Is the Monarch Butterfly Really Endangered by Bt Corn?

Posted by Eric Troy on 01 Dec 2013 21:47

Bt corn (or Bt maize) is a type of corn that has been genetically modified to make a substance called Bt toxin. Bt toxin is a protein made by the soil bacterium Bacillus thuringiensis that kills certain insects, such as caterpillars and beetles, that are pests to plants. A mainstay of GM crop opponents is the widely believed statement that Bt corn crops will endanger the Monarch butterfly population.

There is actually not any credible evidence that Bt corn endangers Monarchs, and the source of the belief is a study that has been considered shoddy, non-reproducible, and since disproved by other studies. However, despite these facts, many organizations and individuals, for whatever reasons, forthrightly assert that Bt corn presents an imminent danger to the butterflies. This assertion is based on a study by J. Losey and others of Cornell University, published in the journal Nature, called Transgenic pollen harms monarch larvae.1 The researchers statement that the transgenic pollen would kill larvae of the Monarch spread like wildfire and was known all over the globe within days.

The only thing that the larvae of the Monarch butterfly eat is milkweed, which grows in and around the perimeter of corn fields, but also, of course, grows in may other places. The researchers figured that transgenic pollen from the modified corn plants might drift onto the milkweed leaves near the corn crop. To find out if this transferred pollen on the leaves, inadvertently consumed by the butterfly larvae, might harm them, they dusted milkweed plants with Bt corn pollen, to see if it was toxic to the larvae. They also dusted other leaves with non-Bt corn pollen, for comparison. They found that the larvae who ate the Bt pollen leaves at less, grew less, and had lower survival rates, compared to those who at the milkweed leaves with regular corn pollen. The authors concluded, therefore, that Bt corn could endanger Monarch populations that fed on milkweed near the Bt corn fields.

Monarch butterfly

Claims of Bt corn's danger to the Monarch butterfly
have yet to prove true.

Monarch butterfly

Claims of Bt corn's danger to the Monarch butterfly
have yet to prove true.

Other scientists, in short order, questioned the validity of the study and pointed out that the results were non-reproducible. They also said that the idea that the larvae would have no other choice but to eat Bt pollen contaminated leaves was not realistic. Also, artificially dusting the leaves with large amounts of pollen produced unnaturally high levels of pollen on the leaves. Followup studies, such as by Mark Sears, et al. 2, found no danger to Monarch populations from Bt corn crops. This does not erase the potential for danger, however, it only underlines the lack of valid evidence of an actual impact on the population.

Perhaps more obvious, but routinely ignored, is that milkweed is an invasive plant in and around corn fields, and farmers don't want it there at all. They try to get rid of it, by the common methods farmers use to get rid of weeds, such as herbicides. The same is true for soybean fields. The point is, a corn field is never a safe place for a Monarch larvae. There are lots of milkweeds in other areas, and there are other things that affect this food source. For instance, there is a lot of milkweed on the sides of roads, and this could be affected by automobile exhaust. As well, monarchs face danger from many other sources, all of which are interrelated, including climate change, increasing populations of exotic birds, and habitat loss. The problems facing the species is much more complex than the simple question of whether Bt corn fields endanger the larvae, but the sensationalized nature of this potential problem has caused many people to completely disregard all the other factors.

The Losey paper, which had been the primary source of "evidence" about the impact of Bt corn crops on the monarch butterfly population, was described by the authors themselves as preliminary. Since 1996, large areas in the U.S. have been used for both Bt corn and cotton, and the monarch butterfly population has thrived. It is, however, under threat from habitat destruction in places in Mexico, where the species winters.3 Despite this, some people seem to believe Monarch butterflies have gone extinct! If this is not a perfect example of preliminary science gone rumor-mill crazy, I don't know what is.

See also What is the StarLink Corn Controversy and How Was Taco Bell Associated?

1. Losey, John E., Linda S. Rayor, and Maureen E. Carter. "Transgenic Pollen Harms Monarch Larvae." Nature 399 (1999): 214-15. Web. 1 Dec. 2013. <>.
2. Mark K. Sears, Richard L. Hellmich, Diane E. Stanley-Horn, Karen S. Oberhauser, John M. Pleasants, Heather R. Mattila, Blair D. Siegfried, and Galen P. Dively. "Impact of Bt corn pollen on monarch butterfly populations: A risk assessment." PNAS 2001 98 (21) 11937-11942; published ahead of print September 14, 2001, doi:10.1073/pnas.211329998
3. Halford, N. G. Plant Biotechnology: Current and Future Applications of Genetically Modified Crops. Chichester, England: J. Wiley, 2006.
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