Firsthand report on a gypsy moth control chemical that works

The high winds of spring carried a host of tiny caterpillars from who knows how many miles away to my garden last year.

The interlopers were larval gypsy moths, descendants of some French ''immigrants'' who came ashore a hundred years ago but who now are as thoroughly Americanized as any group of insects you will find anywhere.

The little critters were a half inch long or longer when I became aware of their all-encompassing presence on the willow tree and, to my extreme dismay, on a neighboring patch of raspberries as well.

The willow, vigorous tree that it is, could take care of itself, but the raspberries would need help if we were to reap the anticipated berry crop. That they produced abundantly is due to a single application of a naturally occurring insecticide known as Bacillus thuringiensis - or Bt for short.

It is the same stuff that will keep your cabbages free of the looper and will even take care of black flies in the larval stage. The beauty of Bt is that it affects only caterpillars, but has no adverse effect on the birds, bees (and other pollinators as well), lady bugs, fish, and a host of other beneficial species, including the valuable earthworm.

In contrast, chemical compounds (Sevin is the most popular and the most effective), which do an effective job in controlling gypsy-moth outbreaks, are also devastating to other insects. If bees are caught in drifting Sevin spray, they will fall from the sky as do raindrops from a thunder cloud.

Parasitic wasps and certain flies which attack the gypsy-moth larvae and the cabbage looper, among others, are equally affected. In short, we eliminate our allies along with our enemies. The net effect of using chemicals is that we become more dependent on them year after year. We become, in effect, pesticide junkies.

Bt is not dramatic in its effects. I sprayed my raspberries and the caterpillars ate on as if nothing had happened. But two hours later all eating had stopped and the caterpillars remained perfectly motionless. A week later only the dried remains of some of the caterpillars still clung to the leaves. The bees came and went in droves and the berry harvest was abundant.

Apparently, I was fortunate in my once-only spray application. For best results it is recommended that the foliage be sprayed when the caterpillars are about one-half-inch long and again 10 days later.

The Massachusetts town of Lexington, about 15 miles from Boston, was one that went the biological-pesticide route last year. Tree warden Paul Mazerall was skeptical when he braced for the anticipated gypsy-moth invasion. Would Bt truly work, he wondered.

''Our town-spraying program covered 75 to 80 linear miles of tree-covered residential area,'' he reports. ''We used Bt and sprayed twice, 10 days apart, during the first three stages of the caterpillars' growth, as we were supposed to.''

The first three growth stages of a caterpillar's life is when it is most vulnerable to biological insecticide, according to Mazerall. In much of the New England area that occurs ''during a two- to three-week period in late May and early June,'' he says. While many communities were badly defoliated, Lexington's trees came through unharmed. So did the the bees.

Bt is available under several brand names from garden centers. Dipel, Thuricide, and Biotrol are some of the names. Communities can buy larger quantities directly from the manufacturers.

A major research center for gypsy-moth monitoring and control is the US Department of Agriculture station at Otis Air Force Base on Cape Cod. You can reach the station at (617) 563-9303. Or try the Audubon Society in your state.