Multiple Toxins in Bt Crops Don't Solve Pest Evolution Problem

New findings could actually improve practices for current biotech crops. Scientists have discovered that multiple toxins in transgenic crops don't help when it comes to pest control.

Many crops have been genetically engineered to produce proteins from the bacterium Bacillus thuringiensis (Bt). This bacterium naturally occurs in soils, and can act as a type of natural pest control. When pests chomp on a Bt crop, they're impacted by the proteins and die. Yet over the past several years, pests have rapidly evolved to resist these Bt crops. That's why many companies have created Bt crops called "pyramids" that produce two or more Bt toxins active against the same pest.

"The idea behind Bt crop pyramids can be explained with a lock-and-key analogy," said Bruce Tabashnik, one of the researchers, in a news release. "The lock on the door is the receptor protein in the insect's gut, and the key is the Bt toxin that binds to that receptor. To be able to kill the insect, the toxin must fit the lock to open the door and get inside. If you have only one key-one toxin-and a mutation has changed the lock-the receptor-then the toxin can't open the door and get inside. The insect is resistant and survives."

That said, no studies have really been conducted to see if pyramids can delay the evolution of resistance by pests. In order to find this out, scientists analyzed data from 38 studies that report effects of 10 Bt toxins used in transgenic crops against 15 insect pests.

What did they find? It turns out that pyramids were only successful in about half of the cases. Contrary to the ideal scenario typically assumed, selection for resistance to one toxin in a pyramid often causes cross-resistance to another toxin in the pyramid.

The findings could help companies decide which toxins to put in their pyramided crops based on data that already exist.

"Our results mean that the keys-toxins-used in Bt crops by farmers worldwide are often not as different from each other as we would like," said Tabashnik. "And that, in turn, has huge implications for agencies tasked with setting standards for the size of refuges to be planted."

The findings are published in the journal Nature Biotechnology.

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