Within the species Bacillus thuringiensis (Bt), there are several types, or subspecies, with specific activity against various insect pests. Going one step further, thousands of strains – genetic lines, or subtypes, exist within the various Bt subspecies.
Each Bt strain displays an individual insecticidal protein toxin mix targeting specific groups of pests. It’s important to understand that Bt quality is determined by, among other variables, the particular strain(s) selected by the original provider
There are four subspecies of Bt that are staples in agriculture and forestry insect management programs:
• Bt kurstaki (Btk): A broad-spectrum subspecies used for worm/caterpillar (Lepidoptera) control for use on vegetables, fruits, nuts, vines, corn, cotton, soybeans, turf, and other crops. Btk is the most widely used biorational product in the world, and controls over 55 caterpillar pests including: leaf-rollers, corn borers, loopers, cutworms, hornworms, and bollworms.
• Bt aizawai (Bta): Another subspecies of Bt used for caterpillar control with specific activity on armyworms (Spodoptera) and Diamondback Moth (Plutella xylostella). Bta is frequently cited as the most effective resistance management tool available as it includes the widest range of Bt toxins.
• Bt tenebrionis (Btt): A subspecies of Bt strain used to control beetle (Coleoptera) larvae, most commonly Colorado Potato Beetle (Lepetinotarsa decemlineata), and Elm Leaf Beetle (Pyrrhalta luteola), among others. Btt is registered for use on potatoes, tomatoes, eggplant, shade trees, and ornamentals.
• Bt israelensis (Bti): A subspecies of Bt strain used to control mosquito, black fly and fungus gnat. This type of Bt also plays an important role in the realm of public health.
Understanding that each manufacturer must obtain and/or maintain its own strains of bacterial sources, from which products are derived, is a key to quality assurance. Growers need to know that the bacterial strains their manufacturer uses will grow and reproduce robustly, will produce the proper proportions and concentrations of proteins, and that the final formulated product will be strong enough, and has the proper tox profile, for the job at hand.