As a kid, I loved riding my bike endlessly around our Connecticut driveway, looping under an almost mythically giant maple. But one spring, my joyrides were tainted by hundreds of caterpillars hanging from the maple on sticky threads. I came to dread that maple and the caterpillars I’d later find creeping through my hair and clothes.
At the time, they struck me as a gross but natural nuisance. Sometimes I’d hold a tin can of kerosene while our neighbor scraped gypsy moth eggs off his trees and plopped them in. I thought it was cruel. Once I understood how those eggs became an obstacle course of dangling caterpillars, I understood his reasons.
Gypsy moth populations peak every 7-10 years; they reached their peak in Connecticut in the early 80s, interrupting my freewheeling bliss. That was 120 years after they were accidentally introduced from Europe to the United States, when a professor brought them to Medford, Massachusetts, hoping to breed a new type of silkworm. The first infestation was reported there in 1869; shortly thereafter, the moths spread throughout the Northeast, reaching New Hampshire by 1905 and Vermont by 1912.
Gypsy moth caterpillars hatch and begin feeding locally in mid-May, primarily on oak, sugar maple, and beech. When populations peak, as in 1981, when they caused over 13 million acres of defoliation in the Northeast, trees are left weakened and susceptible to disease and attack by other insects. Gypsies are especially damaging to forests growing on dry, shallow, or sandy soils, or forests that have experienced recent stress, such as drought or heavy logging.
New Hampshire’s 2001 forest health survey revealed 8,796 acres of gypsy moth feeding damage, up 47 percent from the year before. Damage in 2004 appeared on 5,000 to 10,000 acres, mostly in the Plymouth area, according to Jennifer Bofinger, Forest Health Program Coordinator for the Division of Forests and Lands.
Vermont fared slightly better: only 100 acres were defoliated in 2001. But Trish Hanson, Forest Protection Entomologist for the Vermont Department of Forest, Parks, and Recreation, said that more caterpillars were noticed in 2004 than in 2003.
Unlike some other invasive species, the gypsy moth has predators, and they appear to be controlling gypsy populations. The theory used to be that at low gypsy population levels, animals like white-footed mice controlled populations; at medium levels, parasitoids like bees and wasps moved in; and at high levels, nuclear polyhedrosis virus (NPV) wiped the insect out. When populations boomed, forest ecologists waited for the signs of an NPV epidemic, though population control was often only achieved after damage to trees was done – so pesticides were often employed.
Then, in 1989, gypsy populations started to show signs of an NPV epidemic – but in unusual circumstances: populations were low and not dense enough for NPV. Scientists dissected gypsy moth cadavers and found something unexpected: fungal spores. They came from the fungus Entomophaga maimaga, itself an invasive, brought from Japan to the U.S. in 1910 and 1911 (to kill gypsy moths, ironically), though nobody noticed it in action until 1989, when it took hold in Maine.
E. maimaiga has spread and appears to be effective. The 1999 Vermont Forest Insect and Disease Highlights report notes that the fungus “may be largely responsible for maintaining low gypsy moth populations throughout the Northeast.” Each time an incoming wave of gypsies was spotted on the horizon in these reports, it has just as quickly abated – and the fungus deserves most of the credit, according to Hanson.
The natural predators of the gypsy moth are successful enough in New Hampshire that the state doesn’t take an active role in gypsy moth control anymore. Neither does Vermont, which experienced its last major outbreak in 1990, when 61,270 acres were defoliated. “From time to time,” says Hanson, “we see a lot of mortality of gypsy moth caterpillars that is attributable to the fungus, but we do not apply it for population suppression. Rather, it shows up on its own.”
Unlike the gypsy moth, the fungus’s invasion has been beneficial thus far – except to gypsy moths. It sickens and kills their larvae, and then its spores are actively discharged from the cadavers, spreading via the wind to infect new larvae. Fortunately, scientists have found that the fungus only affects the family that includes gypsy moths and thus affects far fewer species than did the pesticides once used for gypsy moth control.
So what can we expect this year? “If the spring brings dry weather,” says Northam Parr of the University of New Hampshire Cooperative Extension, “we currently have live egg mass counts in the Plymouth area that are high enough to do moderate damage to over 10,000 acres. If we have a wet spring, and the fungus is thus able to flourish, we’ll see less than moderate defoliation over less than 3,000 acres.”