Just 10 miles from Pack Demonstration Forest, a 2,500-acre wood managed by SUNY College of Environmental Science and Forestry, hemlock woolly adelgid is infesting the trees.

It’s too soon to know if the invasive insect is already in the canopy of this forestland in the southeastern Adirondacks. If they are not already there, researchers and land managers believe the question is when, not if, they arrive. 

So to prepare the trees for the coming threat, researchers are conducting a unique experiment, implementing different tree thinning practices. The goal is to get some of the hemlocks more sunlight to see if that bolsters the trees’ health. 

The experiment piggybacks off of silviculture already happening in the southern Appalachian Mountains.

Albert “Bud” Mayfield is a research entomologist with the U.S. Forest Service’s Southern Research Station. He and his colleagues have implemented these forestry techniques and found some success. The Explorer spoke with him for the March/April issue of the magazine for a story about the experiment.

This interview was edited for brevity and clarity. Check out the full story about what’s happening at Pack Demonstration Forest in the March/April issue, which will also soon be online.

Explorer: It feels like it’s always bad news about invasive species, and we’re trying to get a little hope in the magazine. I’m interested to hear about the tree thinning practices going on.

Mayfield: I have been working on silviculture, applied to the hemlock woolly adelgid issue, for about 10 years now. We noticed that trees that were in decline from hemlock woolly adelgid, the parts of the tree that looked the best, were parts that were sticking in the sun, and whether that was a tree that was on an edge, or next to a road, it was like, ‘why does the half of the tree that’s in the road look better than the half that’s in the shade?’ We sometimes would see what we’ve come to call lollipop trees, where you have these big, tall hemlocks, and the only part that’s still alive after woolly adelgid pressure is the very tippy top that’s sticking up in the sun above the main canopy. So that got us thinking about what are the effects of sunlight on adelgid, or, what are the effects of sunlight on the tree that influences adelgid impact? 

In the Northeast, you all have a lot more truly hemlock-dominated stands. Here in the southern Appalachians, a lot of our hemlock is mixed with hardwoods. We would go into these mixed species stands and we started by cutting gaps around the hemlocks. And what we’re seeing through all of this, as you give infested hemlocks more sunlight, they are better able to photosynthesize and produce starch and convert that carbon into new growth. 

So it doesn’t necessarily reduce the populations of woolly adelgid. Sometimes those trees are really heavily infested. But what they’re doing is they’re able to produce new growth because they have more sunlight, more photosynthesis, more stored carbon, and can produce that new growth again in the following spring, even after it has been infested. It’s helping the hemlock tree tolerate the infestation. It doesn’t eliminate the insect. 

And it’s not a silver bullet. What we do think is happening is we’re giving it, temporarily, at least, better crown health in the short-term, 2, 3, 5 years after release. We’re buying time for biological control agents to increase. One of our objectives is to see, can this help reduce the re-treatment interval with insecticides. We’d like to reduce the amount of chemicals that we use to protect trees.

Explorer: Are you using biological controls there? Is that the next step, or is this project trying to keep the tree healthy enough to survive?

Mayfield: It’s not making the trees more resistant necessarily. I would call it tolerance. Think of resistance as more like a genetic predisposition to being able to defend against the pest. This is more of a physiological change in the short-term that gives the tree more tolerance to the infestation. 

One way to think about it is, let’s say you have a relatively healthy hemlock tree, and it’s got lots of new foliage. Then the adelgid comes along and says, ‘ah, that’s a lot to eat.” The tree’s health declines. Now there is a new growth level of the tree with adelgids on it. The tree is in poor health, so the adelgid population drops. The tree gets a break. It’s going to produce some foliage after the break of the adelgid crash. But then, of course, the adelgid is going to come back. That process just goes on and on, but only for a number of times before the tree croaks. And how long that is depends on the environment. Up there in the North, you have colder winters that knock the adelgids back, versus down in the south, it’s more moderate, so we don’t get quite as much winter kill. I think that this gap creation, or thinning, any treatment that would increase the light in a moderate way, could have this effect, a more stable fluctuation of new growth and adelgid over time. And that is good for the predators because if the adelgid population crashes when the tree health crashes, then the predator population also crashes.

So the two beetle predators that have been widely released, one is from the Pacific Northwest, that’s Laricobius nigrinus. The other one is Laricobius osakensis from the Osaka region of Japan. It has been released pretty widely in the south, and we’ve gotten a really good establishment of it. There’s less certainty about how well it establishes in the north. It seems to be well-adapted to the warmer southern climate, which is more like the region of Japan it comes from. We do recover Laricobius nigrinus and Laricobius osakensis from these hemlock trees that we’ve released. 

We also have been releasing the Leucotaraxis, or silver flies, here in the southern Appalachians. We have not had any recoveries of the western silver fly lineages that we have released down here so the jury is still out. 

The silver flies, two species that are being released in New York, from New York to North Carolina, are from the western United States. But, they also occur in the eastern United States. Same species on pine, on pine bark adelgids. But in the west, the silver flies out there are almost exclusively on hemlock. So it was thought that maybe we could release the western lineages of these two silver fly species in the east, since they’re better adapted to hemlock. So that’s the effort to try and establish those western silver flies. What we are finding as we try to recover them, is that the eastern lineage of the silver fly species that are normally on white pine, they’re also using the hemlock woolly adelgid. We find them on hemlocks, too, but just not in levels that are controlling it. 

Explorer: Could the eastern silver fly species naturally adapt to eating adelgid, or do you need to release the western ones?

Mayfield: The thought is we still need to release the western ones because the eastern lineages have been here all along. Hemlock woolly adelgid was first identified in eastern North American and Virginia by the 1950s, so the adelgid has been here for 75 years. The eastern silver flies haven’t controlled it. 

It’s confusing because there are two species. Both of those occur on each coast. So you’ve got two species, two genetic lineages that are geographically distinct. The hope is that the western silver flies that focus almost exclusively on hemlock woolly adelgid in the west, if they were introduced here, they just really hone in and provide population control of hemlock woolly adelgid. 

One of the wrinkles is that the hemlock woolly adelgid we have here in the east is not the same genetic lineage as the one in the west from which the predators came. Our hemlock woolly adelgid is from Japan introduced onto eastern and Carolina hemlocks that had no adelgid species, whereas the western hemlocks coevolved with HWA. Still the same species, but it’s a different genetic lineage. So there’s something about that that makes it more difficult. 

Explorer: The state was very excited that they found establishment of silver fly in the Finger Lakes region, so I’m guessing, then, that that was the western lineage.

Mayfield: That’s right. 

Explorer: But the jury’s still out, like you said, for our area. 

Mayfield:  Yeah, I mean, the recoveries of establishment have been a very small number of insects. So what you want to see is to come back year after year, and yes, we’re finding more. Or, finding it at more sites. You want it to have an impact, and you want it to disperse. 

Explorer: There’s something I meant to ask back when you were telling me about opening up the crown of the tree. We talk a lot about the shade benefits that hemlocks provide, especially for trout streams, fish that need the cold water. I’m wondering whether that impacts that ecosystem service? 

Mayfield: It’s a limitation of the management technique, for sure. Hemlocks not only tolerate deeply shaded conditions, but they also create the deeply shaded conditions. If you go in and start opening up gaps in the canopy, you’re changing that environment and you’re creating, not only differences in sunlight and temperature, but you’re creating the opportunity for shade intolerant species to establish. We looked at this across a couple of different sizes of gaps, and the ways of creating the gap, and the bigger the gap, the more of that growth. So, if you’re going to keep it as an environment that has some light on the hemlock, you will probably have to go in there and retreat or recut. 

We have not done this on streamside hemlocks. This has all been more upland work, so you might not be able to, or even want to do it in those streamside environments. 

Explorer: We mentioned up here the climate is a little different. Is there anything else you’d be interested to see at the Pack Demonstration Forest, besides how the climate may impact the experiment or is there anything else you might expect to happen?

Mayfield: I’m also really interested to see how it works in stands that are a heavier conifer component than ours. If you do this in a primarily hemlock stand, or a mixed hemlock-pine, or mixed hemlock-spruce, the filtered light would be a lot less because those species are going to have canopy year-round. And then, yes, certainly what you said, how effective is it with the colder climate? I imagine you may be able to get even more benefit from it over the long-term in the Northeast because the competing vegetation is going to grow back slower, and the adelgid is going to get knocked back more frequently. 

Explorer: Is it also unique that they’re doing this ahead of an infestation?

Mayfield: Yeah, all of our sites are in places where the adelgid has already been, and for the most part, was for several years if not a decade or more. I don’t think it’s going to prevent the stand from becoming infested, but I think it will help the trees cope once the adelgid gets there.

Explorer: Do you have an overall feeling whether these different management tools provide hope?

Mayfield: It’s hard to feel like we’re winning. I wouldn’t say that. Somebody asked me that at a talk one time, ‘So, are we winning?’ And I was like, ‘Nah.’ But do I have hope? I do. I don’t think any one single management strategy is going to keep hemlocks on the landscape long-term as a viable species that we’re going to manage. But I think an integrated management approach, so some silviculture, biocontrol, judicious use and targeted use of insecticides, especially on high-value trees that are big and old and are seed producers, and with some work on resistance. There’s people working on that. There is not only an effort to find resistance in our natural populations of eastern hemlock, but also some breeding efforts. 

I’m kind of hopeful for this three-legged stool of biological, chemical and silvicultural control, if we can combine those in effective ways, especially if we can get the silver flies on board, that would really help. The laricobius beetles feed really well through the winter and early spring, but they’re one generation per year. So they drop to the soil after they finish feeding. Meanwhile, the adelgid is producing another generation, so that second wave of adults is not being controlled, and that’s the hope for the silver flies. That would help bracket that whole HWA adulthood life cycle.

Explorer: So that seems to be the missing piece.

Mayfield: That’s the missing piece. 

Explorer: My last question I have is in the Adirondacks, especially with the forest preserve being constitutionally protected from any tree cutting, there’s an inherent opposition to things like that. How do you destigmatize that?

Mayfield: I don’t know if you can destigmatize it for everybody. It basically comes from a different philosophical perspective about what forests are and what they’re for. The Forest Service has been more of a ‘let’s manage the forest for the maximum good for the greatest number of people.’ And the Park Service has been, ‘let’s set aside wilderness and areas that are undisturbed by man.’ And I think there’s value in both of those approaches. I’m glad that we have a Forest Service and a Park Service. And that we have state forests, that we have the Adirondack preserve. 

I think one of the things that maybe helps people understand why you want to do this is that invasive species have changed the equation for many of our ecosystems. When you have something come in without its normal complement of natural enemies that keep it in check, or you’re exposing our native trees to an organism to which they are naive, they have no evolutionary history with this. The trees are just really vulnerable. And so we have to decide, we can continue to do nothing, but we’re going to watch trees die. So therefore what can we do with sensitivity to what impact that’s going to have on the ecosystem, but also with the knowledge that if we don’t do something, we may lose this species completely. I think that applies to whatever the management tool is, whether it’s an insecticide or a biocontrol agent. Some people are very against biocontrol agents, and so it’s the same argument. Yes, we could leave it alone, but at what cost?