Caterpillar infestations scattered but locally devastating; trees re-leafing

By Dan Cabot - July 5, 2007

Up-Island, caterpillars continue to plague the woodlands, but the damage is somewhat less widespread this summer than last. Where the worms are now prevalent, the devastation is extreme, but other areas badly infested last year are only lightly affected this year, giving hope that the egg-laying moths or the caterpillars themselves seek fresh food supplies, migrating in each successive season. This is good news, because most trees can survive the loss of their leaves for a year or two. West Tisbury tree warden Jeremiah Brown told town officials last year that three consecutive years of defoliation will kill many trees.

According to Mr. Brown, this year's infestation is mostly larvae from the fall canker worm, a native species which has had population explosions on the Vineyard from time to time over many years. Although recently alarming numbers of the winter moth, a newcomer to North America, raised fears of permanent damage to the Island's trees, the winter moth is far less numerous this year, perhaps succumbing to competition from native caterpillars, to predation, to spraying, or simply to the cyclical nature of caterpillar infestations.

According to Sheriff's Meadow executive director Richard Johnson, a healthy tree can be completely stripped of its leaves (whether by hurricane winds or by an army of caterpillars) and within a month or so grow new ones. Bonsai artists sometimes intentionally remove all of a deciduous tree's leaves in late June because the regrown leaves will be somewhat smaller and conform to the bonsai's miniature scale.

The danger to forest trees comes if the infestation continues year after year. West Tisbury voters this year approved $6,500 to spray trees on town property with Conserve. Conserve, the product most often used on the Vineyard, affects only the caterpillars and not the adult moths. It is not toxic to animals and humans. Mr. Brown reports that the spraying has been done in ten places around town, such as the town hall and Lamberts Cove. Many individual landowners have had their property sprayed.

Sheriff's Meadow executive director Richard Johnson, however, disagrees that spraying is necessary in most cases. None of the Sheriff's Meadow properties has been sprayed. Mr. Johnson says that most trees will survive without spraying, and he points to the trees around the Sheriff's Meadow office on Lamberts Cove Road. They were defoliated last year, he notes, but this year, though there are still a few caterpillars around, the leaves are healthy and people can walk on the property or eat outdoors at the picnic tables. If any species of caterpillar ate leaves from the same trees year after year, it would destroy its own food supply. The worms move on, and most trees will recover.

Mr. Johnson also opposes spraying because, he says, Conserve kills the larvae of all moths and butterflies, many of which are rare and beautiful, some found only on the Vineyard. Mr. Brown adds that daytime spraying may endanger honeybees, as the adults carry the toxin back to the hive where bee larvae may be killed.

Some landowners, including the Polly Hill Arboretum in West Tisbury, have sprayed only a few valuable or specimen trees, a compromise which Mr. Johnson says makes good sense.

One of the places caterpillars have attacked this year is the West Tisbury end of Middle Road. Last year Woods's woods, to the north of Middle Road, were almost completely defoliated by worms, but the south side of the road had only minor infestations. This year, the worms have moved south, and the situation is reversed.

The story is a familiar one to anyone who has suffered an infestation. In late May the understory at Glimmerglass, south of Middle Road, was draped in ten-foot-high shrouds of gray-white webbing, as thick as billowing smoke on a windless day. The dropping worm excrement pattered like rain, and the descending worms and their filaments hung at every level. It was impossible to dodge them all. One picked inchworms and strands of web out of one's hair and off one's skin and clothing. A heavy rain in early June turned the sheets of webbing into disgusting brown residue hanging in festoons on the bushes, but the worms continued to thrive - and to eat.

By the second week in June, the leaves were completely gone from the canopy and most of the understory, and the July sun now penetrates unnaturally, as if on a day in February with the sun mysteriously high and hot in the sky. A grassy woodland path, usually mowed twice a year, now needs to be mowed weekly, as it lies in full sun. Oaks fared especially badly, but the worms devoured almost every tree and shrub, even bull-briers and blackberry roses, which are left with bare green stems bristling with thorns. (One of the plant species that worms don't eat is sassafras. Perhaps we could repel worms by spraying our trees with sarsaparilla.)

Today the infestation at Glimmerglass has run its course. For a time the sandy driveway was blacktopped with droppings and with the corpses of those late-hatching caterpillars that found no leaves to eat, but now the only remnants are the rotten brown webbing. The trees have put out new leaves, and the canopy looks as it did in early May.

If the pattern continues, next year the worms will move on to the south and eat someone else's leaves.