If you noticed many brown moths recently flitting in a zig-zag fashion through your backyard, be warned: They were adult male Gypsy moths seeking a non-flying female secretly waiting in a protected spot in the yard.

Since the female does not fly, the males are attracted to her by a scent known only to them. The males live but a day or two and their search is fast and furious.

After mating, the female attached a brown-fuzzy mass containing 300 to 500 eggs at the base of a tree, under lawn furniture or any place else where they would have some protection from the rigors of winter.

Those eggs should be sought and destroyed before next April when they would hatch. A firm stroke of a broom or stiff brush will disrupt and scatter the egg mass off the trees. For those you cannot reach, spray with a heavy stream of water from the hose.

Areas infested with gypsy moths include most of New England, New York, New Jersey, Pennsylvania, parts of northern Maryland, Delaware, Virginia, a small area in central Michigan, and perhaps other states. A few were trapped in the District of Columbia this year.

Gypsy moth caterpillars were brought to the United States more than 100 years ago by a Massachusetts naturalist who wanted to cross the moths with silkworms. The experiment failed and a few young gypsy moth caterpillars escaped and thrived.

In this country, the moths do much more damage than in some parts of their native Europe and Asia, where parasites keep their numbers low. However, none of the moth's predators accompanied it here.

A single 2-inch gypsy moth caterpillar can eat a square foot of leaves every 24 hours. In heavily infested areas, large trees are virtually stripped of their leaves overnight. In 1979 they stripped 643,000 acres of forests in 12 states.

Depending on the health and vigor of trees, hardwoods usually develop new leaves. But some trees are severely weakened and may die from other insects of disease or from further defoliation. Two or more defoliations can kill hardwoods and one complete defoliation can kill some evergreen trees.

The functions of foilage are to produce food, via photosynthesis. A deciduous tree seems to tolerate up to 50 percent defoilation in a year without much difficulty.

All kinds of physiological changes begin to occur when defoliation exceeds 50 percent, especially if it is severe enough to cause the tree to refoliate in the same growing season.

Growth regulators that control the tree's physiology are changed when the leaves are removed, according to Dr. Phillip M. Wargo, of the USDA Northeastern Forest Experiment Station, Hamden, Conn., who had studied extensively the results of tree defoliation.

"Buds formed orginally for the next year open in about three to four weeks and new leaves begin growing, he says. "Thus new buds must be formed within a shortened growing season.

"The tree metabolizes good reserves (primarily starch in deciduous trees) to maintain its living tissues until the new leaves are formed and produce new food."

Such refoliation may be necessary for the tree to sruvive, but it also results in a significantly altered tree, Dr. Wargo says. "The refoliated tree is completely out of phase with the season. The spring following the season of defoliation, there are fewer and smaller leaves than normal. There is some evidence that leaf size and number decrease with succeeding defoliations.

"The defoliation-refoliation process also causes a drain on the tree's reserve foods. Defoliation usually occurs before a tree has replaced the food reserves that were used during the spring flush of growth. In addition, the defoliation itself reduces the total amount of food produced."

Wargo says the most obvious effect of defoliation is a spring die-back of terminal twigs and branches, caused by winter injury or starvation from too little food during the format months.

"After one defoliation, most buds survive until spring and produce new growth; but the amount of new terminal growth can be significantly reduced. in some instances, twig growth is only 25 percent of that of healthy trees. There is also less radical growth in a defoliated tree.

"A defoliated tree is more vulnerable to the effects of wounding. A wound inflicted on a defoliated tree becomes larger than a similar wound on a healthy tree, because more tissue dies around the original wound. Wounds also close more slowly.

"Many small feeder roots needed for mineral and water absorption die after defoliation, probably from lack of food. This results in poor water and mineral uptake and ultimately in reduced food production in the leaves. In addition, insects can successfully attack the twigs, branches and main trunk of the weakened tree."