Field reports from growers in Iowa of severe rootworm-feeding damage on GM crops engineered to kill the rootworm larvae by producing an insecticidal toxin derived from Bacillus thuringiensis (Bt) alerted Iowa State University researcher Aaron Gassmann.
Subsequent tests on rootworms collected from those fields confirmed it to be the first report of field-evolved resistance by the species, he said.
"Interviews with farmers indicate that [the same type of] Bt maize had been grown in those fields for at least three consecutive years."
There was not the same loss of performance on other types of Bt-resistant maize, however, he pointed out. "That suggests a lack of cross-resistance between Bt toxins."
Insufficient planting of refuges could also have contributed to the breakdown, he added. Growers of the Bt maize are supposed to plant 20% of the field with a non-GM variety for resistance management, but a study has suggested that only 50% of growers comply.
"The results suggest that improvements in resistance management and a more integrated approach to the use of Bt crop might be necessary," Dr Gassmann said.