Hemlock Woolly Adelgid – the invasive aphid-like insect noted for its destruction of Eastern hemlocks throughout the eastern United States – was discovered in Kejimkujik National Park in August of 2018, and now Parks Canada is doing something about it.
To date, the insect has only been detected in Nova Scotia’s five southern counties – Queens, Shelburne, Yarmouth, Digby and Annapolis – according to the Canada Food Inspection Agency. In some regions, such as Weymouth and parts of the Annapolis Valley, trees have already begun to die.
Surveys within Kejimkujik National Park show Hemlock Woolly Adelgid (HWA) has infested roughly 30 per cent of surveyed stands, particularly those around campsites. Park ecologist and co-chair of the HWA Working Group, Matthew Smith, said that if infested trees begin to die in the next 3-10 years, as they gave elsewhere, it will radically alter the appearance of the park, and could come with pitfalls above and beyond the loss of this important tree. With $1.4 million from Parks Canada over the next five years, it’s the intention of Matt, his staff, and partners such as the Canadian Forest Service (CFS), to help Kejimkujik adapt and, where possible, fight back.
“We have some really beautiful old growth hemlock stands,” said Matt, making special note of those surrounding Dennis Boot Lake in the park’s northwest which have, like so many others, become infested with HWA.
Long term solutions for this invasive insect are still forthcoming and park staff are eager to preserve what they can in the meantime, especially significant stands like those lining Dennis Boot Lake. For this, pesticides may play an important role.
HWA kills its host tree by latching onto the base of it needles and slowly bleeding the tree dry of nutrients. In order for pesticides to work against HWA, therefore, the chemical mix in question must be injected into each individual tree. One presently being tested by the CFS in sites near Weymouth, Bear River and more recently in the Tobeatic Wilderness Area, is called TreeAzin, its active ingredients derived from the seed of the neem tree. Results have been promising, but it doesn’t last long in treated tree and would require a dose every year or so. The money received from Parks Canada may allow additional trials within Kejimkujik, environmental assessments permitting.
Some neonicotinoid pesticides have proven effective against HWA in the United States, but are extremely controversial for their collateral damage to pollinators when used in Canadian agriculture. It’s been suggested that injecting these pesticides directly into a tree’s bark or roots would contain their unintended consequences, but Matt said this method is no guarantee of safety, as pesticides might collect in the needles of injected trees then fall to the ground, contaminating surrounding soils. Whether this is the case in Eastern hemlocks is an important question which Kejimkujik and the CFS will soon broach.
Scott Robinson is a Nova Scotian arborist presently offering injections of the pesticide Ima-Jet (active ingredient Imidacolprid) into customer hemlocks; Ima-Jet is the only neonicotinoid presently approved by the Pest Management Regulatory Agency for use against HWA in Canada, and protects trees significantly longer, per dose, than TreeAzin. It’s so expensive, however, that to date no one has paid Scott for this service. He has treated some trees on his own dime, but is eager for cheaper neonicotinoids, such as those applied in the United States and in aspects of Canadian agriculture, to receive approval in Canada so he and customers can afford to protect hundreds or thousands of trees more. A lover of hemlocks, he finds it very difficult to stand by as they wither away.
Eastern hemlocks are well represented across Nova Scotia and in Kejimkujik specifically, providing a suite of ecosystem services from the cooling of streams to the corralling of snow. Numerous species of bird and mammal depend upon the old growth, shade tolerant habitats they provide, and they rank among Kejimkujik’s primary attractions. The consequences of their disappearance over the next decade will be substantial, said Matt.
In response, a significant chunk of the $1.4 million granted to Kejimkujik will be spent on silviculture. Matt said many hemlock stands will be strategically thinned with two major benefits, the first being that remaining hemlocks will have less competition for space and sunlight, and will therefore become stronger. There is reason to believe this strengthening will help remaining hemlocks resist HWA for longer.
A second benefit of these thinnings will be the opportunity to plant a greater diversity of tree species in parts of the park, some of them, such as Red spruce, providing many of the same ecosystem services as the dying hemlocks. This boost in tree diversity will make the overall forest more resilient, and will mean something remains standing after HWA has swept through. This is particularly important near campgrounds.
Matt said the arrival of HWA may cause some woodlot owners to clearcut their hemlock stands, a reaction he cautions against. Doing so will result in very young, uniform and shade intolerant regrowth which, ultimately, won’t be as valuable and won’t support the same ecology. Strategic thinning and the promotion of other shade tolerant species within a woodlot, such as Red spruce and some hardwoods, is recommended.
Matt said Hurricane Dorian blew down a great many hemlocks in the park, blocking trails and threatening infrastructure, and cleaning efforts were significant. The arrival of HWA promises to be much worse, and silviculture could help them get ahead of the cleanup.
The Long Game
While the aforementioned measures may buy time, none promote the overall survival of the Eastern hemlock, resigning it to a similar fate as the American elm or American beech.
“There’s no silver bullet; that’s for sure,” said Matt.
A long term solution being applied in the United States is the introduction of other non-native insects who prey exclusively on HWA. The predators so far introduced – a beetle known commonly as Little Larry (Laricobius nigrinus) and a species of Silver fly (Leucopis argenticollis) – naturally control HWA in British Columbia, allowing this parasite and the Western hemlock to coexist. The survival of these introduced predatory insects on the east coast has been confirmed more recently, and perhaps in time they’ll allow a similar balance to that out west.
Matt said the CFS has been exploring the potential of introducing these predatory insects to Nova Scotia as well, drawing on research conducted in the United States and undertaking more of their own. There is also an effort to identify native Nova Scotian insects which may be preying upon HWA already, though, if such insects exist, Matt acknowledges they’re not doing a very good job. The $1.4 million from Parks Canada will allow Kejimkujik staff to help broach the potential of such “biocontrols.”
Another, more enigmatic solution as yet outside the scope of this funding involves a few Eastern hemlocks in the United States which are, by all appearances, naturally immune to HWA.
Studies thus far conducted on these “bulletproof” hemlock groves by the University of Rhode Island reveal they, and saplings growth from them, resist HWA quite well when compared to your average hemlock, perhaps due to markedly different concentrations of terpene and phenolic compounds identified in their needles. While not yet conclusive, these findings raise the possibility of mass replanting with exceedingly rare, genetically immune Eastern hemlocks.
As HWA continues its siege of Kejimkujik, Matt, colleagues and partners will be undertaking fundamental research to determine if the insect kills trees as quickly in southwestern Nova Scotia as in the United States.
As well, in the United States it’s been observed that as hemlock stands die, the abrupt brightening of their understories can cause an explosion of certain shrub or bush species, suffocating local ecology. In particular Matt is concerned about Glossy buckthorn, an invasive bush which has already established itself in Kejimkujik. If the hemlocks overhead die en masse, this bush could take over significant portions of the park. When it comes to HWA, there are a great many unknowns, which this $1.4 million aims to uncover and address.
“We’re at the leading edge of this in Canada,” said Matt.
Zack Metcalfe is a freelance journalist, columnist and author active across the Maritimes. This article was originally published with Rural Delivery Magazine.