Bacillus_thuringiensis (variety kurstaki) kills more than 180 different species of caterpillars of the moth and butterfly group, the Lepidoptera. This is the general-purpose Bt that is most widely used by homeowners on vegetables, flowers, shrubs, and trees. Some caterpillars may not be affected, because their feeding habits protect them from sprays. These include cutworms in soil, borers in fruit tree limbs, and codling moth larvae tunneling in apples.
Oddly enough, some pest insects that look like caterpillars when they are young—such as currant worms and sawflies—are not harmed by Bt because they not related to moths and butterflies. The bacterium does not release its toxin into the insect unless it encounters a high pH (9.0 or greater) combined with the action of certain digestive enzymes in the digestive tract. Only certain unlucky types of insects meet these criteria.
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When to Use Bt
Authorities agree that best control occurs when you spray or dust newly hatched caterpillars. Therefore, it helps to be aware when unwelcome pest insects arrive in your yard each year. Typically, insects have a predictable schedule, so their larvae will emerge on almost the same day every year, plus or minus a day or two. Bt is most effective in the spring and again in the late summer, when caterpillar feeding activity is greatest.
Spray or dust early in the day when the caterpillars are hungriest. If it rains during the day, plan to replace the Bt on plant foliage when it stops or the very next day.
Caterpillars Controlled by Bt
Some of the common garden pests controlled by Bacillus thuringiensis var. kurstaki include the following:
| Caterpillars Controlled By Bt | |
|---|---|
| Armyworms | Hornworms |
| Bagworms | Inchworms |
| Cabbage worms | Leafrollers |
| Cankerworms | Leaftiers |
| Codling Moths | Loopers |
| Corn Earworms | Parsleyworms |
| Fall Webworms | Sod Webworms |
| Gypsy Moths | Tent Caterpillars |
