Controlling cabbage worms

LAST WEEK, VELVETY

green cabbage worms began to chew my broccoli plants’ leaves to shreds. I stopped the worms, and did so without resorting to any chemical spray. I gave the plants a spray of Bt. Bt is a commonly used abbreviation for Bacillus thuringiensis, a bacterium that causes disease in certain insects. The cabbageworms became sick, stopped eating, and died. Since Bt is toxic only to certain insects, it does not pose a danger to other creatures, such as birds, cats, dogs, humans, and even beneficial insects.

The insecticidal properties raelensis, 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 bee hives. gets sufficiently out of hand to warrant treatment.

All these strains of Bt are available to gardeners. The bacteria are packaged in a dormant condition either as a dry powder, a liquid suspension, or, in the case of Bt var. israelensis, a slow release ring that is floated on water to kill mosquito larvae.

Bt goes under a number of brand names which don’t give a hint of the pesticide’s ingredients, so read the label to make sure of what you are buying. (And do not confuse Bacillus thuringiensis with another biological insecticide Bacillus poppilliae, which is used against Japanese beetles.)

Bt is a living organism, so store it in such a way as to prolong its viability. Kept cool and dry, the bacteria will remain viable in their container for two or three years. Bt works quickly enough so that it has been very obvious to me when I have sprayed spent bacteria on my cabbages and broccolis in an attempt to control cabbageworms.

A friend recently suggested another good reason for careful use of at least the original strain of Bt. Since this strain is toxic to caterpillars, overuse could substantially decrease the caterpillar population and, hence, the num- bers of moths and butterflies. My distaste for celery worm, which has a voracious appetite for carrot, celery, and parsley leaves, is tempered by the beauty of its adult form. Surely the elegant and graceful black swallowtail butterfly fluttering about the garden adds as much beauty as a marigold or rose. of Bt have been known since early in the last 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, which are larvae of butterflies and moths.

Any gardening questions? Email them to me at garden@ leereich.com and I’ll try answering them directly or in this column. Come visit my garden at www.leereich.com/blog.

Bt was first used purposefully against insects to control the European corn borer in Europe in the 1930s. But interest in Bt waned in the late 1940s, when nerve gases used during World War II led to the development of new types of chemical pesticides. In the 1960s, agricultural scientists finally began to take a second look at Bt.

Subsequently, new strains of Bt were isolated which are toxic to insects besides caterpillars. The strain Bt var. is-

Is there some trade-off that must be made when using this apparently benign pesticide?

Some entomologists contend that insect pests eventually will develop resistance to Bt, just as they often do to chemical pesticides. Resistance has been demonstrated in closed systems, such as the laboratory or grain storage bin (the Indian meal moth is the culprit in this case).

Other scientists argue that since Bt is a living organism, it can change along with, and remain toxic to, target insects.

Theoretical arguments aside, the way to lessen the chance of resistance developing in the garden is to use Bt with discretion, applying it at the recommended rate, only to afflicted plants, and only when a pest problem