Cautionary Tale 

Jane Rissler / Nucleus Fall01

New studies published in September in the Proceedings of the National Academy of Sciences (PNAS) add considerably to our understanding of the risks Bt corn poses to monarch butterflies but, contrary to expectations, do not resolve the issue. The corn is genetically engineered to produce the Bacillus thuringiensis toxin to kill insect pests. Two years ago, a Cornell University study demonstrated that Bt corn could also be fatal to monarch butterflies. In that study, monarch larvae died after consuming Bt-corn pollen deposited on milkweed-the larvae's primary food source.

The new research shows that pollen from the two types of Bt corn, which account for most of the Bt-corn acreage (Mon 810 and Bt 11), produces relatively low amounts of toxin. That fact, combined with observations on the amount of pollen typically deposited on milkweeds, allowed

researchers to conclude that monarch larvae were unlikely to consume a lethal dose of toxin even inside Mon 810 and Bt 11 corn fields. As a result, these two types of Bt corn pose negligible risk to monarchs.

Although the new studies go far to allay fears that Bt corn is lethal to monarchs, some scientists have called into question a critical assumption on which the research was based that monarchs eat only pollen and no other corn tissue.

Scientists questioning the PNAS work point out that monarchs consume tissue from anthers--the pollen-producing parts of the corn flower--as well as pollen from Bt corn. Since anthers have been shown to contain considerably more toxin than pollen, they believe that the PNAS studies based on pollen alone may seriously underestimate the toxin dose consumed by monarch larvae in corn fields. These concerns are bolstered by other studies showing that a mixture of Bt 11 pollen and anther fragments has a deleterious effect on monarch larvae and unpublished observations that anther fragments are commonly deposited on milkweed leaves in corn fields. The PNAS studies themselves acknowledge the possibility that larvae may consume anthers as well as pollen.

If additional research were to confirm that the more toxic anthers are indeed part of the monarch diet, this could mean not only that the PNAS conclusion is wrong but that the considerable portion of the US corn acreage planted to Mon 810 and Bt 11 may be delivering a lethal blow to monarch larvae.

While the impact of Bt 11 and Mon 810 on monarchs remains up in the air, the PNAS studies show that a third type of Bt corn is definitely a threat to monarch butterflies even if larvae consume only pollen. Event 176, unlike Mon 810 and Bt 11, produces enough toxin in pollen to kill monarch larvae under field conditions. The company that created Event 176 is taking the product off the market after 2003.

The PNAS studies also did not address longer-term, nonlethal effects of Bt corn, that is, whether monarchs not killed after eating Bt corn might suffer other deleterious effects such as delayed development, impaired reproduction, and altered migration.

The studies published in PNAS were initiated two years ago in response to news that Bt corn could be fatal to monarchs. They were funded by private and public money from the biotech industry and government. A multistakeholder research advisory committee formulated the set of coordinated, multidisciplinary research projects to determine whether Bt corn is lethal to monarchs under field conditions.