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The Spongy Moth in Our Yards and Forests

Spongy moths, an invasive forest pest has recently resurged. Outbreaks of very hungry caterpillars have defoliated trees in the Hudson Valley and beyond. Will this happen again in 2024? Will defoliated trees recover? Can we do anything to protect our trees and forests?

This event features Cary scientists Clive Jones and Charles Canham, who bring deep expertise on the topic. Jones studied the spongy moth for 30 years as part of a long-term project on Cary’s 2,000-acre research campus. Canham has been researching the ecology of Northeastern forests for 40 years. 

Spongy moth caterpillars prefer to feed on oak trees but will defoliate a wide variety of deciduous species, and sometimes conifers. Learn where the spongy moth came from, how it was introduced, how it spread, and the history of outbreaks and defoliation in the US. Discover how outbreaks get started and why they collapse. Find out about forest pest impacts now and in the future. 

The program features a Q&A session where Jones and Canham answer audience questions about the spongy moth.

Resources & Talk Transcript

Cary-led initiative advancing a set of science-based policy recommendations designed to tighten regulations on trade and shipping materials to stop the spread of imported forest pests.

NYS Department of Environmental Conservation (DEC)
Spongy moth fact sheet (pdf)
Spongy moth

Spongy moth
Spongy moth leaflet (pdf)

Clive G. Jones, Richard S. Ostfeld, Michele P. Richard, Eric M. Schauber and Jerry O. Wolff. Chain Reactions Linking Acorns to Gypsy Moth Outbreaks and Lyme Disease Risk. Science, Vol. 279, No. 5353 (Feb. 13, 1998), pp. 1023-1026 (4 pages)

Richard S. Ostfeld, Clive G. Jones and Jerry O. Wolff. Of Mice and Mast. BioScience, Vol. 46, No. 5 (May, 1996), pp. 323-330 (8 pages)

Of Mice and Moths--and Lyme Disease?


Spread: 1900-2007

spongy moth spread

Defoliation: 1972-2007

spongy moth defoliation

Joshua Ginsberg  0:02  
I looked at the list of people who have signed up for this one, and it's mostly, not surprisingly, Northeasterners. You'll know more about why that's not surprising after you hear from our speakers on the spongy moth in our yards and forests. Our speakers tonight are both Cary emeritus scientists.

Joshua Ginsberg  0:18  
Now emeritus means retired, but don't hold your breath; I don't think Cary scientists retire very well. Clive Jones, who will be our first speaker tonight, Clive, you are you are original NYBG; you got here before everybody else. And Clive has been thinking about spongy moths on and off, I had to make sure I cleared this, for over 30 years. And Charlie Canham, who is also an emeritus scientist; we're going to do a hybrid lecture, so Clive will talk in real life and then we will hand off to Charlie, who will talk and then we can answer questions. Charlie has also been part of a project that I think Clive will discuss in some detail.

But as with everything at Cary Institute, we're really interested in how things work, because we like to say you can't solve problems unless you understand them. And so Clive and Charlie and a number of other people spent decades trying to figure out the factors that drive spongy moths, although when they started, it was driving gypsy moths. They outlive the name and the project.

I will just pass to Clive now with a very quick comment that about two years ago, I called Clive, because we were getting, the spongy moths were getting worse and worse, and I thought I better know what I'm talking about. So I knew who to call and I called Clive, and I left New York City. And 49 minutes later, I arrived where I was going and our conversation was just ending. And I thought what a great thing we can do at Cary; a Cary Science Conversation or Friday Night at the Cary. We could do something about this because so many people seem to care. So Clive, it is a pleasure. I'm glad we are doing that. And Charlie, welcome, and we look forward to hearing from you in a little while.

Clive Jones  2:14  
Well, thank you, Josh. I won't talk for an hour! But I'm going to talk about the spongy moths in our yards and forests. And what I'm going to do is tell you where the spongy moth comes from, when it was introduced, and what happened next; then give you a little bit of background in relevant life history and ecology that will help you understand what I then say about the causes of outbreaks and their collapse. Then I'll talk a bit about why this current outbreak and what you can expect next and in the future. And then a little bit about what you can do about the moth. Then I'm going to pass it over to my colleague Charlie Canham, who will talk about what will happen to the trees and the forest.

So the spongy moth is Lymantria dispar and it's native to Europe and Asia. But in North America, it's now classified as a subspecies Lymantria dispar dispar. It was introduced in Medford, Massachusetts in 1868 or 69 by Etienne Trouvelot, a French amateur entomologist and erstwhile entrepreneur who thought he could help develop a silk industry in the cold Northeast by crossbreeding the winter-hardy spongy moth with the cold-intolerant Asian Silk Moth. Now that wouldn't have been possible, but nevertheless, he got some egg masses from Europe, opened the box, put them on the windowsill of an open window when they blew into his garden. He couldn't find them. But he did report it to the authorities.

This is the Medford area 20 years later; the crosshatched areas in the middle are where the defoliation was complete. And the 200-square-mile boundary is where the spongy moth was recorded as being at least present.

So how did they get from Trouvelot's backyard to that 200-square-mile area when the females of the North American subspecies are flightless? Well, there are two ways. First, the early stages of the caterpillars balloon on silken threads. Most of them go a few yards from tree to tree, but occasionally they get caught up in the wind and go much longer distances. And second, and most important, is people move egg masses around on cut timber, on garden furniture when they move homes, and on the underside of vehicles such as the RV that they take on holiday somewhere else in Spring.

It has come a long way in 150 years . This animation from the Forest Service will show its spread more or less annually from 1900-2007, although there are breaks in the data. As you can see, it has spread steadily west, and south, and north, but for some reason not into Canada; I don't know why. No it is in Canada; this is just U.S. Forest Service data! All of that spread is due to larval ballooning and people moving egg masses around. And it's still spreading.

Okay, so the spread has been accompanied by periodic outbreaks, defoliation, and collapse. And by that, I mean there's rapid increases in moth density, and then they come back down again, with defoliation at the peak. Now these outbreaks are not unique to North America, they occur in the native range. This is Serbia from 1945 to about 2015. Itis an obscure axis -- infested area in hectares times 1,000 -- but that peak there represents about 2 million acres defoliated. Outbreaks occur about, and this really approximate, 10 years on average, and that's really an average. They can be relatively synchronous over very large areas, and can lead to extensive defoliation, such as over 9 million acres in the outbreak peak of 1981.

This is going to show you another Forest Service animation. The moving red blobs you see here are defoliation; the data go from 1972 to 2007. Keep your eyes on the 80s...and then again in the 90s. Then as this outbreak wanes away, things stay pretty quiet up until relatively recently.

Clive Jones  7:33  
Okay, so now a little bit of relevant life history and ecology that will help you understand what I later say. In late summer, the females emit a chemical signal, a pheromone, that attracts the males that develop a little earlier over quite long distances. They mate, she lays an egg mass containing a few hundred eggs protected by scale hairs. In low densities, these egg masses are laid at the base of trees often in bark crevices. But as density goes up, egg masses get laid all the way up the tree and on the side branches and on your garden furniture and on the underside of your RV.

The egg masses overwinter and hatch out in spring more or less synchronously with bud break of their preferred host, oaks. And they then go through five stages or instars for males, and six instars for for the larger females, the big one at the bottom. The early stages -- first, second and third instars -- balloon on silken threads as I mentioned earlier. As the density of spongy moths goes up, a greater and greater and greater proportion of the larvae start to balloon and disperse. In low densities, the later stage larvae spent the day at the bottom of the tree resting, going up to feed in the canopy at night. But as density rises, more and more of the caterpillars spend all their time in the canopy feeding nonstop. And when the food has run out, they move across the defoliated canopies and all over the ground. They then pupate at the end of the summer, which takes a couple of weeks; the smaller male is at the top and the larger female beneath. The cycle repeats itself after the female emerges, mate, and lay eggs.

So next I want to tell you a bit about natural enemies of the spongy moth that don't really make a big difference to outbreaks or their collapse. They do kill the moth; they include native species and species we introduced to try and control the moth. There are predators, parasites, and pathogens, and collectively, they attack all the life stages. But their failure to kill doesn't cause outbreaks. And at best, they can help an outbreak that's already collapsing, collapse further. Why is that? Well, they kill too few moths; there's better food elsewhere. And/or they're not very abundant, so even if they do kill moths, they don't kill many. And/or they're reasonably abundant; they do kill moths, but they don't increase in abundance as the spongy moth rises in density. Ans/orr they do all of the above and they do increase, but they do so too slowly to be able to catch up with, overtake and bring down a rapidly rising moth population.

So they include birds such as this cuckoo, one of the few species in North America of birds will eat hairy caterpillars, most will not; but cuckoos are not that abundant. They include most small mammals, with one major exception we'll talk about in a minute. So the smoky shrew, for example, will eat larvae and pupae they find on the ground, but larvae and pupae don't spend most of their time on the ground, and shrews are not that abundant. Chipmunks can go up trees and are very abundant; however, they don't eat larvae or pupae. And finally, there are things like caterpillar hunter beetles, and ants and spiders that will eat larvae and pupae, sometimes in reasonable numbers, but in general, most of those species do not increase in abundance as the spongy moth increases. Then there are parasitic wasps that lay eggs inside eggs inside an egg mass, and that parasitize caterpillars, sometimes again, in reasonable numbers, and they do increase in abundance over time, but they lag well behind the spongy moth; so at best, they can come in towards the end of an outbreak. And finally, most pathogens, with two exceptions we'll come to in a minute, have no effect. So this is the naturally occurring Bacillus thuringiensis; it occurs in soil, so it has little or no contact with the caterpillars. And more importantly, in order to kill a caterpillar the bacterium has to be eaten. This is in contrast to the BT that you spray on the leaves of trees in high doses, which does get eaten and does kill caterpillars.

So let's turn then to what does cause spongy moss outbreaks and then their collapse. There are two causes. The first cause is high female fecundity, which imbues them with the capacity for rapid rises in density. So imagine that every egg in an egg mass survives to adulthood, and there are about 500 eggs per mass. And about half of those eggs are females -- both of these are reasonable numbers --then the maximum theoretical rate of increase of the spongy moth is one times 500 times 0.5 equals 250-fold each year. The maximum we've actually observed in the field is about half that, 125-fold. So think about that. In year one, you have one egg mass; in year two, you have 125; in year three, you have over 15,000 egg masses, and that is more than sufficient to cause complete defoliation which is increasingly likely once you get above 2,000 egg masses per acre. The highest we've ever recorded before collapse of an outbreak was 6,000 masses per acre. So preventing outbreaks requires keeping moth density really low, otherwise it's just going to escape and rise very rapidly.

And the second and important cause of outbreaks is mouse population collapse. This is the white-footed mouse Peromyscus leucopus. It's the most abundant small mammal in our forests and it is a voracious predator on moth pupae; not larvae, not egg masses, just pupae.  And as you can see, they can go up trees to get to pupae.

This shows you how long a female pupa is going to survive in the forest as the percent of pupae remaining over time and as a function of mouse density. It takes 12 to 16 days, about two weeks, for a female to complete development, emerge, mate, and lay eggs. This particular study is done with freeze-dried pupae affixed to burlap panels using beeswax with the panels at the base of the trees. The beeswax leaves characteristic marks of who attacked the pupae; in this case, the tooth marks of the white-footed mouse. We've also done this experiment with living pupae removing the females and any egg masses laid; we get the same result. And we've also done experiments in which we live-trapped mice on large areas -- large grids -- when egg mass density was low, moving  the mice to an area of Cary forest quite some way away, whereupon pupal survival went up and egg mass density went up compared to the control grids where we didn't remove mice. So what this shows is that in '93, where you have seven mice per acre, a moderate mouse density, none of the pupae survive (or persist because they're freeze dried in this case) beyond eight days, which is well short of the time it takes for a female to complete development. In contrast, in '94, when you have one mouse per acre, 50% of the pupae are still remaining 18 days after deployment, well beyond the time it takes for a pupa to develop as a female and lay eggs. So what this means is that moderate-to-high mouse densities keep moth populations low;  can drive them to even lower levels; and, within reasonable limits, can prevent a moth population that has started to rise from rising further. But low-to-very-low mass densities allow outbreaks to start.

Now, mice eat moth pupae, but moth pupae do not directly affect how many mice there are. That's because mice are omnivores. They eat insects and worms and the eggs and nestlings of ground-nesting birds and a tremendous variety of different species of seeds. So moth pupae are just a minor part of the diet, a two-week snack for mice that has major consequences for moths! Instead, the number of mice is determined by the number of acorns produced the previous fall; what's called masting.

Clive Jones  17:32  
So this shows you five of over 30 years that we've been doing this at Cary, on two of six grids that we've been doing this on. “Mast” means some-to-many acorns and “no mast” means no acorns. Okay, so with moderate-to-large acorn crops, as you see here, many mice survive the winter living off acorns they cached, or stored; they start reproducing in late winter/early spring, packing in an extra mouse generation. So by the time the moth pupates, there are high densities of mice. In contrast, when there's no mast or very low acorns, few mice survive the winter. They start reproducing much later, and you get low mouse densities by the time the moths pupate. We've seen this basic pattern for 30 years; we've also done experiments at low moth density, when, in a non-acorn year, we dumped about a million acorns into some of our grids whereupon the next summer the mouse density was increased. At the same time, the increase in mice brought down pupal survival and egg mass density. In the controls where we didn't add acorns, mice did not increase. This basic pattern between acorns and mice has now been demonstrated in a number of locations by other people.

So, I said the moth doesn't directly affect the number of mice, but oak defoliation can reduce acorn production as you can see here with moderate to high and then low defoliation in '80 through '83 and no acorn production during that period. So what that means is that by suppressing acorn production via defoliation, moths can suppress mouse populations and thereby increase the risk of another moth outbreak.

Okay, so let's now move to the causes of collapse. There are three, and the first is the fungus Entomophaga maimaga indicated by this head-down desiccated cadaver to the right of the pupa. It was introduced from Asia in 1910 but failed to establish, but then was reintroduced in the late 1980s and is now quite widely distributed in the Northeast. And if it's established in an area, it will persist for reasons I won't go into. It kills some larvae each year at low moth density. And it can kill many larvae within a year at moderate to high densities, and as a consequence can curtail outbreaks with low-to-moderate defoliation before collapse of the moth population. We don't understand exactly what causes this mass mortality, but we do know that it's more likely to occur when larval density is moderate or greater, AND spring is both wet and cool, where cool means there isn't a period of four or five days where the temperature is more than 80 degrees Fahrenheit, which inhibits the fungus.

The second cause is the Nuclear Polyhedrosis Virus, or NPV, as indicated by this soggy inverted V of a caterpillar that just died from the virus. It's naturally occurring. It's always present. It's mostly sub-lethal in low moth density populations, but it's highly lethal at high moth densities, and it's the primary cause of collapse of gypsy, spongy moth, beg your pardon, in North America. So why this transition from sub lethal to lethal? Well, it turns out that increasing moth density increases larval competition for food and for resting space, which increases the stress on the larvae which decreases their immunity, which increases their susceptibility to the virus and viral mortality.

And the third and final cause of collapse is food limitation. You run out of food and you can't complete development and produce eggs. It's also true, of course, that later-stage larvae do eat non-oaks but their survival and their fecundity is much lower than on oaks. So food limitation can and does bring about collapse, and as I pointed out via the stress it causes on caterpillars, it can boost the efficacy of the virus.

I want to end this part of the session on collapse and outbreaks by showing you all the players together over 30 years at Cary for Julie Hart's benefit. This shows 1980 to 2010, egg masses per hectare on the left and per acre on the right. Each of these divisions is on a logarithmic scale, and is 10 times more than the division below it. The lowest density we've ever seen was one egg mass in about 25 acres of forest. And the highest density we've ever seen was 6,000 per acre just before the collapse. Each of those data points is the year in which the egg masses were laid, so all the action I'm going to talk about occurs in the spring/summer following that year of over oviposition.

So the outbreak ongoing in 1980 led to extensive defoliation; that outbreak collapsed due primarily to food limitation and the virus, with some invertebrates playing a role towards the end when the outbreak hadalready collapsed. There was no fungus. It hadn't yet arrived. An outbreak began in '88 probably due to acorns and mice based on what we now know, but we don’t really know because we didn't start monitoring mice and acorns until '91. That outbreak was curtailed with low patchy defoliation of about 20% due to the fungus. The virus was detected but didn't play a role. From then on, this entire low-density oscillation from around one egg mass per acre to 10 egg masses per acre was due to mice and acorns. When acorns failed, mice failed, moths went up. When acorns were produced, mice went up, moths came down. Throughout that period, the fungus was always detected and occasionally you could see that the virus had killed one or two caterpillars.

All right. So, let me talk about the current outbreak. So this shows acorns, mice, fungus, virus, and defoliation and egg masses from 2019 to 2023. This is reconstructed with the help of colleagues here at Cary. We have data on acorns and mice, but the fungus, virus, defoliation, and egg masses are just based on observations. So there were very high acorns in the fall of 2019 that led to a very high summer mouse population. These are likely the conditions that maintained a low density of spongy moths, as has been the case for a long time. There was low acorn production in 2020, which led tovery low mice the next summer. These are the conditions in which outbreaks get started, and based on the subsequent trajectory, this is when we think the outbreak got started. In 2021, there was a moderate acorn crop and a reasonably high mouse population in 2022, but it looks as if it was too late. Our mathematical models show that if the moth density starts to get much above 10 masses per acre, mice can no longer control it; so that's what we think happened here. Wedetected the virus and the fungus but they didn't really do much, and there was an observed increase in egg mass density. 2022 had very low acorns leading to very low mice the next year. It didn't help, but by then, the moth has already escaped from mouse control. Again, the fungus and virus were detected but had no real big influence. Spring that year was drier than usual, so conditions may not have been as conducive to the fungus as one would hope for. And as a consequence, there was 50% defoliation, and another increase in density. This is the defoliation on the Tea House hill of about 50% in June and complete re-foliation by the end of July. The conifers at the top of the Tea House hill you see there were unaffected.

So what next? So the best guess for Cary is that in 2024 we will have an NPV–induced collapse, plus or minus the fungus depending on the weather. Whether or not there'll be a lot or a little defoliation will really depend on how fast the virus moves through the population. If it moves slowly, there'll be complete defoliation. If it goes through fast, there'll be incomplete defoliation.

Clive Jones  27:28  
And if that doesn't happen, and the density continues to rise or stays at the level it is, then a viral collapse in 2025 has a much higher probability, because by then food limitation and stress will be kicking in. After that, a continued  collapse to low density, and a period of unknown duration when the moth will be rare due to the mice. But you should expect outbreaks in the future with the severity being determined by the efficacy of the fungus, which will, in turn, depend more or less on whether we get more or less cold, wet springs.

I want to end this section with what can you do about the moth. So this is a long list of interventions with those you can't do in New York crossed out in red. You can read all about them. What they rarely tell you is when you might expect them to work. So I'm going to give you four guidelines.

Guideline one: Outbreak stage as years after mouse failure. So, early on, there's low-to-moderate density;  there's potential for having some influence. Egg masses are accessible at the base of trees, so you could remove them or treat them with dormant oil.Larval immigration via dispersal occurs but it's quite limited, so sticky tape and burlap bands around trees also will work.

 In the middle of an outbreak, when you have moderate to very high density, you have very limited options. You can't remove the egg masses because now they're all over the trees where you  also can't treat them with dormant oil. Larval immigration via dispersal is high to very high and a lot of larvae stay in the canopy anyway, which means that sticky tape and burlap bands can play little or no role. So your options are BTK, but you can't use it near water. It requires carefully timing two applications, and you have to hope it doesn't rain just after spraying because that washes the BT off the leaves or, for that matter, it's not too sunny because the UV inactivates the BT too. It's the lowest risk of all the options. The effects on long-target Lepidoptera, which is the only other sort of susceptible tree-feeding insect, are limited because most of those species hatch and develop much later in the season. Acephate systemic tree injection is still allowed; it carries a risk of girdling the trees and killing them. You can only use it on non-bearing trees, which means they mustn't fruit, flower, or produce seed within a year of treatment. It kills most, if not all, leaf-eating insects, and there is a risk of non-target effects on invertebrate and vertebrate predators that then eat poisoned insects. Late in the outbreak, the density is falling or at least not rising. You probably have a lot of fungus and NPV, there are epizootics going on; the outbreak's ending, so you don't need to intervene.

Second, what's your goal? A fully accessible tree or a sapling is much easier to deal with than a lot of tall trees, which is much easier to deal with than a big chunk of forest. Third is the area of intervention. So small areas are easier to influence than very large areas. But you have to bear in mind that surrounding areas can overwhelm whatever local influence you're having due to immigration and dispersal. Fourth, and as a consequence of that, location really matters with respect to the surrounding forests. So isolated trees, you might have an influence on, which is much more viable than trying to deal with the ones next to the forest, and certainly way more viable than those that are deep in the forest.

I want to just point out that Forest Service and New York State DEC no longer intervene except insome very special circumstances. And as a consequence, the spongy moth joins a really long list of introduced forest pests and pathogens that we are unable to control once they arrive. And so with that, I'm going to pass over to my colleague Charlie Canham remotely and who's going to talk about what will happen to the trees and the forest. So over to you Charlie when I press Escape. Magic!

Charles Canham  32:52  
Okay, I'm going to share my screen and just go to my slides. Okay, spongy moth as Clive has said, is just one of a much longer list of the introduced pests and pathogens that are arguably the most pervasive and significant threat to Eastern forests. This is a slide I typically use when giving talks about the future of Eastern forests, and you'll notice that spongy moth is not actually on the list of the pests and pathogens that I consider the greatest threats. In fact, in the chapter in my book, Forests Adrift, about the future of Northeastern forests, I call out spongy moths as being distinctive in that defoliation by the caterpillars is rarely a direct and immediate cause of tree mortality. I'll explain why and mention an exception to that generalization in a minute. Compare that to the first six pests on this list, each of which has caused decimation of the host tree species they attack. I'm sure many of you have dead or dying ash trees on your property or in your neighborhoods. Emerald ash borer has caused the most rapid decline in native tree species of anything on this list, in part because the insect is such an effective disperser. Basically, all of the more than a dozen ash species in North America are currently listed as critically endangered by IUCN. I put spotted lantern fly and beech leaf disease on this list, even though it is far too early to know just how destructive these will be.

And that highlights one of the most enduring challenges of dealing with the emergence of new pests and pathogens. It can take years to do the research; first to know how serious the threat is, and then to find the ways to combat it. The work that Clive has described, and not just his own work, but the work of many others, literally took decades so that we can understand the spongy moth's system. And that highlights the other enduring challenge. How do we stop the emergence of these threats, particularly new pests and pathogens? And I'll talk more about this in a minute. But the bottom line is that we now know that, aside from the very colorful history of spongy moth introduced through entrepreneurial misadventure, there are really two main routes by which they are introduced. For wood-boring insects, the vector's through solid wood packing brought into the country when goods are imported. Other insects and diseases hitchhike on the importation of live plants. So it's no surprise but probably shocking to you the degree to which trade through Northeastern ports has led to such a concentration of nonnative pests. And unfortunately, once here, they inexorably spread across the country.

Let's see. So, spongy mouth has been around for long enough that there is a reasonable body of evidence about its impacts, and Clive has described the incredible detail we have from his work on the ecology of the insect and the factors that control outbreaks. We frankly have much less detailed information on the impacts of defoliation on our tree species. But the bottom line is really very clear, complete defoliation in early summer, even for two years in a row kills very few trees directly. As horrifying as it is to witness the defoliation, to understand why trees are so resilient to defoliation you need to understand their carbohydrate economy. Basically, photosynthesis during the growing season produces the sugars needed to produce new tissues, and the energy those tissues need for their metabolism. But the even more important outcome of a good growing season; essentially the profit left over after meeting those immediate needs, and that profit is in the form of sugars and starches that act as reserves for use next year. As an aside, those carbohydrate reserves are also critical for the cold hardiness of the trees; they essentially act as antifreeze and next spring, those reserves are mobilized to produce the new wood needed to conduct water to new shoots and leaves as well as provided the energy needed for the first stages of stem and leaf growth. And then as the season progresses, the leaves are recouping that investment in reserves and the time required for that varies enormously. But there's little question that complete defoliation in June by spongy moth happens right around the time when the leaves would be starting to produce a profit and replenish reserves. Luckily, it's really very clear that the spring flush of growth and mobilization of those reserves doesn't completely use up last year's reserves and the trees very rapidly produce new leaves, as I'm sure most of you observed on your trees.

And understanding the role of these reserves helps explain, you know, one of the exceptions to this generalization that defoliation rarely kills trees. The Canoe Hills here at the Cary Institute had a really abundant understory of hemlocks 40 years ago. Hemlocks are one of our most shade-tolerant trees, and over time, they would have actually grown up and replaced the overstory oaks. But when we sampled the Cary forests in 1984, right after I joined the scientific staff, virtually all of those understory hemlocks were dead. And we assumed that this was, in fact, a direct result of the defoliation in 1980, '81, and potentially '82. Now hemlock needles, hemlocks are evergreen, and those needles actually last two to three years. But they're also an expensive investment of carbohydrates produced; those tissues are very dense, and by living longer, they can still generate a net profit. But in the heavily under-shaded understory, that profit is actually quite small. And we assume that two or more years of defoliation basically wiped out their reserves and killed them outright either because of hydraulic failure due to inability to produce new wood, or an inability to fight off root rots and other pathogens.

The hardest thing to quantify is whether there are long-term residual impacts of the stress of several years of spongy moth defoliation. In general stresses on trees compound, and ultimately the combination can make them vulnerable to some other more direct cause of mortality; say, something like a bark beetle attack or a root rot fungus. We re-censused the Cary forest plots in 2006 and found an unexpectedly high mortality of chestnut oaks; not catastrophic, but just higher than we would have expected. Chestnut oaks are one of the dominant oaks here in the mid-Hudson Valley on hill slopes. The 25 years after that '80, '81, '82 defoliation saw a number of modest droughts; there were some years with high ozone levels before the Clean Air Act Amendments of 1990 fully kicked in. But it is reasonable to assume that spongy moth defoliation contributed to the higher rates than expected or chestnut oak mortality. So when I think about the impacts of...I'm sorry. So when I think about the impacts of a particular year of defoliation, what I really want to know is how favorable are growing conditions for the new flush of leaves during the rest of the summer. Now, two summers ago, 2022 was the worst summer drought in the Hudson Valley in over 60 years, and I would expect the trees defoliated earlier that year, in the spring, June of 2022, struggled to replenish reserves over the rest of the summer, because, frankly, the leaves had to shut down their stomata because of the low soil moisture, and therefore couldn't photosynthesize.

And that's why I was frankly so happy that this past summer was one of the wettest summers in decades. The trees in my yard had their first complete defoliation this summer. But I feel pretty confident that the heavy summer rains put them in decent shape for what will inevitably, given everything Clive has talked about, and the thousands of egg masses on my trees, is going to be a second year of heavy defoliation this spring. So this very long-winded detour into plant physiology leads to the one general recommendation I have for helping your trees weather a spongy moth outbreak; it's really very simple. If it's dry in mid summer after defoliation, think about watering the rooting zone under your trees. I don't generally favor fertilizing because it's easy to over fertilize and potentially make the trees more tempting targets to other insects. But soil moisture availability has a really strong positive impact on photosynthesis and the ability of the trees to replenish their reserves.

Charles Canham  42:08  
So Clive ended his comments by pointing out that spongy moth is one of this long and really quite depressing list of nonnative pests and pathogens that we have been unable to control once they get established. And as I've said before, I consider these pests and pathogens to be the single greatest threat to Eastern forests. So I want to end my comments by talking about a recent initiative called Tree-SMART Trade started by our late dear friend and colleague Gary Lovett. Many of us here on the staff Clive, Kathy Weathers, myself, and particularly Gary, have spent years studying the ecological and economic impacts of forest pests and pathogens. Around 10 years ago, Gary convened a workshop with experts from the U.S. and Canada on this issue, and led the writing of a very important paper that outlined not just impacts but policy options. And to his everlasting credit, Gary realized that just writing another paper for the scientific literature wasn't enough. And he and others devoted themselves to this new program they called Tree-SMART Trade.

This was a bold thing for a research scientist to do. And it meant Gary started spending a lot of time meeting with industry groups and traveling to Capitol Hill. The bottom line is that the federal agency charged with inspecting and stopping the importation of new pests and pathogens, APHIS, the Animal Plant Health Inspection Service, it's an agency in USDA of really dedicated, very competent people but woefully underfunded, given the flood of shipping containers and live plants that enter the country every day. So we need much smarter ways of dealing with this threat. And I just want to give one example of what I, something I consider a truly brilliant idea that Gary described in his last Lunch Bunch presentation to the staff here at the Cary. Solid wood packing material by international treaty is supposed to be heat treated to kill any insect larvae embedded in it. Treated wood gets a stamp to identify it. But there is clearly some wood that is fraudulently stamped without being treated or simply was not treated sufficiently. And there really aren't any government programs that effectively police the stamping process. Now if a ship, a container ship, arrives in a U.S. port, and an APHIS inspector finds evidence that an insect has emerged from the wood en route, the fully loaded ship is denied entry and sent back to sea. This is an enormous cost to the shipping industry even given the very tiny fraction of shipping containers that can be inspected. So Gary met with shipping industry leaders and together they realized that they needed some sort of commercial private enterprise that would, for a fee from the shippers, guarantee that the solid wood packing was truly all treated. These new entities would take on the liability if fraudulent packing material got onto the ship. So it'd be in their interest to provide effective policing of the treatment program. And it's this kind of creative thinking that the Tree-SMART Trade program can provide. And this is the kind of legacy that any scientist can be proud of. And with that, I'll, I'm going to stop my remarks and try to leave some time for questions. So I will stop the share and turn it over to Josh.

Joshua Ginsberg  45:46  
We can leave Charlie up there. So we're going to do virtual and in-real-life questions as a way of balancing the hybrid nature of this. If you've got a question, we have microphones. No, no microphones. So ask the question. I'll repeat the question so that people both in the virtual audience and Charlie can hear it. And then we'll go on. Please make sure, I always say this, please make sure to ask a question not make a statement. And only one question per person, because we have, despite the brilliant and and incredibly time-conserving presentations, we only have about 12 minutes for questions.

So the question is, what influences acorn production? Charlie, Clive's pointing at you, so can you? Handing off, next one's yours Clive.

Charles Canham  46:46  
Yep. Okay, so acorn production is triggered by cues from the weather. The trees flower over, depending on the oak species, a one-to-two-year cycle. And the energy the reserves stored up over time determine the magnitude, the size of the acorn crop, if it's successful. Now, acorn crops can fail for all sorts of reasons. A summer drought will cause the selective abortion of the developing acorns, for instance, but combination of basically storing up energy for several years following the weather trigger that starts flowering is what determines the size of the acorn crop.

Joshua Ginsberg  47:32  
Great, thank you very much. Question from the virtual audience: I've been watching the infestations moving around the Hudson Valley in New York State the last few years and wonder if you have any insights on the general directions--north, south, east, west or of outbreaks. So do they follow any patterns? Or trends?

Clive Jones  47:53  
The short answer is not really. So if you recall, looking at that defoliation map, you see some general patterns in which a lot of areas go off together. But outside of that, you'll see areas winking on and off. Now what that means is largely that the same basic dynamics we talked about earlier with acorns, and mice, and so on and so forth, are playing out locally. There's no expectation that acorn production, even if it's synchronous over quite large areas will be perfectly synchronous. Nor, that all the mice populations were exactly the same in each of those locations, or that the habitats for them are the same. So the end result is that although this general pattern plays out everywhere, it doesn't play out in the same place and time in the same way. So it might be that it's going Connecticut then Dutchess this time, but maybe it'll be the other way around in the future.

Joshua Ginsberg  48:49  
So it is random, mostly.

Clive Jones  48:53  
It's, there is a lot of stochasticity or variability, driven by the fact that, you know, you suppress, you suppress the moth population, and then it rebounds. But how high did it go? How fast did it escape from the mouse? How low did the mice go that time? So an outbreak can get started here, but not here, and get started here later. So it's the same rules of the game. It's just that the rules of the game are not perfectly timed everywhere.

Joshua Ginsberg  49:24  
Thank you.

Unknown Speaker  49:27  
[ Inaudible ]

Joshua Ginsberg  49:37  
So I'm so glad you pointed that out. For those of you don't know, Rick Ostfeld, who both speakers mentioned, has spent the last 35 years studying Lyme disease, which came about as a result of this project, because he noticed all those ticks on ears and uh, ears of mice and started from there. But I think the point you're making is a really good one, which is we both love and hate mice. We love them because they control the spongy moth outbreaks when they're not escaped. And we don't like them because they control the Lyme outbreaks. And it's really rare for a Cary scientist to be definitive about predicting things in the future. But Rick Ostfeld is pretty good about saying if there are a lot of, of acorns this year, there'll be a lot of mice next year, and the year after that in the spring lookout, there'll be a lot of Lyme. So I think I'm gonna field that question, and then I'll...anything to add, Clive?

Clive Jones  50:39  
No, except, you know, Gary, Charlie, myself, and Rick were there. You know, we started this forest responses to stress and damage project, and a lot of that work went, as a result, went in a lot of different directions, largely because we found out stuff. And you hired Rick originally as a small mammal ecologist, to deal with the mice. And I got Rick into mice early on, because I said, you know, there's some interesting work suggesting mice eat lot ofmoths, are you interested?

Joshua Ginsberg  51:14  
Which is very common around here. People love to collaborate, which is surprisingly less common than you would think, within institutions.

So Charlie, a question for you. Are there certain forest communities that will recover better than others from this process?

Charles Canham  51:33  
Well, as Clyde noted, oaks are sort of the most highly preferred food resource for spongy moths. And so it tends to be the oak forests that are hit the hardest. Oaks are also pretty resilient trees. And so you know, we do expect much lower levels of defoliation than, say, a maple-dominated forest. And so is that a function of their resilience or just that they're not affected? I think the bottom line is that the real answer here is, if there are no other compounding, very serious stresses, all of our forests recover quite well, following a spongy moth defoliation event.

Joshua Ginsberg  52:19  
So I'm going to do a quick little follow up because there was another very closely related question. Okay, so what's killing oak forests?

Charles Canham  52:28  
Well, all sorts of things kill trees; there is no...oak mortality is not elevated in Eastern oak forests over what we would expect. What's happening is that through natural succession, and the lack of the historic lack of fires and suppression of fires, oaks are being replaced by maples throughout the range of oaks in the Eastern U.S. as they age and die. So there's certainly; you know, there's sudden oak death and oak wilt are two diseases that are present in various parts of the East. But they're not, neither of them is common here in the Hudson Valley. So, you know, by and large, I consider all of the oak species in the Hudson Valley to be quite healthy. But over time, over the next 100 years, we fully expect their abundance to decline simply because they're successionally being replaced by more shade-tolerant species in the absence of, you know, selective light ground fires.

Joshua Ginsberg  53:36  
Great. Question from the audience? The question was, when is the best time to wrap your trees with burlap or sticky tape? Or what time of year?

Clive Jones  53:54  
You mean, do you mean time of year? Yeah. So when the larvae hatch out in April, early May, they spend quite a bit of time before, you know, resting and then ballooning around. The behavior for going up and down trees starts in the later instars. So probably within two or three weeks after you see hatch is probably the best time to do it. That way you, the barriers would be in place by the time the larvae start to go up and down tree.

Joshua Ginsberg  54:30  
But if you wait too late, it's as it were.

Clive Jones  54:34  
I said that's, yes, assuming it's feasible and viable. That's what I'm saying that you should earlier rather than later. It doesn't do any harm to put a sticky band around a tree in early May; it's just that you won't be catching any larvae until later.

Joshua Ginsberg  54:52  
But to reiterate, Clive, you think this summer, because... Oh, forget it. That's clear, a declarative statement from a Cary scientist. Forget it.

Clive Jones  55:02  
It depends on where you're sitting, because for some people, the outbreak has already collapsed. In Connecticut, there are parts of Connecticut, it's collapsed, you don't need to do anything. Other places, the density may not be that high. So you need to go out into yard, look at how many egg masses there are, look at where they're distributed on the trees. If you see them everywhere and a hell of a lot of them, then the chances of you having an effect are very low. If you see a few, then you have a chance of intervening.

Speaker 3  55:35  
I live about two miles from here. And all of my oaks have taken masses. And I have a rather adolescent attitude towards what to do about it. I take a propane torch. For about five seconds each, it seems to be sufficient in my point of view, who knows? But it seems to be sufficient to do. Am I'm doing more harm than good?

Joshua Ginsberg  56:01  
Clive, let me just repeat it: Is hitting your egg masses with a blowtorch helping or hurting your oak trees?

Clive Jones  56:14  
Yeah, as long as you don't scorch the bark or set the tree on fire. They used to use blow torches back then, they also used to spray lead arsenate, and they used to have people climbing up to the top of 200-foot trees using ladders picking egg masses off, okay. So if you can reach all of the egg masses, it's very easy to remove them assuming that there's not a vast number. So you take, you remember a milk gallon jug, the plastic of a milk gallon jug, you take a piece like that, you roll it into a cone, you tape it across there, you put a plastic bag on one end with a rubber band. And then with the funnel part you just scrape the egg mass; they all fall in at the bag at the end. Take the bag, dump it in hot soapy water. And it'll do a fine job. So I would do that if you could reach them, unless, of course, you have one of those really big flamethrowers!

Joshua Ginsberg  57:17  
One more from the audience. And then I'll do one more from the virtual.

Clive Jones  57:27  
No, don't dump them on the ground. It's certainly true that if you break up the scale hairs that removes some of the thermal protection. But depending on what time of the year you do it and the weather, they're very hardy. So most of them may actually hatch out. So put them into the water, hot, soapy water. And yeah, leave them there for a day or so. And then you can compost them if you want.

Joshua Ginsberg  57:54  
Okay, and Charlie, you can answer the last question: What is the one thing every person watching this presentation should do to get invasive plants and insects under control?

Charles Canham  58:12  
A nice simple question. And my answer to these sorts of questions is, that there are how many billion of us on the planet, and there are so many things that need doing; find the thing that makes sense to you and do it. There are lots of organizations that can help guide you. You know, land trusts, conservation organizations, USDA, the Forest Service, Soil Conservation Service, etc., the Farm and Home Center. They all can provide information and your local nature preserves can always use volunteers to help control invasives. And then support programs like Tree-SMART Trade, and, you know, support efforts by all organizations to stop the importation of new ones. You know, once they're here, we're facing millions of dollars needed to even just minimize what are already pretty devastating impacts. So find the thing that makes sense to you and do it.

Clive Jones  59:12  
I was just gonna say, you know, Tree-SMART Trade; call your legislator.

Joshua Ginsberg  59:14  
That's right. So thank you very much. Before we go, I just want to say our next event is a fully virtual event on February 15. It's a Thursday, we decided not to do it on Valentine's Day in hopes that you have something better to do. We are going to hear from Cary Institute wildlife biologist Mike Fargione, who, for the last decade has been putting out camera traps on our property with a scientific goal of understanding the dynamics of the deer on our property. But also it gives us real insight into all the critters that live on our property and live in the Hudson Valley. I have seen some of these pictures; I can guarantee that it will be entertaining and enjoyable. And you can even watch the show and have dinner at the same time. So that is 7 o'clock on the 15th of February. And if you sign up virtually you'll get a link. And if you miss it, you can watch it later. Go to our website and you'll see a link to all the previous videos and there will be one to this one. So if any of your friends ask you about what to do about spongy moths, you can say not much, or watch the video.

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