Do Bt Crops Affect Monarch Butterflies?

What are Bt crops?

"Bt", or bacillus thuringiensis, is naturally occurring bacterium that is found in soil and works as an insecticide. Using the proteins in Bt, scientists developed a gene in corn and potatoes that triggers the production of the Bt protein within those plants. The result is that these modified plants produce this Bt protein, which enables the plant itself to resist insect pests.

Are Bt crops safe?

All Bt plants commercially available in Canada have been thoroughly assessed for food use, livestock feed, and unconfined environmental release. The CFIA does environmental release and feed assessments, and Health Canada does food assessments. Examples of the factors used in livestock feed and environmental release assessments can be found on the CFIA Web site in the decision documents for plants with novel traits.

The history of use and extensive literature indicate that Bt proteins should not pose a risk to humans, other mammals, fish, birds, and most insects.

Why do farmers use Bt corn?

Farmers use Bt corn for the same reason they use any pest management tool—to grow better crops to make their farms run more efficiently. Some farmers, including those who prefer to use organic agricultural practises, spray Bt on their crops. In some situations, insecticides produced inside a plant, as with Bt corn, may work better than topically-applied insecticides, so some farmers choose this method of insect control.

Why do people talk about monarch butterflies and Bt corn?

In 1999, researchers at Cornell University did a preliminary lab study on the effects of Bt corn pollen on monarch caterpillars. The lead researcher, Dr. John Losey, sent a description of the study to the editors of the science journal Nature (Volume 399, 20 May 1999, page 214).

In Dr. Losey's study, monarch caterpillars in a laboratory were fed milkweed leaves that had been dusted with pollen from Bt corn. This was done because wind-borne corn pollen can settle on the leaves of milkweed plants, and milkweed is all that monarch caterpillars eat. Milkweed often grows in meadows or untilled fields and can be found in or near corn fields. Dr. Losey wanted to determine whether pollen from Bt corn would affect monarch caterpillars. His study found that " … larvae of the monarch butterfly on milkweed leaves dusted with transgenic Bt-corn pollen ate less, grew more slowly, and suffered higher mortality than those fed leaves dusted with untransformed corn pollen or leaves without pollen."

Some people understood the results of the lab study to mean that Bt corn harms monarch caterpillars, but other scientists pointed out that the study may not accurately reflect what would happen in a field of Bt corn. They noted that:

  • there were higher amounts of Bt pollen on the milkweed leaves in the lab than there would be found in a field;
  • in the lab, caterpillars were limited to eating only leaves covered in corn pollen, whereas in a field, caterpillars may be able to avoid pollen-coated leaves.

As a result of Dr. Losey's findings, some scientists decided to combine their research on this topic and produced a large body of peer-reviewed work on monarch butterflies and Bt corn, which was published in the Proceedings of the National Academy of Sciences (PNAS). These studies concluded that monarch butterflies exposed to Bt corn in the environment are not subjected to any significant risk. More information on these studies can be found on the Web site of the U.S. Department of Agriculture, Agriculture Research Service.

The CFIA, in co-operation with Environment Canada, commissioned a study that became part of that body of work, called, "Final Report on the Ecological Impact of Bt Corn Pollen on the Monarch Butterfly in Ontario". This study concluded that the risks to monarch butterflies from Bt corn pollen is less than 1/100 of 1 per cent. 

Can insects develop resistance to Bt?

Over time, insects can become resistant to any insecticide, so scientists have long been interested in insects developing resistance to Bt. This interest now includes crops expressing Bt. For this reason, the CFIA has mandated insect resistance management plans (IRM plans) suited for the biology of each target insect pest. The IRM plan is an agreed upon, common plan that developers are required to implement.

For example, for the European corn borer, the Canadian Corn Pest Coalition (CCPC) developed the standard IRM plan, and this was adopted by the CFIA as a regulatory requirement. The U.S. Environmental Protection Agency later adopted the same requirement. More information about the Coalition and its activities—including the roles of farmers and developers with respect to IRM plans—can be found on the CCPC Web site.

What about Bt and soil health?

The health of soil can be an indicator of the overall health of the environment. Scientists can determine how healthy a soil is by examining its flora and fauna and by observing and measuring its physical and chemical conditions.

While Bt crops can release Bt proteins into soil, it is known that soil organisms are already exposed to Bt proteins that originate from the naturally occurring Bt bacteria, which are present in soil, or as a result of Bt being applied as an insecticide. For many years, Bt insecticides have been used without reported negative effects on soil health. Many published scientific papers show that Bt proteins do not significantly affect earthworms or other small invertebrates.

For more information on Bt and soil health, refer to the Government of Canada's response to a petition received on this topic.

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