Last weekend I was in the Finger Lakes area of New York State attending a Hemlock Woolly Adelgid (HWA) detection workshop. I wanted to have a look at the pest that has been killing entire Hemlock forests from Georgia to Maine over the past few decades, but just as important was meeting the people who are trying to save some of the hemlocks, and hearing about their strategies. There are lessons there for Ontario, where HWA has already been detected.
The pest itself is a tiny sucking insect that can be recognized on Hemlock twigs from January to June, by its white woolly protective covering. While it superficially looks similar to some other forest insects and their byproducts (especially spider ovisacs) it was good to realize that I will be able to recognize it when I see it in Ontario. There are a few tell-tale signs that distinguish it:
- It is attached directly to the twig, not the needle
- It is soft and woolly / waxy, not wet like a spittle bug or tough like a spider ovisac
- It is usually not solitary, there will typically be at least a few on an infested branch or twig, often in a row
HWA has been spreading in the Finger Lakes since it was first detected in 2008, and trees have already started dying. It’s a story that has been repeated many times now in the southern half of Hemlock’s range, with predictably depressing results. But while HWA has been spreading and adapting to cold climates, researchers and land managers have also been learning and adapting; we now have more options for managing HWA than we used to. Specialized predators that eat the adelgid have been introduced from the west coast, and seem to be effective at controlling HWA in some forests. A small HWA-eating beetle called Laricobius nigrinus (lari for short) has become established and bred in forest stands, and Hemlock trees have sometimes started to recover. More recently Silver Flies (Leucopius spp), which feed on HWA eggs, have also been released.
The problem with this strategy is that breeding enough of the predators to control HWA before Hemlocks die, and especially enough to keep up with the rapid expansion of HWA infestations (averaging 15 km/year), is likely impossible. This doesn’t mean we have to give up on having Hemlock on the landscape, it does mean we need to act now, and we have to prioritize which Hemlocks should live while many around them die. Caroline Marschner of the NY Hemlock Initiative identifies the next management steps as:
- Develop region-wide priorities for managing specific stands: choose stands to concentrate efforts on. These would include old-growth forests, forests that are stabilizing steep slopes, or areas of high recreational, aesthetic or educational value.
- Maintain genetic resources: protect trees with systemic insecticides while predator populations establish. Generally large, old trees which are the most valuable genetic reservoirs would be treated while some younger trees would be left untreated, allowing predator populations to grow.
- Early detection and monitoring: in the face of a rapidly advancing front of HWA infestation it’s important to detect HWA as soon as possible after it arrives in a forest. This expands management options and increases the number of trees that may survive.
- Produce predators in the field: predators are in limited supply, so sites are needed around the region to grow predator populations for dispersal. Hemlock hedges work best, so Marschner says they are looking to partner with people who have existing hedges, but they are also planting field insectaries near areas where the predators are needed.
In Ontario we have an advantage, which we could quickly lose, that HWA is not widely established here so we have some time to prepare. For us, early detection and monitoring is going to be critical so that we can catch HWA as soon as it starts to spread in the province. Almost certainly that will be in the Niagara Peninsula, because HWA was already detected (though hopefully eradicated) in the Niagara Gorge in 2013. HWA could easily still be there or elsewhere in the Niagara region and we should be vigilant. We’ll also need to keep a close eye around the east end of Lake Ontario and the Thousand Islands over the next few years, as HWA is moving that way and often makes unpredictable jumps. Since Hemlock forests are more continuous in the Frontenac area, this may become the more important corridor of HWA colonization.
Marschner points out that there are often two factors determining where HWA is detected – one is the movement of the insect itself, the other is public awareness that often lags behind. HWA often goes undetected for years, so if we are actively looking for it before it arrives we stand a much better chance of saving Hemlocks in some of our high value forests. Areas such as the Niagara Gorge, Twenty Valley, or Twelve Mile Creek Headwaters could lose a lot of Hemlocks if we don’t detect HWA and respond quickly. At this stage a response would likely include systemic insecticides around the area of a detection. Biocontrol is something we need to be planning as well, but the essential first step is detection.
Right now is the season to be looking, so if you’re out walking in the Niagara Peninsula before leaf-out, plan your walk to include a Hemlock forest and check some branches when you can reach them (use our monitoring page for tips). Ancient Forest Exploration & Research is doing a study right now to map Hemlock distribution and identify some of the highest value Hemlock forests, and we are also seeking input from those on the ground, so let us know of areas that you think are important.
The approach of identifying high value forests and actively monitoring them for several years before HWA arrives has paid off in Pennsylvania, where some of the most impressive and well known Hemlock forests remain. Volunteers and park staff were monitoring Cook Forest and old-growth forests in the Allegheny plateau for years before detecting HWA in 2013. Because they caught it very early and there was a plan in place to deal with it, some of the most important old-growth forests will likely be preserved for future generations… not just as museum pieces (though they will be more rare than ever) but as genetic reservoirs and seed sources to colonize the landscape.
The final, essential step to keep Hemlock on the landscape as more than a chemically-preserved museum, is biocontrol. We can’t keep treating trees with insecticides forever, so we need to establish predator populations. Forward-thinking individuals in Ontario, Michigan, the Adirondacks, and elsewhere will want to consider planting Hemlock hedges (using local Hemlock stock) which can act as field insectaries to breed HWA-eating beetles and Silver Flies. In the Finger Lakes area they are doing this now, and as Jessi Lyons from the Cornell Cooperative Extension points out, it’s far more difficult and expensive to plant large trees with root balls than small bare root trees. Additionally, trees need time to establish before they can be infested with HWA and then predators. Planting hedges now could make a real difference to Hemlock survival.
Pennsylvania and New York are our models for how to respond to HWA in Ontario, and there is a time limit to get it right before adelgids reach Muskoka and Algonquin Park. Once that happens we’re going to lose a lot of Hemlocks regardless, but being prepared may mean the difference between losing many, most, or nearly all of our Hemlocks. And we shouldn’t be complacent, Hemlock Woolly Adelgid may already be here.