Impact of Bacillus thuringiensis Corn Pollen on Monarch Butterfly Populations

摘要

Nearly four years ago, in 1999, a note submitted to the editor of Nature from scientists at Cornell University claimed that pollen from a Bt corn hybrid harmed larvae of the monarch butterfly (1). They had sprinkled an unspecified amount of pollen from a Bt11 corn hybrid on milkweed leaves that they fed to small larvae of monarchs. The ensuing international reaction from anti-biotechnology activists created the impression that this pollen and its underlying technology might cause significant damage to populations of non-target insects such as the monarch as well as detrimental effects to the environment. A number of scientists in Canada and the United States who had evaluated Bt corn for efficacy, economic viability and its impact on non-targets in the agricultural environment gathered to initiate a collaborative investigation into the impact of Bt corn pollen on monarchs. As a group, we felt that, due to the naive methodology and inconclusive results of the reported laboratory study and the unjustified statements from activist groups, further appropriate and scientifically sound investigations were warranted. We approached this process with the objective of obtaining information on toxicity and exposure of monarch larvae to Bt corn pollen and to develop a classical risk assessment using this information. We performed a series of replicated bioassays using both purified toxins and pollen samples from a number of corn hybrids created with different transformation events. In these, we measured the effect of increasing doses of Bt pollen on feeding behavior and weight gain of first-instar monarch larvae. We established a no-observable-effect level (NOEL) from these results and compared that dose with the density of pollen that we estimated to occur on milkweed leaves in and around corn fields to determine the likelihood of encounter with an effective dose. To validate our laboratory observations and estimates of impact, we performed a series of field experiments over two years in three different locations within the Corn Belt. Monarch larvae were either caged on milkweed plants or fed milkweed leaves from plants in and around corn fields that had previously received corn pollen during pollen shed. Larvae were allowed to feed on the milkweed for the period of their development and their weight gain and survival monitored. Treatments were carried out on milkweed plants placed in fields of Bt corn, in conventional corn fields and in areas removed from corn altogether. Concurrently, we examined populations of monarchs in several locations within the Corn Belt to determine the synchrony of larval development in relation to pollen shed activity in corn fields. In addition, we attempted to estimate for the entire Corn Belt the amount of milkweed found in com fields, other agricultural fields and in non-agricultural land to determine the proportion of monarch larvae that could receive significant amounts of pollen in their diets. Response of monarch larvae to Cry proteins produced from various events used to create commercial corn hybrids was variable. Bt corn pollen from commercial hybrids at levels exceeding 1000 pollen grains/cm~2 of leaf area did not cause measurable impact to monarch larvae in bioassay trials. Only one type of pollen produced any impact, from Event Bt176 corn, and hybrids using that event never exceeded 2% of the production area and no longer are registered for use in North America (2). In every field trial, no impact was measured to larvae exposed to pollen from any Bt corn hybrid, except those developed from Event 176 technology. Even in subsequent trials, no monarch larvae have shown a measurable reaction to Bt corn pollen at field densities.