The Local Climate – The Mighty are Falling

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The Eastern hemlock is a force of nature in Nova Scotia.

Not only is it among the most common trees in the province, it’s invariably the oldest. It was routinely ignored by foresters past and present in favour of more valuable species like red spruce, white pine and hardwoods more generally. As a consequence the vast majority of our remaining old growth forests are dominated by hemlocks, some older than European settlement, sheltering between them provincially rare biodiversity.

What’s more, hemlocks are ecosystem engineers. Because their needles cast a steady shadow year round, they have the effect of cooling streams which flow underneath to the benefit of spawning fish. As well their firm canopies collect and pile snow in a way which creates shallow walking paths for wildlife in the throes of winter. The number of species to who they provide habitat directly, especially birds, is difficult to quantify.

Anyone who’s spent enough time in the remaining ancient forests of Nova Scotia has fallen in love with these trees – tall and straight, steely and dignified, old and strong, and in 10 years they could all be dead.

Hemlock Woolly Adelgid (HWA) is an aphid-like insect native to Asia which, in the long tradition of invasive species, was accidentally introduced to Virginia in the 1950s where it first encountered the Eastern hemlock, a tree completely ill-equipped to resist the advances of this veracious insect.

Over the last 70 years, HWA has raged across the eastern United States fatally infecting hemlocks in every age group. It attacks its host by attaching to the base of the needles and bleeding the tree dry over 3-10 years. Photos are not hard to find of hemlock forests, entire oceans of green, shriveling to a dead grey.

This invasive’s first sojourn into Canada took place in Ontario, in 2011 and again in 2013, both attempts turned back by the careful application of pesticides. We wouldn’t see it again on our side of the border until July of 2017, near Weymouth, Nova Scotia. Followup surveys have since revealed its thorough infestation of our province’s five southern counties – Queens, Shelburne, Yarmouth, Digby and Annapolis – where it has thus far been contained by restrictions on the moving of wood products. It’s even been found in the old growth stands of Kejimkujik National Park and the Tobeatic Wilderness Area, promising to kill the oldest trees in the Maritimes.

We’re facing the ubiquitous destruction of our most important tree. There’s no hope of eradicating the insect outright, and so government agencies, regional charities and academics on both sides of the border have been scrambling for solutions. We have some very difficult decisions to make.

Pesticides are an obvious response to this infestation, but they have their drawbacks. First, given the sucking nature of this insect, the most effective pesticides would have to be injected directly into each individual tree, immediately making them more costly, and making landscape scale applications impractical. What’s more, when this insect arrived in Nova Scotia, there were no pesticides on the market approved for use against HWA in Canada.

The neonicotinoid Ima-Jet has since been added to the list, but is so expensive to apply that, to date, no one has purchased the service in Nova Scotia, at least not that I’ve heard. Neonicotinoids are extremely controversial for their killing of pollinators when used in Canadian agriculture, and so we aren’t likely to approve others for use against HWA anytime soon. It’s possible injecting neonicotinoids directly into the tree will prevent its contamination of the surrounding environment and make them a relatively safe response to HWA, but this is a contention we have yet to study in depth.

Another pesticide, called TreeAzin, is presently being put through its paces by the Canadian Forest Service in sites near Weymouth, Bear River, the Tobeatic and perhaps soon in Kejimkujik. Botanically derived from the seed of the neem tree, it’s an environmentally sensitive alternative to neonicotinoids, but it doesn’t last in treated trees nearly as long and so would require more frequent injections. Again, we run into barriers of cost and scale. At best, these pesticides will allow us to preserve our oldest and most significant hemlock groves for a while longer, but will do nothing for the species as a whole.

The long term solutions presently on offer are big and ambitious, the first being biological pest control. HWA, you see, has been on the western coast of North America a few thousand years already, but it’s not a problem for resident Western hemlocks because several predatory insects keep HWA in check, eating them into balance. If those same predatory insects were taken from the west coast, in this case British Columbia, and introduced here, they might control our HWA infestation and allow some kind of ecological balance. Maybe.

Two of these predatory insects – a beetle known commonly as Little Larry (Laricobius nigrinus) and a species of Silver fly (Leucopis argenticollis) – have already been introduced to some New England states and seem to have successfully established themselves. Similar introductions are being considered in Nova Scotia.

Another potential long term solution has been identified more recently. Now that HWA has killed billions of trees across the United States, it’s been discovered that a very few hemlock stands are, by all appearances, naturally resistant to this insect. Research already conducted by the University of Rhode Island suggests this might be because of markedly different concentrations of terpene and phenolic compounds in their needles, a genetic quirk which seems to shield them from the worst of HWA. While not yet conclusive, these findings raise the possibility of mass replanting with genetically resistant Eastern hemlocks. It won’t save Nova Scotia’s old growth forests, but it could save the species from extinction.

It’s difficult to know exactly what to do. Some jurisdictions, such as Kejimkujik, are undergoing the thoughtful thinning of hemlock groves so the ones they leave standing have better access to space and sunlight, strengthening them for their fight against HWA. In the margins they’re planting a greater diversity of trees such as Red spruce, strengthening forest health overall and ensuring something will remain standing after HWA runs its course.

The future of our Eastern hemlocks is very uncertain, but we can’t give up on them just yet. Perhaps one of the aforementioned solutions will come through, or we’ll uncover altogether different options. Over the coming years, it’ll behoove us to keep one eye on the canopy.

Zack Metcalfe is a freelance journalist, columnist and author active across the Maritimes.


Shown above is a stretch of Nova Scotian old growth forest dominated by Eastern hemlocks, some predating European settlement. (Zack Metcalfe photo)