Many forest insects are mistaken for tree killers

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Insects — they can get a bad rap. Many of our humankind categorically view them as pests — agents of uncleanliness, nuisance or destruction. Sure, it’s hard to appreciate houseflies, ticks, mosquitos and yellow jackets, but the vast majority of them — nearly 100,000 known insect species in the US alone — are simply going about their business and doing no harm to us. Many are even providing services that we take for granted, such as disposing of detritus and cycling nutrients.

The same goes for insects in the forest. At the Extension Service, we receive many photos and samples from people who suspect that insects are killing their trees. However, if you have a dead or dying tree, chances are that even though it is full of insects and their tunnels, it’s usually a case of correlation, not causation.

Insects feed on all parts of trees — there are foliar feeders, cone and seed bugs, root weevils and more — but bark beetles are what seem to strike fear in the hearts of tree owners. Bark beetles are problematic because their larvae feed on a tree's cambium, or inner bark, creating channels or galleries as they go and eventually girdling the tree.

There are only a few species of bark beetles of concern in western Oregon, and they each are associated with a particular host tree. For example, there’s the Douglas-fir beetle for Douglas-fir (obviously), the California five-spined ips for ponderosa pine and the fir engraver for true firs.

However, bark beetles are attracted to trees that are already under some other stress — whether from water stress, root disease or mechanical injury. So, aside from the rare local outbreak, accusing a bark beetle of tree murder is like condemning an accessory to the crime while ignoring the ringleader.

Bark beetles are usually quite small and inconspicuous. On the other hand, the samples and photos that come into the Extension office are usually large and/or colorful insects — something more likely to catch the eye.

We often note the commonality among these insects — they all inhabit dead or dying trees, meaning that they are secondary pests. They find trees that are already dead or dying and then begin the decomposition process, recycling nutrients and perhaps becoming a woodpecker’s lunch.

Landowners need not worry about these insects “spreading” or “wiping out” a stand of timber. They are the turkey vultures of the insect world, coming in after the kill. The Oregon Department of Forestry has a nice fact sheet on some of our more common woodboring beetles.

There are some important exceptions: invasive, non-native bark beetles and wood borers. Some of these are tree killers because our native trees did not evolve with natural defense mechanisms against them and there are fewer natural enemies in their introduced environment.

We worry about potentially major impacts of the emerald ash borer, the Asian longhorned beetle and the gold-spotted oak borer, among others. You’re unlikely to find one, but if you think you might have, you should send in a report to the Oregon Invasive Species Hotline.

To sum it up: more often than not, abiotic (non-living) stress factors such as drought or injury typically play a leading role in triggering tree decline. Bark beetles may or may not show up to finish the job. Then, wood-boring insects — the ones that we usually observe — come in to be the tree recyclers. They are important components of the forest ecosystem and often beautiful to look at.

Now that you know about these non-tree killers, Extension still welcomes your forest insect photos (high-resolution please!) and specimens for identification, as we often learn about new insects this way. Another useful tool for crowdsourcing insect identification is iNaturalist. And, don’t forget, if you’re concerned about something being invasive, use the Oregon Invasive Species Hotline.

Original story published with more photos on the TreeTopics blog.

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