Bt

Bacillus thuringiensis is a rod-shaped, gram-positive bacterium that produces crystal proteins toxic to the larvae of lepidopterans (butterflies and moths) and some coleopterans (beetles). The bacterium lives in soil worldwide and has been used as a commercial pesticide since the 1950s. When an insect larva ingests Bt spores and toxin crystals, the alkaline pH of its gut dissolves the crystals, releasing proteins that bind to midgut cell receptors and create pores in the gut wall, causing paralysis and death within hours.

Commercial Bt products contain either whole spores and crystals (as a powder or liquid concentrate) or recombinant Bt toxins produced in fermentation vessels. Different Bt strains produce different crystal protein types, designated Cry1Aa, Cry1Ab, Cry2Aa, and others. Cry1Ab and Cry1Ac are effective against most lepidopteran pests including European corn borer, diamondback moth, and cabbage looper. Cry3Aa targets Colorado potato beetle. Cry1F and Cry1Ca target armyworms. The choice of strain determines which pests will be controlled on a given crop.

Application timing is critical because Bt kills only feeding larvae, not eggs or pupae, and toxins break down in sunlight within 3 to 14 days depending on UV intensity. Spraying must target the window when larvae are actively feeding. On field crops, Bt is often tank-mixed with adjuvants to improve leaf coverage and adherence. Conventional formulations require 3 to 5 applications per season in high-pressure situations. Genetically modified Bt crops (corn, cotton, soybeans) produce Cry proteins in plant tissue throughout the growing season, reducing or eliminating the need for foliar sprays.

Resistance and Limitations

Insect populations exposed to sustained Bt pressure can develop resistance through selection for genetic variants that lack susceptible midgut receptors or produce detoxifying enzymes. Diamondback moth, Indian meal moth, and Colorado potato beetle have all shown documented field resistance. Best management practice requires rotating Bt products with different Cry toxins, using non-Bt controls in non-cropped areas to maintain susceptible alleles in the broader population, and monitoring field performance.

Bt has no known toxicity to mammals, birds, or beneficial insects at agricultural use rates because their digestive systems lack the specific receptor proteins and neutral pH gut environment required for toxin activation. This profile made Bt the foundation of organic pest management systems and reduced-input agriculture starting in the 1970s. Today it is one of the most widely used biopesticides globally, deployed on food crops, ornamentals, and turf.