Effect of Bt Corn on Nontarget Insects: The News Isn't All Bad

June 16, 2000
Bt corn and other transgenic crops have received so much negative press within the past year that one begins to wonder if anything positive will ever see the light of day. The potential adverse effects of Bt-corn pollen on monarch butterfly caterpillars, based initially on one preliminary laboratory study, received an inordinate amount of press coverage, and most of it was negative. We wonder if a more recently conducted study in Illinois will receive a similar amount of press coverage.

The study to which we are referring was conducted by C. L. Wraight, A. R. Zangerl, M. J. Carroll, and M. R. Berenbaum in the Department of Entomology at the University of Illinois. The objective of the research was to determine whether mortality of early-instar eastern black swallowtails, Papilio polyxenes, was associated either with proximity to a field of Bt corn or by levels of Bt-corn pollen deposition on host plants. The results of the research were published recently in the Proceedings of the National Academy of Sciences USA and were published online before print June 6, 2000. The article is entitled "Absence of toxicity of Bacillus thuringiensis pollen to black swallowtails under field conditions" and can be viewed at http://www.pnas.org/cgi/content/full/130202097v1.

Black swallowtail caterpillars occur throughout North America east of the Rocky Mountains. They feed almost entirely on plants in the celery or parsnip family, several of which are found in pastures and along roadsides and edges of cultivated fields. Consequently, these caterpillars may be found feeding on their host plants near Bt cornfields. Wraight et al. used arrays of potted wild parsnip plants (five rows of five potted plants per row) as hosts for the caterpillars (10 first instars per plant). The potted plants and caterpillars were placed next to a Bt cornfield (Pioneer 34R07; event MON810, Cry1Ab gene) 24 hours after the initiation of pollen shed. The amount of pollen falling on each plant was estimated with microscope slides covered with a thin coat of Vaseline. The number of live larvae on each plant were recorded daily for 7 days; the condition of surviving larvae was determined by weighing each larva at the end of 7 days. In addition, Wraight et al. conducted bioassays in the laboratory to determine the range of toxicity of pollen from Bt corn (both Pioneer 34R07 and Novartis Max 454 [event 176, Cry1Ab gene]) and non-Bt corn (Pioneer 3489).

We won't bore you with detailed results from the study; you can peruse the results yourself at the aforementioned website. However, the authors of the paper summarized their results as follows: " . . . there was no relationship between mortality and proximity to the field or pollen deposition on host plants. Moreover, pollen from these same plants failed to cause mortality in the laboratory at the highest pollen dose tested (10,000 grains/cm2), a level that far exceeded the highest pollen density observed in the field (200 grains/cm2). We conclude that Bt pollen of the variety tested is unlikely to affect wild populations of black swallowtails. Thus, our results suggest that at least some potential nontarget effects of the use of transgenic plants may be manageable."

Wraight et al. also stated: "Larvae of the black swallowtail, by virtue of their multivoltine life history and broader host range in the Midwest, are as, if not more, likely to encounter corn pollen between late June and mid-August during its 8- to 10-day period of anthesis than are larvae of the monarch butterfly, yet under actual field conditions no mortality directly or indirectly attributable to ingestion of endotoxin-containing corn pollen could be detected in our study. This is not to say that monarch butterflies are unaffected by Bt corn pollen; however, field studies as well as appropriately controlled laboratory studies are necessary before such a conclusion can be drawn."

Potential effects of Bt-corn pollen on nontarget insects continue to generate a great deal of interest. In fact, considerable field research regarding the effects of Bt-corn pollen on monarch butterfly caterpillars was initiated in 1999, and even more will be conducted in 2000. Results from a lot of these studies will help us determine whether Bt-corn pollen will affect populations of nontarget insects such as monarch butterflies in the field. These "real-world" studies should help us address the most important questions. The findings from the Wraight et al. study suggest that Bt corn is not the ecological disruptor it is claimed to be by many anti-biotechnology groups. We challenge the media to provide an amount of coverage regarding these findings similar to the coverage they gave the monarch butterfly story.--Kevin Steffey and Mike Gray

Author: Mike Gray