Caterpillars Munch Away on Island


Armies of caterpillars made their annual spring emergence on the Vineyard this week, munching away on Island foliage and swinging down on silken threads to stick to unsuspecting passersby.

But reports from across the Island suggest that the caterpillar population is down significantly from past years, and that pockets of infestation have moved to different areas. The pattern relieves concerns that repeated defoliation could have caused widespread tree mortality on the Vineyard. That no longer appears to be a threat.

"The numbers are orders of magnitude smaller than last year," said Matthew Pelikan, Islands program director for The Nature Conservancy. "You really have to look hard to find any damage."

In another positive sign, Island ecologists are reporting that they are seeing mostly native fall cankerworms, and have witnessed a significant decline in the troubling winter moth caterpillar - a non-native European species that was first seen in Massachusetts only a few years ago.

"In our up-Island location, it's all fall cankerworm as far as we can tell," said Timothy Boland, executive director of the Polly Hill Arboretum in North Tisbury. "And they seem to have moved. They aren't attacking trees they defoliated the year before."

Caterpillar outbreaks are a relatively common cyclical occurrence, with insects typically appearing in very large numbers for a few years before subsiding rather dramatically. They usually defeat themselves by overfeeding. Also, a change in ecological conditions might cause a rise in a particular fungus or parasite that controls the caterpillar population.

State entomologists are still not sure what caused the recent outbreak - which was unique in that it involved several different species sustained over several seasons - or what led to the decline this year. But University of Massachusetts entomologist Robert Childs this week said caterpillar populations were significantly down on the mainland as well, suggesting that the lower numbers were likely caused by natural forces.

Meanwhile, a number of Vineyard conservation leaders have raised concerns about the amount of pesticides sprayed on the Island over the last few years.

About a half-dozen tree service and landscaping companies have been busy spraying private properties in recent weeks. Voters in Tisbury and West Tisbury also approved tens of thousands of dollars in taxpayer money this year to spray trees in public places such as Lambert's Cove Beach and Main street in Vineyard Haven. Though at least one company is using a moderately toxic pesticide, most of them are spraying a substance whose active ingredient, spinosad, is derived from a naturally occurring bacterium.

The conservation groups agree that these so-called biorational pesticides are better than traditional insecticides, and that the judicious application to certain trees may be appropriate. But they also note that any type of pesticide will likely kill other insects, including some potentially rare species.

Mr. Pelikan noted that a significant number of the rare animal species found on the Vineyard are moths and butterflies - many of which have survived on the Island in part because the Vineyard has traditionally rejected widespread pesticide spraying.

"It's the old throwing out the baby with the bathwater," said Richard Johnson, executive director of Sheriff's Meadow Foundation, a conservation agency which has refused to spray on its more than 2,000 acres. "You're killing other caterpillars and rare insects. You don't know what you're putting into the system," he added.

"Almost by necessary, with any pesticide there's going to be bycatch or collateral damage. So you've got to be cautious," said Martha's Vineyard Commission water resource planner William Wilcox, who underwent extensive pesticide training when he worked as the Dukes County Extension agent. "I don't think anyone knows what the impact of spinosad is on benign insects, which serve a purpose in the environment that might be obscure but of some importance."

This week marked what would have been the 100th birthday of Rachel Carson, the late author and marine biologist whose book Silent Spring is widely credited with launching the modern environmental movement. Published in 1962, the book exposed the dangers of indiscriminate use of pesticides, and encouraged carefully managed use with an awareness of impacts to the entire ecosystem.

Mr. Wilcox shared a caterpillar anecdote from last summer that echoed the title of Ms. Carson's book.

"I know someone in Vineyard Haven who had their neighborhood sprayed last year. He said all the caterpillars were gone, but the rest of the summer was really eerie and quiet," Mr. Wilcox said. "He started to wonder whether anything else was still alive out there."