In response to last week’s post on the value of dead wood in the forest, I received this e-mail from a landowner:

“We’ve never left much on the ground in the way of dead wood…not during logging, but wind damaged, etc.  Our thought has always been that these rotting logs increase the insects in the forest, both good and bad. Is this a valid concern and if so, where is the balance between bugs and wildlife?”

He raises a point worth exploring. While calling an insect “good” or “bad” is a matter of perspective, for the purposes of this discussion let’s assume that “bad” insects are those that cause economic or environmental damage, and “good” insects are those that don’t. The vast majority of insects that inhabit western Oregon forests fall into the “good” category…with a few notable exceptions.

One of these “bad” bugs that the e-mailer might have in mind is the Douglas-fir beetle. This time of year, the adult beetles are flying around in search of Douglas-fir trees where they lay their eggs underneath the bark. Their favorite targets are large diameter, freshly downed logs—or standing trees that are weakened from another cause (root disease, soil compaction, etc.). Through the summer and winter, the eggs hatch and the larvae grow as they tunnel around under the bark (this activity is what kills the tree). The following spring, they have become adult beetles, and they fly away in search of new homes. If they can’t find another weak tree or fresh log, they will go after a healthy tree.

Douglas-fir beetle (Dendroctonus pseudotsugae ) on Douglas-fir (Pseudotsuga menziesii ) - 1587008
reddish bark dust in bark crevices is a sign of the Douglas-fir beetle (Photo: Elizabeth Willhite,

Healthy trees can withstand a low-level Douglas-fir beetle attack, and in normal circumstances there are rarely enough beetles around to cause concern. The problem arises when the beetle population builds up and lots of them infest a healthy tree at once. When does that happen? In situations where there is a lot of freshly downed or damaged wood on the ground for them to target initially – like after a winter windstorm.

Here’s a true story for illustration.

In fall 2009, a landowner in the Coast Range was hit hard by beetle kill to his otherwise healthy, 100+ year old forest. Why? Here’s how we think this may have played out.

  • The stand is adjacent to a sawmill.
  • The big windstorm of December 2007 created lots of blowdown along the coast, though this particular stand was too far inland to be damaged.
  • Some of the coastal blowdown was not salvaged until summer 2008…too late, because Douglas-fir beetles had already found them during the spring.
  • The salvaged logs were brought to the mill, along with the beetle larvae living under the bark.
  • Then, in late 2008 the recession hit and the mill curtailed operations. The logs sat in the deck…and the beetles matured.
  • In spring 2009, they emerged and flew off to the neighboring stand, where they attacked the healthy, mature trees.

It was a sad situation, especially since the landowner had to cut more trees to avert further beetle damage, and in a poor market.

So, back to the e-mailer’s question: does retaining snags and downed wood for wildlife create a forest health risk? The take-home messages are these:

  • Most insects are not forest health risks.
  • In western Oregon, the Douglas-fir beetle (the “baddest” dead wood-inhabiting insect) only thrives in FRESHLY dead or downed trees. Once the snag or downed wood has been dead for more than a year, it is no longer a target. Instead, it will become inhabited by the dozens of “good” bugs that feed wildlife.
  • There needs a LOT of this fresh down wood to pose a forest health risk – like after a storm. According to Oregon Department of Forestry, a good rule of thumb is that fewer than 3 FRESH down logs/acre does not present a hazard.

After a windstorm or other stand-damaging event, yes, prompt salvage is important in order to prevent a beetle infestation. But, when scattered trees gradually die in a stand from other causes, it is hard to imagine when this would create a risky situation with respect to bark beetles. And during harvest activities, you can be strategic about how much dead wood is left behind, and in what conditions.

We appreciate it when readers respond to our blog posts. We like your e-mails, but you can also respond by commenting directly on the blog, where other readers can contribute to the conversation.

Amy Grotta

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3 thoughts on “Bugs, the good and bad (dead wood revisited)

  1. I commend Maser, _Trees, Truffles and Beasts_ to the emailer’s attention.

    While the book looks primarily at two kinds of old forests (In US and Australia), its roots (so to speak) reach back into two research threads: the recovery of blast areas following the 1980 eruption of Mt. St. Helens, and basic research by the authors (and others) conducted during the peculiar circumstances of the ‘old growth’ debate in the PNW. It’s fairly well-written if text-booky (and preachy) but it neatly summarizes a chunk of research.

    What allowed rapid recovery in the apparently lifeless blast zone and where it happened first post-eruption was largely a function of remaining structure. One example is that down logs (and the inside of down material) shielded small areas in their lee (or their interior) from the blast, which not only provided a source of plant seed and rootstock for rapid recolonization, but also microbes, reptiles, amphibians, insects, worms and crawly things and small mammals. There were tree-frogs where water accumulated adjacent down logs first year post-blast, and insects within reasonable travel distance to pollinate plants as they recovered.

    In the process of trying to sort out what the heck an ‘old growth’ forest is during that era, researchers made some remarkable connections between the micro and macro life of the forest. One example (beloved of Maser) is the northern flying squirrel consuming the fruiting bodies of mychorrhizal fungi (truffles) as well as douglas-fir seeds, and flying about at night pooping fir seeds prepped for planting, in a perfect little packet of balanced fertilizer and mychorrhizal innoculum.

    Microbes (notably nitrogen-fixing bacteria and mychorrhizal fungi) and insects (and the microbes in insect guts) play an enormously important role in forest ecosystems of all kinds. The ‘below ground’ portion of a forest is vast. The root biomass of a 50-year-old douglas-fir forest is about 20% of the total tree mass. And that doesn’t include any of the other living or formerly living bits: shrub roots, groundcover roots, decaying twigs and chunks, accumulating humus, and a whole _lot_ of insects, worms, decay bacteria and the fungi which extend those roots’ effectiveness to many times their actual length.

    Forests do not exist without “rot”. The rot part of a forest’s life cycle (whatever the stand is managed for) is fascinating – and absolutely critical to maintaining energy and nutrient cycling on a site.

    One of the things which makes managing a forest distinctive to me is the sheer complexity of using a naturally-functioning and largely self-renewing system to produce stuff for us humans to use. How much and how little fiddling ‘ought’ I do?

    I have no idea what some future ‘harvester’ will think the “products” of the forest I am planting are. Pretty “woods” to look at and wander in? germplasm reserve? cellulosic feedstock? fuel? mushroom farm? squirrel refuge? source of miracle cure? hot tub? If I fiddle too much, will I be taking the really important thing out?

  2. Hi I was reading your blog about beetles. I’ve had several trees die and others continue to die. I think the Douglas fir beetle is the culprit for my firs but what could be killing my hemlock. I had the extension service come out almost two years ago and the state forester. They did not believe that bugs were killing the trees. They thought overcrowding, or compacted soil from my two horses. I’m having the property logged and thinned but more trees keep dying.
    I’m outside of Colton Oregon. Clackamas county.
    Any ideas would be helpful. I did find a yellow and black insect, beetle today.

  3. Dawn,
    You may want to contact the Extension office in Clackamas County to follow up on your situation. Ask for Glenn Ahrens, who is the Forestry Extension Agent. Trees die for many reasons, but most insects do not kill trees. Many only go after trees that are already under stress (like from compacted soils or overcrowding). Thinning sounds like a good idea, but will take a while for trees to respond. Keeping your horses away from the trees is another good step.

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