Bacillus thuringiensis (Bt) is a rod-shaped bacterium found in soils used as a biological pesticide in both conventional and organic agriculture. Bt is highly targeted, providing an effective way to manage specific insect pests while minimizing the impact on the environment and non-target species. Its insecticidal properties are linked to the production of specialized protein crystals.
Understanding Bt Strains and Specificity
The term Bacillus thuringiensis refers to many strains, each producing unique insecticidal proteins that target distinct insect groups. These protein crystals, known as Cry proteins or delta-endotoxins, must be ingested by the susceptible pest to be effective. This requirement for specific ingestion and activation gives Bt its remarkable specificity.
For instance, Bt kurstaki (Btk) targets caterpillars (Lepidoptera), the larval stage of moths and butterflies. Bt israelensis (Bti) is effective against fly larvae (Diptera), such as mosquitoes and black flies. Bt tenebrionis targets certain beetle larvae (Coleoptera). Bt aizawai also targets caterpillars, often used for pests like the wax moth and those affecting stored grains.
Specific Insect Pests Targeted by Bt
The most common strain, Bt kurstaki (Btk), controls a wide range of leaf-eating caterpillars. These include common garden and crop pests such as the Cabbage Looper, Tomato Hornworm, and Diamondback Moth larvae. Btk is also employed for forest pests like Gypsy Moth larvae and Tent Caterpillars.
Another widely used strain, Bt israelensis (Bti), specializes in controlling aquatic fly larvae. This strain is highly effective against Mosquito Larvae, Black Fly Larvae, and Fungus Gnats, which are common greenhouse and indoor plant pests. Products containing Bti are applied to standing water where these pests breed to control them before they develop into flying adults.
For beetle control, the Bt tenebrionis strain is used to manage pests like the Colorado Potato Beetle larvae and the Elm Leaf Beetle.
How Bt Kills Insects
The insecticidal action of Bt begins after the pest consumes the bacterial spores and their associated crystal proteins. These protein crystals are initially inactive protoxins. For the toxin to become active, the insect’s digestive system must meet two conditions: the toxin must dissolve, and it must be processed by gut enzymes.
The first condition is met by the highly alkaline environment of the insect’s midgut, which typically has a pH ranging from 9.0 to 10.5. This high pH dissolves the insoluble protein crystals, releasing the protoxin. The protoxin is then cleaved by protein-digesting enzymes (proteases) found in the insect gut, converting it into the smaller, active Cry toxin.
The activated toxin then binds to specific receptor sites on the epithelial cells lining the insect’s midgut. This binding is species-specific, explaining why a Btk toxin is harmless to a beetle larva. The toxin inserts itself into the cell membrane, forming pores that disrupt the cell’s osmotic balance. This causes gut paralysis, and the insect immediately stops feeding, eventually dying from starvation or systemic infection.
Non-Target Organisms: What Bt Does Not Kill
Bt is defined by its safety profile for organisms outside the specific target range, reinforcing its role as a selective biopesticide. The specific mechanism of action means that most non-target organisms are unaffected. Beneficial insects, such as honeybees, ladybugs, parasitic wasps, and predatory mites, are generally not harmed by Bt applications.
This safety is due to the lack of necessary physiological conditions for toxin activation. Mammals, birds, fish, and other non-target organisms, including humans, have highly acidic stomach environments, which prevents the Cry protein crystals from dissolving and activating. Furthermore, these organisms lack the specific gut receptors required for the activated toxins to bind and cause damage. The toxin is simply passed through the digestive system without effect.