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

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 U.S. 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.

Larval galleries of wood boring insects in a dead ponderosa pine tree

The same goes for insects in the forest. We in Extension receive many photos and samples brought to us from people who suspect that insects are killing their trees. However, I’m here to tell you that if you have a dead or dying tree, then 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 the cambium, or inner bark, of the tree, 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. Below is a sampling of what has come my way recently. Some of these I could identify on my own, but for others I needed to turn to an expert entomologist for help.

Clockwise from top left: 1. Cedar tree borer (Semanotus ligneus), feeds on dying and dead cedar and juniper and often found in firewood; 2. Larva of a horntail woodwasp, which lives on dying or dead conifers; 3. Green bark-gnawing beetle (Temnoscheila chlorodia), found on dead or dying pines where it is a predator of other wood-boring beetles; 4. Golden buprestid (Buprestis aurulenta), inhabits recently dead or dying trees and logs, sometimes emerging much later out of structural timbers; 5. California prionus (Prionus californicus), larvae feed on dead and dying conifers. Photos are not all at the same scale.

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. 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. Because none of these insects is known to be in Oregon, 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, I still welcome your forest insect photos (high-resolution please!) and specimens for identification, as I often learn new insects myself 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.

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