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‘Pyramid’ approach to bug control may be too optimistic

Selection for resistance to one Bt toxin often causes cross-resistance to another

caterpillar on a cotton plant leaf

University of Arizona scientists say developers of transgenic insect-resistant crops may be a bit too optimistic about their success.

Since 1996, more than a cumulative total of a billion acres worldwide have been planted with crops incorporating the Bacillus thuringiensis (Bt) toxin to control rootworm and other pests.

Writing in the journal Nature Biotechnology, the scientists say crop developers have recognized the potential for the pests to become resistant to Bt and used a “pyramid” approach of producing two or more different Bt toxins. If the insect becomes resistant to one toxin, one of the others will still kill it. They compare it to a “lock and key” approach — if you lose the key to your front door, you can still get in the house if you have a key to a different lock on the back door.

“So, if you can’t kill the insect one way, you can kill it another way. That’s how pyramids work. It’s like having two different keys, so the insect needs two different mutations to become resistant,” study co-author Bruce Tabashnik says in the university release.

However, he says the theory may not work as well in practice, and that selection for resistance to one toxin in a pyramid often causes cross-resistance to another.

The scientists say their findings could improve management practices for current biotech crops and promote development of new varieties that are more effective and more durable. One goal of the study is to help biotech companies decide which toxins to put in their pyramided crops based on data that already exists, rather than by a time-consuming process of trial and error.

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