Bacillus thuringiensis, commonly called Bt, is a naturally occurring soil bacterium that produces proteins toxic to specific insects. It is one of the most widely used biological pesticides in the world, applied both as a spray on crops and engineered directly into genetically modified plants. Bt works by targeting narrow groups of insects while posing minimal risk to humans, other mammals, and most beneficial species.
How Bt Kills Insects
Bt bacteria produce crystal-shaped proteins during their spore-forming stage. When a susceptible insect eats plant material coated or infused with these proteins, the crystals dissolve in the insect’s alkaline gut. The dissolved proteins bind to specific receptors lining the gut wall and punch holes through it, causing the gut contents to leak into the body cavity. The insect stops feeding within hours and typically dies within one to three days from a combination of starvation and infection as gut bacteria invade the body.
This mechanism is highly specific. Mammals, birds, and fish have acidic stomachs that break down Bt proteins before they can do anything. Without the right gut chemistry and the matching receptor on the gut wall, the proteins are simply digested like any other protein. That specificity is the main reason Bt has been a cornerstone of pest management since the 1960s.
Which Strains Target Which Pests
Different subspecies of Bt produce different crystal proteins, and each set of proteins is effective against a narrow range of insects. The major strains used commercially break down like this:
- Bt kurstaki and Bt aizawai target caterpillars of moths and butterflies (the larval stage of Lepidoptera). These are the strains most commonly sprayed on vegetable gardens and field crops to control pests like cabbage loopers, tomato hornworms, and corn borers.
- Bt israelensis targets the immature (larval) stages of mosquitoes, black flies, and fungus gnats. It is widely used in standing water and wetland areas for mosquito control programs.
- Bt tenebrionis, Bt san diego, and Bt japonensis target beetle larvae (Coleoptera). Colorado potato beetle is the most well-known target in this group.
This strain specificity matters for practical use. A product containing Bt kurstaki will do nothing against mosquito larvae, and a Bt israelensis dunk dropped in a rain barrel won’t affect caterpillars on your tomato plants. You need to match the strain to the pest.
Bt as a Spray vs. Bt in GMO Crops
Bt is used in two fundamentally different ways. As a spray, it has been an organic farming staple for decades. The bacterium (or its purified protein) is mixed into a liquid and applied to leaves. Insects that feed on the treated foliage ingest the toxin. Because Bt proteins break down quickly in ultraviolet light, sprayed Bt typically remains active on leaf surfaces for only a few days before it degrades. Reapplication is necessary after rain or prolonged sun exposure.
In genetically modified crops, scientists have inserted the gene responsible for producing a Bt crystal protein directly into the plant’s DNA. Bt corn and Bt cotton are the two most common examples. Every cell of the plant produces the insecticidal protein continuously throughout the growing season. This eliminates the need for repeated spraying and provides protection even to parts of the plant that are hard to reach with a spray, like the inside of a corn ear where borers tunnel.
The trade-off is that constant exposure to the toxin creates stronger evolutionary pressure on pest populations to develop resistance, a problem that has required careful management strategies.
Managing Insect Resistance
When insects are exposed to Bt proteins generation after generation with no break, resistant individuals survive and pass that trait to their offspring. To slow this process, the EPA requires farmers planting Bt crops to maintain “refuge” areas of non-Bt plants nearby. The idea is straightforward: susceptible insects breeding in the refuge mate with any resistant insects emerging from the Bt field, diluting the resistance gene in the next generation.
The size of the required refuge depends on the crop, the region, and how many Bt proteins the plant produces. For Bt corn with