Bacillus thuringiensis is a Gram-positive bacterium found widely in nature, belonging to the genus Bacillus. Its most famous property is its ability to produce protein crystals that are highly toxic to the larvae of certain insects. This property has made Bt the most widely used and successful microbial insecticide worldwide, and the core gene source for the development of insect-resistant biotechnology crops.
Different strains of Bacillus thuringiensis differ primarily in the types of insecticidal crystal proteins (ICPs, commonly referred to as Cry and Cyt proteins) they produce. These differences in proteins determine the specificity of each Bt strain in targeting different pests. Simply put, differences in Bt strains equate to differences in the types of pests they can kill.
1. Core Difference: Diversity of Insecticidal Proteins (Crystal Proteins)
When forming spores, B. thuringiensis produces one or more insecticidal protein crystals. Scientists have divided these proteins into several families and classes based on their amino acid sequence structure and insecticidal specificity.
- Cry proteins (Crystal proteins): This is the most important and diverse class of insecticidal proteins. They exhibit highly specific toxicity against specific insects (e.g., Lepidoptera, Coleoptera, and Diptera).
- Cyt proteins (cytolysin proteins): These proteins are toxic to Diptera larvae (e.g., mosquitoes) and can lyse the cells of a variety of insects and non-insects, in some cases preventing insects from developing resistance to Cry proteins.
- Vip proteins (vegetative insecticidal proteins): These proteins are produced during the vegetative growth phase of bacteria, rather than during spore formation. They also possess insecticidal activity and are a common gene source for new-generation genetically modified crops.
Key Point: A single Bt strain may contain genes encoding one or more insecticidal proteins. Therefore, strains containing different combinations of proteins have different insecticidal spectra (the range of pests they can kill).
2.Main strain types and their targeted pests
| Strain serotype/subspecies | Main insecticidal protein | Main targeted pest types | Specific pest examples | Application form |
| Bt var. kurstaki | Cry1Aa, Cry1Ab, Cry1Ac, Cry2A | Lepidoptera larvae | Cabbage looper, diamondback moth, cotton bollworm, corn borer, pine caterpillar | The most commonly used biological insecticide, used for vegetables, fruit trees, forestry, etc. |
| Bt var. israelensis | Cry4A, Cry4B, Cry10A, Cry11A, Cyt1A, Cyt2B | Diptera larvae | Mosquitoes, midges, black flies, and midges | Used to prevent mosquito breeding in water bodies, it is an important tool for public health |
| Bt var. aizawai | Cry1Aa, Cry1Ba, Cry1Ca, Cry1Da, Vip3A | Lepidoptera larvae (especially effective against diamondback moths) | Diamondback moth, beet armyworm, tobacco budworm | Often used to combat pests that have developed resistance to kurstaki |
| Bt var. tenebrionis | Cry3Aa, Cry3Bb, Cry7A, Cry8A | Coleoptera larvae | Potato beetle, yellow flea beetle | Used to control beetle pests. |
| Bt var. japonensis | Specific Cry proteins | Coleoptera larvae (underground pests) | Grubs (beetle larvae) | Used to control ground grubs on lawns and golf courses. |
| Bt var. sotto | Cry1Aa | Lepidoptera larvae | Silkworm | Note: This strain is highly toxic to silkworms and is primarily used for research. |
3. Deeper Reasons for Differences: Genes and Evolution
- Genetic Differences: Different Bt strains carry different insecticidal protein genes (cry, cyt, and vip genes) in their genomes. This is the most fundamental difference between strains. Through genetic engineering, specific genes can be introduced into crops to create insect-resistant transgenic crops (such as insect-resistant corn and cotton with Cry1Ab/Cry1Ac genes).
- Synergy: Many highly effective strains (such as Bt israelensis) are highly effective and resistant to resistance because the multiple proteins they produce (such as Cry and Cyt proteins) exhibit synergistic effects.
- Evolutionary Adaptation: Different Bt strains were isolated from different geographical environments and insect hosts. This natural selection led to the evolution of toxins specific to specific hosts.
4.Which Bacillus thuringiensis is most common?
Among Bacillus thuringiensis, the most widely used and common one is:Bacillus thuringiensis subsp. kurstaki (Bt kurstaki)
1.Importance of Target Pests: Bt kurstaki specifically targets the larvae of Lepidoptera pests (caterpillars of various moths and butterflies). These pests are among the most serious and prevalent in agricultural production, including:Pieris rapae (caterpillars of the white butterfly)，Plutella xylostella，Cotton bollworm, tobacco budworm，Cornal borer，Rice leaf roller，Various fruit sawflies and leaf rollers, etc.
2.Wide Applications: Due to the widespread nature of its target pests, Bt kurstaki is widely used in:Vegetable cultivation (such as cabbage, cauliflower, and tomatoes)，Field crops (such as cotton and corn)，Fruit crops (such as apples and pears)，Forestry (for pine caterpillars)
3.Market Dominance: Bt kurstaki products are the largest-volume and highest-selling biopesticide in the global market. The main active ingredient of most Bt powders and liquids you can buy on the market for controlling vegetable caterpillars is Bt kurstaki.
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