http://www.nature.com/nsu/021202/021202-2.html

 Corn could make cotton pests Bt resistant

 Insects offered GM-free refuges are eating modified crops further
 afield.

 4 December 2002

 TOM CLARKE

 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.

 References

 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). |Article|

 (c) Nature News Service / Macmillan Magazines Ltd 2002