Disappointing but not unexpected is a leading UK scientist’s reaction to news of what is claimed to be the first instance of in-field resistance to the GM-derived insect defences of a crop.

“The sky won’t fall in but it’s disappointing,” said Ian Denholm, head of Rothamsted Research’s Department of Plant and Invertebrate Ecology.

He was commenting on an Independent newspaper report on research at the University of Arizona which found bollworm moth caterpillars resistant to the toxin from the Bacillus thuringiensis (Bt) gene that is widely used in creating pest-resisting GM species.

“Evolution in action”

According to the report, the resistant caterpillars were found in 12 cotton fields in Mississippi and Arkansas during surveys from 2003 to 2006, and research team leader Bruce Tabashnik is quoted as saying: “What we are seeing is evolution in action. This is the first documented case of field-evolved resistance to a Bt crop.”

Since introduced in the early 2000s Bt crops have become widely grown throughout the world, noted Dr Denholm. “They have been spectacularly successful.

“We have always known that it was relatively easy to select for resistance [to the Bt toxin] in the laboratory. But we haven’t seen it in the field because these crops are very carefully regulated.”

This included the requirement to provide refuge areas allowing susceptible species to survive and mate with any becoming resistant, he explained.

More than one available

As with resistance to conventional insecticides, the development of resistance to the Bt toxin was not unexpected, he said.

“But it’s not the only one available. There are three or four others, although they are all based on Bt.

“If this latest finding is confirmed there will need to be some kind of re-think on strategy and possibly a change to some of these others.”

Monsanto’s Colin Merritt points out that Randy Luttrell, of the University of Arkansas, measured the susceptibility of cotton bollworm to the Cry1Ac protein (in Bollgard cotton) between 1992 and 2007.

Interpretations

“Dr Luttrell presented his data at the recent 2007 Entomological Society of America meeting and concluded there is variation in bollworm response to the Bt protein, but the data do not support a conclusion of resistance.”

Dr Tabashnik’s review of the data is published in the February 2008 issue of Nature Biotechnology.

“Entomologists in the cotton industry disagree with the interpretation of the data Tabashnik has made,” says Dr Merritt.

“Our understanding is that Environmental Protection Agency also disagrees with his interpretation.

“During the use of Bt as a spray for many decades, as well as in GM crops for 10 years, isolated strains of insects have shown slight variations in response. But the fact that field performance has not been affected shows the technology remains a robust alternative to chemical methods.”

Picture courtesy of Clemson University, Department of Entomology.