Impact of Bt corn pollen on monarch butterfly populations: a risk assessment

Abstract

A collaborative research effort by scientists in several states and in Canada has produced information to develop a formal risk assessment of the impact of Bt corn on monarch butterfly (Danaus plexippus) populations. Information was sought on the acute toxic effects of Bt corn pollen and the degree to which monarch larvae would be exposed to toxic amounts of Bt pollen on its host plant, the common milkweed, Asclepias syriaca, found in and around cornfields. Expression of Cry proteins, the active toxicant found in Bt corn tissues, 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 utilizes milkweed stands in and near cornfields, and the current adoption rate of Bt corn at 19% of North American corn-growing areas. This 2-year study suggests that the impact of Bt corn pollen from current commercial hybrids on monarch butterfly populations is negligible.

Figure 1

Conceptual model of components of risk assessment of the impact of Bt corn pollen on populations of the monarch butterfly.

Figure 2

Percent growth inhibition at 96 h for monarch larvae exposed to pollen from event 176 corn hybrids and for larvae exposed to pollen from Bt11 corn hybrids. These two response curves are compared with the average cumulative proportion of pollen on milkweed leaves as recorded within Iowa and Ontario cornfields during 1999–2000.

Figure 3

Joint probability curves that compare the likelihood of exposure of monarch larvae to events 176 or Bt11 corn pollen with the percent of larvae demonstrating inhibition of growth. The estimated area under the curves provides a comparative measure of the impact that exposure to each type of pollen poses.