One prominent concern about genetically modified crops is the possibility of environmental impacts from the
movement of fitness-enhancing traits to wild plant populations. Decisions to deregulate Bt crops in the USA
have relied strongly on arguments that these crops will not interbreed with wild relatives in the permitted
growing regions. Limited attention therefore has been directed to analyses of the consequences of gene flow.
To provide a transparent evaluation process for risks associated with insecticidal transgene escape, we crafted
a series of questions designed to guide this aspect of the risk assessment. We then explored the current
knowledge base available for answering such risk-related questions for three Bt crops (cotton, rapeseed, and
rice). First, we generated a list of wild relatives of these crops. A definitive list of potential transgene recipients
is not yet possible for some crops. Sufficient data are not available for some crops to eliminate certain related
plant species from consideration of fertile hybrid formation, thus making lists for these crops subject to
speculation. Second, we queried the HOSTS database (UK) to obtain a worldwide listing of lepidopteran species
that feed on these crops and their wild relatives, and to determine the host range of the larvae. To our
knowledge, this list of 502 lepidopteran species is the first such list published for these crops and wild crop
relatives. Third, we used a data set maintained by the Canadian Forest Service to assess Bt toxin susceptibility
for these lepidopterans. Only 3% of those species have been tested for susceptibility; and the literature
suggests that generalizations about susceptibility among taxa are difficult due to the variability within families.
Fourth, we consulted the literature to interpret what is known about the ability of lepidopterans to regulate plant
fitness or invasiveness. We could not eliminate the possibility of ecological release due to plant resistance
against lepidopterans. In fact, there is strong experimental evidence that lepidopteran herbivores do limit the
distribution and/or abundances of at least some wild plant species. Neither could we eliminate the possibility
that non-target lepidopterans might have important functions in the ecosystem as pollinators or alternate hosts
to natural enemies of pest species. This study suggests that crucial data are lacking for the development of a
credible scientific basis to confirm or deny environmental risks associated with the escape of Bt transgene
constructs to wild relatives. Given the absence of information on the identity, level of susceptibility, and
ecological roles of lepidopterans exploiting specific wild relatives of Bt crops, we suggest that new efforts be
directed to assessing possible consequences of lepidopteran mortality on resistant wild relatives.