In several U.S. states, Western corn rootworms are developing resistance to a Bt protein produced by certain varieties of genetically modified Bt maize. In response, twenty-two leading scientists in the field of pest control in maize farming addressed a joint letter to the U.S. Environmental Protection Agency (EPA) at the beginning of March. Among other things they criticize the lack of adequate resistance management and the marketing strategies of the big companies. The scientists call for integrated pest control that includes the cultivation of Bt maize.

In 2011, scientific papers were published for the first time describing discoveries of resistant Western corn rootworms in the Midwest. This development came as no surprise to scientists because they had known for a long time that the concentration of Cry3Bb1 – a Bt protein that targets the Western corn rootworm – in the GM maize varieties in question is relatively low. This means there is a risk that not all the pests will be killed and that the surviving insects will develop resistance. Other Bt proteins in other maize varieties, e.g. Cry1Ab in MON810 plants, are produced in concentrations that ensure all the pests will be killed. Used in combination with sufficiently large refuge areas, this has prevented the pests developing resistance to Bt proteins for the last 15 years.

In their letter to the EPA, written in March 2012, the 22 scientists stress that since it was introduced, Bt technology has been valuable in terms of reducing insecticide use and increasing farm income. It is therefore in principle an effective, environmentally friendly pest control method. One of the reasons why the Western corn rootworm is developing resistance is, according to them, the fact that farmers often grow varieties with the same Bt protein year after year. Another reason is the fact that refuges are too small. In 2003, some US scientists called on the EPA to prescribe refuge areas of 50 per cent for Cry3Bb1-producing maize varieties, but the EPA prescribed only 20 per cent. During the introduction of maize varieties with, which also contain a second Bt protein that targets the Western corn rootworm, the prescribed area for refuge was reduced still further and is now 10 percent.

The scientists also criticize the fact that the latest maize varieties with the highest potential yields are available almost exclusively as GM variants with one or more Bt genes. This, they claim, is one of the main reasons why Bt maize is also grown in regions where pest infestation is low and there is actually no need to grow Bt crops. This encourages the development of resistant pests. The economic advantage of Bt maize will be undone if farmers have to use insecticides as well.

The 22 scientists write that it is not possible to rely on one single method of pest control in the long term. They call for integrated pest control, including the cultivation of Bt maize alongside other measures like crop rotation and the use of insecticides.