Bt Kills Garden Pests, Won’t Harm Other Creatures

The velvety green cabbageworms that chew leaves of broccoli and cabbage to shreds can be stopped by making these insect pests sick. And the way you do this is by spraying the plants with Bt - short for Bacillus thuringiensis, a bacterium that causes disease in certain insects. The sick cabbageworms stop eating, then die.

Since Bt is toxic only to certain insects, it does not pose a danger to other creatures, such as birds, cats, dogs, humans, even beneficial insects.

The insecticidal properties of Bt have been known since early in this century, when the bacterium was discovered as a silkworm pest by Japanese researchers. The originally discovered strain of this bacteria is toxic only to caterpillars, so it is also effective against tomato hornworm, gypsy moth and corn earworm. More recently, additional strains have been discovered that are toxic to insects besides caterpillars. The strain Bt var. israelensis, for example, is toxic to larvae of black flies and mosquitoes. Another strain, Bt var. san diego, is toxic to Colorado potato beetles, and Bt var. aizawai is deadly to the wax moth of beehives.

You can purchase any of these strains of Bt. The bacteria are packaged in a dormant condition either as a dry powder, as a liquid suspension, or, in the case of the Bt var. israelensis, as a slow release ring that is floated on water to kill mosquito larvae. Since Bt goes under a number of brand names that do not give a hint of the pesticide’s ingredients, read the label to make sure of what you are buying. (And do not confuse Bacillus thuringiensis with another bacterial insecticide Bacillus poppilliae, which is used against Japanese beetles). Bt is a living organism, so store the package where it is cool and dry; then the bacteria should remain viable for two or three years. If you cannot find Bt locally, it is available through the mail from suppliers such as Gardens Alive! (812-537-8650) and Necessary Trading Co. (703-864-5103).

Is there some trade-off that must be made when using this apparently benign pesticide? Insects can develop resistance to Bt, just as they do to to chemical pesticides, lessening its effectiveness. Limit the chances of this happening by using Bt with discretion and applying it at the recommended rate. Use it only on plants already attacked by a susceptible insect pest, and only when the problem gets sufficiently out of hand to warrant treatment.

There is another reason to be careful in using Bt, at least the original strain.

Because this strain is toxic to caterpillars, overuse could substantially decrease the caterpillar population and, hence, the numbers of moths and butterflies. Temper your dislike for the celeryworm, a caterpillar that has a voracious appetite for carrot, celery, and parsley leaves, with the beauty of its adult form.

Surely the elegant and graceful black swallowtail butterfly - the adult form of the celery worm - adds as much beauty to the garden as does a marigold or rose.