Monarchs and Bt corn: A research update

During the past 2 years, considerable controversy and debate have surrounded the impact of Bt corn pollen on monarch butterfly survival. A series of scientific studies that address this issue were recently published in the Proceedings of the National Academy of Sciences USA. Rick Hellmich and Les Lewis, USDA-ARS Corn Insects Lab, in Ames, conducted some of this research. Listed below are the article titles and a brief summary of the major findings of this research. The complete articles are available online.

Temporal and spatial overlap between monarch larvae and corn pollen. This study assessed the likelihood that monarch larvae will be exposed to Bt corn pollen in or near cornfields. It was found that monarchs use milkweed in cornfields and insect densities per plant were as high or higher in agricultural habitats as in nonagricultural habitats. There was a temporal overlap between susceptible monarch stages and corn anthesis in the northern part of their breeding range. Most of the monarchs produced in the upper Midwest are likely to originate in cornfields and other agricultural habitats.

Corn pollen deposition on milkweeds in and near cornfields. This study measured naturally occurring corn pollen densities on milkweed. Pollen density was highest in the cornfield and averaged 171 grains/square centimeter and was progressively lower from the field edge outward, falling to 14 grains/square centimeter at 2 meters. The highest pollen density observed was 1,400 grains/square centimeter during a rainless anthesis period. A single rain event can remove 54 to 86 perecent of pollen on leaves. Leaves on the upper portion of milkweed plants, where young monarch larvae tend to feed, had only 30 to 50 percent of pollen density of middle leaves.

Monarch larvae sensitivity to Bacillus thuringiensis-purified proteins and pollen. Laboratory tests established the relative toxicity of Bt toxins and pollen from Bt corn to monarch larvae. Pollen bioassays suggest that pollen contaminants, such as anthers, which are an artifact of pollen processing, can dramatically influence larval survival and weight gains and produce spurious results regarding pollen impacts. The only transgenic corn pollen that consistently affected monarch larvae was from Cry1Ab event 176 hybrids, which is currently less than 2 percent corn planted and for which reregistration has not been applied. Results from other types of Bt corn suggest that pollen from Cry1Ab (events Bt11 and Mon810) and Cry1F, and experimental Cry9C hybrids, will have no acute effects on monarch larvae in the field.

Assessing the impact of Cry1Ab-expressing corn pollen on monarch butterfly larvae in field studies. Survival and growth of monarch larvae after exposure to pollen from three Cry1Ab events were examined in field studies. First-stage larvae exposed to low doses (22 grains/square centimeter) of event 176 pollen gained 18 percent less weight than those exposed to Bt11 or Mon810 pollen after a 5-day exposure period. Larvae exposed to 67 pollen grains/square centimeter on milkweed leaves from within an event 176 field exhibited 60 percent lower survival and 42 percent less weight gain compared with those exposed to leaves from outside the field. In contrast, Bt11 pollen had no effect on growth to adulthood or survival of first or third instars exposed for 5 days to 55 to 97 pollen grains/per square centimeter, respectively. Similarly, no differences in larval survivorship were observed after a 4-day exposure period to leaves with 504-586 (within fields) or 18-22 (outside the field) pollen grains/square centimeter collected from Bt11 and non-Bt sweet corn fields. The effects of Bt11 and Mon810 pollen on survivorship of larvae feeding 14 to 22 days on milkweeds in fields were negligible.

Impact of Bt corn pollen on monarch butterfly populations: A risk assessment. A formal risk assessment was developed for the impact of Bt corn on monarch populations. Expression of Cry proteins, the active toxicant found in Bt corn, differed among hybrids, and especially so in the concentrations found in pollen of different events. In most commercial hybrids, Bt expression in pollen is low, and laboratory and field studies show no acute toxic effects at any pollen density that would be encountered in the field. Other factors mitigating exposure of larvae include the variable and limited overlap between pollen shed and larval activity periods, the fact that only a portion of the monarch population uses milkweed in and near cornfields, and the current adoption rate of Bt corn at 19 percent of corn-growing areas (although it is much higher in many Iowa counties). This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch populations is negligible.

This article originally appeared on pages 13-14 of the IC-488 (2) -- February 18, 2002 issue.

Updated 11/27/2006 - 10:47am