Bt Corn Not a Threat to Monarchs
Entomologist Rich Hellmich (right)
and technician Randy Ritland
collect milkweed leaves near
Last summer, when entomologist Mark K. Sears was researching whether
Bt corn posed a risk to monarch butterflies, he noticed that
two monarchs had hit his car's windshield as he drove to test fields
around Ontario, Canada.
"That's probably more monarchs than were lost that day because
of Bt corn, according to findings in our studies," says
Sears, with the Department of Environmental Biology at the University
Sears is part of a group of scientists coordinated and partially funded
by the Agricultural Research Service
who have spent 2 years investigating whether Bt corn is a threat
to monarch butterflies.
Student aid Stacy Van Loon
releases a monarch butterfly
into a breeding cage. The
butterflies consume artificial
nectar from the flower-shaped
Bt corn contains genes from the bacterium Bacillus thuringiensis
so that the plant will produce proteins to protect itself against insect
pests such as the European corn borer. This reduces the amount of insecticide
farmers need to apply. Since Bt corn was introduced to the marketplace,
use of the insecticides recommended for European corn borer control
has decreased from 6 million acre treatments to slightly over 4 million
in 1999, a drop of about one-third, according to the Environmental Protection
Although Bt corn was approved in 1995, new concern about possible
risk was raised when a note published in a May 1999 issue of Nature
suggested that Bt corn could harm monarch butterflies when the
caterpillars were given no choice but to feed on milkweed leaves heavily
coated with Bt corn pollen.
Monarch caterpillars feed exclusively on leaves of milkweed plants,
which grow in and around cornfields. But during the 1 to 2 weeks a year
that corn pollen is shed, it can be blown onto the milkweed leaves.
A large monarch caterpillar
feeds on a common milkweed
In response, ARS organized a series of workshops that encouraged butterfly
biologists, corn researchers, ecologists, entomologists, and other experts
to work together to determine whether a risk actually existed. During
a February 2000 workshop, a group of scientists from government, universities,
industry, and environmental groups prioritized specific research needs.
The idea was to ensure that all the most important questions were covered.
In addition to funds already assigned, ARS contributed $100,000 to a
grant pool, which was then matched by industry, to fund the research.
The collaborations established at the workshops continued throughout
the research process. For example, the group agreed early on to use
similar experimental designs and methods, such as how to handle the
pollen. This ensured that data collected by different scientists would
"Being able to pool data gave us much larger, more reliable sample sizes, so we could develop the best scientific answers to the question of risk," says ARS entomologist Richard L. Hellmich, with the Corn Insects and Crop Genetics Research Unit, Ames, Iowa. Hellmich was the lead ARS scientist on the project.
Cooperation even extended to how the research was published. All the researchers funded from the special grant pool got together and divided the data into logical sections and agreed to submit all manuscripts together to a single scientific journal. Publishing the exposure, toxicity, and risk-analysis studies at one time in one journal provided the most complete picture possible of whether any risk actually existed.
Entomologist Les Lewis
(left) and technician Keith
Bidne observe a group of
newly emerged monarch
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.
When Hellmich and his colleagues began their Bt corn/monarch
butterfly laboratory studies, they encountered a much higher level of
Bt protein in the pollen than expected. They checked several
possible explanations, including inspecting their methods for pollen
collecting and preparation.
"We found that some of the pollen we were using in our tests was
contaminated by ground-up or fractured anthers," Hellmich says.
Anthersthe organs at the end of plant stamens that produce pollenhave
a much higher level of Bt protein than does pollen itself. "But
simply passing the pollen through a fine screen before using it removed
the anthers," he adds.
The problem of anther contamination may explain the toxic results in
some earlier studies, Hellmich points out. But that left the question
of whether caterpillars might eat the high-Bt-containing anthers
under natural conditions.
"Anthers have commonly been found on milkweed leaves within cornfields,
but none of them were fractured. Fractured anthers appear to be an artifact
of pollen processing in the laboratory," Hellmich says.
Then they looked at caterpillar-anther interaction. "Our preliminary
results show that small larvae avoid anthers. With the size difference,
it would be like a person trying to eat a city bus," Hellmich points
out. "Wind and rain also readily dislodge anthers from milkweed
leaves, making it less likely that caterpillars will encounter anthers."
"Bt Corn Not a Threat to Monarchs" was published in the February 2002 issue of Agricultural Research magazine.