(March 24, 2013 Beyond Pesticides) A study by an entomologist at Iowa State University in Ames Iowa found that western corn rootworm is now resistant to two varieties of Bt-incorporated genetically engineered (GE) maize and that resistant insects are likely to be cross-resistant. This study adds to the growing scientific literature that shows insect resistance to Bt crops is making certain GE technologies obsolete, which could lead to an increase in insecticide use.
The study, Field-evolved resistance by western corn rootworm to multiple Bacillus thuringiensis toxins in transgenic maize, conducted by a team led by Aaron Gassmann, PhD, adds to the growing scientific literature that that finds western corn rootworm is resistant to varieties of Bt-incorporated GE maize. In 2009, farmers in Iowa observed severe injury to Cry3Bb1 maize —one of the three varieties of Bt-incorporated GE maize- from larval western corn rootworm in the field. Subsequent laboratory assays reveal that this injury is associated with rootworm resistance to Cry3Bb1. This study finds that injury to Cry3Bb1 maize because of rootworm resistance persisted beyond 2011 and expanded to include mCry3A maize, a second variety of Bt-incorporated GE maize.
Laboratory analysis of western corn rootworm from these fields finds resistance to Cry3Bb1 and mCry3A and cross-resistance between these toxins. Cross-resistance is important because it means that when generations of corn rootworms are resistant to one of these varieties they are also often resistant to a second form. To slow injury to crops from resistance, biotech companies are pyramiding, or stacking, Cry3Bb1 and mCry3A varieties with Cry34/35Ab1, the third form of Bt maize and has as yet exhibited resistance.
However, according to the study, the presence of resistance to one toxin in a pyramid diminishes the effectiveness of a pyramid to delay resistance, and may hasten the evolution of new resistance. These results demonstrate that insects can evolve resistance rapidly to Bt crops -resistance shows up in Iowa fields an verage of 3.6 years after Cry3Bb1 is introduced -and raise concerns about the adequacy of current resistance management strategies and the ongoing effectiveness of non-GE Bt strains used widely in organic agriculture.
Several previous studies have also documented growing corn rootworm resistance to Bt maize. In 2011, Dr. Gassmann published “Field-Evolved Resistance to Bt Maize by Western Corn Rootworm,” a study verifying the first field-evolved resistance of corn rootworm to a Bt toxin. The researchers documented resistance to the Bt toxin Cry3Bb1. The study found the western rootworm’s ability to adapt is strongest in fields where Bt corn is planted for three consecutive years and suggests that insufficient planting of refuges contributes to the problem.
This study was cited by a group of 22 prominent entomologists who submitted formal comments to EPA on their concerns about the viability of Cry3Bb1 corn. Recently, even the Environmental Protection Agency (EPA) concluded that, “Corn rootworm may not be completely controlled by Cry3Bb1 in certain parts of the corn belt.” However, after this release, EPA did little to mitigate resistance beyond announcing that Monsanto had committed to conduct grower education programs demonstrating the value of crop rotation.
This publication also adds to the growing literature of cross-resistance. A 2013 study, “Potential shortfall of pyramided transgenic cotton for insect resistance management,” by Thierry Brévaul, PhD and colleagues, found that stacking several Bt-incorporated traits does not stop resistance. Researchers assumed that caterpillars resistant to the first Bt toxin would survive on the on-toxin plants, but die when consuming two-toxin plants because they had not yet developed resistance to the new formulation. However, caterpillars selected for resistance to one toxin survived significantly better than caterpillars from a susceptible strain.
Growing insect resistance to GE technology may lead to an increase in insecticide use. According to a report by the Wall Street Journal in 2013, insecticide sales soared in 2013 as target insects have developed resistance to genetically engineered insecticide-incorporated crops. Pesticide manufacturers American Vanguard, FMC Corp, and Syngenta have all reported higher sales in 2012 and 2013 than in previous years. Syngenta alone reported doubling sales in 2012. Similarly, American Vanguard reported soil insecticide revenues rose by 50% in 2012.
For more information on the hazards associated with GE technology, visit Beyond Pesticides’ Genetic Engineering webpage.
Continue the conversation at Beyond Pesticides’ 32nd National Pesticide Forum, “Advancing Sustainable Communities: People, Pollinators, and Practices,” in Portland, OR April 11-12. Among the featured speakers, George Kimbrell, senior attorney at Center for Food Safety, will speak on his spearheading litigation on USDA’s deregulation of genetically engineered crops and the campaign to label food with GE ingredients. The Forum will focus on improving farmworker protections along with solutions to the decline of pollinators and other beneficial organisms, strengthening organic agriculture, and creating healthy buildings, schools and homes. Space is limited so register now.
All unattributed positions and opinions in this piece are those of Beyond Pesticides.