EFFECTS OF FENOXYCARB ON THE SECRETORY ACTIVITY OF THE PROTHORACIC GLANDS IN THE 5TH INSTAR OF THE SILKWORM, BOMBYX-MORI

Effects of fenoxycarb on the secretory activity of the prothoracic gla nds (PGs) of the silkworm, Bombyx mori, were investigated. Fenoxycarb inhibited the secretory activity of PGs in vitro throughout the fifth instar. Incubation of PGs in the presence of 1 mu g of fenoxycarb resu lted in a similar to 50% inhibition of their secretory activity. This inhibition of fenoxycarb was dose dependent. Stimulation of the PGs by recombinant prothoracicotropic hormone (rPTTH) reached a high activat ion ratio of 15.9. Simultaneous presence of fenoxycarb and 1 ng rPTTH significantly decreased the stimulatory effect of rPTTH on the PGs, an d the activation ratio never exceeded 3.65. Early in the fifth instar, fenoxycarb strongly inhibited the secretory activity of the PGs and i nduced a developmental arrest as well. After the onset of pupal commit ment, its action toward the PG secretory activity was inhibitory. Howe ver, fenoxycarb treatment was not able to induce a developmental arres t in intact animals after the pupal commitment although it could do so in head-ligated animals. On the contrary, application of fenoxycarb a fter the wandering behavior resulted in a significantly increased secr etory activity by the PGs of both intact and head-ligated larvae. (C) 1996 Academic Press, Inc.