Bacillus thuringiensis (Bt) is a naturally occurring soil bacterium that gardeners use to target pesky insects — mainly caterpillars — without harming beneficial species. However, successful application requires careful timing and product selection, according to University of Missouri Extension horticulturist Tamra Reall.
Different Bt products target different insects. For example, Bt kurstaki treats vegetable garden caterpillars, while Bt israelensis targets mosquito and fungus gnat larvae.
"It is important to choose the correct product for the right pest," Reall said. "It’s not a ‘one spray fixes everything’ kind of product."
To be effective, the insect must ingest the Bt. Applications work best on young, actively feeding larvae and will likely fail on mature caterpillars. Gardeners should accurately identify the pest before selecting a Bt strain and should never apply it randomly as a broad-spectrum pesticide.
Environment also plays a role. Ultraviolet light from the sun degrades the bacteria quickly, and heavy rain washes it away. Gardeners must ensure thorough coverage on all plant surfaces — especially leaf undersides — and reapply as needed after heavy rain or extended sunlight.
