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Monarch butterfly larvae exposed to anthers from Bt maize ate less and gained less weight.

Manipulated Organism: Maize (Zea mays L.).

Inserted Transgene and Intended Effect: crylAb gene derived from Bacillus thuringiensis (Bt). This gene gives transgenic Bt maize (also called Bt corn) the ability to produce insecticidal delta-endotoxin that can kill the larvae of the European corn borer and other insect pests feeding on the corn.

Goal of This Study: Investigate how Bt maize may affect a non-target insect, the larva of the monarch butterfly (Danaus plexippus). In a laboratory experiment, monarch butterfly larvae were raised in Petri dishes containing the leaves of common milkweed; some dishes contained, in addition, either anthers from Bt corn or anthers from unmanipulated corn.

Results of This Study: "Larvae exposed to Bt anthers fed less and gained less weight than larvae exposed to non-Bt or no anthers, yet there was no evidence of feeding on anthers" (p. 228). That they fed less and gained less weight was not because the larvae moved around more.

The authors conclude that their "results suggest that larvae exposed to Bt anthers behave differently and that ingestion may not be the only way Bt can affect nontarget insects like the monarch butterfly" (p. 228). They do not know what might be causing this change in feeding behavior.

Additional Comments: An initial laboratory study by Losey et al. indicated that Bt corn pollen can harm Monarch butterfly larvae (Losey, J. E., L. S. Rayor, and M. E. Carter (1999). "Transgenic Pollen Harms Monarch Larvae," Nature vol. 399, p. 214). This study received widespread attention and particularly careful scrutiny by other scientists. In a collaborative effort, a group of scientists including the USDA's R. Hellmich, who is a co-author of the study reported here, carried out studies that were published in 2001 and can be accessed at These studies indicated, on the whole, that there may be little risk to Monarch larvae in the wild due to feeding on Bt maize pollen. The study reported here shows that the issue is not yet resolved.

Source: Prasifka, P. L., R. Hellmich, J. Prasifka, and L. Lewis (2007). "Effects of CrylAb-Expressing Corn Anthers on the Movement of Monarch Butterfly Larvae," Environmental Entomology vol. 36, pp. 228-33.

Author Affiliations: Department of Entomology, Iowa State University, and the USDA-ERS, Corn Insects and Crop Genetics Research Unit.

Funding: Grants from the USDA-ERS, U.S. Environmental Protection Agency, and Agricultural Biotechnology Stewardship Technical Committee.

Product Status: Commercialized crop, Monsanto's Bt maize (MON810).

Copyright 2008 The Nature Institute.

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