Bacillus thuringiensis (Bt)- Various Types
There are three different strains of Bacillus thuringiensis (B.T. for short). Each of the strains controls specific pests.
Bt Kurstaki strain controls caterpillars, or the larvae of Lepidopterous
Bt must be ingested by the insect in order to achieve control. After ingestion, insects stop feeding within hours but may not actually die for several days.
Using a feeding attractant, such as the commercial product Entice, greatly improves the effectiveness of Bt. Or a mixture of one tablespoon to two tablespoons of brown sugar or molasses to a gallon of water will also work.
Bt degrades rapidly in sunlight and persists on foliage for only 24 hours to four days.
Bt is ineffective against pest eggs.
Bt is less effective in cool weather (below 45° F.) Insects must be actively feeding for best control.
Thorough and uniform coverage of Bt is essential. Use a high-volume of water. Spray until mixture begins to drip off leaves. Spray both upper and lower surfaces of the leaves as well as the interior of the plants.
Best results are achieved if Bt is applied with a spreader sticker. There are many spreaders, stickers, and spreader-stickers available commercially. Studies indicate greater effectiveness of Bt when mixed with an insecticidal soap. Use 1/3 to 1/2 the labeled rate of the insecticidal soap when mixing with Bt (usually one tablespoon to two tablespoons per gallon of water). The soap/Bt mixes are stable up to a week but best when applied as soon after they have been mixed.
Use Bt within one year to two years of purchase. Reduced effectiveness has been reported after two years of storage. Store in a cool, dry, place out of direct sunlight. Do not expose stored Bt to freezing temperatures or to extreme heat (particularly the liquid formulations).
Bt is most effective on young larvae (first instar through third instar). Large worms or those that have burrowed into fruit or have curled up in the leaves are harder to control.
Bt is ineffective against adult moths, butterflies, beetles, flies, aphids, and mites. It is specific to particular pests. READ THE LABEL carefully to make sure your target pest is susceptible to Bt before applying.
Bt is washed off by rain or irrigation. Wait at least 48 hours after an application before irrigating. Re-application may be necessary if 1/2 inch to 1 inch of rainfall occurs within 12 hours to 24 hours after application.
Prepared by Helen Atthowe, Missoula County Horticulturist; Missoula MT 59808 - Updated for 2006