Caterpillar Invasion Returns for Encore, Stripping Trees, Shrubbery, in Our Hair

By IAN FEIN

They dangle down on silken threads and stick to your shirt or in your hair. They crawl up the sides of homes and infest your trees. You can hear them munching away on leaves, their frass falling to the ground like rain.

Caterpillar tales are spreading around the Island like horror stories, as the Vineyard - like other parts of southeastern Massachusetts - undergoes one of the most memorable caterpillar outbreaks in decades. More than a quarter million acres of trees were defoliated statewide last year, and already this summer large swaths of trees off Lambert's Cove Road in West Tisbury and near the blinker in Oak Bluffs have been decimated.

The sticky topic arose in selectmen's meetings in three different towns this week, with both Tisbury and West Tisbury boards discussing the possibility of pursuing massive townwide spraying.

Anyone who has stepped outside on the Vineyard this week will attest to the inconvenience and discomfort posed by the army of caterpillars, but many arborists and entomologists are concerned about widespread overreaction to what is almost certainly a temporary natural cycle.

"People certainly have a natural aversion to the overpopulations, but this type of thing has repeated itself before," said Tim Boland, executive director of the Polly Hill Arboretum, which hosted two caterpillar forums this winter and spring. "I would urge people to be conservative in their approach and not be too aggressive in spraying. If we were to globally spray everything, we would lose a good portion of our native biodiversity."

Although caterpillar outbreaks are a relatively common cyclical occurrence, the current situation is somewhat unique in that it involves several different species sustained over multiple seasons.

Massachusetts Natural Heritage and Endangered Species Program restoration ecologist Tim Simmons said yesterday that significant declines in the caterpillar populations may not be seen for another decade.

"We're going to see massive defoliations for at least another ten years at least. It should take that long for natural or introduced parasites to catch up," Mr. Simmons said. "It's going to be a while."

There are five different types of caterpillars feasting on Vineyard trees, including three native species: the eastern tent caterpillar, the fall cankerworm and the forest tent caterpillar, which has not seen such outbreak numbers for more than 100 years.

Two non-native species - the gypsy moth and winter moth caterpillars - are also present on the Island. While the gypsy moth has been around for more than a century and reached outbreak proportions statewide in the 1980s, the European winter moth was first seen in the region only a few years ago. Some state entomologists studying the current outbreak have theorized that the introduction of the winter moth may have distracted some of the predators of native caterpillars, leading to an overabundance of multiple species.

Most of the caterpillar types typically appear in very large numbers for a few years before subsiding rather dramatically. They usually defeat themselves by overfeeding, or a change in ecological conditions might cause a rise in a particular fungus or parasite that controls the caterpillar population.

The winter moth situation is slightly more concerning to entomologists because there are no known predators here at this time. A nonnative predatory fly species has been identified and is now being studied for possible release, but some biologists have warned against it. Another parasitic fly that was released on the Vineyard to control the gypsy moth population in the 1980s is now partially blamed for the decline of the threatened imperial moth.

Over time, the caterpillar feasting could eventually have an impact on the Vineyard tree population. Many of the younger trees will likely re-leaf later in the summer, and although a season without leaves does not pose an immediate threat to a tree, those that are defoliated year after year can die within a few years.

Island biologists have noted that many of the caterpillar populations appear to be rotating around the Vineyard over the last few summers, which might prevent some repeated defoliation. Also, some tree deaths might not be a bad thing for the Vineyard environment; it could open the canopy for smaller native plant species that have been blocked out by oaks.

At least four different tree service companies have been busy spraying on the Island over the last few weeks. While most have been using an organic substance called Conserve SC, there is not unanimity on the potential impacts that the compound might have on other plant and animal species. And at least one company has been using a moderately toxic pesticide.

Sheriff's Meadow Foundation - a conservation organization and the largest private landowner on Martha's Vineyard - decided not to spray any trees on its more than 2,000 acres this year, because the solutions currently available are not species specific and might harm native moths and butterflies.

Mr. Boland said it is probably too late for spraying to have much of an effect on the caterpillars at this point anyway, and he recommended that homeowners should regularly water the defoliated trees they want to protect over the long term. At Polly Hill they sprayed Conserve SC on some of their oldest trees that they consider irreplaceable, but Mr. Boland specifically warned homeowners against spraying anywhere near native pitch pines or frost bottoms, which some rare moths are known to inhabit.

Although the caterpillar species are on somewhat different schedules, many are expected to migrate into the soil within in the next week to spin a cocoon and pupate, only to reemerge as moths later on.

Mr. Boland suggested that people try to take the long view when thinking about the caterpillars.

"Nature does have the ability to rebound, and that's the thing to remember," he said. "Things will eventually even out."