from the Latest Monarch Study"
A recent paper titled, "Field deposition of Bt transgenic corn pollen: lethal effects on the monarch butterfly", by Laura Hansen and John Obrycki from Iowa State University, was published online in the European journal Oecologia. It has been described as the first field study which describes the effect of certain Bt corn pollen deposits on milkweed leaves and their effects on larvae of the monarch butterfly. This report has received considerable media attention and we believe it is important to offer some comments.
We believe careful examination of the methods, conclusions and discussion of this paper are warranted, and that such examination raises serious questions about what, if any, conclusions can be realistically drawn from this study. As field biologists we realize the difficulty in conducting quality field research relevant to real world situations, but we believe this study does little to help understand potential risks of deploying Bt plants in the field.
In order to assess the deposition of pollen in the field, the authors placed potted milkweed plants in or at various distances from the edge of corn fields during pollen shed. Samples of milkweed leaves were brought to the laboratory and newly hatched larvae of Monarch butterflies were placed on leaf disks cut from the leaves and fed for 48 hours. While we appreciate the difficulty of setting up these tests, these method make the assumptions that the placement of the potted plants simulates the natural distribution of plants in and near corn fields and that adult monarchs would find and lay their eggs on these plants. These assumptions, as well as the assumption that Monarch populations occur at the same time as pollen shed, go to the heart of the matter of whether Monarch larvae will be exposed to lethal concentrations of natural depositions of Bt pollen. Furthermore, we are aware of work indicating that Monarch butterflies avoid laying their eggs on leaf tissue with high deposits of pollen. These factors, along with several others, make us question the "field" aspect of this paper.
In order to assess the dose of pollen required to cause mortality of newly hatched monarch larvae, another set of experiments was conducted. Pollen was collected from the field and applied to leaves in the laboratory at three different doses (14, 135 and 1300 grains per square centimeter). Small larvae were placed on disks and fed for 48 hrs. Mortality and development and growth characteristics were assessed. The point is, only the collection of leaf samples with pollen and the collection of the pollen itself was carried out in the field.
Field trials are those that are conducted entirely under the conditions prevalent in the field during the experimental period. Because of this, the results will be affected by factors such as moisture on the leaf surfaces, variable temperature and humidity, degradation of the pollen by sunlight, moisture, microorganisms, rainfall, wind, natural dispersal and behavior of monarch larvae, predation on monarch larvae, and a host of other ecological factors, not to mention the adult Monarch choosing whether or not to lay her eggs on a pollen-infested plant. Some or all of these factors will have a direct result on the measurement of mortality and may completely overshadow the effects of the Cry 1Ab toxin expressed in the pollen. True field trials are necessary to understand the nature of pollen deposition on milkweed plants and the possible effect on monarch larvae or any other species of caterpillars.
Field trials are underway in an international comprehensive program involving scientists from Maryland, Iowa, Nebraska, Ontario, Canada and other places and the first year results have been discussed at several scientific meetings. The data from the comprehensive effort provide information on many of the important questions not addressed in this study such as whether natural populations of monarch are actually exposed to lethal effects of Bt corn pollen in the field. We believe great caution should be exercised in assessing the Hansen and Obrycki paper until publication of the scientific data from the international programｹs efforts, which we hope will be done in a timely fashion. But for now we must address our concerns with the present study.
A misleading statement by the authors and media implied that the results implicate all Bt corn types (or events expressing Cry 1Ab toxin). In the study, only pollen from Event 176 Bt corn showed any consistent lethal effect. Event 176 Bt corn represented about 2% of the total Bt corn acreage planted in North America in 1999 and probably is no more than 1% of the acreage in 2000. The pollen from Event 176 is considerably more toxic than pollen from Bt11 or MON810, the leading Bt corn products. It is misleading to imply that toxicity associated with that product is relevant to Bt hybrids planted by the vast majority of farmers.
The conclusions of the authors go far beyond the extent of the data presented. They imply in their discussion that significant amounts of pollen could be distributed within and up to 10 meters outside of cornfields such that significant mortality to monarch larvae would occur. Their own data do not support this speculation. Nowhere have they reported on the density of milkweed plants in or around fields or in different habitats in the area, nor have they provided any information on the phenology of the monarch populations in relation to the pollen shed period of the Bt corn hybrids. These are the points being addressed in the international comprehensive program. Without this information, it is improper to speculate on the risk associated with the Cry 1Ab toxin found in some Bt corn hybrids.
A more detailed discussion of study findings and procedures, some of which raise questions, follows:
The examples given above illustrate that the study does not duplicate a field environment, and they raise questions about whether the tested material was more toxic than pollen that would occur in the field. The lack of a dose response is particularly troubling, as is the small sample size. The study does confirm what has been seen in various field studies - that pollen density drops off rapidly at the edge of a cornfield.
The results of these experiments do not represent the potential impact of Bt pollen from all events, as the authors suggest, across the extensive range of the monarch butterfly in North America. We await the results of the comprehensive international study which includes studies conducted over a two year period (1999-2000) in the field and whose preliminary results, already presented at several scientific meetings, provide a much more complete and balanced assessment which indicates that Bt corn does not pose a significant threat to Monarch populations as implied in this study or as portrayed in some of the press.
** NOTICE: In accordance with Title 17 U.S.C. Section 107, this material is distributed for research and educational purposes only. **