The researchers, led by Aaron Gassman of the University of Iowa’s Department of Entomology, found​ that resistance to the natural insecticidal toxins derived from the bacterium Bacillus thuringiensis​ (Bt) – used in Monsanto’s GM corn – had developed in four Iowa fields. The insects’ resistance to the toxin developed after farmers had planted Bt corn on the same field three years in a row – a relatively rarely occurrence, as farmers tend to rotate crops more regularly.

Nevertheless, the researchers said that evidence of insect resistance could cut short the benefits of Bt maize, which include reductions in pesticide use and localized suppression of some key agricultural pests.

They wrote: “These results suggest that improvements in resistance management and a more integrated approach to the use of Bt crops may be necessary…Even with resistance management plans in place, sole reliance on Bt crops for management of agriculture pests will likely hasten the evolution of resistance in some cases, thereby diminishing the benefits that these crops provide.”​

The researchers also found that it was possible for the insects to pass on Bt resistance to their offspring.

They said this is thought to be the first time an insect has developed resistance to a GM crop trait.

In response to the study, Monsanto said it was too early to tell whether there were implications for growers in the field.

The company said that farmers are still getting good results from the technology, and recommends that they rotate crops to include non-Bt varieties.

It said: “These products continue to perform very well for growers in 2011, providing the expected level of WCR [western corn rootworm] control on more than 99% of the acres planted with this technology. However, we take performance and efficacy of our products seriously, and we are collaborating with Dr. Gassmann to better understand his initial data and to determine if and how they impact our IPM [integrated pest management] recommendations to growers.”​

Bt maize accounted for about 45 percent of the corn crop planted in the United States in 2009.