No morphological difference was found in the epithelial cells of midgut between the normal and the diflubenzuron-treated last instar larvae of the cabbage armyworm, Mamestra byassicae L. Diflubenzuron and polyoxin D strongly inhibited chitin synthesis in the peritrophic membranes of the midgut when glucosamine or N-acetylglucosamine was applied, as a precursor of chitin, either inside or outside the midgut. When UDP-N-acetylglucosamine was injected into the midgut with polyoxin D, chitin synthesis was blocked, but not with diflubenzuron. The results lead to the following speculation: UDP-N-acetylglucosamine would be formed from glucosamine or N-acetylglucosamine in the epithelial cells and then move to the outer face of the biomembranes of the cells to form chitin. Diflubenzuron may act as an inhibitor of the transport system of UDP-N-acetylglucosamine across biomembranes, since an accumulation of UDP-N-acetylglucosamine is known in insects treated with diflubenzuron. Na+-K+ ATPase and Cat+Mg2+ ATPase were not affected by the insecticide.

No abstract

Diflubenzuron was found to inhibit chitinous cuticle formation in Mamestra in vivo. When epidermis from last instar larvae was cultured in vitro in the presence of diflubenzuron or polyoxin D, both compounds inhibited chitin synthesis with 150 of 8.9 x 109M for diflubenzuron and 8.8 x 10 M for polyoxin D. When larvae were injected with 14C-GA after treatment of diflubenzuron, 14C-UDP-AGA was found to accumulate in vivo. Apparently diflubenzuron blocks the terminal polymerization step in chitin synthesis. Chitin synthetase activity in cell-free extracts was inhibited by polyoxin D but not by diflubenzuron. Yet the enzyme preparations from the animals treated with diflubenzuron were about one-tenth as active as those from the controls.