Once again, an important war is being waged at Gettysburg - one that affects the North and the South.
But this time, unlike the 1863 battle that helped turn the tide of the Civil War, it isn't men in blue or gray uniforms who are on the attack. Instead, it is gypsy moth caterpillars - hundreds of billions of them.
Last year, a record 12.8 million acres of woodland from Maine to West Virginia were defoliated by the tiny pests. By one estimate, as much as $28 million in damage to timber may have been done in the process. And this year the wave of caterpillars is headed straight for the rich hardwood forests of the South. Estimated arrival time: perhaps five years.
James Nichols, chief of the Pennsylvania Division of Forest Pest Management, has a warning for states not yet infested: ''Get ready! It's going to become a national problem.''
Already foresters in Tennessee, the No. 1 hardwood-producing state, are worried that visitors to Knoxville's World's Fair from infested states may be carrying gypsy moth caterpillars on cars, trailers, and camping equipment.
Mr. Nichols and his colleagues from other infested states and from the forest-products industry meet periodically to discuss gypsy moth control methods and to review research. The next meeting is tentatively scheduled for December in nearby Harrisburg.
A number of defenses, man-made and natural, are combining to hinder the march of the gypsy moth, or at least minimize its damage:
* Chemical sprays have proved effective in controlling the spread of the caterpillars, but not eradicating them. Spraying can cost upwards of $10 an acre , and at least two applications are necessary because not all caterpillars hatch at the same time.
Moreover, chemical spraying is inexact, sometimes kills fish and useful insects, and is loudly opposed in some areas as a potential hazard to human health. Environmentalists prefer the use of bacterial sprays, but foresters consider these inexact, too.
* Natural predators, such as mice, wasps, flies, and some birds, can help control local caterpillar populations. But it normally takes two years before the predators reach sufficient numbers to do much good. In the meantime, a newly hatched caterpillar, clinging to a single silken thread it has spun, can be blown up to 20 miles away.
* Healthy trees in moist soils usually will restore themselves by growing new leaves within a few weeks. But doing so robs them of vital energy reserves. Tree growth slows down by as much as 30 percent and wood quality is adversely affected, according to the American Forest Institute in Washington.
* There is growing support for forest management as a tool in combatting the spread of gypsy moths. The Glatfelter Pulp Wood Company of Carlisle, Pa., one of the major forest products companies in Pennsylvania, has thinned out some timber stands near Gettysburg so that as little as 50 percent of their leaf canopy remains - a practice which encourages the growth of younger and smaller trees for future harvesting by allowing generous amounts of sunlight to reach the forest floor. It also deprives feeding caterpillars of much of the shade they seek during the day.
On a tour of infested woodlands here, Wilbur Wolf, senior area forester for the Glatfelter Company, showed reporters how a narrow band of trees missed by a chemical spray was being heavily defoliated while stands on either side that had been sprayed were not greatly affected. His company has spent $30,000 on aerial spraying this year.
Areas that depend on tourism in late spring and early summer, such as national and state parks and other recreation sites, could feel the impact of the infestation, since few visitors want to look at or pitch their tents under trees that have been stripped of vegetation.
Gettysburg, slowly filling up with visitors to the national military park and the site of Abraham Lincoln's famous ''Four score and seven years ago . . . '' address, is at the leading edge of this year's gypsy moth invasion. Even the battlefields are reported to be slightly infested.
Surrounding wildlife could also be affected if deer, squirrels, and other species cannot find the food supplies they need. An oak that has refoliated after a gypsy moth attack produces few or no acorns in the fall in order to save its energy for the following growing season.
Mr. Wolf and other professional foresters are urging researchers to develop and share new information on the impact of gypsy moth devastation on woodlands. Only then, he says, can areas not yet infested be better protected.