By Amy Grotta, OSU Forestry & Natural Resources Extension – Columbia, Washington & Yamhill Counties

It is shaping up to be another exciting year in forest health here in northwest Oregon. Fortunately, neither of the two defoliating insects currently on the scene are serious threats to forest or human health, but they are certainly causing a stir.

Right now, Columbia County is in the midst of the largest documented western tent caterpillar outbreak that Oregon has seen in two decades, according to the Oregon Department of Forestry. I first noticed a few tent caterpillar clusters on one site in the area two years ago. Last summer, our Extension office received many calls as the caterpillar population built up.  Aerial surveys done a few weeks ago show that at least 13,000 acres are affected in the county this year.

Map and aerial view showing extent of western tent caterpillar defoliation, early June. Affected areas are brown in the photo. Source: Oregon Department of Forestry
Map and aerial view showing extent of western tent caterpillar defoliation, early June. Affected areas are brown in the photo. Source: Oregon Department of Forestry

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Winter storm damage
Winter storm damage

Storm damage may lead to beetle problems in ponderosa pine

By Brad Withrow-Robinson, OSU Forestry & Natural Resources Extension – Benton, Linn & Polk Counties


Not to be a fear-monger, but there is talk about last winter’s storm damage leading to some future beetle problems for ponderosa pine in the Valley.

Now, bark beetles are generally weak predators of trees.  Damage is often limited to marginal sites, with beetles usually attacking trees weakened by other stresses, such as drought or flooding.  Generally this does not pose a great  threat to the other, healthier trees in the area.

But I recently spoke to a couple landowners concerned about bark beetle attacks in their ponderosa pine.  Continue reading

By Amy Grotta, OSU Forestry & Natural Resources Extension, Columbia, Washington & Yamhill Counties

Our Master Woodland Manager trainee class wraps up next week, after sixty-plus hours of learning and sharing together over the past six months. As an instructor, what I’ve enjoyed the most of this experience is that over half of our instructional time has been spent in the field, doing hands-on activities designed to encourage critical thinking about the day’s topic.

Root excavation
Root excavation

For each class session, we’ve tried to hone in on a few key themes. One theme for our forest health session was the importance of being observant in diagnosing a pest or disease. Our instructor, Dave Shaw, had the class look methodically for signs and symptoms at each site we visited. Where on the tree is the problem noticeable? Was there a pattern to the damage across the stand? What’s going on at the ground level? Any signs of chewing, wounding or scraping? What else is going on in the area?

Twice that day, we learned how easy it can be to prematurely jump to conclusions about a forest health problem, without pausing to get the full picture of what’s going on around you. We went to one site where there was extensive mortality of young Douglas-fir trees. Knowing that laminated root rot is the #1 most common disease in the area, when we walked into the stand and saw symptoms consistent with laminated root rot, most of the group who had seen it before agreed that it was the likely cause. But it was not until we started excavating around some of the dead trees that we found signs of two other root diseases, as well. Continue reading

By David Shaw, OSU Extension Forest Health Specialist

western oak looper
Top left: western oak looper caterpillar; Top right: oak leaf eaten by looper; Bottom left: affected tree appears dead; Bottom right: caterpillars aggregating on a fence post. Photos: Dave Shaw

Insects of Oregon white oak are causing some damage this summer, and you may be seeing trees that look completely brown or have scattered dead branches (distinct brown foliage clumps all through the crown).  There are two different issues that have emerged around the Valley this summer:  whole tree defoliation/leaf eating by the western oak looper (Lamdina fiscellaria somniaria); and scattered branch death caused by the combination of a twig gall wasp (Bassettia ligni) and the western grey squirrel.

The western oak looper (a type of inchworm) is a flashy defoliator that is a native in the Willamette Valley.  The caterpillars are messy feeders, hang by silken threads, and leave browned mostly consumed leaves all over the tree, giving the tree the appearance of being dead.  But the oak trees usually come back the next year.  Historically, the outbreaks have been of short duration in any single area (typically one or two years) and the oak trees rarely suffer long term damage even though the defoliation may be spectacular.  However, when conifers such as Douglas-fir are intermingled with affected oaks, they can also be defoliated in an outbreak and impacts to the Douglas-fir may be more severe. Continue reading

drought stressThe phone has been ringing off the hook lately with calls from people describing sick and dying Douglas-fir and other conifer trees. The trees are of a wide range of ages and in many environments and settings, although most calls have been coming from within the valley margin and have to do with young trees.

So far, the answer is generally: “It is drought stress”.  Huh, in May? Well it has been a dry winter and spring, … but that is not the issue.

My best explanation is that we had a pretty hard end of summer last year. Remember that? NO rain until mid-October then, Boom, it was winter. By then, many trees had started running out of water, killing tops or branches, and leaving leaders and branches susceptible to attack by various opportunistic pests.

We started seeing a few classic signs of drought stress (tops dying and branches “flaring out”) at the very end of the season last year, but late enough that many did not have time to show up before the weather turned. Injuries had occurred, so it was just a matter of time before they expressed themselves, which is happening now. The recent hot weather seems to have made it more sudden and dramatic.

This happens from time to time. Here are two good articles a few years back by the ODF Forest Health team explaining Dead tops and Branches (with Good pictures), and about Drought and Mortality.

It is important to keep in mind that the Willamette Valley can be a challenging climate for trees. Many of our soils in the valley are poorly drained, which is hard on most of our conifers, and other soils are fairly shallow and cannot hold much water. Also our summers are hotter and drier than in the mountains. Heat and drought stress can kill trees outright, or more often just put the trees under stress, which can then lead to pest problems (as explained in the two publications above). From what I am seeing and hearing, the major cause of the problem now seems to be drought stress. Insect or diseases which able to take advantage of a stressed tree’s condition may sometimes be involved, but they are generally not the cause of the problems.

Finally, weather can be more stressful when trees are overcrowded, so thinning stands to keep trees vigorous with adequate growing space may be helpful in the long term. Right now, we just have to wait it out, and hope we get some serious rain this year, or we will see this problem intensify.

We all better get out there and wash the car…..

Brad Withrow-Robinson

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

I recently got a call from a guy selling some woodland property in the Coast Range. A prospective buyer recently told him that he had Swiss needle cast (SNC) and so was not interested in buying the property. It is not hard to find the disease in western Oregon. It is a native disease of Douglas-fir and is wide spread from the coast into the Cascades. But this fellow was calling for some guidance about how to respond to this concern. Was it reasonable? How can he gauge its impact on his young forest stand?

Phot credit: SNC Coop

He already knows how to recognize SNC when he sees it: from a distance it makes a tree look paleand sparse. This is because the fungus is developing in the needles, gradually clogging the stomates, which is where the leaf exchanges water vapor, carbon dioxide and oxygen. Up close with a hand lens you can look on the underside of a diseased needle and see tiny black dots in neat rows where healthy white dots (the stomates) should be visible. In some places or during seasons when the disease is severe, this causes many needles to turn yellow, and eventually to drop (to cast), giving the recognizable symptoms. If enough stomates get plugged, and or enough needles are cast the disease begins to affect photosynthesis, and possibly growth, the crux of the caller’s question.

“The key to understanding the impacts from Swiss needle cast,” says Dave Shaw, OSU Extension Forest Health Specialist and Director of the SNC Cooperative, “is whether the needle retention on the tree is good or not. If the tree is retaining around 3 years of needles, then growth should be close to normal. The impacts occur when needle retention is below three years, and especially when it drops to 2 years or less.”

Photo Credit: SNC Coop

So, the question for the caller is: “What is your average needle retention in these stands?” If near 3 years, he can tell folks that yeah, the disease is around but the stand is doing ok.

To count needle retention, use binoculars and cruise the stand, taking the needle retention from the mid crown, south side of tree, and not the apical stem, but the 4-yr and older side branches. There is a good illustration of branching and needle cohorts on page 3 of Swiss needle cast of Douglas-fir in Oregon. This time of year is good. Even if the stand is discolored a little, needle retention is the key factor.

More information about SNC can be found on the SNC Cooperative website, which has aerial survey data, research findings and even a Stand Assessment Tool that provides a more quantitative approach to assessing impacts on growth.

Brad Withrow-Robinson

Submitted by Glenn Ahrens, OSU Extension Forester, Clackamas, Marion & Hood River Counties

Alder flea beetles are particularly active this summer as they go about their business of skeletonizing leaves on red alder trees. I have seen this come and go over the years, and generally flea beetles are not a serious threat. Flea beetle flare-ups in the forest usually run their course after 2-3 years, after which their population crashes – similar to western tent caterpillar infestations. Healthy alder trees with good vigor are usually not seriously affected. Stressed alder trees – particularly those in dense thickets – may die after 2-3 consecutive years of flea beetle infestation. If you like your red alder trees and want to promote “good vigor” the best way is to 1) avoid growing alder on poor alder sites and 2) keep trees well-spaced to ensure large healthy crowns.

The western tent caterpillar is a native insect to our forests. It population is cyclical. Over a period of two to three years, the population builds up and then crashes as natural parasites and diseases kill them off. Then we don’t see them again for maybe 8 – 10 years.

This week I saw some tent caterpillars in a recently planted site near Clatskanie (see photo). Last month, I saw lots more over by Sisters, where they were all over the bitterbrush around our family’s camping spot (“eew”, proclaimed my 9-year-old). Here on the westside, they prefer hardwoods such as alder, cottonwoods and willows.

While they may look alarming and can substantially defoliate the trees they infest, there’s little cause for alarm. The caterpillars are done feeding by late June, and the affected plants typically regrow a new set of leaves later in the summer. There’s no reason to spray insecticide – the best strategy is to wait it out and let nature take its course.

My guess is that we’ll see more western tent caterpillars in some localized areas next year.

Summer must be coming to an end. I say that not because the kids are going back to school or the tomatoes are (finally) starting to turn red, but because today I got my first call of the year about a strange and striking looking insect.

This is a banded alder borer. It is a native wood boring insect, but it is not considered a forest pest because it generally infests dead or downed wood (people often find them on their firewood piles). This insect is often confused with the Asian Longhorn Beetle, which is one of the nation’s most un-wanted invasive pests. If you find a large, black and white insect with long antennae, chances are it’s the banded alder borer (the good guy), but to be sure, look for a white head with a large black dot on it. See the photo above.

I am not sure about the banded alder borer’s life history, but I think the adults must be most active in August and September because that’s when the calls and emails start to come in.