Q&A: Bt Corn and Monarch Butterflies
Research findings from the Agricultural Research Service
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Allowing science to guide decisions
There is no significant risk to monarch butterflies from environmental exposure to Bt corn, according to research conducted by a group of scientists coordinated by the Agricultural Research Service (ARS), U.S. Department of Agriculture. This research was published in the Proceedings of the National Academy of Sciences (PNAS).
That Bt corn might present a risk became a matter of scientific and public concern when a small experiment in 1999 indicated caterpillars suffered when given no choice but to feed on milkweed leaves heavily dusted with Bt corn pollen.
The issue focused on the pollen of Bt corn because it, like any corn pollen, can blow onto milkweed leaves, which are the exclusive diet of monarch caterpillars.
Two major questions needed to be answered to determine whether there was any actual risk to monarch caterpillars from the Bt pollen:
How much Bt corn pollen does it take before there are any toxic effects on caterpillars?
What is the likelihood that caterpillars might be exposed to that much pollen?
The studies in this project showed that monarch caterpillars have to be exposed to pollen levels greater than 1,000 grains/cm2 to show toxic effects.
Caterpillars were found to be present on milkweed during the one to two weeks that pollen is shed by corn, but corn pollen levels on milkweed leaves were found to average only about 170 pollen grains/cm2 in corn fields.
Reports from several field studies show concentrations much lower than that even within the cornfield. In Maryland, the highest level of pollen deposition was inside and at the edge of the corn field, where pollen was found at about 50 grains/cm2. In the Nebraska study, pollen deposition ranged from 6 grains/cm2 at the field edge to less than 1 grain/cm2 beyond 10 meters. Samples collected from fields in Ontario immediately following the period of peak pollen shed showed pollen concentrations averaged 78 grains at the field edge.
The cooperation between researchers from many separate institutions was extraordinary, from planning through publication, to address this issue that came up suddenly and which had drawn such public concern. The way in which this research was done is being considered as a model for conducting risk assessment research.
Research priorities were set at an ARS-organized workshop in Kansas City, MO, in February 2000 through discussions among scientists from government, universities, industry, and environmental groups. The idea was to ensure that all of the most important questions were covered.
Exchanges of information between butterfly biologists, corn experts, pest specialists and others helped ensure that studies reflected how monarchs actually interact with Bt corn. Through the workshops and other discussions, the scientists also standardized many of their methods so that data could be easily pooled to provide the most complete picture possible.
For example, by standardizing how collected pollen was treated, data from several scientists' studies were compatible, providing a much larger sample size and therefore more reliable results.
In another example, scientists from different areas were able to combine data they collected about when pollen is shed from Bt corn in various geographic regions. This allowed them to construct a complete picture of the overlap of the time when pollen is shed across the major corn growing areas of the United States and Canada and the hatching of monarch butterfly eggs.
Even publication of this research was handled in an unusual, coordinated fashion. The researchers themselves pooled their data and results, divided the data in logical sections, and agreed to submit all manuscripts together to a single scientific journal for peer review. By publishing exposure, toxicity and risk analysis studies at one time in one journal, it allows scientists and policy makers to more easily evaluate the facts.
All of the studies published by this group have undergone critical peer review by independent experts to ensure the validity of the scientific methods, analysis and conclusions.
This type of cooperative research and scientific publication is being seen as a model for conducting risk assessment research. By working together, the researchers were able to share expertise, more data were accumulated more quickly, and they were able to present a more complete assessment of the potential for risk.
A small, preliminary study done at Cornell University, and reported as a note in Nature in June 1999, indicated that monarch butterflies under laboratory conditions might be harmed by eating pollen from Bt corn plants. That experiment used a small number of caterpillars and gave them no choice about avoiding eating leaves that had been treated with a thick layer of Bt corn pollen. It did not attempt to duplicate real world environmental conditions.
A large, informal group of scientists came together in workshops held by ARS to discuss the questions posed by the note in Nature. This included scientists from ARS, universities, industry and environmental organizations.
In December 1999, ARS and the Agricultural Biotechnology Stewardship Technical Committee (ABSTC) each contributed $100,000 to a grant pool to fund research. ABSTC is a consortium of agricultural biotechnology companies and associations including Aventis CropScience, American Crop Protection Association, Biotechnology Industry Organization, Dow AgroSciences, Monsanto, Novartis Seeds, and Pioneer Hi-Bred International.
Grants were selected by a steering committee consisting of Adrianna Hewings, Midwest Area Director, ARS-USDA; Eldon Ortman, Purdue University; Eric Sachs, Monsanto; Mark Scriber, Michigan State University; and Margaret Mellon, Union of Concerned Scientists.
The senior authors on the scientific journal articles published in the Proceedings of the National Academy of Sciences coordinated the analysis of data and the writing of the papers:
Many other scientists contributed to each paper and to the important discussions that helped advance the research.
One Bt corn variety or eventBt 176was found to have some negative effects on monarch caterpillars with pollen concentrations of only 10 grains/cm2. Bt 176 was the earliest developed Bt corn and was quickly supplanted by other types; it has never been planted on more than 2 percent of all the acres planted with corn. It likely will be phased out by 2003.
The U.S. Environmental Protection Agency considered data on Bt corn's impact on nontarget organisms according to their 1995 fact sheet (U.S. Environmental Protection Agency Publ. No. EPA731-F-95-004) and found no threat. But the note published in Nature raised new concerns, especially with the public. This research provides reliable scientific information on which any new regulatory decision should be based.
The use of Bt corn needs to be considered compared to the alternatives, rather than in a vacuum. Bt corn was developed as a way of providing corn with natural resistance to some pests, particularly the European corn borer and to a lesser extent the corn earworm, the southwestern corn borer, and the lesser cornstalk borer.
Before the commercial introduction of Bt corn in 1996, the European corn borer was only partially controlled using chemical insecticides. Timing of applications for control was difficult, so farmers either accepted yield losses or used more applications. In addition, pesticides often damage nontarget insects.
Since Bt corn was introduced, use of pesticides recommended for European corn borer control decreased from 6 million to slightly over 4 million acre treatments in 1999, a drop of about one-third, according to the Environmental Protection Agency.
When the research began, scientists believed that monarch butterflies laid their eggs on milkweed that grows on the edges of cornfields. This was the reason for studying how much and how far corn pollen is blown by the wind outside of the fields.
They were surprised to find that many monarch butterflies lay their eggs on milkweed plants growing within the fields. So researchers then had to check pollen concentrations on leaves within the cornfields to see if pollen accumulations were higher there.
Another unexpected finding was that pollen used for studies could be contaminated by ground up anthers (anthers are the organ at the end of the plant stamen that produces pollen) unless specifically handled to avoid the contamination. Grinding the anthers released a much greater amount of Bt protein, making the pollen appear to be more toxic to caterpillars during laboratory studies.
While anthers are found on milkweed leaves in the environment, the anthers are not ground up. Young caterpillars are too small to eat anthers, which means they are not exposed to the higher level of Bt protein. ARS entomologist Richard Hellmich compares a young caterpillar trying to eat an anther to a person trying to eat a car.
When scientists simply sieved the pollen to remove the contaminating anthers, the detrimental effects on caterpillars disappeared.
There is always a next question for researchers to answer. But the question of whether Bt corn pollen poses an immediate significant risk to monarch butterflies has been answered.
Studies are now under way to determine if there are subtle effects that could occur when caterpillars are exposed to Bt corn pollen for longer periods. In addition, studies are also being done to check whether older caterpillars unintentionally eat corn anthers that fall onto milkweed leaves.
Habitat destruction; mowing of highway rights-of-way, ditches, and pastures, which destroy milkweed; collisions with cars and trucks; and insecticides all reduce monarch populations. The most common fatality for monarch caterpillars is being eaten by other insects. Fewer than 10 percent of caterpillars make to it to adulthood.
All of the factors, including Bt pollen, should be weighed in proportion to the documented mortality that they cause.
Proceedings of the National Academy of Sciences:
Bt corn and European Corn Borers
Butterflies and Bt Corn: Allowing Science to Guide Decisions
EPA Biopesticides Registration Action Document: Bacillus thuringiensis Plant-Incorporated Protectants
Monarch and Migration, Science Museum of Minnesota