Bacillus thuringiensis

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Bacillus thuringiensis (Bt) is "a naturally occurring bacterial disease of insects." [1] Because Bt produces a toxin that kills insects, it is used as an insecticide in agriculture. Bt is an especially important insecticide in organic agriculture.

A major category of genetically modified organisms (GMOs) are Bt crops. These are crops that are genetically engineered to produce the same toxin as Bt in every cell of the plant, with the goal of protecting the crop from pests. Due to widespread use of Bt crops, Bt resistance has now been observed in some pests. This is a major threat to organic agriculture, as it would render one of organic agriculture's most powerful pest control tools useless if Bt-resistance to pests evolved on a large scale.

Insecticidal Use

Discovery and Commercialization

"Bt is a naturally occurring bacterium common in soils throughout the world."[1] The insecticidal properties of Bt were first discovered in 1911. In the 1950s, Bt became commercially available for use as an insecticide.

Properties

"Unlike typical nerve-poison insecticides, Bt acts by producing proteins (delta-endotoxin, the "toxic crystal") that reacts with the cells of the gut lining of susceptible insects. These Bt proteins paralyze the digestive system, and the infected insect stops feeding within hours. Bt-affected insects generally die from starvation, which can take several days.
"Occasionally, the bacteria enter the insect's blood and reproduce within the insect. However, in most insects it is the reaction of the protein crystal that is lethal to the insect. Even dead bacteria containing the proteins are effective insecticides."[1]

Impact on Non-Target Species

Because Bt is lethal only when ingested and because each strain only works on certain types of insects, Bt is generally safe to non-target (i.e. insect species that are not pests) and beneficial species (such as pollinators).[1]

Persistence in the Environment

When Bt is applied, it does not persist very long in the environment because it susceptible to degradation by sunlight. After application, Bt will remain on foliage for less then a week, and sometimes only for 24 hours.[1]

Strains of Bt

There are several different strains of Bt, each with different properties:

  • Aizawai strain (Bta): This strain kills wax moth larvae in honeycombs.[2]
  • Israelensis strain (Bti): This strain kills fly larvae. It is used against larvae of mosquitoes, black flies and fungus gnats.[1]
  • Kurstaki strain (Btk): This strain will kill only leaf- and needle-feeding caterpillars.[1] It is the most common strain.[1]
  • San Diego strain (BTSD): This strain works against leaf beetles, such as the Colorado potato beetle and the elm leaf beetle.[1]
  • Tenebrionis strain (Btt): This strain kills leaf beetles, such as the Colorado potato beetle and the elm leaf beetle.[1]

Insects Controlled by Bt

Bt and GMOs

Bt is used in genetically modified organisms (GMOs) by inserting the genes to produce the Bt toxic crystals into other species. In some cases, the genes are inserted into other bacteria that can live longer on leaf surfaces than Bt itself.[1] However, the more common use of Bt in GMOs is the insertion of Bt genes into the crops themselves, so that every cell of the plant produces the toxic crystals. For more information, see the article on Bt Crops.

Insect Resistance

Prior to the use of Bt in GMOs, issues with Bt resistance evolving in pests were unlikely due to the brief and infrequent exposures to Bt pests experienced. Following an application of Bt, the majority of pests would die, and then the Bt would degrade within a week. A farmer might apply Bt one more time, to kill any insects that hatched after the first application of Bt. Still, insects were not exposed to Bt for a very long period of time, giving them little opportunity to evolve resistance.

However, with the advent and widespread use of Bt toxin by GMOs, pests have more opportunities to evolve resistance to Bt. Bt resistance in corn pests was first observed in 2011. For more information, see the article on Bt-Resistant Insects.

Safety of Bt Crops

A second concern about Bt crops is their safety to humans, animals, and the environment. While Bt is marketed as a safe insecticide, when it is applied instead of produced by GE crops, it breaks down in the environment long before humans consume the crop.

A 2010 study published in the journal Reproductive Toxicology found that "3-MPPA and Cry1Ab toxin are clearly detectable and appear to cross the placenta to the fetus."[3] 3-MPPA, or 3-methylphosphinicopropionic acid, is a metabolite of the pesticide gluphosinate ammonium, which is used in some genetically engineered crops. The Cry1Ab is the insecticidal protein produced by Bt crops. The study found that the "Cry1Ab toxin was detected in 93% and 80% of maternal and fetal blood samples, respectively and in 69% of tested blood samples from nonpregnant women." The study authors speculated this was due to consumption of contaminated meat, i.e meat from animals fed Bt corn that had retained the Cry1Ab protein in their flesh.

Articles and resources

Related SourceWatch articles

References

  1. 1.00 1.01 1.02 1.03 1.04 1.05 1.06 1.07 1.08 1.09 1.10 1.11 1.12 1.13 W.S. Cranshaw, "Bacillus thuringiensis," Colorado State University Extension, December 2008, Accessed September 8, 2011.
  2. Bt Crop Spraying, Accessed September 8, 2011.
  3. Aziz Aris and Samuel Leblanc, "[somloquesembrem.files.wordpress.com/2010/07/arisleblanc2011.pdf Maternal and fetal exposure to pesticides associated to genetically modified foods in Eastern Townships of Quebec, Canada]," Reproductive Toxicology, February 13, 2011.

External resources

External articles