The gypsy moth was introduced into Massachusetts in 1869 by a French naturalist who was experimenting with silkworms. Some of the insects accidentally escaped. Without their natural enemies -- predators, parasites, diseases -- they thrived, multiplying to enormous numbers and causing defoliation of forest, shade and orchard trees.

The problem was confined to the northeastern United States until recent years but now threatens the oak-hickory forest of the eastern half of the country and parts of the Midwest. The moth is now a pest in parts of Maryland and Virginia. This year for the first time it was reported to have defoliated 100 acres in Michigan, and Delaware was damaged for the first time.

This year's gypsy moth damage in the Northeast was only half of that of last year, and forests in Pennsylvania -- plagued for the past eight years with serious defoliation -- were largely spared, according to preliminary figures of the U.S. Department of Agriculture.

The figures, compiled each year from aerial surveys, show approximately 643,000 acres were defoliated in the Northeast, the principal infested region, down from 1,271,990 acres in 1978. The pests stripped approximately 8,500 acres in Pennsylvania this year, down from nearly 450,000 acres in 1978 and 1.3 million acres in 1977.

A combination of factors caused the dramatic drop in damage in Pennsylvania, according to James O. Lee, deputy administrator of the department's Animal and Plant Health Inspection Service (APHIS).

Among the causes were the wet, cold weather this spring, diseases of the caterpillars, parasites released through state and federal programs, and treatment (spraying) of areas where heaviest damage was expected.

However, the insect's populations increased in some other areas -- parts of Massachusetts, for example -- and Lee said this points up the need for continued state and federal cooperation in developing a comprehensive integrated management approach to deal with the pest. Many new tools, including the gypsy moth virus, a bacterial insecticide, the sex lure, large-capacity moth traps and highly specific pesticides, can be used in such a program.

The gypsy moth has one generation a year. The insects pass the winter as eggs in velvetlike, buff-colored masses attaching to stones, tree trunks, logs, lumber, fences and other objects. These masses each contain 400 to 500 eggs. The eggs hatch in late April or early May. The young caterpillars spin silken treads and may be borne long distances by the wind. While young they become larger by devouring all green leaf material available to them. In about seven weeks, around early July, they are fully grown. They spin a few strands of silk and change to the pupal or resting stage.

In about 7 to 17 days the moths emerge. The female lays her eggs during late July or August.

Low temperatures during the egg stage may cause considerable mortality in unprotected situations. Forest areas with an abundant population of birds and small mammals are less likely to suffer serious damage from the gypsy moth caterpillars, as such animals feed on them. Insects that are parasites, many of which have been imported from Europe and Asia, attack various stages of the gypsy moth and help to limit increases. Disease producing organisms also are potent weapons against them.

But with all of it, man has not been able to manipulate them to his needs, and insecticides must still be used to prevent serious damage by the pest. So far there has been only limited success with them.

Defoliation three years in succession may kill hardwoods but one complete defoliation may be fatal to conifers.