Monarch Butterflies and the Potential Impact of Bt Pollen

May 27, 1999 6(10):61-62

John Wedberg and Joe Lauer
Entomologist and Agronomist

Bt (Bacillus thuringiensis) is a naturally occurring soil bacteria that has been used as an insecticide to control caterpillars in the insect family Lepidoptera for decades. It either kills susceptible larvae or severely inhibits their growth. Young larvae are more susceptible to the toxin than older larvae. Transgenic corn plants that produce Bt protein in various plant tissues (including pollen) were released during 1996 to control larvae of the European corn borer, a pest of corn that can cause severe damage on an annual basis and historically has required annual intervention with insecticides to prevent crop loss. The conventional insecticides used are broad spectrum and kill beneficial and pest insects and require careful handling because of the potential for toxicity to humans using such materials. In comparison, Bt has a very narrow spectrum of control and has never been proved to be harmful to humans or other vertebrates. In fact, the insecticide formulations of Bt have been approved for organic farming.

Recent releases of laboratory results have prompted concern about the impact of Bt pollen if it falls on milkweed plants, upon which larvae (caterpillar) of the Monarch butterfly feed exclusively. Because Monarchs are in the family Lepidoptera they can be intoxicated if they feed on pollen containing the Bt protein. The question is where does milkweed grow in relation to Bt corn pollen? Jerry Doll (UW Weed Scientist) has performed a perennial weed survey 5 times since 1977. In 1994, 73% of survey respondents rated milkweed as a slight problem in conventional tillage and 54% of survey respondents rated it as a slight problem in no-till systems. If milkweed is a problem weed it is usually found in "no-till" fields. Milkweed is ubiquitous in Wisconsin, but is typically found in line fences, roadside ditches, pastures, CRP and public lands. Those who walk cornfields know that milkweed typically does not grow in grain fields. Cultivation and herbicide application is used to control milkweed and other weeds within cornfields.

Most corn pollen falls within 50 feet of the plant on which it is produced. Hybrid corn seed fields must be isolated by 660 feet, so that contamination from other pollen sources does not occur. Milkweed located between 50 and 660 feet can potentially have pollen dusted onto milkweed. Research conducted (Hansen and Obrycki, Proceedings 51st Annual meeting, North Central Branch - Entomological Soc. of Amer.) at Iowa State University during 1999 examined the relationship between the proximity of Bt expressing corn and milkweed plants, and the extent of Monarch caterpillar mortality. They examined the amount of corn pollen deposited on milkweed leaves within and adjacent to a Bt cornfield at 0 m, 1 m and 3 m. The highest levels of pollen deposition were found on plants within the cornfield, and lowest levels found at 3 m (approx. 10 ft) from the edge of the cornfield. Leaf samples taken from within and at the edge of the cornfields were used to assess mortality of first instar (newly hatched) caterpillars. Within 48 hours, there was 19% mortality in the Bt corn pollen treatment compared to 0% on non-Bt corn pollen exposed plants and 3% in the no pollen controls. The Bt fields are only a threat to the caterpillars during pollination and most cornfields shed pollen for 8-10 days. How long the pollen is potentially hazardous (if it isn't flushed from the plant by rain or irrigation) to Monarchs needs to be examined.

Monarchs do not overwinter in the Midwest. Each year they migrate here from forests high in the mountains of Mexico. Monarchs west of the Rocky Mountains migrate from small groves of trees along the California coast. In the entire world, no butterflies migrate like the Monarchs of North America. We normally anticipate their first arrival in Wisconsin by mid-July. This will vary with the seasons and we know of one record of May arrival. They produce 1-2 generations during the summer before migrating back to the Transvolcanic Mountains of Mexico during early to mid-September.

Human activities constitute a threat to the continued existence of the Monarch, and although it appears that other human activities are of much greater danger than Bt corn, the threat to this and other non-target caterpillars is being studied by entomologists. The greatest threat to the Eastern Monarch (the one that migrates to the Midwest) is logging activities in the mountains of Mexico which annually destroys the Monarchs overwintering sites. In California, Monarchs roost where people like to build houses and this destroys Monarch habitat. Monarchs are most vulnerable in their overwintering sites, because there is where huge populations of butterflies are concentrated in small areas. Within the Midwest urban sprawl, highway construction and farming activities annually destroy habitat that supports milkweed populations. These transformations of the natural landscape may eventually make it impossible for the Monarch to live here.

The alternative to using Bt corn is insecticide use. Conventional insecticides can cause high mortality to Monarch caterpillars located on milkweed plants within and on the edge of cornfields. ECB is a serious pest in corn causing an estimated $1 billion dollars damage annually.

If you would like to subscribe (or unsubscribe) to updates during the growing season, click here.
©  1994-2024