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Monarch butterflies and Bt corn

The impact of transgenic crops on the environment has stirred much interest in some circles. Thus the report by Losey et al. (1999) that Monarch larvae were poisoned by pollen from Bt corn was guaranteed to spark debate.

The Monarch larva is the caterpillar life stage of one of North America's most colorful and familiar natives, the Monarch butterfly. The larvae feed only on the leaves of milkweed plants, which are commonly found in both natural habitats and cultivated fields. Because their diet does not include corn plants, Monarchs are non-target organisms in the scheme of insect pest control in a corn field. Yet as members of the insect order Lepidoptera (butterflies and moths), they are susceptible to poisons that affect Lepidopteran pests. One such Lepidopteran pest, the European corn borer, is the main target of the Bt Cry1 toxins in transgenic corn varieties.

Monarch butterfly on alfalfa flower
Photo: Marlin E. Rice

Does the effort to kill European corn borers imperil Monarch butterflies as well?

Losey and two fellow researchers suggested that it might. In laboratory tests, they dusted Bt corn pollen onto milkweed leaves and then gave the leaves to Monarch caterpillars to eat. Additional caterpillars were fed on leaves dusted with conventional corn pollen or leaves without pollen. The caterpillars eating leaves dusted with Bt pollen ate less and grew more slowly. Over four days, nearly half of the caterpillars on Bt-dusted leaves died while no caterpillars died in the other two groups. Five replications of the treatments provided enough data to indicate that the results were statistically significant.

Larva of a Monarch butterfly
feeding on a milkweed leaf

Losey and his co-workers reported their findings to the weekly scientific journal Nature, and a brief story with graphs showing the reduced feeding and the number of deaths was published in May 1999. The reaction was immediate and extreme. Opponents of transgenic crops hailed the report as proof that Bt corn was an environmental nightmare, while supporters of transgenic crops criticized the research methods and dismissed the study as "preliminary" and "flawed." Agencies of the U.S. and Canadian governments, state agencies, growers groups, and the big agrobiotechnology companies--Aventis, Dow, du Pont, Monsanto, and Syngenta--jointly funded a set of studies in Canada and the United States in the summer of 2000 to explore the main issues:

  • Does Bt corn pollen kill Monarch caterpillars in laboratory tests and/or in field tests?

  • How likely is it that Monarch caterpillars will be exposed to corn pollen under natural conditions?

Six teams of researchers published their results in the Proceedings of the National Academy of Sciences in the fall of 2001 (Hellmich et al., 2001; Oberhauser et al., 2001; Pleasants et al., 2001; Sears et al., 2001; Stanley-Horn et al., 2001; Zangerl et al., 2001). According to these reports, Bt 176, the transgenic component in one kind of Bt corn, produces pollen that is highly toxic to Monarch larvae and to other butterfly species such as the black swallowtail.

Black swallowtail butterfly
Photo: Jacalyn Loyd Goetz

But this transgene is used in a very small (and decreasing) portion of the corn acreage in North America, less than 2 percent by one estimate. The transgenic components in the corn varieties most commonly grown in North America, MON 810 and Bt 11, do not produce much of the Bt toxic protein in pollen. Larvae can eat large amounts of this pollen, more than 1,600 grains of pollen per square centimeter of leaf surface, without showing ill effects.

Table 1 shows the amounts of pollen from Bt 176, Bt 11, MON810, and several other Bt corn sources that were tested on butterfly larvae.

Bt 176 corn is more toxic to butterfly larvae than other versions of Bt corn because the promoter used in Bt 176 is very effective in causing Bt protein to be produced in the corn pollen. Table 2 shows how Bt 76 differs from Bt 11 and MON810.

Because the vast majority of the Bt corn now grown in the U.S. does not produce pollen that kills Monarch butterflies, experts conclude that the risk to Monarch populations from Bt corn is low.

Other factors contributing to the low risk estimate:

  • Corn pollen is shed for only about two weeks of the summer, whereas Monarchs produce several generations of offspring over the course of a summer, so some generations of caterpillars will not be exposed to pollen.
  • Rain washes some of the pollen off the leaves, so caterpillars may be exposed to much less than the potential maximum amount of pollen, depending on the frequency and severity of rain during the period of pollen shed.
  • Most of the pollen produced in a corn field falls within the field or nearby. Concentrations of pollen fall rapidly as you move away from the corn field, so caterpillars feeding on milkweed plants at a short distance from corn fields are unlikely to encounter any pollen.

Table 3 shows the concentrations of pollen deposited on leaves at various distances from corn fields. The highest amount of pollen measured within a corn field in these studies is harmless to Monarch larvae if the corn type is Bt 11 or MON810.

Factors contributing to an increased risk for caterpillars:

  • In U.S. study areas, Monarch butterflies seemed to show a preference for laying their eggs on milkweed plants inside corn fields (Oberhauser et al., 2001), where the caterpillars might be exposed to heavy doses of pollen if they are feeding during the two week period of pollen release. However, in the Canadian study area, Monarch seemed to prefer natural habitats over corn fields.
  • Survival of young caterpillars was higher in corn fields than in other habitats.

Thus, it seems likely that most of the Monarch butterflies seen in the Midwest begin their lives in corn fields (Oberhauser et al., 2001).

Scientists continue to discuss the issues raised by Bt corn. A good exchange of letters on this topic is available in the November 2001 issue of the journal BioScience. The citation is BioScience 51(11):900-906.

The USDA has a question-and-answer page on Monarchs and Bt corn at http://www.ars.usda.gov/is/br/btcorn and a discussion of the controversy at http://www.ars.usda.gov/sites/monarch.

For a review of the controversy surrounding Monarchs and Bt corn see

Page last updated : March 11, 2004

Copyright Department of Soil and Crop Sciences at Colorado State University, 1999-2004. All Rights Reserved.
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