One of the most successful strategies in pest control is to endow crop plants with genes from the bacterium Bacillus thuringiensis, or Bt for short, which code for proteins that kill pests attempting to eat them.

But insect pests are evolving resistance to Bt toxins, which threatens the continued success of this approach. In the current issue of Nature Biotechnology, a research team led by UA Professor Bruce Tabashnik reports the discovery that a small modification of the toxins' structure overcomes the defenses of some major pests that are resistant to the natural, unmodified Bt toxins.

“A given Bt toxin only kills certain insects that have the right receptors in their gut,” explained Tabashnik, head of the UA’s entomology department in the College of Agriculture and Life Sciences. “This is one reason why Bt toxins are an environmentally friendly way to control pests,” he said. “They don’t kill indiscriminately. Bt cotton, for example, will not kill bees, lady bugs, and other beneficial insects.”

Unlike conventional broad-spectrum insecticides, Bt toxins kill only a narrow range of species because their potency is determined by a highly specific binding interaction with receptors on the surface of the insects’ gut cells, similar to a key that only fits a certain lock.

“If you change the lock, it won’t work,” Tabashnik said. “Insects adapt through evolutionary change. Naturally occurring mutations are out there in the insect populations, and those individuals that carry genes that make them resistant to the Bt toxins have a selective advantage.”

The more a toxin is used, the more likely it is pests will adapt.  Bt toxins have been used in sprays for decades.  Crops that make Bt toxins were commercialized 15 years ago and covered more than 140 million acres worldwide in 2010, according to Tabashnik.

In a joint effort with Alejandra Bravo and Mario Soberón at the Universidad Nacional Autónoma de México (UNAM), Tabashnik’s team set out to better understand how Bt toxins work and to develop countermeasures to control resistant pests.

“Our collaborators developed detailed models about each step at the molecular level,” Tabashnik said, “what receptors the toxins bind to, which enzymes they interact with and so on.”

Previous work had demonstrated that binding of Bt toxins to a cadherin protein in the insect gut is a key step in the process that ultimately kills the insect.  Results at UNAM indicated that binding of Bt toxins to cadherin promotes the next step - trimming of a small portion of the toxins by the insect's enzymes.  Meanwhile, Tabashnik's team identified lab-selected resistant strains of a major cotton pest, pink bollworm (Pectinophora gossypiella), in which genetic mutations altered cadherin and thereby reduced binding of Bt toxins.