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

Isolated whole integuments from L. cuprina larvae rapidly incorporate radioactivity from both N-acetyl[I-'4C]glucosamine and [1-'4C]glucosamine into alkali-insoluble material, a reaction which does not require preincubation of the tissue with /i-ecdysone. The labelled product was degraded to N-acetylglucosamine during digestion with chitinase, establishing that it consists mainly of chitin. Incorporation was inhibited by polyoxin-D (Iso, 6 x 10- 7 M) and diflubenzuron (Iso, 7 x 10- 7M) but was not inhibited to any marked extent by isoprothiolane, Vetrazin or a-methyl-DOPA. The effectiveness of diflubenzuron as an inhibitor of chitin synthesis in this system (Iso, 7 x 10- 7 M) correlates well with its potency as a larvicide (LDso, 2·1 x 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 (Iso, 6 x 10- 7 M) than as a larvicide (LDso, 2·0x 10- 5 M). It was established that the final intermediate of chitin biosynthesis (UDP-N-acetylglucosamine) was formed in the isolated integuments in the presence of diflubenzuron and polyoxin-D. It seems likely therefore that both compounds interfere with the final polymerization step of the chitin biosynthesis pathway.