Two Big Questions
To determine whether the concern about Bt corn was valid, two
major questions needed to be scientifically answered: "Exactly
how much Bt pollen does it take to cause toxic effects in monarch
caterpillars, and what are the chances caterpillars will encounter that
dose under natural conditions?" Hellmich says.
First, the scientists assessed the feeding behavior of monarch larvaecaterpillarsto
see whether Bt's presence on milkweed leaves influenced their
weight and survival. Pollen from six Bt corn typesBT11,
MON810, CBH351, DBT418, TC1507, and BT176was tested along with
no-pollen and non-Bt-corn-pollen controls.
"We looked at larval weight and larval survival and found it took
large amounts of pollen to get any statistically significant effect,"
Eating leaves with pollen coating densities below 1,000 grains/cm2
had no effect on caterpillars' weight or survival rate. Above 1,000
grains/cm2, caterpillars were smaller than those from the
control treatments, but their survival rate was no different from that
One type of Bt cornBT176did show some harm to larvae
at pollen levels of 10 grains/cm2. BT176 was the earliest
Bt corn developed and was quickly supplanted by other types.
It has never been planted on more than 2 percent of all corn acres and
is likely to be completely phased out by 2003.
Once the scientists knew how much Bt corn pollen it took before
monarch caterpillars showed any ill effect, the second question was
how often are they exposed to pollen levels above 1,000 grains/cm2
under natural conditions?
To find out, the researchers established corn pollen density and distribution
patterns on milkweed leaves near cornfields. Hellmich's team set up
lines of collecting devices at seven different fields, from the edge
of the field to 600 feet away, in all four compass directions.
The researchers measured pollen deposition three ways. They put out
tubes holding cuttings of milkweed stems with two leaves, whole potted
milkweed plants, and microscope slides coated with glycerin. Sampling
lasted about 10 dayscovering peak pollen production periods.
"We found that, on average, less than 30 percent of the pollen
that corn produces ends up on milkweed leaves, even when conditions
are perfect, and most of that gets deposited on milkweed within the
cornfield," Hellmich says of his field studies in Iowa.
In Ontario, Sears conducted similar field studies of pollen deposition
and found the same pattern. Other pollen studies by University of Maryland
and University of Nebraska researchers also confirmed the pattern and
extent of pollen distribution.
Data pooled from Iowa, Nebraska, Maryland, and Ontario showed that
the average Bt corn pollen density on milkweed leaves inside
cornfields was about 170 grains/cm2, and it rarely went above
"These pollen densities mean monarch caterpillars inside cornfields
will encounter pollen levels exceeding 1,000 grains/cm2the
lowest observable effect doseless than 1 percent of the time,"
Hellmich points out.
Many factors contribute to keeping pollen density low. Corn pollen
is relatively heavy, so it doesn't blow far; higher milkweed leaves
tend to shelter lower leaves; and rain washes pollen off of milkweed
leaves easily, Hellmich says.
Given the low toxicity of Bt corn pollen and the low rates of
exposure, the effect of Bt corn pollen from common commercial
hybrids on monarch butterfly populations is negligible. "Furthermore,
you need to compare the potential for risk to monarchs from Bt
corn with the alternative, which is chemical insecticide use,"
Hellmich says.By J.
Kim Kaplan, Agricultural Research Service Information Staff
This research is part of Crop Protection and Quarantine, an ARS
National Program (#304) described on the World Wide Web at http://www.nps.ars.usda.gov.
Richard L. Hellmich is
in the USDA-ARS Corn
Insects and Crop Genetics Research Unit, Iowa State University,
Ames, IA; phone (515) 294-4509, fax (515) 294-2265.