A severe caterpillar outbreak expected, putting foliage at risk

Area communities can expect to be hit next month with another massive outbreak of leaf-eating caterpillars, which have defoliated thousands of acres of trees in Southeastern Massachusetts over the past few years, said officials who track such infestations.

Deborah C. Swanson, a horticulturist with the Plymouth County Cooperative Extension office in Hanson, which educates people about agriculture and horticulture, said she expects an especially heavy outbreak of gypsy moth caterpillars because of the number of eggs that have been laid.

It could be the worst hatching since the period between 1979 and 1981, when millions of the voracious, 2-inch, hairy caterpillars munched their way across the state from the Connecticut River to the ocean.

Swanson said two other pest caterpillars that have left their mark over the last few years -- the winter moth and forest tent caterpillar -- also could have strong years, based on the numbers of eggs laid. Because all three species are expected to reproduce in large numbers this year, the caterpillar invasion could be the biggest in recent memory, she said.

The gypsy moths lay tan or buff-colored oval egg masses, which are about the size of a quarter or half-dollar, and are covered with fine hairs. Each mass contains about 500 eggs.

In Hanson, Swanson said she recently saw a 20-foot branch of an oak tree covered with the egg masses. ''It looked like spray-on insulation," she said. ''It's just unbelievable."

What could check the caterpillars' advance is a wet spring, said Joseph S. Elkinton, a professor of entomology at the University of Massassachusetts at Amherst. When it's wet, a naturally occurring fungus in the soil kills large numbers of the caterpillars.

No one is sure what triggers the massive outbreaks. The number of caterpillars south of Boston has been steadily increasing since the 1990s, according to specialists.

If their predictions for this spring are accurate, hordes of the hungry caterpillars will strip trees and shrubs of leaves when they appear, leaving them bare. The effect will be seen across acres of forest and on solitary backyard trees. People will be plucking the bugs from their hair and sweeping them off their decks until the infestation runs its course sometime in June.

About 34,000 acres of trees were defoliated last year in Eastern Massachusetts. Statewide, about 250,000 acres were defoliated by gypsy moths and tent caterpillars and 21,000 acres by winter moths.

The three caterpillars look different, hatch at different times, and prefer different foods.

The winter moth caterpillar is greenish, about an inch long, and moves like an inchworm. The European bug landed here in the 1990s.

With no known predators, its population has exploded south of Boston and across Cape Cod. Eggs hatch in late March or early April as buds are swelling but not yet open, and the caterpillars eat foliage and some flower buds of oaks, ash, and blueberry and crabapples, according to information from the UMass Extension service.

The gypsy moth caterpillar, introduced to Eastern Massachusetts in the 1860s, is 2 to 3 inches long, dark and hairy. It has five rows of paired blue dots on its back, followed by five pairs of red dots. The bug prefers oak leaves, but will eat hundreds of other plants and sometimes pine trees, feeding from early May until mid- or late June.

The forest tent caterpillar, blamed for much of the defoliation in Freetown, Lakeville, Middleborough, and Halifax, grows to 2 or 3 inches and has a series of keyhole-shaped spots down its back. It feeds until early to mid-June and prefers oaks and maples, but will eat a variety of other leaves as well.

A similar pest caterpillar, the Eastern tent caterpillar, can be identified by the long white stripe running down its back.

All of the caterpillar species can be treated with products containing Btk (bacillus thuringiensis kurstaki) or insecticides containing spinosad. Other chemical treatments are also available.

Before they hatch, the eggs can be scraped off trees and disposed of. But Swanson said the eggs can still hatch if they are simply thrown on the ground.

In an experiment, entomologists released a species of parasitic flies in Wompatuck State Park in Hingham last year, hoping the flies would kill winter moth caterpillars and help curb their explosive growth.

It could be a decade before the result of the experiment is known.

Residents and garden professionals can learn more about the caterpillars and how to protect trees and shrubs at these upcoming events:

An all-day, $65 seminar for plant professionals on moths and invasive species will be held March 29 at the Radisson Hotel in Brockton.

A free lecture on moths and caterpillars geared for residents will be held from 7 to 9 p.m. March 30 at the Indian Head School gym in Hanson.

For more information about either event, call the Plymouth County Cooperative Extension office at 781-293-3541.

Matt Carroll can be reached at mcarroll@globe.com.