Bacillus thuringiensis kurstaki

Bacillus thuringiensis subsp. kurstaki (Btk) is a group of bacteria used as biological control agents against lepidopterans (moths and butterflies). Btk, along with other B. thuringiensis products, is one of the most widely used biological pesticides due to its high specificity; it is effective against lepidopterans, and it has little to no effect on nontarget species. During sporulation, Btk produces a crystal protein that is lethal to lepidopteran larvae.[2] Once ingested by the insect, the dissolution of the crystal allows the protoxin to be released. The toxin is then activated by the insect gut juice, and it begins to break down the gut.[3]

Bacillus thuringiensis kurstaki
Scientific classification
Domain:
Bacteria
Phylum:
Bacillota
Class:
Bacilli
Order:
Bacillales
Family:
Bacillaceae
Genus:
Bacillus
Species:
B. thuringiensis
Subspecies:
B. t. kurstaki
Trinomial name
Bacillus thuringiensis kurstaki
Bulla et al. 1979[1]

Btk is available commercially and is commonly known as "Garden Dust" or "Caterpillar Killer", both of which are produced by Safer Brand. Other Btk-producing companies include Bonide and Monterey.

Effects on nontarget species

Btk is generally regarded as environmentally safe, as its toxicity is essentially limited to its target pest; humans, wildlife, and beneficial insects are regarded as unaffected by the pesticide. Even so, in 2012, a regulatory review of several strains of Bt by the European Food Safety Authority stated that although there was data supporting the claims of low toxicity, the data was insufficient to prove the claims conclusively.[4]

Humans

In studies of the effects of Bt on humans, most subjects were unaffected when exposed to Bt. Some individuals reacted with irritation of the eyes and skin. Other subjects with hay fever reported more significant effects, including throat irritation, upset stomach, and difficulty sleeping.

Trichogramma wasps

Trichogramma is a genus of parasitoid wasp whose females lay their eggs in the eggs of their hosts; after killing the host, an adult wasp emerges. For this reason, Trichogramma are often used as a biological control agent, and it can even be used in conjunction with pesticides like Btk. In November 2015, a study was conducted examining the effects of Btk on T. chilonis wasps. The study showed that high doses of strains of Btk containing δ-endotoxins were acutely toxic to the wasps, while Btk without these toxins had no adverse effects. It further showed that low doses of Btk, regardless of the strain, led to significantly greater longevity in the wasps.[5]

Soil

In a 1998 study, Btk was added to different types of soil in order to determine how the type of soil affected the persistence and concentration of Btk.[6] The results of the study showed that insecticidal activity started to decline after only a month in one soil, while in another toxicity was still high after six months. The authors of the study noted that even though Btk is considered non-toxic to nontarget species, the accumulation and persistence of the Btk toxins could eventually lead to environmental hazards or the selection of Btk-resistant lepidopterans.

References
  1. Bulla LAJ, Davidson LI, Kramer KJ, Jones BL. (1979). "Purification of the insecticidal toxin from the parasporal crystal of Bacillus thuringiensis subsp. kurstaki". Biochem Biophys Res Commun. 91 (3): 1123–1130. doi:10.1016/0006-291x(79)91997-1. PMID 526269.
  2. Adang, MJ; Staver, MJ; Rocheleau, TA; Leighton, J; Barker, RF; Thompson, DV (1985). "Characterized full-length and truncated plasmid clones of the crystal protein of Bacillus thuringiensis subsp. kurstaki HD-73 and their toxicity to Manduca sexta". Gene. 36 (3): 289–300. doi:10.1016/0378-1119(85)90184-2. PMID 3000881.
  3. Choma, Christin; Surewicz, Witold; Carey, Paul; Pozsgay, Marianne; Kaplan, Harvey (February 1990). "Secondary structure of the entomocidal toxin from Bacillus thuringiensis subsp. kurstaki HD-73". Journal of Protein Chemistry. 9 (1): 87–94. doi:10.1007/BF01024989. PMID 2340079. S2CID 24749422.
  4. European Food Safety Authority (2012). "Conclusion on the peer review of the pesticide risk assessment of the active substance Bacillus thuringiensis subsp. kurstaki (strains ABTS 351, PB 54, SA 11, SA 12, EG 2348)". EFSA Journal. 10 (2): 66 pp. doi:10.2903/j.efsa.2012.2540.
  6. Tapp, H; Stotzky, G (15 April 1998). "Persistence of the insecticidal toxin from Bacillus thuringiensis subsp. kurstaki in soil". Soil Biology and Biochemistry. 30 (4): 471–476. doi:10.1016/S0038-0717(97)00148-X.
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