Local group aims to save hemlock trees

A hemlock forest. (Photo by Jim Longbotham)

If you’ve ever gone cross-country skiing or snowshoeing in the Shawangunks, you know that a grove of snow-laden hemlock trees is one of our region’s unsurpassable winter beauties, creating an aura of magic of the sort that we associate with fairy tales set somewhere in Bavaria. But the Eastern hemlock, Tsuga canadensis, is more than just a picturesque landscape feature. It’s what ecologists call a “climax species,” meaning that it’s a sign of a mature, healthy forest, and a “foundation species,” meaning that it creates a unique habitat.

Hemlocks favor steep slopes and rocky streambanks, their shallow, branching root systems serving to stabilize soil against erosion. They like to grow densely, their shady canopy keeping flowing water cool enough to support native trout populations and suppressing the seeds of invasive plant species from sprouting. Hermit thrushes, warblers and vireos favor their branches for perching and nesting. Porcupines, deer and moose depend on their soft needles and inner bark as food sources.

The Eastern hemlock is the third most common tree in New York State, and more of them grow here than in any other state. But its reign is now threatened by an insect pest native to southern Japan, imported to the US in the 1950s and to New York in the 1980s, very likely via infected nursery stock. It’s called the hemlock wooly adelgid, commonly abbreviated HWA, and it is slowly strangling the life out of millions of native hemlock trees in our region. That’s the bad news. The good news, according to Charlotte Malmborg of the New York State Hemlock Initiative (NYSHI), a project of Cornell University who gave an illustrated lecture on the subject at SUNY New Paltz last Thursday, is that scientists are doing something about it – and you can help.

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As might be deduced from the binomial nomenclature of this species, Adelges tsugae is adapted to eat only one thing: hemlocks. Malmborg explained that hemlock species in many other parts of the world had HWA evolve alongside them, prompting natural predators to co-evolve over the millennia. Until recently, our Eastern hemlocks had no such codependent neighbors. Consequently, they are easy prey for HWA now that the insects have arrived.

Malmborg described the tiny adelgid as essentially “a big beachball body attached to a long sucking mouthpart.” It lives to suck out a hemlock tree’s nutrients, and is only able to move during one brief stage of its life cycle, called a “crawler” by entomologists. It spends most of its existence dug into the underside of a hemlock twig, spinning a white, wooly, waxy mass about itself that resembles a tiny cotton ball. They’re easy to identify, fortunately – especially from late fall to early spring.

It’s not the HWA’s vampiric habits that spell eventual death for the tree, so much as the fact that the tree keeps trying to patch up the minute punctures that the insects inflict. “Blocking the wound stops the flow of nutrients” within the tree’s circulatory system, Malmborg explained, leading to die-off of affected branches. And as branches die, subsequent generations of the adelgids – which, being asexual, don’t need to go downtown and find a mate to lay eggs – migrate to higher branches, until finally the whole tree dies.

This process can take anywhere from four to 20 years from the time that a particular tree is infested. Colder climates tend to slow down the process, which works in our favor here in the Northeast. A sudden severe cold snap can set a local population of adelgids back significantly (though the survivors will be tougher than their forebears, because that’s how evolutionary adaptation works). If you spot a hemlock tree whose lowest branches are turning grey, showing no “tennis-ball green” new growth in springtime, according to Malmborg, you’ve got a sick tree on your hands. But it may not be too late to save it.

There are two chemical pesticides currently available to halt the progress of HWA in an infested tree. Dinotefuran, which is patented (and therefore costlier) and sold under the brand names Safari and TransTect, is fast-acting and has a short life after application. The best intervention for a tree that is badly infested and in imminent danger of death, dinotefuran can only be legally used by a certified applicator, and only works during the spring and fall, “when the trees are transpiring.” Applied directly to the basal bark, it’s less likely to impact nearby water supplies than the other widely available option: imidacloprid, which anyone can purchase and apply as a soil drench around the tree’s roots. Imidacloprid is a slow-acting, systemic insecticide that will persist in the tree for four to seven years, so is regarded as the best choice for only lightly infested trees that have more time to spare before they’re overwhelmed by the pest.

Both insecticides are classified as neo-nicotinoids, which means that they are potentially hazardous to pollinating insects like honeybees. Fortunately, notes Malmborg, “Hemlocks are wind-pollinated,” and don’t have flowers attractive to bees. Moreover, she says, neo-nicotinoids “selectively bind in insects and not vertebrates,” so don’t present a major problem of bioaccumulating up the food chain. Still, chemical pesticides rightly make people nervous, and there are studies going on in the Great Smoky Mountains – which has worse HWA problems than New York State – to monitor the effects of dinotefuran and imidacloprid on non-target organisms, and to identify best practices to “prevent cascading ecological effects.”

Closer to home, testing of “integrated pest management” biocontrols by NYSHI has been underway for more than a decade. Malmborg and her colleagues have been breeding and releasing two natural predators of adelgids that co-evolved with hemlock species in the Pacific Northwest. One is a beetle, Laricobius nigrinus, which is prey-specific – when offered a variety of food choices, HWA was “the only thing they wanted to eat” – and has a life cycle that syncs well with the invasive pests. The other consists of two species of silver fly of the genus Leucopsis, whose larvae feed on adelgid eggs.

Just last year, NYSHI scientists successfully bred the silver flies in their laboratory for the first time. Leucopsis has been released at ten HWA-infested hemlock stands on public land in New York State since 2015. Laricobius beetles have been released 21 times since 2009 (including in Minnewaska State Park last year); the release sites are monitored annually, and the Adelgidbusters were recently heartened to discover that five of the original sites now have self-sustaining beetle populations.

To hold the line against further encroachment by HWA, NYSHI needs citizen scientists to get involved, says Malmborg. The first step, if you’re a property owner, is to inventory your own land. Find out exactly where you have Eastern hemlocks. Note any that look greyish, dull and dry, or are dropping branches. Check the undersides of twigs, on all sides of each tree, for cottony masses. Then comes management: Treat infested trees as needed. And let NYSHI know if you have a problem, or if you spot infested hemlocks in other locales, so they can add your data to their mapping efforts, and perhaps use that site for chemical or biocontrol treatment. If you have a hemlock hedge that appears to be in perfect health, they want to know that too, as baseline data – and a possible future battleground.

A wealth of guidance on how to exercise vigilance against the onslaught of the hemlock wooly adelgid in your own back yard can be found at https://blogs.cornell.edu/nyshemlockinitiative, www.lhprism.org and www.nyimapinvasives.org. The latter even provides an app that can be used to identify invasive species in the field using your cellphone camera.

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