Researchers at the University of Valencia in Spain and PlantGenetic Systems in Belgium have discovered why mothsexposed in the field to Dipel, a Bacillus thuringiensis (Bt)-based insecticide, become resistant to it.

The findings may result in improved insecticides. Companiesdeveloping Bt-based insecticides include Ecogen Inc., EntotechInc., Monsanto, Mycogen Inc. and Sandoz Ltd.

Different strains of Bt bacteria produce protein crystals thatkill different groups of insects. The crystals are thought towork by binding to and disrupting the insect's mid-gutmembranes. Normal diamondback moths are susceptible tocrystal toxins CryIA(b), CryIB, and CryIC.

Diamondback moths, isolated from an area in the Philippinesexposed to Dipel for many years, were resistant to CryIA(b),but were killed by CryIB and CryIC, an article in the June issueof the Proceedings of the National Academy of Sciencesreports. Dipel contains CryIA(b), but does not contain CryIB orCryIC. Thus, the moths apparently built up a resistance toCryIA(b) upon extended exposure.

The scientists were unable to detect any binding of CryIA(b) tothe mid-gut membranes of resistant moths. CryIB and CryICbound to the membranes as tightly as to normal mothmembranes. The scientists propose that resistant moths haveeither deleted or modified CryIA(b) receptors on their mid-gutmembranes so that CryIA(b) can no longer bind to themembranes and kill the insects.

-- Carol Talkington Verser, Ph.D. Special to BioWorld

(c) 1997 American Health Consultants. All rights reserved.