Are GM crops destroying America's regal inhabitant?

September 29, 2000

As Greenpeace protesters are acquitted of criminally damaging GM crops, Julian Morris says it may be morally wrong not to take a risk that gives third world peoples hardier, life-saving crops. JOHN OBRYCKI AND LAURA HANSEN (right) present evidence that shows the harmful effects of GM corn on the monarch butterfly, while Mark Sears (far right) argues that their science is flawed

John Obrycki and Laura Hansen present evidence that shows the harmful effects of GM corn on the monarch butterfly.

The monarch butterfly is one of the most widely recognised insects in North America. Reared by thousands of schoolchildren each year, probably no other insect species in the United States or Canada is as admired by the public.

Each year, countless monarchs migrate from sites in Mexico to spring and summer breeding areas across eastern North America. Yet they are now facing a potential new man-made threat from corn that has been genetically modified to produce insecticidal toxins derived from genes from the bacterium, Bacillus thuringiensis (Bt). Several interacting factors create a relatively high risk for the butterflies from pollen from the GM crop.

First, half of the over-wintering adults in Mexico originate from the central US, the major corn-growing area of North America. In addition, monarch larvae in the northern US and southern Canada prefer to live on the common milkweed. This weedy species frequently grows in and around the edges of corn fields. The milkweed are colonised and fed on by monarchs. And finally, corn pollen, which can be dispersed up to 60m by the wind, is produced at about the same time as the butterfly larvae are feeding over large areas of the US Midwest. Thus, monarchs, milkweeds and Bt corn pollen overlap spatially and temporally in the central US.

A recent study conducted by John Losey and colleagues at Cornell University showed that monarch larvae reared for 96 hours on milkweed leaves dusted with pollen from Bt corn in the laboratory suffered significantly higher mortality (44 per cent) than did larvae reared on leaves dusted with untransformed corn pollen (0 per cent) or leaves without any pollen at all (0 per cent).

In our field study, recently published online in the journal Oecologia, Bt corn pollen was naturally deposited on milkweed leaves within and adjacent to a field of the GM crop. Levels of pollen deposition were highest on plants within the corn field, and this decreased to very low levels 10m from the edge.

Leaf samples taken from within and at the edge of the field were used to assess the impact on young monarch larvae. Within 48 hours, there was 20 per cent mortality of larvae in the Bt corn pollen treatment compared with 0 per cent on non-Bt corn pollen exposed plants and 3 per cent in the controls without pollen.

Based on our studies, we predict that the planting of Bt corn will have an effect on monarch butterfly populations for three reasons. First, larvae will be in or near GM corn fields during pollination. Second, we have demonstrated significant larval mortality resulting from exposure to pollen concentrations representative of field depositions. Finally, as we exposed larvae to transgenic pollen for only 48 hours, our results represent an estimation of the minimum effects of field exposure to transgenic pollen.

Monarch larvae developing in late summer are likely to be exposed for most of their development, and thus mortality may be higher due to cumulative exposure.

In our analysis of the pros and cons of Bt corn developed to protect crops from the European corn borer - an insect in the same group as the monarch butterfly - we conclude that its benefit for pest management may not outweigh its ecological and economic risks. We suggest a more thorough review, based on ecological interactions, should be considered in the registration and use of transgenic insecticidal crops. The widespread and unquestioned acceptance of Bt corn by the agricultural community is similar to the rapid adoption and use of synthetic insecticides in the early 1950s. A balanced examination of Bt corn is required to incorporate this technology into an integrated control framework, to prevent the promotion of another "silver bullet" for pest management.

John Obrycki and Laura Hansen, department of entomology, Iowa State University, United States.

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