Bacillus thuringiensis (Bt) is a naturally occurring, spore-forming bacterium found in soils. This microbe is used extensively as a biological pesticide for managing pest populations. Bt formulations provide a highly targeted method of insect control in agriculture and home gardening. This specificity comes from the unique way the bacterium produces insecticidal proteins, making it harmless to most other forms of life.
How Bt Achieves Selective Killing
The selective toxicity of Bt requires the target insect to ingest the bacterium’s components. During its spore-forming stage, Bt produces crystalline proteins, known as delta-endotoxins or Cry toxins, which are stored within the cell. These Cry toxins are initially produced as inactive protoxins.
The protoxin only becomes active once consumed by a susceptible insect and enters the midgut. The highly alkaline digestive tracts of target insect larvae solubilize the ingested protein crystals. Once dissolved, specific enzymes, called proteases, cleave the inactive protoxin, transforming it into the active toxin.
This active toxin is then able to bind to specific protein receptors located on the surface of the insect’s midgut epithelial cells. The presence of these receptors is the primary factor determining which insects are susceptible to the toxin. Binding of the toxin causes pores to form in the gut lining, which severely disrupts the insect’s digestive function. The insect rapidly stops feeding, and the disruption of the gut wall ultimately leads to septicemia and death.
Bt Subspecies and Their Primary Insect Targets
The specificity of Bt products is related to the different subspecies used, as each produces a unique set of Cry toxins that bind to receptors in different insect groups. Manufacturers utilize these subspecies to target specific pest issues. The three most common subspecies target insects in the orders Lepidoptera, Diptera, and Coleoptera.
Bacillus thuringiensis var. kurstaki (Btk)
B. thuringiensis var. kurstaki (Btk) is the most commonly used subspecies, primarily controlling insects in the order Lepidoptera (moths and butterflies). Btk is effective against a wide range of common garden and forest pests. Specific targets include the cabbage worm, the tomato hornworm, and the gypsy moth caterpillar. It is also used to manage populations of the diamondback moth and the European corn borer. Btk must be consumed by the feeding larvae after application to foliage to be effective.
Bacillus thuringiensis var. israelensis (Bti)
B. thuringiensis var. israelensis (Bti) is specifically toxic to the larval stage of insects in the order Diptera. The toxins produced are highly effective against aquatic-dwelling larvae and are frequently used in public health and pest control programs. Bti is the primary biological agent used to control mosquito larvae in standing water, targeting species such as Aedes, Anopheles, and Culex. It is also effective against the larvae of black flies and fungus gnats, and is often applied directly to water sources.
Bacillus thuringiensis var. tenebrionis (Btt)
B. thuringiensis var. tenebrionis (Btt) targets certain insects in the order Coleoptera. This strain produces toxins specific to the gut receptors of certain beetle species. Btt is used less frequently than the other two strains but is important for controlling specific agricultural pests. The main pests targeted by Btt are the larvae of the Colorado potato beetle and the elm leaf beetle.
Organisms Bt Does Not Affect
The selectivity of Bt is one of its most valuable attributes. The mechanism of action, which relies on specific conditions and receptors, ensures that most other forms of life are unaffected. This makes Bt a safe option for use around humans, pets, and beneficial wildlife.
Vertebrates, including mammals and birds, are not susceptible to Bt toxins because their digestive systems are fundamentally different from those of target insects. The highly acidic conditions in the stomachs of vertebrates prevent the initial solubilization and activation of the protoxin crystals. Furthermore, vertebrates lack the specific glycoprotein receptors in their gut lining that the activated toxins must bind to in order to function. The Cry proteins are simply digested by humans and animals like any other dietary protein without causing any toxic effect.
This selectivity also extends to most beneficial insects. Adult pollinators, such as bees and butterflies, are not harmed because they do not ingest the product and they lack the necessary gut receptors. Natural predators, including ladybugs, lacewings, and parasitic wasps, are similarly unaffected by Bt. The result is a pest control method that targets specific destructive larvae while preserving the populations of insects that help to keep the ecosystem balanced.