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Beetles and feeding damage found in Bt maize fields

First resistant corn rootworms found in the USA

There are indications in the USA that the Western corn rootworm could develop resistance to genetically modified Bt maize. Scientists in Iowa, Illinois and Minnesota have found living specimens of the beetle and evident corn rootworm feeding damage in Bt maize fields. The precise extent of the resistance is not yet known. So far, only Monsanto maize varieties have been affected. The company has spoken of isolated incidences, but intends to carry out further research in collaboration with the scientists.

Western corn rootworms on a maize root
Photo: Mihaly Czepo

Feeding damage on the roots of maize plants on a field in Illinois that has evidently been caused by corn rootworm larvae.
Photo: Michael Gray, University of Illinois

Once the roots have been damaged, the plants become unstable and topple over.
Photo: Michael Gray, University of Illinois

Third larval stage of the Western corn rootworm

L3 larva of a Western corn rootworm
Photo: K. Gloyna,
BTL Bio-Test Labor

From 2005 to 2008 scientists at the University of Göttingen investigated the development of resistance to Bt maize (MON88017) in the Western corn rootworm.

They discovered that a few larvae did survive in the Bt maize crop. The quantity of Bt protein produced, especially in the roots, is so small that it mainly kills the first larval stage (L1).

Corn rootworm larvae can also feed on other plants, including grasses. If the L1 larvae feed on these other plants first and only switch to the Bt maize afterwards, partially resistant pests can develop, which could lead to the development of full resistance over several generations.

At the end of July 2011, the online journal PloS ONE published a paper by Aaron Gassmann, an entomologist at Iowa State University. The paper described for the first time Western corn rootworms that have developed field resistance to the Bt protein Cry3Bb1. This protein specifically targets the corn rootworm. In 2009, Gassmann had collected adult Western corn rootworm beetles on four fields in Iowa on which Bt maize from Monsanto which produces Cry3Bb1 had been planted. In laboratory tests the scientists demonstrated that the beetles were capable of surviving a much higher dose of Cry3Bb1 than other corn rootworms.

At the same time as the paper was published, Monsanto issued a press release stating that the maize variety in question effectively controls the pest in 99 per cent of cases. The company nevertheless claimed that it would be taking the findings seriously and conducting further studies and working together with Aaron Gassmann. In an interview, a press spokesman said that there had been isolated cases which had been confined to a small area of less than 10,000 hectares.

At the end of August 2011, Michael Gray, an entomologist at the University of Illinois, wrote in a farming journal that he had found Western corn rootworms on Bt maize fields in two counties in Illinois. The beetles need to be studied in the laboratory before it can be established whether they are resistant. Since then, several farmers are said to have contacted Gray to report instances of Diabrotica and related feeding damage on their Bt maize plants. Ken Ostlie, an entomologist at the University of Minnesota, has reported typical Diabrotica damage on Bt maize fields in Minnesota.

This development comes as no surprise to scientists. Stefan Vidal at the University of Göttingen told GMO Safety in an interview back in 2003 that Western corn rootworms could develop resistance in the USA in the foreseeable future because the Bt maize varieties in question produce relatively low levels of Cry3Bb1. This concern was shared by a number of scientists who were consulted by the Environmental Protection Agency during the crop’s authorisation in the USA. They advocated the establishment of 50 per cent refuges. In the end, however, the agency prescribed refuges of only 20 per cent. A study by the Center for Science in the Public Interest showed that in 2003 over 90 per cent of the farmers growing Bt maize respected the refuge requirements. By 2008 only three-quarters were complying with them.

Aaron Gassmann shares the view that insufficient refuges could have contributed to the development of resistance. Gassmann has also called for integrated pest control, using measures like crop rotation in addition to Bt plants. While instances of resistance remain isolated, and the frequency of the resistance genes within the pest population is low, this does not present a problem. The real problem arises when the pest population consists predominantly of resistant individuals. It is not yet clear to what extent resistance has actually developed in the Western corn rootworm in the USA.