Eastern Hemlock, the Hemlock Woolly Adelgid, and Mycorrhizal Fungi: Citizen Science?

Many of you know that I am located in Pennsylvania, but you may not be aware that the state tree of Pennsylvania is the Eastern Hemlock (Tsuga canadensis). Hemlock is not high on the list of valuable trees – the wood is moderately important in the pulp/paper and woodworking industries and it was once highly valued for tannins optimal for leather treatment.  As a symbol for the state of Pennsylvania and for its ecosystem services – which include being a carbon sink, habitat for wildlife, and foundation for ecosystem water retention and purification – the tree is extremely important.

forest service photo

Eastern Hemlock is facing a serious threat to its existence: the Hemlock Woolly Adelgid (Adelges tsugae).  The Hemlock Woolly Adelgid is a Hemipteran insect (think tube feeders, like Aphids, which are somewhat closely related) so the pest typically feeds by sucking on plant juices from the phloem, which drains water and photosynthate from the plant.  Like other tube feeding insects, the Adelgids are sloppy eaters and spill plant juices all over the place, so the plant looses more nutrients and water than just what the insect feeds on.The woolly part of the Adelgid protects its body and eggs and maintains a microclimate to survive harsh temperatures and weather.  This protective hairy covering is also somewhat impermeable to pesticides and biocontrol agents.

photo 1

When I was on the West Coast of the US working with the Forest Service, we frequently came across a native Woolly Adelgid, which occupies the Pacific rim.  Adelgids were only first identified on Eastern Hemlocks in 1951 near Richmond, Virginia, probably introduced from nursery stock –like many other invasive species.  Using phylogenetic techniques researchers have identified that the Hemlock Woolly Adelgid populations introduced in the Eastern US trace to those from Southern Japan, not from the Pacific Northwest of the US.

EH range

The mortality of Eastern Hemlock has become so severe that Congress has become interested.  Just this past week (see also here and here), Glenn Thompson, a Republican from Pennsylvania’s 5th District, Dean of Penn State’s College Of Agricultural Sciences Bruce McPheron, Kurt Gottschalk from the US Forest Service, and Penn State Entomology professor Kelli Hoover held an open public forum to discuss the Hemlock Woolly Adelgid on Pennsylvanian Eastern Hemlocks.  Many are worried that the Hemlock Woolly Adelgid (along with other aggressive invasive pests such as the Gypsy Moth, Sirex Wood-Wasp, Asian Long-horned Beetle, and the Emerald Ash Borer) will enter the category of pests that have progressed to complete destruction of a host with little to no choice except to cut down dead trees (i.e. Chestnut Blight and Dutch Elm Disease).  Obvious discussions have centered on the fact that the cost of treatment or management greatly exceeds what some deem the cost of the commodity.

american forests photo

Perhaps you can see – knowing a little about my research interests – where I am going with this: with massive tree mortality I am interested in what is happening to the obligate mycorrhizal symbionts of the Eastern Hemlock.  There’s not much data on fungal specificity for the Eastern Hemlock – most of what I know is either anecdotally passed on to me or based on my own observations.  Collectively, we really don’t have much understanding on the magnitude of fungal diversity in general, but we’re completely at a loss regarding the below-ground fungal diversity associated with Eastern Hemlock.  Lactarius purpureus, one of my favorite mushrooms from a genus near and dear to my heart, is obligately associated with Eastern Hemlock; it forms mycorrhizae with no other plants.  There is a handsome purple-capped white-stemmed Russula, something close to R. xerampelina, but not exactly like it, that is also found only associated with Eastern Hemlock.  Is anyone certain of any others?

One report in a recent meeting proceedings described five genera, which included Lactarius (16 species), Amanita (13 species), Russula (six species), Tricholoma (four species), and Cortinarius (four species), that were all collected in large mono-dominant Eastern Hemlock forests.  Whether or not these fungi are obligate symbionts of Eastern Hemlock have yet to be determined, but that list is a start, nonetheless.

I have been collecting under Hemlock in one form or another for about 15 years –not a long period of time I admit – but I have been hard pressed to find some of these fungi in the same locations in recent years.  These mushrooms seemed common in past years in some specific locations, but I haven’t collected Lactarius subpurpureus for many years now.  I have observed Hemlock Woolly Adelgid in all of these locations.  Trees that have insect infestations do not have extra photosynthate to provide to the fungal symbionts.  A lack of carbon for the fungus should translate into a lack of, or at least lessened, reproductive fruiting.  An opposing argument is that trees that are losing photosynthate are more reliant on symbionts for nutrient uptake to compensate for nutrient loss, so there would be a greater reliance on, and therefore more frequently observed, mycorrhizal roots.


There’s a lot of stochasticity involved with finding mushroom fruiting bodies.  I am just one observer, observing one location at a time – and let’s not forget huge variations in local weather and timing of rainfall during the short window when most fungal sporocarps fruit.  It’s not uncommon for some fungi to go decades uncollected at the same location.  As a result, I spent some time, about 5 years ago, trying to access public foray records over the past 50 or so years to see if there has been a noticeable decrease in the collection of Hemlock associated macrofungi.  The problem is that there really isn’t much in the way of public foray records to access on the Internet and I gave up on the idea of documenting recent fruiting records under Eastern Hemlock.  Some researchers, such as Pat Leacock, David Lewis, & Greg Mueller at Chicago’s Field Museum, have done a great job working with national amateur mushroom societies, such as NAMA’s Voucher Collection Project, and many small local groups to make foray data available.  There’s just not enough data available to address the types of questions I am interested in here concerning the fungi associated with Eastern Hemlock.

This recent public interest in the demise of Eastern Hemlock has again kick-started my thinking about fungi associated obligately with Hemlock.  Amateur mycological and botanical societies could play a huge role in documenting species presence at forays and weekend gathering, as well as playing a leading role in establishing multi-year surveys of hemlock fungal symbionts.

Would anyone like to provide any input here?

Are there resources that I am unaware of that are provided by amateur scientists?

Would anyone like to start a citizen science brigade with me to survey in and around Eastern Hemlock for fungi (particularly Lactarius subpurpureus)?


UPDATE: I have received a few questions about the photos posted here.  If you click on a photo, the link will take you to the source of the photo.

The two photos of the Hemlock Wooly Adelgid infestations on Eastern Hemlock (the photo where I am showing you the underside of the hemlock branch and the photo just about this comment showing the top portion of the branch) were taken from a tree right outside my office on the campus of Penn State.  Some of these trees have been sprayed with a pesticide (not sure which one) in previous years, but the Hemlock Woolly Adelgid infestations have continued.  The future of these campus trees is uncertain.

Check out the re-blog on the great site: The Hyphal Tip!

  • David

    I have heard that Dutch Elm disease is caused by acid rain. There have been bugs and beetles for hundreds of millions of years, so one insect destroying ALL trees of a certain species doesn’t make much sense. The trees have defences against these kinds of attacks.

    The theory proposed was that acid rain had caused the soil acidity to increase to such an extent that all the elm trees were fighting to survive, and it was in this weakened state that the beetles were able to cause so much damage.

    • This is an interesting theory you have on acid rain and Dutch Elm disease. I’m not sure how acid deposition would affect the ability of the American Elm, or the Hemlock, to respond to disease; although I would agree acid rain would influence the ability of a tree to survive.

      I do think many of our tree diseases appear to arise from the effects of globalization, whether it’s an insect (in the case of Hemlock) or fungus (in the case of the American Elm); new diseases seem to appear from other locations. It seems that there has been enough time for genetic differences to occur in specialization so that when new pathogens are introduced to host trees, the trees seem not to be able to fend them off. At least, as is the case with the American Chestnut, the Chestnut blight fungus will eventually kill both the resistant Chinese Chestnut and the susceptible American Chestnut, but it kills the American Chestnut with extreme speed.

      Thanks so much for reading and taking the time to comment!