The May publication in Nature of a peer-reviewed Scientific Correspondence paper indicating that transgenic pollen kills larvae of the monarch butterfly provoked contrasting responses around the world. While environmental interest groups seized on the findings as demonstrating the harmful effects of genetically modified crops on 'non-target' species in agricultural regions, plant scientists and representatives of life science companies among others criticized the work as premature, incomplete, and unconvincing.In the controversial experiments, monarch larvae fed on milkweed leaves dusted with pollen from N4640 Bt corn (a commercial variety containing a gene for an insecticidal Bacillus thuringiensis protein as a defense against infestation with the European corn borer) ate less, grew more slowly and had higher mortality than those fed plain leaves or leaves dusted with pollen from a non-transgenic corn line. After four days, su-vival of larvae exposed to N4640 Bt corn was 56% compared with 100% for the pollen-plus and pollen-minus controls. The monarch, whose larvae feed exclusively on milkweed leaves, is regarded as a particularly sensitive indicator of environmental disturbance.Critics of the work have particularly highlighted a number of weaknesses of the paper. The first is that it is unsurprising that a lepidopteran species such as the monarch should be affected when fed plant material that contains a protein used precisely because of its lepidopteran-specific killing properties.Willy De Greef, the worldwide head of regulatory and government affairs for Novartis Seeds (Basel, Switzerland), the company that developed the N4640 Bt variety, would have liked to see a greater stringency with respect to the Cornell University (Ithaca, New York) group's experiments. The forced feeding of a potentially toxic compound to larvae does not really represent the reality in the field. More convincing, he says, would have been the results of 'choice' experiments where, as in the field, the monarch larvae are exposed simultaneously to milkweed leaves with and without Bt-maize pollen. The preliminary Cornell experiments reported in Nature indicate that the presence of both transformed and untransformed pollen reduces leaf consumption by the larvae, something that might be expected to have a bearing on larval survival. De Greef says that the 'choice' experiments would explore the monarch's pollen avoidance strategies.