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The Hills Are (Too) Alive: Lymantria Among Us


Female gypsy moth, Lymantria dispar dispar, Dorsal side. Photo by Didier Descouens,

Back in the early 1990s, standing in my small Allegany County woodlot, I was ordered into the trenches. Only three miles to the north, the first wave of Lymantria dispar dispar (LDD), commonly known as the gypsy moth, infestations was crashing over the ancient glacial hilltops. Hundreds of acres of mostly hardwoods in the Rattlesnake Hill Wildlife Management Area, more than 5,000 acres, had been stripped bare. Even a casual visitor to the area couldn’t be spared this reality, not least because the epicenter of the defoliation was on a steep, south-facing hillside above a state highway.

I was getting panicky about my 17 acres. Invasive egg masses were multiplying, and it was obvious the following spring would see the hardwoods on my place — largely red and sugar maple with yellow birch, beech, cottonwood, and a smattering of black oak — getting badly chewed up. So I launched a one-person eradication campaign: scraping egg masses carefully into bags with a very dull knife, then transporting the stuff back home for ritual destruction safely indoors.

All this hit home again last year. Just west of Canandaigua Lake, in Ontario County, a frighteningly efficient LDD outbreak struck hundreds of acres. Ontario County Park, in the town of South Bristol, was the epicenter. I witnessed the initial loss of foliage — almost total in some patches — with alarm. Local news media covered the story. Individual landowners in the affected area looked mighty depressed. There hasn’t been a comparable outbreak near my land for the past 30 years. But, though I remain vigilant and quick to scrape, I don’t give myself much credit for preserving the foliage. And learning more about the Hundred-Fifty Year War against Lymantria dispar dispar in North America has tempered my passions toward the invader and refocused my efforts.

I’ve even developed some grudging respect for this unwelcome insect. First, I’m going with a name change. Calling it the gypsy moth is so 19th Century. Such exoticism is no longer acceptable. Even the Entomological Society of the US has formally rejected the ethnic stereotyping, and the moniker Lymantria dispar dispar — generally rendered as LDD — is now de rigueur. (Updating the name to “Roma moth,” or the like, is obviously no solution.) Let’s hope no battles in America’s Culture Wars break out on this front. If this happens, organic folks will hopefully line up on the cultural left.

Then there’s the matter of “invasive.” I used to almost celebrate my anger at the presence of LDD by imagining myself in the defensive vanguard. Then I learned my allies had “surrendered” by recategorizing the pest as “naturalized.” Yes, science often conflicts with morale. But what are we going to do about the pest? LDD, since its introduction to Massachusetts in 1869 by an entrepreneur forcing the marriage of LDD and silkworms, has been spreading west, southwest and north. Things came to a head regionally during the same years Rattlesnake Hill got hammered, but this is a cyclical phenomenon we’re dealing with, and here we are again.

Scientists tell us LDD operates in “four-phase” cycles. An “innocuous phase” can last for years, followed by a one-to-two year “release phase, then an “outbreak phase” of the same duration. But this schedule is highly dependent on weather and other variables. Outbreaks tend to collapse, for example, when, unsurprisingly, food for moth larvae disappears. And a fungus called Entomophaga maimaga (EM) plays a crucial role in controlling the spread — though the thirsty fungus, in turn, can be thwarted by extended dry weather.

Good news: When LDD came here, it had few natural enemies. But as time goes on, the pest appeals to more palates, including those of some predatory wasps and flies, blue jays and robins, white-footed mice, raccoons, and more.

Scientists have unleashed — experimentally, with appropriate controls — some Old World LDD parasites, with some success. And Bacillus thuringiensis kurstaki (Btk) is useful, but this requires good planning so other moth and butterfly species are not harmed. Btk on a large scale also is expensive, since aerial application is involved. In our region, I’ve heard of only one such application, on private land in one of our most upscale wooded areas, near Canandaigua.

The entrepreneur of 1869 wanted a silkworm that would be more resistant to cold than Asian species. LDD thrives under varied conditions. Its Old World range extends quite far north, while in North America it seems headed toward most of temperate Eastern Canada, the Eastern states and Upper Midwest. (A simple rule of thumb: the species will do well in almost any habitat in which oaks, Quercus genus, are well-represented.)

But the cold barrier is weakening. Reportedly, even Northern Minnesota with its historically cold winters is now on the hit list. I recall that when I bought my Southern Tier land 40 years ago, subzero temperatures were commonplace — down occasionally to -20F or lower. These days, well, you know the story. Often the ground doesn’t even freeze.

Oaks may be LDD’s favorite cuisine, but they’re far from alone on the menu. LDD caterpillars dine happily on dozens of locally-prominent species, including broadleaf trees like maple, birch, beech, etc. They will even go after conifers like pine and hemlock, and when they do, the results are unfortunate, since conifers don’t regenerate their food factories the way oaks and others do. Which often means quick mortality for the conifers, while the broadleaf trees can spring back with new foliage.

As they say, it’s hard to make predictions, especially about the ecological future in a world experiencing a climate crisis, vast extinctions, and mass organized violence.

The warm months of 2020-21 hereabouts have made some of us nervous. LDD has affected parts of several counties, and some of our Monroe County parks and preserves sport all too many LDD egg masses on beautiful, old hardwoods. With a bit of luck, a lot of cold winter nights and generous precipitation, 2022 will bring only spotty occurrences locally.

And may the EM fungus prosper!

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