Induction of dauer larvae by application of fenoxycarb early in the 5th instar of the silkworm, Bombyx mori.

ABSTRACT

Fenoxycarb application from 0 h until 60 h of the 5th instar of Bombyx mori induced 100% dauer larvae. Between the 60 and 78 h, the ratio of fenoxycarb-induced dauer larvae decreased, and the ratio of supernumerary instar moulting larvae increased. After application of fenoxycarb at the 48 h of the 5th instar, the haemolymph prothoracicotropic hormone (PTTH) titer was higher in fenoxycarb-treated larvae than in control larvae. Furthermore, brain-corpora cardiaca-corpora allata (Br-CC-CA) complexes from fenoxycarb-treated larvae released higher amounts of PTTH in vitro than the Br-CC-CA complexes of control larvae. Prothoracic glands (PGs) of fenoxycarb-treated larvae at the 48 h of the 5th instar exhibited basal and PTTH-stimulated secretory activities similar to that of control PGs until the 72 h of the 5th instar. After that time point, both basal and PTTH-stimulated secretory activity of PGs from fenoxycarb-treated larvae significantly decreased and remained low for the rest of the investigated period. The combined results suggest that the application of fenoxycarb affects the ability of the PGs to be stimulated by PTTH and the induction of dauer larvae in Bombyx mori is not due to inhibition of PTTH release from Br-CC-CA complexes.