Web Page Available on Bt Corn Risk to
Monarch Butterflies By
October 5, 2001
Information about Bt corns impact on monarch butterflies
is now available on a web page (www.ars.usda.gov/is/br/btcorn)
from the Agricultural Research Service.
The core of the web page is research coordinated by ARS and recently published
in the Proceedings of the National Academy of
That Bt corn might present a risk became a matter of scientific
and public concern when a small study in 1999 indicated caterpillars suffered
when given no choice but to feed on milkweed leaves heavily dusted with Bt corn
pollen. Bacillus thuringiensis (Bt) is a soil bacterium used as an
effective alternative to chemical insecticides for controlling moth pests.
Two major questions needed to be scientifically answered to
establish whether Bt corn actually posed a threat to monarch caterpillars--the
direct toxicity of Bt pollen for caterpillars and the likelihood that
caterpillars might be exposed to that much pollen, according to entomologist
Richard L. Hellmich with the ARS
Corn Insects and Crops
Genetics Research Unit in Ames, Iowa.
The studies found monarch caterpillars are not very sensitive to
pollen from most types of Bt corn, and that caterpillar exposure to Bt pollen
is low. It took pollen levels greater than 1,000 grains of pollen per square
centimeter (cm2) before there were any toxic effects in monarch
caterpillars, and even greater levels before the effect was significant.
Caterpillars were found on milkweed in cornfields during the 1-2
weeks pollen is shed by corn, but corn pollen levels on these plants were found
to average only about 170 pollen grains per cm2. Less than 1 percent
of the milkweed leaves in cornfields had pollen levels exceeding 1000 grains
per cm2 during pollen shed.
One variety of Bt corn--Bt 176--did have a toxic effect with
pollen doses as small as 10 pollen grains per cm2. Bt 176 is one of
the earliest forms of Bt corn and has never been planted on more than 2 percent
of the corn acres. It will be completely phased out by 2003.
ARS is the U.S. Department of
Agriculture's chief scientific research agency.