Summer's slinky visitors decline, but why?
By Janet Hefler
Published: June 5, 2008
What happened to the caterpillars? For the past two summers, the foliage-munching marauders slinked their way across the Island, leaving behind acres of defoliated trees and bushes and covering every outdoor surface with their frass, which resembled a sprinkling of fine pepper.
Adding insult to injury, caterpillars rained down from trees into the hair and onto the clothing of hapless humans, while inchworms launched a parallel assault, parachuting through the air on invisible gossamer threads.
Although caterpillars are back this year, the perception that there are fewer of them is not an illusion, according to entomologist Robert D. Childs, an extension educator in the Department of Plant, Soil and Insect Sciences at UMass.
"Population numbers for all of these pests have dropped noticeably, much to most people's delight; researchers and those in the pest management business may look at it differently," Mr. Childs said in an email response to questions from The Times.
Caterpillar Utopia
The combination of just the right conditions led to the past years' infestations, Mr. Childs explained. With a quality food source such as healthy trees, coupled with mild winters and fewer natural controls, such as parasites, predators and microbes, all of the Northeast's more than 2,000 known species of Lepidoptera caterpillars (butterfly and moth larvae) have the potential to go into "outbreak phase," he said.
Over the past few years, the Vineyard and many areas of eastern Massachusetts experienced "outbreak numbers" of several active caterpillar species, including the gypsy moth, eastern tent caterpillar, forest tent caterpillar, fall cankerworm and winter moth.
"There may be other factors involved that are more subtle and perhaps not well understood," Mr. Childs wrote. "Whatever the cosmic events were that supported the outbreak of all the above-mentioned species in Massachusetts over the past several years is not well known. It was rather rare to have all of those caterpillars active in huge numbers at the same time in the same place but Nature is often random and unpredictable."
Many Islanders fought back against the caterpillars, hiring tree experts to spray their trees with pesticides. Last summer, although the damage Island-wide was less widespread than the previous year, several areas up-Island suffered another season of devastation, such the West Tisbury end of Middle Road.
The decision on whether or not to treat trees with pesticide proved difficult, as well as costly, for homeowners and town officials alike.
In 2007, West Tisbury voters approved $6,500 and Tisbury voters $47,500 to treat trees on town property for caterpillars. Polly Hill Arboretum limited the use of bio-rationale pesticides to rare trees and ones that would be difficult to replace, due to concern about possibly harming native butterfly and moth species.
Some people decided to forego spraying, gambling that nature would take its course and the caterpillar epidemic would continue to subside. This year, it appears that Mother Nature did step in.
Photos by Ralph Stewart
Slinky retreat
Outbreak numbers seem to have peaked, with native caterpillars such as the eastern tent, forest test, and fall cankerworm, and the gypsy moth, not native but naturalized after 140 years, mostly controlled by naturally occurring parasites, such as flies and wasps, and microbes, such as a fungus that kills forest tent caterpillars and gypsy moths, Mr. Childs said. However, the reason for the reduction in the number of winter moths, which have no natural enemies, remains unknown.
One speculation is that since adult winter moths emerge from the soil, mate, and then lay eggs in late November and December, heavy snowfall in eastern Massachusetts right after Thanksgiving may have trapped them in the ground and killed them.
Soil temperature also played a role, according to Plymouth County extension educator Deborah Swanson. "We have had snow in December in previous years, however, the ground was not frozen," she said, "and I believe the winter moths were able to pupate and crawl out in the space between the ground layer and snow." Ms. Swanson continues to monitor the winter moth population after being the first to discover it in her county several years ago.
The winter moth population may not be as well established on the Vineyard as on the mainland, Mr. Childs said. Many of the small green inchworm caterpillars that may have been identified as winter moths actually were fall cankerworms, which are similar in appearance and timing of activity. In years past, one species was more prevalent on one end of the Island than the other, he noted.
Winter moth menace
However, Mr. Childs cautioned, "While we have mostly stopped worrying about the native caterpillars for the moment, we remain vigilant of the winter moth, which has no known natural enemies here. This pest, like the gypsy moth 140 years ago, has the potential to explode in numbers despite having had one bad season."
Joseph Elkinton, a forest entomologist and professor at UMass-Amherst, continues his work with the imported fly parasite Cyzenis albicans, the winter moth's only known predator, Mr. Childs said. The parasitic fly lays its eggs on the foliage of plants the winter moth caterpillars are likely to eat. They ingest the fly eggs, which hatch inside them and kill them.
The fly was introduced in Nova Scotia in 1954, effectively bringing the winter moth population under control in about six years. Mr. Elkinton collected the flies in Nova Scotia, bred them in cages at Otis Air Base, and released them in several locations across Massachusetts in the spring of 2006.
"His earlier releases have resulted in over-wintering success and the finding last year of parasitized winter moth caterpillars on Cape Cod," Mr. Childs noted. "Yearly releases will need to continue for about 10 years but if this fly becomes established it could very easily become the controlling factor for winter moth in the Northeast."
Island caterpillar sightings
In the meantime, although Islanders are enjoying the shade of many leaf-canopied trees, not all of them are caterpillar-free. In today's Times, garden columnist Abigail Higgins reports a caterpillar hot spot in Chilmark between North Road and Vineyard Sound.
"In a lot of places that got hit hard last year, there are hardly any," said Tom Robinson, a professional arborist who owns of Island Timber. "I haven't seen any up at Spring Point where they first showed up four years ago, or around Makonikey and Pilot Hill." He added that he has noticed some caterpillars in the Manuel F. Correllus State Forest, but only in the northwest corner.
Caterpillars hatched out in West Tisbury about a week to 10 days ago, according to Polly Hill Arboretum Executive Director Timothy Boland. "At least at the arboretum and in my neighborhood, they are in smaller numbers and isolated pockets than last year," he said. "I don't know about the rest of the Island, but Vineyard Haven would be about four to five days ahead of us, because it's a little warmer there, based on growing days by temperature."
However, Tisbury's tree warden and public works director Fred LaPiana said this week he hasn't noticed many caterpillars, which he attributes to spraying.
Information on caterpillars is available on UMass Extension's web site, umassgreeninfo.org. Click on fact sheets, and then "insects and mites," where caterpillar types are listed alphabetically under "defoliators."
