Bt Corn & Monarch Butterflies
From PA IPM News, Volume 4, No.
1, Winter 2001
As we move into the new millennium, biotechnology is revolutionizing everything from what we eat to how we treat diseases. In agriculture, genetic improvements such as Bt corn are being developed that can form their own protection against pests. Currently, Bt corn is being grown on about 25 percent of the corn acreage in the U.S. and that number is expected to rise despite concerns Bt corn may cause harm to non-targeted insects such as the Monarch butterfly.
In the development of Bt corn, a gene from the bacterium, Bacillus thuringiensis, is added to make the hybrid resistant to the European corn borer and other pests. The European corn borer, the targeted pest of Bt corn, costs U.S. corn growers more than $1 billion annually in yield losses and crop protection costs. Once the targeted pest ingests the tissue of the plant containing the crystalline protein, it releases the toxin, causing the gut lining of the insect to break down killing the insect.
There are three major lines of corn with the gene that codes for the Cry 1A(b) toxin commonly planted in the U.S.: 1) Bt 176 (Knockout - Syngenta and NatureGard trademarks - Mycogen), 2) Bt-11 (YieldGard trademark - Northrup King/Sygenta), and MON 810 (YieldGard trademark- Monsanto and associated seed companies). Each type produces different concentrations of the toxin in its pollen and other tissues. One particular event of Bt corn, Bt 176, produces a high level of these toxins in its pollen. Since pollen is wind blown, there is fear that non-targeted pests, such as our national insect, the Monarch butterfly, could be affected. In a study published last year in Nature magazine, Dr. John Losey, Cornell University, collected pollen from Bt 11 and exposed the Monarch butterfly to unknown concentrations. He used a visual measure of concentration from observations of pollen on weeds found in cornfields. His study did not provide evidence that monarchs in the field are substantial impacted by pollen from Bt-corn. However, in the lab he found 40 percent of the butterflies died after exposure to the highest concentration of pollen. After being exposed to pollen lower concentrations of Btu-11 pollen, the mortality rate decreased but growth of the Monarchs slowed down.
According to Dr. Dennis Calvin, Associate Professor of Entomology at Penn State, the USDA and EPA determined that field studies should be conducted to assess the degree of impact on the insects under field conditions. "A number of research scientists were called upon to collect data and obtain answers to several questions: are butterfly larvae in the area at the same time when corn pollination is occurring, and if the butterflies are there at the right time, are they at the right location to be exposed, and finally do they come in contact with lethal or sub-lethal pollen?"
Calvin and his colleagues from the USDA/ARS and other universities found that in some northern areas of the country Monarch butterfly migration and pollination occurs at the same time. They also found that Bt corn pollen could be wind blown for long distances, but lethal concentrations of the most commonly planted Bt, MON810, occurred only up to one meter away from the field. These hybrids represent approximately 95% of the Bt-corn planted in the North America. Lethal concentrations of the Bt-176 occurred up to 8 meters from the field edge, but these hybrids represent only about 2% of corn planted in North America and are being phased out in favor of hybrids with lower toxin concentrations in the pollen. "Past one meter, Monarchs would have virtually no chance of being exposed to the lethal or sublethal pollen concentrations," Calvin says.
Calvin's colleagues then looked to see where the Monarch's hosts, milkweeds, are typically found. "In the state of Iowa, 60 percent of all milkweed plants were found along roadways and non-croplands, while only 40 percent were found in corn and soybean fields," Calvin states. However, they also found that 70 percent of Monarch butterfly eggs are laid in the fields, typically because they tend to be better quality plants. In fact, the herbicide programs used to control milkweed, the monarchs only host, delayed plant development making the milkweeds better hosts than those found in roadways and non-croplands. Milkweeds found along roadways and in non-croplands were of lower quality due to more advanced development (leathery leaves) and dust accumulations. This would suggest the possibly that Bt-hybrid pollen would have a significant impact on monarch populations. However, there are several off-setting factors.
and his associate also found that there is a short time
period in which Monarchs could be exposed to Bt corn
pollen, about three days. This three days represents the
time period during which corn pollen concentrations in a
cornfield exceed lethal and sublethal levels on milkweed
leaves. Monarch females are depositing eggs over a 30 day
period; so only a small percentage of eggs are deposited
in the field during this time. In addition, only 25% of
all cornfields are planted to Bt-corn hybrids, thus 75%
monarch larvae are not exposed to Bt-toxins in 75% of
fields. "In that three day time period, very few
Monarch larvae were exposed, with one percent being the
worst case scenario," Calvin explains. In fact,
studies in the field revealed that Monarch eggs placed in
a Bt cornfield at pollination had a similar survival rate
to those found on roadside milkweed plants. "There
is a 97 percent natural mortality rate in Monarchs from
the time eggs are laid until adults emerge. If Bt corn
pollen doesn't kill them, something else will,"
Calvin says. From his data, Calvin concludes that Monarch
butterflies in Bt cornfields have an equal chance of
surviving to those in other habitats.
Calvin says many people don't realize they are already consuming GMOs (genetically modified organisms), as they are now used to produce pharmaceuticals, insulin and even cheeses. New biotechnologies are making existing foods safer and are also producing better products. "I think we'll see more biotechnologies in the future, with more companies investing in them and seeing the benefits," Calvin says.
Extracted from PDF file @ http://paipm.cas.psu.edu/pdf/Winter2001.pdf -- by Roger Lovejoy -- 04 January, 2002