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Rootworms developing resistance to pesticidal transgenic corn

Fri, 03/21/2014 - 4:09am
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As Dr. Ian Malcolm says in "Jurassic Park," “Life finds a way.” Even when humans don’t want it to.

Rootworms capable of destroying corn crops had been thought by farmers to have been defeated by using transgenic corn called Bt corn. The Bt corn is named for the pesticide-producing gene from the bacteria Bacillus thuringiensis and, according to the US Department of Agriculture (USDA), 75 percent of corn crops in the country have this type of corn to kill pests without destroying other crops or useful animals.

However, farmers are now having a problem familiar to hospital doctors: Evolution of resistance. The few corn rootworms with a resistance to the Bt toxin have the whole field to themselves to eat and reproduce in. Soon enough, the whole field has only the Bt-resistant rootworm.

Part of the reason the rootworm in particular has been successful in evolving resistance is that the Bt corn that can target the western corn rootworm does not produce a very high dose of the toxin, according to a 2014 paper by Iowa State University entomologist Aaron J. Gassman.

“These first cases of resistance by western corn rootworm highlight the vulnerability of Bt maize to further evolution of resistance from this pest,” Gassman et al. wrote in the paper, “and, more broadly, point to the potential of insects to develop resistance rapidly when Bt crops do not achieve a high dose of Bt toxin.”

Would making transgenic corn with the ability to make higher doses of the Bt toxin be a potential solution then? Probably not, at least not in the long term; like the issue of antibiotics, taking smaller doses definitely helps shore up resistant organisms faster, but they all eventually adapt.

An Environmental Protection Agency (EPA) panel convened in 2002 recommended that at least half of each corn farmer’s fields be devoted to non-Bt corn with non-resistant rootworms; that way, if resistance develops in the Bt fields, the resistant rootworms can be bred with the non-resistant rootworms to make non-resistant offspring.

Seed companies resisted this recommendation being put into law. The EPA eventually set voluntary refuge guidelines at between 5 and 20 percent; many farmers don’t even do that.

Now that resistance is developing, entomologists are concerned that farmers will return to using chemical insecticides.

“When insecticides overlay transgenic technology, the economic and environmental advantages of rootworm-protected corn quickly disappear,” several entomologists wrote in a letter to the EPA in 2012.

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