Many letters and phone calls have reported that thousands of tent caterpillars appeared suddenly in gardens in Anne Arundel, Prince Georges and Montgomery Counties. Since it seemed unlikely they could be tent caterpillars, which hatched from eggs about mid-May, or bagworms, which will hatch about mid-June, an inspection was made in four cases.

In all four cases they were Palmerworms (Dichomeris ligulella). What is unusual is that these caterpillars (worms) show up in measurable numbers only every 60 over 70 years.

The last population peak of palmerworms in this area was about 1880, according to Dr. John A Davidson, Unversity of Maryland insect specialist and professor of entomology. Heavy infestations of adult palmerworm moths (which themselves are harmless) were noted this year during mid-May, says Davidson. The caterpillars that are present now came from eggs laid by those moths.

The caterpillar are about one-half inch long, and are olive-green with two white stripes along the side and two narrow white lines on the back.

If you find large numbers of such caterpillars on your property, don't panic, Davidson says. They will do little or no damage. Remember, you're seeing something that won't appear again for 60 to 100 years, based on past history.

The palmerworms prefer oak tree leaves. Any leaves they devour will soon be replaced by new growth.

Tent caterpilalrs do little permanent damage because the leaves they eat also ar soon replaced. Those consumed by bagworms may not be replaced because it is late in the season, but they confine themselves mostly to wild cherry.

The only practical way to attempt to control tent caterpillars and bagworms is to destroy their over-wintering nests. Once they hatch, they seldom do enough damage to justify an effective elimination effort.

Because the worm is so seldom seen and is not notoriously destructive, very little is known about the palmerworm, says Dr. Ronald W. Hodges, chief of the systematic entomology laboratory, USDA Agricultural Research Service, Beltsville. In fact, he says, the U.S. Environmental Protection Agency doesn't even have specific pesticide label directives for its control.