Effects of Several Larvicidal Compounds on Chitin Biosynthesis by Isolated Larval Integuments of the Sheep Blowfly Lucilia cuprina

Abstract

Isolated whole integuments from L. cuprina larvae rapidly incorporate radioactivity from both N-acetyl[1-14C]glucosamine and [1-14C]glucosamine into alkali-insoluble material, a reaction which does not require preincubation of the tissue with .beta.-ecdysone. The labeled product was degraded to N-acetylglucosamine during digestion with chitinase, establishing that it consists mainly of chitin. Incorporation was inhibited by polyoxin-D (I50 [median inhibition], 6 .times. 10-7 M) and diflubenzuron (I50, 7 .times. 10-7 M) but was not inhibited to any marked extent by isoprothiolane, Vetrazine or .alpha.-methyl-dopa. The effectiveness of diflubenzuron as an inhibitor of chitin synthesis in this system (I50, 7 .times. 10-7 M) correlates well with its potency as a larvicide (LD50, 2.1 .times. 10-6 M), providing additional support for the proposal that this compound kills larvae by interfering with chitin deposition in the cuticle. Polyoxin D was much more effective as an inhibitor of chitin synthesis (I50, 6 .times. 10-7 M) than as a larvicide (LD50, 2.0 .times. 10-5 M). The final intermediate of chitin biosynthesis (UDP-N-acetylglucosamine) was formed in the isolated integuments in the presence of diflubenzuron and polyoxin D. Both compounds apparently interfere with the final polymerization step of the chitin biosynthesis pathway.