Technology and the Monarch

By Ron Dieter, Sunnyfield Greenhouse & Gardens

August 18, 1999

Earlier this summer when our butterfly bushes burst into bloom, I watched as lovely swallowtails and painted ladies visited the flowers and sipped the nectar from deep inside. It occurred to me that the most popular butterfly, the monarch, was nowhere to be seen. Usually the bushes attract a good number of monarchs.

About the same time, there was a short article in a popular garden magazine mentioning the possible toxic effect of Bt corn pollen on the caterpillar of the monarch butterfly. I know there is Bt corn planted around my place and I began to wonder if I was seeing what the article was alluding to.

Before I go on, I’m happy to report our butterfly bushes are presently being visited by a good number of monarchs. It is possible that I was looking for them too early in the season. At any rate I’m glad they’re here again.

On a recent garden tour, I mentioned this topic to Martha Smith, our area extension horticulturist. She was not aware of any problem, but this week she sent me an article discussing the situation.

Many gardeners are already familiar with Bacillus thuringiensis, or Bt. It is a naturally occurring soil organism that produces a toxin fatal to many plant-eating caterpillars. Bt is the active ingredient in Dipel and some other insecticides. Vegetable gardeners apply Bt insecticide to cabbage and broccoli plants to control the caterpillars that eat holes in the leaves. If not controlled, the cabbage loopers will devour most of the plant. The cabbage looper is the larval stage of the little white butterflies that flutter about the vegetable garden. Other forms of Bt are applied to ponds and pools to control mosquito larvae.

Genetic engineers found a way to insert Bt genetic material into corn, effectively making the plant toxic to the European corn borer, a terrible pest for corn producers.

The monarch caterpillar feeds on milkweed, not corn plants, so what’s the problem? Pollen drifting from cornfields into fencerows and natural areas where milkweed grows may be a problem for monarch larvae, as well as other beneficial insects. Studies suggest that some monarch caterpillars are killed when they eat milkweed leaves dusted with Bt corn pollen.

So does planting Bt corn mean maximum corn and minimum monarchs? I don’t think so. The monarch butterfly is not an endangered or threatened species. But the preliminary studies have called for more research, which is now in progress. There are a lot of factors that come into play and we need to look at the big picture.

Farmers plant Bt corn to reduce the need for chemical insecticides, some of which are toxic to monarchs and other beneficial insects. Bt corn reduces our exposure to these chemicals and keeps them out of the environment

The preliminary studies were made in experimental conditions and further research is being done in actual field trials to answer important questions. How much pollen must drift to milkweed plants to be toxic to monarchs and how long does it remain? Which Bt corn varieties produce the least toxic pollen? Does pollen production and drift coincide with the feeding stage of the larvae? What cultural practices can be used to reduce the exposure of the monarch larvae to Bt pollen?

I am greatly encouraged that researchers are looking for answers to these questions. Bt corn has reduced arial spraying in rural areas, a benefit to farmers and rural village dwellers as well. In a short time, we’ll find out if the monarch is really at risk from Bt corn, and if so, how we can insure its safety.

For more about Bt corn and the monarch, check out our timely topics section.