Genetically modifying cotton promises to reduce the use of chemicals and, potentially, create a better environment for harmless insects and other animals. For the last decade, some farmers in Arizona have been planting cotton engineered to contain a toxin that kills pests such as the pink bollworm. A study of randomly chosen cotton fields reveals that although this genetically modified cotton did reduce pesticide use, it did not reduce use of herbicides nor did it improve biodiversity when compared to unmodified strains.

Ecologist Yves Carriere of the University of Arizona and his colleagues randomly selected 81 cotton fields--split between unmodified and transgenic cotton breeds--over the course of two growing seasons. The scientists gathered data on pesticide use, herbicide use and all the ants and beetles they could find in pitfall traps placed in the fields, as well as other information. "The idea here is to look at not only the possible effects of transgenics but also all the other factors," Carriere says.

The data confirmed that farmers applied pesticides less often to transgenic fields--and used more precisely targeted chemicals when they did. But use of such targeted pesticides on modified cotton did rise in the fields selected during the second year of the study, perhaps due to the need to control pests unaffected by the engineered toxin, the authors speculate. And herbicide use remained the same no matter whether the cotton in question was unmodified, toxin-producing, or toxin-producing and herbicide resistant. "My guess is that they use herbicide resistance as more of an insurance policy," Carriere says.

Nor did genetic modification seem to have an effect on ant and beetle biodiversity; no matter which type of cotton was grown, ant populations declined and beetles boomed in farmed fields compared to adjacent unfarmed fields. Other factors such as soil type, seeding rates and amount of rain played a bigger role in determining population dynamics, according to the paper in Proceedings of the National Academies of Science.

The researchers will continue to refine their analysis of the data, looking for differing impacts on predatory and plant-eating insects as well as an economic analysis of the costs and benefits of genetically modified cotton. "You cannot simply assume that you will get across-the-board benefits," Carriere notes. "One thing I was a bit surprised to find is that if you control some pests with [transgenic] cotton, others become more of a problem."