Mistletoe Mystery

Hemlock Dwarf Mistletoe (Arceuthobium tsugense)
Hemlock Dwarf Mistletoe (Arceuthobium tsugense) growing on Western Hemlock (Tsuga heterophylla).

I’ve been looking for Dwarf Mistletoe (Arceuthobium sp.) at Miracle Beach Provincial Park for years now – I know that it is there, growing on the Western Hemlock (Tsuga heterophylla). My wife Jocie has found it several times and has brought it back to the house for me to photograph but for one reason or another I’ve never managed to get around to taking a photograph of it.

This Monday I finally found some! We’ve had a couple of days of very strong winds and that always brings down trees and branches in the park. While out for a walk I spotted a large branch of Western Hemlock lying on the road with some strange growths on the wood. You may see large clumps of “witches broom” that are a sign that the mistletoe is growing on the tree but Mistletoe is generally so high up that you rarely get to see it up close.

Hemlock Dwarf Mistletoe (Arceuthobium tsugense)
Hemlock Dwarf Mistletoe (Arceuthobium tsugense) is parasitic on Western Hemlock.

So the next challenge is identifying it to species, notoriously difficult with mistletoes. Pojar and MacKinnon lists Western Dwarf Mistletoe (Arceuthobium campylopodum)as parasitic on a “wide range of conifers… [but] most commonly on western hemlock” so there is a good fit there. eFlora also lists A. campylopodum as a good candidate and the range map shows records of this species on Vancouver Island. The other two species listed in eFlora (A. douglasii and A. americanum) prefer different host trees (Dougla-fir and Pinus sp. respectively) and show no records on Vancouver Island. The simple key in Illustrated Flora of BC also tends to favour A. campylopodum in that the stems are over 2 mm thick and over 3 cm long.

So – Western Dwarf Mistletoe it is, right? Well, a little more research and a suggestion from Jamie Fenneman leads to the likelihood that this mistletoe is actually Hemlock Dwarf Mistletoe (Arceuthobium tsugense) which makes sense since it is parasitic on Western Hemlock – Jamie has identified his sample picture for A. campylopodum as A. tsugense in eFlora. Washington State University has a good overview on several mistletoes that is useful for identification. Again, the size and colour of A. tsugense, as well as its host tree, distinguish it from other mistletoes in British Columbia. However, Calflora states that A. tsugense is undergoing name changes and is now called A. campylopodum (!) so we’re back where we started!

Hemlock Dwarf Mistletoe (Arceuthobium tsugense)
Is it or isn't it? Hemlock Dwarf Mistletoe (Arceuthobium tsugense) or Western Dwarf Mistletoe (Arceuthobium campylopodium)?

This has turned out to be more complicated than I thought! I think that part of the problem is that these mistletoes are going through name changes and new species are being classified due to genetic research. At this point, I’m happy to call this mistletoe¬† Hemlock Dwarf Mistletoe (Arceuthobium tsugense) due to its association with Western Hemlock.

I love walking after wind storms, you never know what you’ll find and what you’ll learn. There’s lot going on up in the forest canopy!


  1. Hi, Dave. I enjoyed your excellent photographs. As a long-time forest pathologist and researcher, I’ve been interested in dwarf mistletoes for many years. Hemlock dwarf mistletoe is our current major interest. We’re doing several monitoring projects with Interfor looking at spread of the mistletoe from residual trees into young trees, particularly in patially cutover blocks.

    The best person to explain all of the taxonomic complexities and controversies is Robert Mathiasen who has published several papers on the subspecies of A.tsugense. My understanding is that the epithet campylopodum has been used by some based on the general morphological features of many of the mistletoes. I prefer to follow the scheme proposed by Frank Hawksworth and colleagues , including Bob M., where species are differentiated on the basis of detailed morphological attribures as well as host specificity, periodicity of flowering and other features. Based on Bob’s recent work, there are three subspecies of A. tsugense based on major hosts: subsp. tsugense on western hemlock, subsp. contorta on shore pine and subsp. mertensiana on mountain hemlock. We covered quite a bit of the taxonomic references in our recent USDA literature review. I’ll dig up the reference and send it separately.

  2. Thanks John,

    I’d love to take a read through these papers. I actually returned to the branch with the mistletoe to get a few more shots with a little better depth of field and sharpness – haven’t processed them yet!

  3. Sorry for my delay in responding, Dave. We’ve been pre-occupied with non-mistletoe concerns. I’ll look for my copy of the USDA report and send you the url.

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