28 - Bt Corn and the Monarch Butterfly: Research Update

Abstract

This chapter discusses the effects of Bacillus thuringiensis (Bt) corn on the monarch butterfly, Danaus plexippus. In a laboratory investigation conducted by Losey et al., young monarch larvae, which were given no choice but to feed on milkweed, Asclepias curassavica, leaves dusted with pollen from a Bt corn hybrid ate less, grew more slowly, and had a significantly higher mortality rate than larvae feeding on leaves dusted with nontransgenic pollen. The amount of pollen dusted onto the milkweed leaves in the Losey et al. study was not quantified, and as a result, the dose of Bt protein consumed by the larvae could not be quantified. To formulate a quantitative risk assessment, the level of toxicity must first be determined. Although toxicity of Bt pollen and the toxins expressed by pollen can be clearly documented and could represent a hazard, the risk to monarch populations from exposure to Bt corn pollen is also a function of exposure. For monarchs to be exposed and potentially affected by corn pollen, larval development must coincide with corn anthesis. Exposure to the pollen, may be longer depending on the duration of pollen availability on milkweed and the rate of Bt protein degradation. Another important factor in assessing exposure of monarch larvae to Bt pollen is milkweed distribution. Behavior of monarch larvae and adults also can influence exposure to Bt pollen.