VORACIOUS gypsy moth caterpillars will chew through more trees in the United States this year than at any time since 1981.In that peak year, the insects defoliated 12.9 million acres of forest in the northeastern United States. This year, they are likely to strip an estimated 10 million acres - up from 7.3 million last year and 3 million in 1989. "The trend since 1984 has been up," says Daniel Twardus, an entomologist at the US Forest Service in Morgantown, W.Va. "It's been a year of increasing populations in the Northeast." Parts of New England were hit particularly hard this year. Big concentrations of gypsy moths attacked southern Maine, southern New Hampshire, and various parts of Massachusetts, says Dennis Souto, an entomologist at the Forest Service's Northeastern Area in Durham, N.H. Vermont, Connecticut, and Rhode Island fared much better. It was their second quiet year in a row. Pennsylvania, which was hit very hard last year, appears to have escaped the worst of the infestations this year. A number of factors conspired to increase the destruction, entomologists say. One was the weather. "This year was an unusual year because it was very dry in May and June," says Mr. Souto. "We didn't get reports of a lot of larvae mortality." The dryness is an important factor because scientists have discovered a fungus that attacks gypsy moths. Researchers surmise that the fungus thrives in wet weather, so it was probably less effective against the pests this year because of the dryness. But Souto is encouraged that the fungus is still prevalent, allaying fears that it might disappear completely after a dry spell. The second factor behind the increased defoliation is the continued spread of the gypsy moth. North Carolina saw enough gypsy moths that it started a project this year to suppress it, he adds. Next year, Georgia is planning its own eradication project. "It's moving rapidly south," Twardus says. "It's moving faster west than we anticipated it would." When one Forest Service official calculated the pest's historical rate of movement, he forecast that by 2015 it will have spread to a huge triangle stretching from South Carolina to Nova Scotia to eastern Iowa. Unwittingly, man has contributed to the gypsy moth's wanderings. Since 1981, the insects have moved well beyond the Northeast into Canada, the Midwest, and the Southeast. This year, they have spread into four northeast counties of Ohio - probably tripling last year's defoliation in that state, Mr. Twardus says. In recent years, the caterpillars have shown up in Utah, Oregon, and British Columbia. Scientists suspect that cross-country campers and recreational vehicles may have picked up gypsy moth eggs in the Northeast and spread them in the West unknowingly. This year, federal authorities are investigating a Pennsylvania nursery company in Quakertown, Pa. The authorities believe the nursery shipped trees infested with gypsy moths to at least 11 states. Meanwhile, logging companies in the Pacific Northwest are asking for permission to import Siberian logs to the US. One of the biggest threats of the plan, however, is that the logs could be infested with gypsy moths. When US scientists visited the Soviet port near Vladivostok that would handle the logs, they were shocked to find major gypsy-moth defoliation. "We expressed a fair amount of concern...," says Donald Goheen, a plant pathologist at the Forest Service's Northwest Region in Portland, Ore. The team found a number of troublesome insects. One of its major concerns was the Asiatic gypsy moth. The female is a very strong flier, while its US counterpart is essentially flightless. Should the Asiatic strain infest the US, it has the potential to spread the pest very rapidly, he says. The team's report will go to the US Agriculture secretary. Mr. Goheen ex pects some official action by the end of this year.