Difficulty in inducing extra larval ecdysis in the silkworm, Bombyx mori, by repeated administration of ecdysone.

Abstract

Induction of extra moults in 4th instar larvae of Bombyx mori by ecdysone or 20-OHecdysone applied by an oral or a haemolymph injection was examined and the results were compared with the effects of dietary application of ecdysone. Precocious ecdysis was effectively induced by the haemolymph and oral injection of ecdysone at doses of more than 5 and 10 ,ug/larva, respectively, to larvae on day 1 of the 4th instar. In case of 20-OH-ecdysone 2.5 ug for haemolymph injection and 10ug for oral injection were sufficient for inducing the advanced ecdysis. By the 2nd oral injection of ecdysone some of larvae ecdysed to the 6th instar, but morphological abnormalities were observed. Moreover, the 2nd larval ecdysis never occurred by the application of 20-OH-ecdysone. When the precocious 5th instar larvae induced by ecdysone injection were reared on the ecdysone containing diet, a large fraction of larvae ecdysed to the 6th instar, and some of them ecdysed to the 7th instar. On the basis of changing patterns of ecdysteroid titer in the haemolymph of the larvae, the failure of the induction of normal advanced ecdysis by the injection methods may be ascribed to a too rapid increase of ecdysteroid titer in the haemolymph.