6 December 2002
CORN COULD MAKE COTTON PESTS BT RESISTANT
Insects offered GM-free refuges are eating modified crops further afield.
Nature, 4 December 2002
Moths in the United States are feeding on corn all summer before flying south to munch cotton in the autumn, new research shows1. The annual exodus could stymie future efforts to stem pests' resistance to genetically modified crops.
Where the moths move between generations had been hotly debated. The information is crucial to preventing them from damaging economically important produce.
Fred Gould of North Carolina State University in Raleigh and his colleagues used techniques developed by archaeologists to track the corn earworm (Helicoverpa zea) also known as the cotton bollworm and tomato fruitworm.
Of the cotton in the southern United States, 40-60% is engineered to contain the Bt gene from the bacterium Bacillus thuringiensis. This enables the plant to produce a toxin that is lethal to many pests. The technology has reduced crop losses in this $40-billion industry.
The corn earworm develops resistance to Bt in the lab, raising fears that if it were exposed for several generations in the field, the pest would become inured to the toxin.
So, in an effort to ensure that some pests remain susceptible, the amount of Bt cotton and corn grown in the south is strictly controlled. Farmers must plant 'refuges' of non-engineered crops between fields of genetically modified plants.
Gould's finding that the same moths feed on corn in America's Midwest and cotton further south hints that the refuge strategy alone might not hold back the spread of resistance.
For the time being, the discovery is actually good news. Today, only about 25% of Midwestern corn contains Bt - current varieties are not as effective against insects as is modified cotton.
So the remaining 75% is helping to keep earworms susceptible to Bt cotton. "Corn is serving as a great refuge," says entomologist Bruce Tabashnik at the University of Arizona in Tucson, who describes the new results as "very exciting".
But if the percentage of Bt corn increases - which, with new varieties in development, could happen soon - then moths feeding on Bt corn all summer may rapidly come resistant.
Flying south to feed on cotton later in the year, their offspring could well be immune to the effects of Bt cotton as well. "If 90% of corn in the Midwest becomes Bt then we could be in real trouble," says Gould.
Scale of the problem
Because caterpillars feed on several crops and moths move around, "it's difficult to know where a moth grew up as a caterpillar", Gould explains.
Fortunately, moths are what they ate as caterpillars. Gould's team analysed the ratio of two different types of carbon, 12C and 13C, in the dusty scales on the wings of moths collected in the cotton fields of Louisiana and Texas. This approach is usually used to estimate the age of ancient archaeological remains.
Cotton and corn plants contain slightly different ratios of carbon isotopes, as they have slightly different ways of photosynthesizing. This distinction is recorded in the tissues of insects that ate grasses such as corn when they were caterpillars, and those that munched broad-leafed plants such as cotton or soybeans.
Many of the moths that the researchers collected in October - late in the cotton season - had grown up on grasses.
"They can't be coming from Texas or Louisiana," says Gould: corn is harvested in the south much earlier. The moths must have migrated from the cornfields of the Midwest.
1.Gould, F. et al. Bacillus thuringiensis resistance management: Stable isotope assessment of alternate host use by Helicoverpa zea. Proceedings of the National Academy of Sciences, published online, doi:10.1073/pnas.242382499 (2002).
© Nature News Service / Macmillan Magazines Ltd 2002
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