in the German cockroach, Blattella germanica

Changes in ecdysone titre of the larvae of the German cockroach, Blattella germanica, exposed continuously to the juvenile hormone (JH), or to the insect growth regulator (IGR) with JH activity, can be correlated with the nature of the substance applied, its dose, and the time of application. The younger larvae exposed to the high dose of the IGR die in the next ecdysis, whereas the same treatment induces a diapause-like stage of developmental arrest in the last larval stage. The affected larvae have very little or no ecdysone, the synthesis of which takes place in the second part of the instar. The same treatment after this period has a lesser effect. The extent of the effect is correlated to the amount of ecdysone synthesized before the application of IGR. Last instar larvae exposed to the lower dose of the IGR or JH lack the peak of ecdysone normally found in the controls at the end of the second third of the instar when metamorphosis takes place. In these insects the first rise of the ecdysone titre begins towards the end of the instar, and ecdysis into the supernumerary larval stage is initiated when the ecdysone titre reached a level permitting ecdysis.A direct or indirect antagonism between these hormones, both fundamental to insect development, can explain the morphogenetic, inhibitory, and lethal effects observed in insects treated with JH or IGR with JH activity.