By Mark K. Sears, Professor and Chair, Department of Environmental Biology,
University of Guelph
Despite news reports
and repeated misgivings voiced by those opposed to transgenic crop technology,
pollen from Bt corn hybrids grown in Ontario and throughout the Corn Belt of North
America poses a negligible threat to caterpillars of monarch butterflies. This
conclusion was reached by scientists from Ontario (University of Guelph) and locations
across midwestern U.S. (USDA-Agricultural Research Service in Ames Iowa, Iowa
State University, University of Nebraska, University of Minnesota, University
of Maryland, Cornell University, Pennsylvania State University, and University
of Illinois) following more than two years of research to investigate the impact
of Bt corn pollen on the health of monarch caterpillars that may be exposed to
pollen from corn that dusts their food plant, common milkweed. This conclusion
is in direct contrast to previous findings by scientists at Cornell University
and at Iowa State University that were interpreted as evidence that monarch caterpillars
were killed or severely affected by pollen from all Bt corn hybrids.
Our conclusion is based on two important findings. First, the amount of Bt toxin
found in corn pollen is far less than that found in the rest of the corn plant
tissues, and the toxin has no activity on monarch caterpillars at typical levels
found on milkweed leaves located in and around cornfields. Second, only a small
portion of the monarch population is exposed to pollen from corn plants throughout
North America, let alone pollen from Bt corn. The combination of these two factors,
the toxicity of pollen and the likelihood of exposure to pollen, indicates that
risk is negligible. It is important that both of these factors be considered before
any discussion of risk can be properly undertaken.
Development of these findings has been supported by research from a series of
experiments carried out over the last two seasons, in both laboratory and field.
Experiments on the toxicity of pollen were carried out primarily in laboratory
tests. Each type of pollen from a variety of hybrids was tested. Not all types
or ‘events’ of Bt corn produce the same amounts of Bt protein or toxin
in the pollen of various corn hybrids. Most corn hybrids utilize the Mon810 or
Bt11 events, and we were able to demonstrate that any measurable effect on caterpillars
that were fed pollen on milkweed leaves occurred at a dose above 1000 pollen grains/cm2.
This dosage is far above the average amount of pollen found on milkweed leaves
of plants directly in cornfields – about 170 pollen grains/cm2 – and
occurred in less than 1% of the samples collected. One Bt corn type, event 176,
one of the first Bt corn hybrids to be marketed, showed a toxic effect at a dose
of 10 pollen grains/cm2. Event 176 has not represented more than 2% of the acres
planted with Bt corn and will be phased out of production through 2003. Even if
it had been grown much more widely, it still would not have proven a significant
threat to monarchs.
Field trials carried out at five separate locations throughout the Corn Belt and
in Ontario confirmed that insufficient pollen exists on milkweed leaves in cornfields
to demonstrate any measurable effect on monarch caterpillars. Caterpillars were
exposed to pollen that had fallen on milkweed leaves on plants in the fields and
at various distances from the edge of fields. Despite pollen deposition during
the peak of the pollen shed period in all locations, no mortality or more subtle
growth effects were observed with larvae in the field or on leaves brought to
a laboratory setting for closer examination. One important observation concerned
the nature of the pollen that was found on leaves in the field. The only other
plant material from the corn was the occasional anther or male flower parts found
on the leaves. Anthers are far too large for a small caterpillar to consume and
do not impact on their livelihood.
We examined milkweed leaves within cornfields and leaves on plants outside cornfields
to some distance for deposits of pollen. More than 90% of the corn pollen fell
within 5 meters of the edge of a field: the amount dropped off drastically as
the distance from the edge increased. Only about half as much pollen is found
on leaves of plants that are only 1 meter from the edge, making it unlikely that
exposure to these leaves poses any threat to monarchs. It is clear that only monarch
larvae feeding on milkweeds within cornfields will have any chance of being exposed
to enough Bt pollen of any of the events to cause a detrimental effect.
The second factor needed for risk assessment is that of the probability of exposure
to pollen. We examined three important elements of this probability: the area
planted to corn in a given
1