How does it work?
Bt proteins are produced by bacteria found in the soil, called Bacillus thuringiensis. The bacteria kill the insect using these proteins, and then colonize the insect’s remains. Each Bt protein, also known as a Crystal or “Cry” protein, can affect a narrow spectrum of hosts, which makes them well-suited to put into plants to fight insects that are crop pests, while not significantly affecting insects that do not eat the plants. These proteins are extensively tested before being genetically engineered into the crop.
In the bacteria, the proteins are not dissolved due to a certain section of the protein which allows it to condense into a crystal. For the protein to work properly in plants, this must be removed. The versions engineered into plants are truncated (shortened) to remove these regions and allow the Bt protein to dissolve in the cell.
Each Bt protein has a similar mechanism of action, but Cry1Aa provides a good window into how these proteins work. To be active, several copies of the Bt protein must be linked together in a specific way. Processing by the caterpillar’s digestive system is what allows this to happen. The end result of this processing is that several copies of this protein link up to form the active toxin. The toxin is then passed to a transporter protein, which the toxin uses to settle into the gut and pop the cells.
When enough of the Bt proteins pop the cells, holes form in the gut. These holes allow the contents to spill out, and bacteria in the gut enter and colonize the body. This eventually causes the death of the insects.
This process requires specific receptor proteins on the surface of the gut cells, and different Bt proteins are specific for receptors from different kinds of insects. This makes the Bt proteins very specific for certain kinds of insects, such as caterpillars or beetles, while not affecting other kinds of insects or other animals such as humans.
What crops are modified?
Corn was the first commercialized crop engineered to produce Bt, followed by Bt cotton which represent the two most widespread applications of this trait. Bt soy has also been approved in South America, while Bt Brinjal (eggplant) is currently approved in Bangladesh. Bt rice is under development. Bt-producing potatoes have been produced, but are not currently commercialized.
Which insects are targeted?
The insects that feed on these crops vary around the world, and Bt crops control different pests in each location. Western Corn Rootworm and Pink Bollworm are considered worldwide pests, but there are other, major pests that are included in this list. The list is not exhaustive.
- Corn:
European Corn Borer
Corn Earworm (also attacks cotton and soy; has different common names on these plants)
Fall Armyworm
Western Corn Rootworm - Soy:
Soybean Looper
Velvetbean Caterpillar - Cotton:
Pink Bollworm
Corn Earworm (Cotton bollworm, AKA Helicoperva zea) - Eggplant:
Brinjal Fruit and Shoot Borer - Potato: (not commercialized)
Colorado Potato Beetle
Benefits
Growers have seen benefits from Bt crops in the form of decreased loss of their crops due to insect feeding, but also increased quality of their crops as measured by mycotoxin reduction.
Bt corn has increased the yields for farmers by reducing insect pressure. This has reduced the need for broad spectrum pesticide sprays. Pests such as European Corn Borer and Western Corn Rootworm which were once key pests have declined in importance as Bt crops have been adopted. As a result, the use of broad-spectrum insecticide sprays has declined for Bt crop growers.
Many of the insects that feed on crops also introduce fungi which can produce mycotoxins that cause some rather serious health effects. Although many management strategies have been devised to combat these mycotoxins, Bt crops provide an additional measure of protection by preventing feeding insects from introducing them. The exact level of reduction depends on many factors, both environmental and biological, but the reduction is economically significant for many mycotoxins. Furthermore, Bt crops are compatible with biological control programs which target mycotoxin producing fungi.
Challenges
With any pest control measure, resistance can become an issue because management puts evolutionary pressure on a pest to evolve. Bt growers use a strategy called “high-dose/refuge” (H-D/R) which combines crops that produce a high enough dose of Bt to kill the insects with “refuge” plantings of non-Bt crops to maintain populations of non-resistant insects that will breed with any resistant insects that evolve. This strategy has been generally successful, but has been hampered by lack of refuge planting among growers. Current measures to solve the problem involve incorporating the refuge in the bag, although how effective this is compared to the standard refuge is unclear. In areas of the world where the refuge strategy has not been enforced, resistance to some Bt proteins has emerged, and newer varieties with multiple “stacked” Bt proteins increase effectiveness and slow resistance.
Although Bt has reduced the need for broad-spectrum insecticide sprays, this lack of pesticide also creates challenges for growers. Some minor pests were controlled by these sprays, and narrow-spectrum control methods allow these pests to rebound. In many regions, these pests include Plant Bugs (Hemiptera: Miridae), Stink bugs (Hemiptera: Pentatomidae), and Thrips. For many of these pests transgenic control options are also possible, although their biology will present additional challenges for the development of biotech crops.
The issues of resistance and replacement are not unique to Bt crops, and these have been documented in other types of control programs.
References
- Abbas, H. K., Zablotowicz, R. M., Weaver, M. A., Shier, W. T., Bruns, H. A., Bellaloui, N., … & Abel, C. A. (2013). Implications of Bt traits on mycotoxin contamination in maize: overview and recent experimental results in Southern United States. Journal of agricultural and food chemistry, 61(48), 11759-11770.
- Catarino, R., Ceddia, G., Areal, F. J., & Park, J. (2015). The impact of secondary pests on Bacillus thuringiensis (Bt) crops. Plant biotechnology journal, 13(5), 601-612.
- Cullen, E. M., Gray, M. E., Gassmann, A. J., & Hibbard, B. E. (2013). Resistance to Bt corn by western corn rootworm (Coleoptera: Chrysomelidae) in the US corn belt. Journal of Integrated Pest Management, 4(3), D1-D6.
- Lu, Y., Wu, K., Jiang, Y., Xia, B., Li, P., Feng, H., … & Guo, Y. (2010). Mirid bug outbreaks in multiple crops correlated with wide-scale adoption of Bt cotton in China. Science, 328(5982), 1151-1154.
- Marvier, M., McCreedy, C., Regetz, J., & Kareiva, P. (2007). A meta-analysis of effects of Bt cotton and maize on nontarget invertebrates. science, 316(5830), 1475-1477.
- Pardo-Lopez, L., Soberon, M., & Bravo, A. (2013). Bacillus thuringiensis insecticidal three-domain Cry toxins: mode of action, insect resistance and consequences for crop protection. FEMS microbiology reviews, 37(1), 3-22.
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- Citation: Layla Katiraee, Joe Ballenger, Karl Haro von Mogel, Anastasia Bodnar. Insect Resistance: Bt Traits. Version 1.0. Biology Fortified, Inc. Feb 14, 2017.
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