Bt Toxin

Image: USDA

A bacteria species called Bacillus thuringiensis, which occurs naturally in the soil, manufactures a poison called Bt toxin. It seems to target plant pests, such as the European corn borer, almost exclusively. Corn and cotton have also been genetically engineered to express the Bt toxin, making the plants poisonous to pests but not to humans. Genetically engineering the gene for Bt toxin into cotton decreased by 78,000 tons the amount of insecticides used in China in 2001; similar results are seen in other areas where Bt crops are planted. It was found not to pose any health risks to humans; in fact, for more than five decades organic farmers have safely used Bt-toxin-based sprays as pesticides (Ronald, 71-4).

Bt toxins are proteins. Because they only target cell types that only a few species of insects possess, Bt toxin does not pose a danger to mammals, including humans. It is not allergenic or otherwise harmful to humans. The fact that organic farmers have safely used Bt-toxin-based pesticides for decades — and continue to do so — might make wary consumers feel better about plants that have been engineered to contain the Bt gene. Once the Bt protein hits our digestive tracts, it is quickly (within zero to seven minutes) destroyed by our acidic (low pH) gastric fluids and converted into its constituent amino acid components (Mendelsohn).

Other than concerns regarding human safety, many people worry that pollen from Bt crops can contaminate wild plants, propagating the gene in the wild. Luckily, in the case of Bt corn, cotton, and potatoes, this is not a likely outcome. Wild species in the United States that are related to these plants are either not genetically compatible (i.e., incapable of breeding with the GM crops), flower at different times (i.e., will not be receptive to pollen at the time that pollen carrying the Bt gene might be floating in the air), or are geographically separated from Bt crops. There are exceptions: Pollen flow between Bt cotton and wild relatives of cotton might be possible in Hawaii, Florida, and Puerto Rico, and the sale and distribution of Bt cotton is restricted in these areas (Mendelsohn).

Bt toxins are crystal proteins that kill insects by lysing (destroying) the epithelial cells in their guts. It affects a few orders of insects, including Lepidoptera, in which the alkaline (high pH) environment in the midgut activates the toxin, allowing it to connect to receptors on the surface of the epithelial cells. At this point, the toxins destroy the cells by forming pores in their membranes. It is thought that this leads to insect death either by inhibiting the insect’s ability to eat or by allowing the proliferation of harmful bacteria (Broderick).

This might sound quite gory, but the fact remains that a narrow-spectrum pesticide like Bt toxin results in far fewer insect deaths than broad-spectrum pesticides. A greater diversity of insects can flourish in agricultural areas when highly selective pesticides are used in lieu of pesticides that cause the deaths of a wider variety of insects.

In 1999, after a preliminary (and methodologically flawed) report that pollen from Bt corn was harmful to monarch butterfly larvae, the USDA embarked on an investigation. Their final word is that Bt pollen from corn is generally not harmful to the larvae of monarch butterflies. In natural conditions, it is improbable that toxic levels of Bt pollen would blow onto the monarch’s natural diet of milkweed leaves; even within cornfields, the amount of Bt pollen found on milkweed leaves failed to approach toxic levels. Only one variety of Bt corn, Bt 176, caused harm to monarch larvae at concentrations likely to be found in real-world conditions. However, Bt 176 is an older strain and has been phased out (USDA, Mendelsohn). There are also rare circumstances in which Bt pollen from corn can be harmful, such as when the pollination of a crop is temporally and spatially coincident with the larval-growth stage of monarch butterflies (Borrell). Bigger risks to monarch butterflies include habitat destruction and the loss of milkweed, conventional insecticides, and predation (USDA). Other members of Lepidoptera, the order of insects primarily affected by Bt toxin, do not necessarily inhabit areas in which plants genetically engineered to express Bt toxin are grown (Mendelsohn).

There is also controversy as to whether Bt toxin poses a risk to other non-target insects. A 2007 meta-analysis in Science provides evidence that non-target insects and other invertebrates are more likely to be found in fields of Bt cotton or Bt corn than in nontransgenic cotton or corn fields in which insecticides are used. However, when compared with fields in which insecticides are not used, some non-target species may be less abundant in Bt fields. This seems to indicate that Bt crops are less harmful to non-target organisms than crops in which insecticides are sprayed. However, a certain Bt corn crop (the aforementioned phased-out Bt 176, containing the Cry1Ab gene) does seem to have a harmful effect on non-target organisms, in the order Lepidoptera, that are closely related to target organisms (Marvier). However, fields planted with Bt 176 corn were safer habitats for monarch butterfly larvae than were non-GMO fields sprayed with certain pesticides (UC Biotech).

Spraying fields with insecticides has a more adverse impact on insect populations than does the presence of genetically modified Bt crops. Narrow-spectrum pesticides will protect crops from damage while harming fewer insects — and crops modified to express the Bt gene reduce the need for broad-spectrum pesticides by producing their own highly selective pesticide that can only interact with the cellular structures of a small variety of insects.

Insect species certainly might evolve resistance to Bt crops – however, the same can be said in relation to plants that have been conventionally bred for pest resistance. Since the beginning of agriculture, farmers and plant breeders have struggled to stay one step ahead of the opposing forces of nature. Is genetic engineering one of many weapons in our arsenal?

References

Borrell, B. (2011). Food Fight. Scientific American, 304, 80-83.

Broderick, N.A., Raffa, K.F., Handelsman, J. (2006). Midgut bacteria required for Bacillus thuringiensis insecticidal activity. Proceedings of the National Academy of Sciences of the United States of America, 103(41), 15196-15199. Obtained from http://www.pnas.org/content/103/41/15196.long

Marvier, M., McCreedy, C., Regetz, J., Kareiva, P. (2007). A Meta-Analysis of Effects of Bt Cotton and Maize on Nontarget Invertebrates. Science, 316, 1475-1477.

Mendelsohn, M., et al. (2003). Are Bt crops safe? Nature Biotechnology, vol. 21, No. 9, 1003-1009. Obtained from http://www.cof.orst.edu/cof/teach/agbio2010/Readings%202010/are_bt_crops_safe_nat_bio_2003.pdf

Ronald, P.G., Adamchak, R.W. (2008). Tomorrow’s Table: Organic Farming, Genetics, and the Future of Food. Oxford University Press.

UC Biotech (no date given). Can Genetically Engineered Crops Cause Adverse Effects on Nontarget Organisms? Obtained from http://ucbiotech.org/answer.php?question=40

United States Department of Agriculture, Agricultural Research Service (2004). Q&A: Bt Corn and Monarch Butterflies. Obtained from http://www.ars.usda.gov/is/br/btcorn/

One thought on “Bt Toxin