It was in 1999 that professors at Cornell University sounded the alarm: Research had found that monarch butterfly larvae eating pollen from Bt corn hybrids were injured or killed by the pollen. If this involved an unattractive moth or a lesser known butterfly the news probably would have gone unnoticed by the press and the public. But the news spread far in a matter of days, largely because the monarch butterfly is one of the best known, loved butterflies in the world; its brilliant orange and black coloration make it easy to recognize. There hadn’t been a hint of Bt corn pollen affecting monarch butterfly larvae in any previous scientific literature, nor was it among the concerns cited even by people opposed to genetic engineering. In 1999 corn hybrids with the Bt trait (which produced resistance to the European corn borer) were just becoming popular. At the time these hybrids represented less than 20 percent of all field corn planted in the U.S., so this was in the early years of genetically modified organisms (GMO). This trait was rapidly becoming much more popular, however, because it was highly effective in protecting corn plants from the European corn borer.
Corn borer moths lay eggs on the underside of corn leaves. The eggs hatch and following several developmental stages (instars) the larvae become large enough to bore into the corn stalks, creating feeding cavities in the stalk that weaken it and result in lodging. (Lodging refers to corn plants breaking off, thereby becoming difficult or impossible to harvest.)
We did two years of research at Miner Institute (Chazy, New York) comparing replicated strips of Bt and non-Bt corn, with dramatic results: We found that 25 percent (first year) and 46 percent (second year) of the non-Bt corn plants were invested by corn borers, whereas in the Bt hybrid, which was genetically identical except for the Bt trait, there wasn’t a single corn borer in any plant we examined. This was true for both years of the study. I also remember how much work it was for our research staff to harvest and then split the stalk of each corn plant examined, looking for corn borer damage. This was necessary since a casual examination of the plant may miss the borer hole.
Milkweed is the only food of monarch butterfly larvae, so the Cornell researchers dusted corn pollen from a corn hybrid with the Bt-corn borer trait onto milkweed plants, looking for signs of toxicity in the larvae. Their reasoning was that milkweed is commonly found in waste areas near corn fields and the pollen from the corn plants with the Bt trait could drift from the field onto the milkweed leaves. They found that compared with larvae feeding on milkweed leaves dusted with corn pollen from non-Bt corn, the monarch larvae feeding on milkweed dusted with Bt corn pollen had decreased feeding, growth and survival rates. Their conclusion: Bt corn could threaten monarch butterfly populations feeding on milkweed growing near these corn fields.
Other entomologists challenged the validity of these results, pointing out that the amount of pollen dusted onto milkweed leaves was far in excess of anything that would be found in nature. Under natural conditions much of the corn pollen landing on milkweed leaves is blown off by the wind or washed off by rain, but under laboratory conditions there was neither wind nor rain.
What this study did, however, was to cause entomologists from other land grant universities to conduct studies of the interaction between Bt corn and monarch butterfly larvae. Extensive research has found that survival of monarch butterfly populations are not threatened by the planting of corn with the genetically engineered Bt trait. A study in Maryland better represented natural conditions; it examined the survival rate of monarch larvae exposed to Bt and non-Bt corn pollen in a corn field. Survival rates of the larvae ranged from 80 percent to 93 percent, with no difference in survival rates between the Bt and non-Bt plots.
Some people were concerned that milkweed growing as “weedy invaders” in corn fields would be especially subject to corn pollen depositions. However, farmers work hard to eliminate milkweed from corn fields, usually with good results. One change that has had a negative impact on monarch butterfly populations is the conversion of vast acreages of continuous hayland and permanent pasture in the Midwest – prime sites for milkweed – into rotated cropland growing corn, soybeans and other grain crops. With these cropping changes a greater portion of milkweed is now found along roadsides, resulting in a lot more monarch butterflies meeting their fate on the grills of passing cars than from exposure to Bt corn pollen.
Recently there’s been good news regarding monarch butterfly populations, which have been greatly increasing from their lows of several years ago. In fact, between 2015 and 2016 the number of monarch butterflies overwintering in Mexico (their natural winter habitat) tripled. These butterflies are also getting a helping hand from the U.S. Department of Agriculture through its Natural Resources Conservation Service (NRCS).
In November 2015 NRCS announced a conservation effort in 10 states in the Midwest and the southern Great Plains aimed at helping farmers provide food and habitat for monarch butterflies. NRCS is providing technical and financial assistance to help farmers and “conservation partners” plant milkweed along field borders, in buffers along waterways or around wetlands, in pastures and other suitable locations where they won’t interfere with normal farming practices. NRCS is also helping farmers manage their pastures to increase populations of milkweed while not decreasing the productive capability of the pasture. Therefore, the situation has changed from where farmers growing Bt corn were wrongly implicated in the decline of the monarch butterfly, to the current program where farmers are assisting USDA efforts to expand populations.