Bt Cotton Team

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One key insect pest controlled by the Bt cotton and much less effectively controlled by foliar insecticide sprays is the pink bollworm, Pectinophora gossypiella. This pest grows inside the developing cotton boll and therefore is protected in the larval stage from insecticides sprayed onto the plant. However, having the Bt toxin produced by the plant means that the pink bollworm larvae consume the Bt toxin.

A problem is that continual or repeated exposure to particular insecticides or insecticide groups tends to develop resistance over generations of insects. The insects acquire mutations, become genetically altered, and are able to survive exposure to the insecticide. This has happened frequently in the past. The risk of developing insecticide resistance can be reduced by limiting the exposure of the insects to the insecticide and/or by ensuring that there are plenty of non-resistant insects so that susceptible genes are not maintained in the population. The strategy of providing a refuge ensures a continuous supply of non-resistant insects, thereby helping combat the increase in resistant insects in the population.

Refuges

Combatting resistance in Bt cotton is accomplished by using Refuges, plots of land on which non-Bt cotton is grown so insects are not exposed to the Bt toxin. For an explanation of how and why this works, see Refuges. In short, the refuges must be sufficiently large and located close enough to fields of Bt cotton so that resistant insects are likely to mate with susceptible insects.

Pink Bollworm Resistance to Bt toxin in Arizona Bt Cotton Fields
Bt cotton was introduced commercially in Arizona in 1996. Statewide field monitoring of Pink Bollworm resistance to Bt toxin started in 1997 and has continued annually since that time.

Compliance of Growers with the Refuge Strategy
Arizona cotton producers have benefitted an average of $15,000 per farm per year from use of Bt cotton. From this it is clear that preserving Bt cotton is in the economic interest of our producers. However, refuges of non-Bt cotton can be highly damaged by pink bollworm and other pests. Producers must be willing to except damage in refuges in order to achieve the desired production of susceptible insects.

Compliance with refuge requirements (size and placement) is documented systematically in Arizona by the Arizona Cotton Research and Protection Council. Locations of Bt and non-Bt cotton fields are recorded annually. These data are analyzed by the Carrière Laboratory using geographic information systems software and the proportion of Bt cotton fields with refuges of appropriate size and distance from their reference Bt cotton fields is recorded. These analyses provide objective analyses of the degree of compliance with the Bt cotton resistance management program in Arizona. Perhaps more importantly, these analyses will provide powerful retrospective inferences, once resistance develops, to determine the Bt cotton use patterns that yielded resistance problems. No resistance to Bt cotton has been found in field populations as of March 2004.