Tiny insect continues to take down hemlock forests across eastern US

Severe damage to hemlock trees by the hemlock woolly adelgid

Alternate blog title: When is it time to give up on one of your favorite species?

Hemlock trees in eastern North America have been hit hard by a non-native insect, the hemlock woolly adegid. I’m fond of hemlock trees and would hate to see them go, but right now it seems inevitable that they are on their way out. This year has been a good one for the insect. Many new forests have been colonized by them, and trees that were already infested have seen a greater decline in health. Although it is too early to say with certainty, the warm winter and dry summer conditions seem to have been two direct punches in the gut at the same time.

Hemlock Woolly Adelgid Infestation

You can find a lot about the biology and spread of the hemlock woolly adelgid (often shortened to “HWA”) elsewhere, so I won’t go into detail here. Take a look at the links at the bottom of this page, especially the overview from the NY Invasive Species Clearinghouse. First identified in the 1950s in Virginia, the unintentional introduction of this Asian native has spread to its current distribution today, covering states from New England to Lake Erie and southward just about as far as hemlock trees live.

Older hemlock forests are spectacular places to spend time. The evergreen trees tower over the ground like giant architectural columns, creating a microclimate that is dark, moist, and cool. This microclimate is one reason why hemlock forests are so unique. Where hemlock trees exist along small streams, the temperature of the water is lower, which benefits fish and other aquatic organisms that thrive in colder temperatures. In addition to this, few plants can grow in the deep shade beneath these trees, so the understory is fairly open. The acidic soils and cooler microclimate tend to reduce the rate of decomposition, which leads to a thicker organic soil layer.

In addition to their ability to alter microclimatic conditions, hemlock trees can dominate large areas of a forested landscape, which is why some people call it a “foundation species”. Losing a foundation species from an entire landscape will certainly alter how these ecosystem function and change the habitats available to other organisms.

There are several reasons why we might have to let go of losing this species to a non-native insect. One is the unfortunate fact that hemlock trees don’t carry a clear economic value. Its wood is not as strong or sought after as oaks or even other conifers, and we don’t get syrup from it like sugar maples, the poster child for the campaign to stop the spread of the Asian longhorn beetle in eastern North America.

Another reason we might have to accept the decline of hemlock is that we don’t have a feasible way of stopping the spread of the adelgid insect. You can treat trees with fairly harmless horticultural oils each year or with pesticides every few years. However, it is much too expensive and tricky to treat an entire forest this way. Researchers are trying to identify a method of biological control. There are several organisms (especially beetles) from Asia that may be possible as well as a fungus, but no one has been able to demonstrate widespread success.

Hemlock forests have seen hard times in the distant past. Around 5500 years ago, they nearly vanished from the landscape during a period known as the mid-Holocene hemlock decline. The cause of this decline was likely a pathogen. Some researchers point to a fungus. Others have evidence that the hemlock looper (another insect) was to blame. There is also good reasoning (and some evidence) that climate changes at the time were involved, or at least there was an interaction between climate change and insect damage. Hemlocks eventually bounced back to become an important component of North American forests, but it took more than 1000 years to do so.

In graduate school, I spent several summers searching for old-growth stands of hemlock in northwestern Wisconsin, where hemlock trees reach their westernmost limit. If you travel farther westward, climate conditions are too dry for hemlock to get established and grow. It was a perfect situation for me to study how climate changes in recent ecological history (the past 2000 years) may have led to the expansion and contraction of hemlock’s distribution. Using pollen trapped in the mud of very small ponds (actually, tiny wet areas beneath the canopy of a forest) I helped develop a picture of the movement of the edge of a species range. In the case of hemlock it appears that small, outlying colonies developed first and then expanded rapidly hundreds of years later, perhaps during periods when climate conditions were conducive.

It’s funny that this appears to be how the adelgid spreads too. It is often found in forests beyond its range limit (probably traveling on the feet of birds), then spreads to fill in the space around it.

During my time stomping around in forests as a graduate student, I developed a deep fondness for hemlock trees. Today, when I pass one in a forest I take note of it. I look at where it’s growing, notice what other species are living nearby, and, of course, I turn over some of the branches to check for the adelgid. When I moved to Massachusetts in the late 1990s, the adelgid had just arrived in the state too. Now, 12 years later, the adelgid has been found in Vermont and northern New York.

Harvard Forest research scientist Dave Orwig has been studying the spread and impact of HWA in hemlock forests in New England since the mid-1990s. Along with others, he observed that the spread of the adelgids has been slower in Massachusetts. The trees have been hanging on longer when they do become infested compared to those just south in Connecticut. Many people say that the adelgid has reached its climatic limit and is showing signs of slowing down. However, the adelgid keeps popping up farther and farther north. Just a few years ago it would have been hard to find an adelgid in the Harvard Forest’s 3500 acres. This year, field crews are finding it in many of their research plots. This past winter’s warm temperatures may have had an impact, allowing more of the adelgids to survive and lay eggs well into the spring.

Cold temperatures may be the one thing that is going in our favor. Even though they are protected in winter by a woolly sac (its namesake), lab studies demonstrate that when temperatures dip below -30 degrees Celsius (-22 F) only 3% of the population survives. All of them die between -30 and -35 degrees C. But it’s tricky to put a number on it. A long cold snap of -20 degrees C may do the trick, at least to slow them down.

Even so, over time the adelgid populations may become better able to survive colder temperatures as the result of microevolution. Even if 95% of their population dies over the winter, there’s still 5% left that will produce offspring, which may be resistant to colder temperatures. In addition to this, a warmer future climate will allow them to continue their northward.

Elongate hemlock scale infestation

If this weren’t enough, another non-native insect(again from Asia) is also infesting hemlock trees – the elongate hemlock scale. It apparently interacts with HWA and competes for the trees resources, often doing better. The impact of the scale insect will likely still lead to tree death, but the process might take a little longer.

When I started teaching at Westfield State University, I started some small research projects with students to see what we were dealing with in western Massachusetts. Hemlock trees comprise a considerable fraction of the forest remnants around our campus and are a significant part of forests in the region. In one forest remnant we have been studying, HWA infestation was much higher in 2005-2008 than today. Since then, the elongate hemlock scale has become more common. The trees continue to die, but most of them are hanging on even after eight years of observations. Forests at higher elevations in our watershed are less likely to be infested, but over time we continue to find that adelgids are spreading. Some forests seem to be much more infested than others, even when they aren’t far from each other, which is a mystery. This year there seems to be a lot of personal observations of the adelgids showing up in places they hadn’t been seen before.

So, as a summary … here’s the status of hemlock and the adelgid in northeastern North America:

  1. The speed at which HWA has been spreading is slower than in mid-Atlantic and southern states, probably because of colder winter temperatures that kill some of the larvae.
  2. Although colder temperatures may be slowing its spread, the adegid continues to expand northward and may have done so very rapidly this year (2012) as a result of the warmer winter. In addition, over time the HWA populations may evolve to become more cold-hardy, and warmer temperature in the future may allow HWA to continue its spread.
  3. The elongate hemlock scale also infests hemlock trees. It tends to show up in forests following HWA infestation and may be less damaging than the adelgid (though it will still likely lead to tree death).
  4. Controlling HWA is possible with chemicals/soaps, but only cost-effective for a few trees (not entire forests). Biocontrol agents have had little success so far, but may be possible in the future.

All of this has led me to ask myself how much time we should spend working on an environmental problem that seems unsolvable. We have so few resources and time at our disposal and there are a lot of other environmental changes to work on. I don’t like being defeatist, but I do like to be realistic.

Still, I also like hemlock trees.


A few online resources …

US Forest Service Hemlock Woolly Adelgid website

Harvard Forest’s Hemlock Woolly Adelgid research page

NY Invasive Species Clearinghouse webpage on the Hemlock Woolly Adelgid

David A. Orwig, Jonathan R. Thompson, Nicholas A. Povak, Megan Manner, Donald Niebyl, David R. Foster. 2012. A foundation tree at the precipice: Tsuga canadensis health after the arrival of Adelges tsugae in central New England. Ecosphere 3(1): article 10. Published: January 26, 2012

Monique N. Lustenhouwer, Liza Nicoll, Aaron M. Ellison. 2012. Microclimatic effects of the loss of a foundation species from New England forests. Ecosphere 3(3): article 26. Published: March 26, 2012

As noted above, two photos originate from Nicholas_T at Flickr