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United States Department of Agriculture

Agricultural Research Service

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Butterflies and Bt corn
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Butterflies and Bt corn. Allowing Science to Guide Decisions.
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Other factors affecting monarchs

The public will be pleased to learn that Bt corn pollen does not harm monarchs living in the Corn Belt. Monarchs are threatened by many factors, including harsh weather, predatory insects, and habitat loss in overwintering sites and summer breeding grounds. Monarch caterpillars developing on milkweeds within agricultural areas also may be adversely affected by the application of conventional insecticides targeting crop pests or herbicides that affect weed species including milkweeds.

The harm posed to monarchs by insecticide use, especially in agricultural situations, is the final piece in assembling the true picture of risk assessment. The potential for harm from the use of genetically modified corn cannot be evaluated by itself. It must be considered in light of the alternative to planting Bt corn, which is insecticide use by farmers.

An experiment in a sweet corn field (Stanley-Horn et al., 2001) demonstrated that monarchs were adversely affected by treatments of lambda-cyhalothrin, a broad-spectrum insecticide commonly applied to non-Bt sweet corn to eliminate pests and damage. It is important to note that Bt corn can dramatically reduce the number of insecticide treatments typically applied to sweet corn and field corn (U.S. EPA, 2000; Gianessi et al., 2002). Bt corn eliminates the need to apply conventional insecticides for corn borer protection because of its built-in insect protection made possible through biotechnology.

In the future, new Bt corn products are expected to transform the way growers control rootworms, the No. 1 insect pest of corn. This biotech corn could help growers significantly reduce overall insecticide use, resulting in much less impact on the environment. This does not mean that corn rootworm Bt corn should not be scrutinized for impact on non-target organisms such as the monarch butterfly. But any risk found also should be weighed against harm done to those non-target insects and other animals by unintended exposure to chemical insecticides.

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Woman with backpack surrounded by and covered with Monarch butterflies

“The pollen issue – Does Bt corn pollen pose a significant risk? – has been answered. But what other risks are there in this case of monarchs? All of these things need to be assessed in terms of risks, benefits and comparative risks.”

— Dr. Mark Sears, chair of the Department of Environmental Biology, University of Guelph, Ontario

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Last Modified: 11/29/2015
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