SUSSEX COUNTY-Much as Thin Lizzy once warned about the boys being back in town, so, too, are gypsy moths, those pernicious insects whose larvae eat tree foliage like unwelcome mice eat cheese. The New Jersey departments of both environmental protection and agriculture say that a hot and relatively dry spring is to blame. But the real threat to area forests won't reach full bloom until next year, when this year's larvae hatch from their nests and reproduce. Forestry and agriculture experts already are making plans for a counteroffensive, and local communities next year will have to decide whether they want to let the caterpillars munch on the forest canopy or pay for aerial spraying to keep them in check. "We're anticipating a larger spraying program next year than we've had in the past few years," said Joe Zoltowski, a bureau chief for the state agricultural department's division of plant industry. "Barring no treatment at all, two to three years of serious defoliation would usually cause trees to die." Gypsy moths are normally held in check by a fungus, entomophaga maimaiga or, in scientific shorthand, E. maimaiga. The fungus infects caterpillars, which pass it to other caterpillars through contact. Growing inside the animals, the fungus eventually kills them before they can turn into breeding adults. But the fungus needs moist weather to be active, and this year's spring was too dry. Without the fungus, the caterpillars are thriving and many trees n particularly oak n in affected areas have had their canopies reduced to lace-like remnants. Most of the caterpillars have already begun to pupate, gluing themselves with silk to the bottom of leaves. By the middle of August, the pupas will have turned into moths n brown-colored males that fly and white females that are flightless n mated and laid their egg masses, which will hatch out new hordes of larvae next year. "It's because it was so dry (this spring)," Zoltowski said. "The last two springs, it rained almost every day. This year, it's generally been dry. There's a lot that's not known about the fungus. But being a fungus, it needs moisture, we all know that." The caterpillars hatch by late April and begin eating their way through entire forests unless something stops them. And if the fungus isn't revived, the recourse is aerial spraying, Zoltowski said. "What we do is first (conduct) our aerial defoliation survey, and then we use that information to send out to the township officials (involved)," Zoltowski explained. "And we ask them if they want us to do a ground survey in the fall in order to get an accurate egg mass count. "What we then do is propose areas for treatment and ask them if they want to participate or not," he added. Following that, those interested usually contract with a vendor/sprayer, paying the full amount up frontand if there's federal funding available, "then we get money and we reimburse half the spray costs to the township," the bureau chief said. Zoltowski conceded that the spring of 2006 "could" produce a bigger strain on any available federal reimbursement. The program is strictly voluntary. The spraying agent used to combat gypsy moths is bacillus thuringiensis, a "minimal-risk pesticide" that disrupts the moths' stomachs, thus causing them to stop feeding, Zoltowski said. Also known simply as Bt, the pesticide is actually a bacteria that targets caterpillars and makes them starve to death. One year of defoliation doesn't kill trees, he went on."What happens with the trees is because they're defoliated, they've used up their resources in the root systems, and they're then using their reserves to bring in another patch of leaves," explained Zoltowski. "The reason the caterpillars are bad is that they weaken trees one year, and weaken it again the next year." Repeated defoliation puts the tree under excessive stress within a few years, leading to its premature death. Zoltowski said that aerial surveys place Sussex County third statewide in gypsy moth damage with 4,144 defoliated acres, trailing only Ocean and Burlington Counties. Montague has been hit the hardest. Vernon, Wantage, Wallpack, Stillwater, Hampton Township and Sandyston have also suffered defoliation damage, Zoltowski said. Milford, Pa. is also affected, he added. Last year, spraying cost about $28 per acre, with federal subsidies cutting the final cost to communities that chose to spray to $14 per acre.