transgenic hosts must be planted on each farm to produce 500 susceptible pest individuals for each resistant individual produced in the Bt crop.
In the case of cotton, EPA-registered transgenic plants do not produce a high dose of Bt toxin for the cotton bollworm, so a large proportion of partially resistant individuals could survive on the Bt cotton. This registration was not appropriate according to the EPA Scientific Advisory Panel, and current requirements for on-farm, non-Bt acreage are not expected to produce the desired 500:1 ratio. Only a substantial increase in the refuge acreage could ameliorate this problem. While the EPA may not increase the on-farm refuge requirements, concern over resistance evolution in H. zea due to an inadequate resistance management program was reemphasized by the most recent EPA Scientific Advisory Panel (EPA 2000b). If the stacked trait cottons such as the one approved by the environmental assessment discussed here are commercially successful, they could increase regionwide adoption of Bt cotton, further accentuating the risk of rapid evolution of Bt resistance in H. zea.
This case study identifies only two negative environmental effects that could be caused by the interaction of two transgenic traits. If outcrossing to weedy relatives was more of a problem with cotton, the interaction between herbicide tolerance genes and Bt genes could exacerbate an additional risk—the transfer of the Bt trait to noncrop plants. A Bt gene inserted into a crop along with a herbicide tolerance gene could be transferred to wild relative populations much faster than in cases where the Bt gene was inherited separately from herbicide tolerance. A potential scenario is as follows: Pollen for the stacked trait cultivar crosses with a weedy relative in a cotton field. The next year progeny from the cross as well as other individuals of the weed species germinate in the cotton field. The farmer sprays bromoxynil. This kills most of the weeds without the herbicide tolerance gene, but those with the gene increase in frequency. Although the Bt gene confers no direct advantage with regard to survival against herbicide spray, there is a major increase in the frequency of weeds with the Bt gene because it is linked to the herbicide tolerance gene. This results in a large fraction of weeds that are now protected from insect feeding. If APHIS reviews transgenic plants with weedy cross-compatible relatives in the United States, such as canola, with stacked herbicide tolerance and insecticidal genes, it would definitely need to consider this interaction.
The APHIS environmental assessment indicated that the agency received no responses to its Federal Register announcement of this petition