Insects laughing at Bt toxin? Try this

To combat insect resistance to the widely used pesticide Bt, an international research team has announced a new way to restore the pesticide’s punch.

The insect-killing Bt toxins take their name from Bacillus thuringiensis, the bacterium that makes them. Genetic engineers have borrowed the bacterium’s toxin-making genes and inserted them into cotton, corn, and other crops so that the plants can make their own pesticides.

Farmers, especially in North America, have planted Bt crops in abundance, exposing so many insects to the toxin that entomologists say it’s just a matter of time before significant pests evolve resistance.

In an effort to stave off that day, Mario Soberón and Alejandra Bravo of the National Autonomous University of Mexico in Cuernavaca and their colleagues have been tinkering with the toxin genes. The researchers collaborated with Bruce Tabashnik at the University of Arizona in Tucson to study the Cry1A family of the Bt toxins, as they make their fatal attack on the guts of caterpillars.

Enzymes in the insect’s midgut snip Cry1A into pieces. The researchers now conclude that when these snippets bind to a protein called cadherin, they lose a molecular fragment. The loss initiates a series of reactions that end with holes in the gut wall.

Tabashnik knew from lab experiments that pink bollworms evolve resistance by developing a balky version of cadherin, which doesn’t bind well with the Cry1A pieces. Thus, Bt’s attack falters at this step.

To work around that roadblock, the Cuernavaca researchers remodeled Cry1A so that its segments don’t need to be clipped, obviating cadherin’s role. The new version of Cry1A can indeed kill Tabashnik’s formerly resistant bollworms, the researchers report in an upcoming Science.