Q. I thought that gypsy moths were no longer a problem in our area, but they have almost killed two pine trees in my yard. I didn't realize what was happening until a few days ago. They are all over my house like the bad old days of the late 1980s. Is there anything I can do to control these large caterpillars?

A. While gypsy moths (Lymantria dispar) have been pretty much under control in our area for the past 15 years or so, they will never be completely eradicated. They were introduced into Massachusetts in 1869 by Leopold Trouvelot for his silkworm-breeding program. A number of the insects escaped, and by the early 1900s were responsible for major defoliation of forests in New England. They arrived in Western Pennsylvania in the late 1980s and have continued their westward movement as far as Minnesota. The common name "gypsy moth" comes from the way they move from area to area: on the wind. Their progress across the country has been slowed by our prevailing winds from the west.

A combination of well-timed insecticide sprays, introduced pathogens and predators and parasites worked to bring gypsy moths under control here after their arrival. Their populations build to outbreak proportions every five to 10 years, depending on weather conditions.

A big factor in reducing gypsy moths has been a fungus that infects and kills the pest, Entomophaga maimaiga. Years of plentiful rain help the fungus spread, while dry years cause the fungus to die out.

Young gypsy moths are gray-brown and very hairy caterpillars. Their hatch coincides with the leafing out of their favorite food, white oaks. Adult caterpillars are also hairy, and are characterized by five double rows of blue spots followed by six double rows of red spots on their backs.

While young caterpillars stay in the tree and feed day and night, older caterpillars feed only at night. They rest under leaf litter on the ground and in bark crevices near the base of host trees to avoid desiccating or being eaten by predators. Only mature gypsy moths feed on conifers, such as your pine trees.

Mature caterpillars are beginning to pupate, so the window for controlling them is closing fast. You may get control with Sevin (carbaryl) or Bayer Advanced Power Force Multi-Insect Killer (cyfluthrin). If your pines are very large, you should hire a tree service with a certified arborist on staff to spray them.

State foresters monitor gypsy moth populations on state and federal forestlands, and use their findings to target regions for aerial spraying. A total of 221,831 acres in 27 counties in northeastern and central Pennsylvania was treated in 2008. The aerial sprays used by the state are biological controls, either Bt (Bacillus thuringiensis) or Gypchek (gypsy moth nucleopolyhedrosis virus). They are both very safe for people, pets, birds, wildlife and even most other insects. Both are most effective on young caterpillars.

Residential property owners must contract with an arborist to have trees treated, but landowners in forested areas may qualify for spraying by the state. They must contact their county gypsy moth program coordinators by July 15 to have their property evaluated for possible inclusion in the state's 2009 gypsy moth suppression program. Allegheny County's program coordinator, Andy Baechle, can be reached at 412-350-2460. For other counties, go to the state Department of Conservation and Natural Resources Web site at www.dcnr.state.pa.us.