Wasps in the Woods

By Melissa Fischer, Northeast Region forest health specialist, Washington State Department of Natural Resources, melissa.fischer@dnr.wa.gov

As many of you may have noticed, there were a lot of angry wasps flying about this past season. I was stung on multiple occasions while working in the woods.

I really thought I was going to get away with not stepping on a nest this year, but those hopes were dashed in late fall after I stepped on one over in Loomis State Forest. One of those stings left a welt unlike any I’ve had before (Image 1).

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Image 1. Welt on the back of my leg from a yellow jacket sting. (Photo: Melissa Fischer)

I am originally from Pennsylvania and was quite surprised at how aggressive the yellow jackets are here in Eastern Washington. I’ve been stung plenty of times in Pennsylvania, but not aggressively hunted in the woods as I am here. I also noticed that in some years (2015, 2018), yellow jackets seem more numerous and extra-aggressive. Wasps are not a focal insect for forest entomologists. I found myself curious as to why they are so aggressive some years and not others, so I decided to do a little research …

What are wasps?

Wasps are insects in the order Hymenoptera, which also consists of ants and bees. The wasps we will focus on are in the family Vespidae.

Sometimes people refer to wasps as bees, but they are not actually bees. Although the two have some similarities, they also have some significant differences. Some species of both wasps and bees are social; they live and work together, and both wasps and bees can be beneficial pollinators. Unlike bees, wasps are also predators and scavengers, killing pestiferous insects such as defoliating caterpillars and house flies.

Bees and wasps both consist of species that are capable of stinging. Only females sting though, as the stinger is actually a modified ovipositor (body part used to deposit eggs).

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Image 2. Morphological differences between wasps [Western yellow jacket (Vespula pensylvanica) on the left (Image: Buck et al. 2008)] and bees [sweat bee (Halictus farinosus) on right (Image: Top: Laurence Packer, 2014/ http://www.discoverlife.org and Bottom)
Although bees and wasps may have a similar morphological appearance, bees tend to be more hairy and stocky, whereas wasps are shiny, have much less hair, and have slimmer bodies with a petiole or “waist” (Image 2). Many species of wasp build paper nests from wood pulp; bees build their nests from a waxy substance. There are no wasp species that produce honey.

Yellow jackets

There are several species of wasp in Washington, the most aggressive of which are yellow jackets. Yellow jackets are about half an inch long, they are black and yellow, and feed on insects and sweet nectar. Although most people think of yellow jackets as one particular species, there are actually several, including the common (Vespula vulgaris), western (Vespula pensylvanica), aerial (Dolichovespula arenaria) and German (Vespula germanica) yellow jacket.

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Image 3. Four species of yellow jacket that can be found in eastern Washington; the common (Vespula vulgaris), western (Vespula pensylvanica), aerial (Dolichovespula arenaria) and German (Vespula germanica) yellow jacket (Images: Buck et al. 2008)

Aside from some morphological differences (Image 3), there are also some differences in the nesting habits of these yellow jacket species. The western and common yellow jacket build papery nests located underground. The aerial yellow jacket builds small, round papery nests above ground (Image 4). These nests are commonly located on roof overhangs and other protected exterior building surfaces.

The German yellow jacket, which was introduced into the northeastern U.S. in the 1970s and arrived in Washington in the 1980s, build above-ground nests as well, but they are not round, they are typically larger than that of the aerial yellow jacket, and they are often located within wall voids or attics.

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Image 4. Paper nest of the aerial yellow jacket (Dolichovespula arenaria) (Image: Buck et al. 2008)

Paper wasps

Similar to bees, paper wasps are often mistaken for yellow jackets. They are morphologically similar to yellow jackets with black and yellow coloration, very little hair, and slim bodies, but there are some noticeable differences.

Paper wasps tend to be slightly larger (about three-quarters of an inch long) and slimmer, and rather than black on the upper side of the antennae, they have orange (Image 5). Paper wasps fly with their legs hanging down, while yellow jackets tuck their legs against their bodies during flight.

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Image 5. Morphological differences between the western yellow jacket and the European paper wasp. Note that the upper side of the yellow jacket’s antennae is black and the paper wasps’ is orange.
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Image 6. Typical European paper wasp nest (Image: Buck et al 2008).

Two common paper wasps species in Eastern Washington include the golden paper wasp (Polistes aurifer), which is native, and the European paper wasp (Polistes dominula), which was introduced into the U.S. in 1981 and reached the Pacific Northwest by 1999.

Similar to yellow jackets, paper wasps are beneficial predators that also feed on nectar. They do not scavenge non-living food as yellow jackets do though. They make paper nests, but their nests are built so that the comb cells are open to view from below (Image 6). These nests are often found in protected sites such as under eaves, and typically contain 20 or fewer cells.

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Image 7A. Bald-faced hornet (Dolichovespula maculate) worker and B. typical bald-faced hornet nest (Images: Buck et al 2008).

Bald-faced Hornets

The bald-faced hornet (Dolichovespula maculate) is another common wasp species that occurs in Eastern Washington. The bald-faced hornet in not really a hornet; hornets are in the genus Vespa and none actually occur in Washington. Bald-faced hornets are about three-quarters of an inch long and black and white/pale yellow (Image 7A).

Bald-faced hornets build a large papery nest that may have leaves and twigs on the outer nest wall (Image 7B). They are considered beneficial, as they feed almost entirely on living insects, including yellow jackets.

The wasp lifecycle

In the winter, mated wasp queens overwinter in protected areas, such as under bark or on the ground in weedy areas. The paper wasp may overwinter in attics or other parts of buildings, and can become stinging pests when temperatures inside the house increase.

Wasp queens will emerge in the spring, beginning to feed on nectar and build new nests. (Old nests are not reused.) Yellow jackets and bald-faced hornets work as individual queens to construct their own nests. European paper wasps sometimes work together as cooperating queens to make a shared nest.

A single egg is deposited in each cell. The queen cares for her larvae by feeding them chewed-up insects. The larvae emerge as adult worker wasps about one month after eggs are laid. The workers are infertile females; males are not produced until September.

As summer progresses, the workers that are produced maintain the nest, forage for food, and tend to the larvae; the queen just lays eggs. Bald-faced hornets and some species of yellow jacket may have nests consisting of hundreds to thousands of workers by late August or early September. Paper wasp nests are substantially smaller, seldom exceeding 100 individuals and often comprising fewer than 20.

In the fall, new reproductive males and queens are produced. These reproductives leave the nest and mate. The old queen dies, and the colony begins to die out thereafter. All workers and males die with the first freezing temperatures.

Back to last summer … 

So what happened this past season? Why were there so many wasps? To start with, neither the Washington State Department of Natural Resources, nor the U.S. Forest Service (nor anyone else for that matter), release Vespidae wasps to control bark beetles or any other insect. Although wasps are beneficial predators, they are generalists and would not even begin to put a dent in the bark beetle population. On top of that, because bark beetles spend most of their lives under tree bark, they are largely inaccessible to Vespidae wasps.

There are many species of parasitoid wasps that are insect predators, some of which are specific to bark beetles. These wasps use their ovipositors as they were originally intended — to deposit eggs. Some parasitoids have very long ovipositors that allow them to deposit eggs under tree bark and into bark beetle tunnels. These eggs hatch into larvae that feed on bark beetle eggs, larvae, and sometimes adults.

Some species of parasitoid wasp deposit eggs inside insect larvae and feed on the larvae from the inside out. Others deposit eggs inside eggs, and the larvae feed on the eggs from the inside out. If you’ve ever wondered where people come up with alien scenarios in horror movies, you can look to parasitoid wasps!

Parasitoid wasps have been released for control of mostly invasive, nonnative insect species. One of the reasons nonnative species have the ability to become invasive is because they do not have natural enemies in their new habitat.

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Image 8.  The parasitic wasp, Agathis pumila, released for control of the larch casebearer (Image: Roger Ryan USFS PNW Station/ forestryimages.org).

If eradication of an invasive species does not work and other controls measures are not working, sometimes entomologists will look for native enemies (often parasitoids) of these invasive species in their home habitats. These natural enemies are collected and studied in quarantine labs. If, after many rigorous tests, they are found to be safe to release in the U.S., they may be released as biological control agents. A good example would be the release of Agathis pumila (Image 8) for control of the larch casebearer (Coleophora laricella).

Biological control agents are rarely released for control of native species because native species typically have a suite of natural enemies with which they have evolved.

So if DNR and U.S. Forest Service didn’t release the wasps, why were there so many? Insect populations in general are largely controlled by the weather, and wasps are no exception. Spring weather in particular largely determines if we will have wasp problems. Mild springs allow overwintering queens to survive, whereas cold, rainy weather may reduce the likelihood that queens can build a nest and collect enough food to feed her offspring.

Summer weather can also contribute to wasp issues because warm summer weather accelerates wasp metabolism and growth rates, resulting in more wasps at a faster rate.

Wasps become most troublesome in late summer and early fall as colonies reach their maximum size. Yellow jackets are more readily provoked into stinging during this time, as natural foods become scarce and workers aggressively scavenge food scraps. Additionally, workers are more likely to vigorously defend their nests as new reproductive queens and males are produced.

Protection/ Control

For general protection around the house, you may want to keep lids on trashcans, clean up fallen fruit, and avoid wearing perfume. You don’t want to leave soda cans and food laying around. Wearing white or tan clothes can be helpful, whereas mosquito and tick repellents are not, as they don’t work against wasps.

If you do happen upon a ground nest, run away!

Keep running until the wasps stop their pursuit. Yellow jackets may chase you several feet. Wasps can sting you approximately four or five times before they run out of venom (unlike honey bees which can only sting you once).

If any land on you, flick them off. Do not swat or crush their bodies, as this will prompt them to release alarm pheromones that then stimulates a mass attack from other workers.

Although yellow jackets traps do work to trap yellow jackets (they do not attract paper wasps or bald-faced hornets), remember that there is no way you will control the yellow jacket population using them. Workers may be flying from as far as 4,000 feet away, and there may be many nests with hundreds of wasps in the surrounding area.

Traps are mainly useful for drawing yellow jackets away from areas where people congregate. Therefore, if you do invest in one, hang it away from these areas.

Homemade traps can work rather well, too. One such trap consists of a container (such as an old dishwasher tub) filled with water and dish detergent and a board laid over top with a piece of meat hanging beneath the board. Yellow jackets will remove a piece of meat and try to fly away with it, but the meat will make them heavier than normal, causing them to fall into the water.  The detergent will act as a wetting agent, trapping the yellow jacket and making it unable to fly, leading to its death via drowning.

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Image 9. Banty rooster feeding on yellow jackets from a homemade yellow jacket trap.

I made one myself last year using fish skin for my bait. At first, I thought the trap didn’t work at all, as I was coming home from work and hardly any yellow jackets were in it.  But I quickly realized that one of my banty roosters was using it as his personal snack bar (Image 9).

Thereafter, I used vegetable oil instead of dish detergent as the wetting agent. I figured dish detergent may not be good for him. The vegetable oil seemed to work just as well.

If you need to destroy a nest, hiring a professional would be my first suggestion. If you choose to do it yourself, be sure to read the label of any insecticide you decide to use prior to treatment. Many are pyrethroids, nerve poisons that can be hazardous to humans, pets, and wildlife.

If you need to eradicate aerial nests, aerosol jet sprays and foam sprays are available. Dry dust insecticides would be best for German yellow jackets located in wall voids. For ground nests, use a liquid drench. Never use gasoline, as this will contaminate the soil and could contaminate ground water.

If you would prefer not to use insecticides in the soil, flooding a ground nest with water rarely works, but vacuuming can be effective (and should only done by a professional).

Treat nests after dark when flight activity is minimal. Do not shine a flash light directly at the nest. Spray the insecticide into the nest entrance first, then spray the nest surface, then soak the entire nest. Leave it for a day or two before removing, as any workers that were not in the nest may come back and come into contact with the insecticide.

Given the fact that wasps are ecologically beneficial, I would only use control if they are a direct threat. Pay attention to what species you are considering eradicating. While yellow jackets are indeed a rather aggressive species, bald-faced hornets are less so and paper wasps are actually quite docile (although both will sting to defend their nests).

Let’s all hope next season is less exciting in the world of wasps!

References

Akre, R.D. and Antonelli, A.L. 2003. Yellowjackets and Paper wasps. Revised by Peter Landolt and Arthur L. Antonelli.  Washington State University Cooperative Extension, EB0643. 7p.

Bechinski, E., Merickel, F., Stoltman, L. and Homan, H.  2009. Homeowner guide to Yellowjackets, Bald-faced Hornets, and Paper wasps.  University of Idaho Extension, BUL 852. 16p.

Buck, M., Marshall, S.A. and Cheung D.K.B. 2008. Identification Atlas of the Vespidae (Hymenoptera, Aculeata) of the northeastern Nearctic region. Canadian Journal of Arthropod Identification No. 5: 492 pp.   Available online at doi: 10.3752/cjai.2008.05

Bush, M.R. and Murray, T.A. 2014. The European Paper Wasp. Washington State University Extension Fact Sheet, FS152E. 5p.

Does Chipping Pine Attract Bark Beetles?

Lop and scatter method of slash disposal
Lop and scatter method of slash disposal. Photo: USDA

As many forest landowners already know, pine slash has a tendency to attract pine bark beetles. There are several recommendations for slash disposal to mitigate this problem. Two common methods — lop and scatter, and chipping — focus on eliminating bark beetle habitat while retaining the nutrients contained within the slash on site.

The lop and scatter method involves cutting slash into smaller pieces (less than 12 inches in length, less than 3 inches diameter) and scattering the pieces throughout the site. Chipping involves running slash through a chipper and scattering it throughout the site, similar to lop and scatter. While there is no chance that bark beetles can infest the chips, it is still possible for bark beetles to infest the pieces in the lop and scatter method. The chances of the beetles being able to successfully produce brood in these pieces is pretty low, as they will likely dry out before the larvae are finished developing. Both methods are a reasonable substitute to piling and burning when burning is prohibited, BUT…

Anytime you cut pine, you are releasing volatiles. Volatiles emitted from pine contain chemicals called monoterpenes. Some monoterpenes have been shown to attract bark beetles and are often included as part of the lure when trapping for bark beetles.

female bark beetle
A female bark beetle eating the phloem of a tree. Photo: USFS

If you are choosing between lop and scatter and chipping, which is best? You would think that chipping would be the better choice since there is no chance that bark beetles can infest the chips. But a study by Fettig et al. (2006) showed that chipping in ponderosa pine stands attracted bark beetles more so than the lop and scatter method, particularly in the spring. Why would this be? As it turns out, directly following treatment chipping releases much larger quantities of attractive monoterpenes than the lop and scatter method. If large quantities of attractive monoterpenes are released at a site where the slash has been chipped and bark beetles arrive, where are they going to go? They cannot infest the chips. They are going to go to the standing, residual trees. Although the act of thinning should increase the health and vigor of the residual trees, it will take at least a year for those trees to respond, therefore, you may wind up with bark beetle infested leave trees.

So, does this mean you cannot chip? Not necessarily. Some landowners prefer to begin thinning treatments on their property in the spring; it is finally warm (but not too warm), the snow is melting off, and they are ready to go outside and get things done. Other landowners find that they have to thin in the spring for various reasons (the snow is too deep during the winter; fire season often prohibits the use of machinery in the summer; contractors cannot fit everyone in during the bark beetle “off” season). Unfortunately, as Fettig et al. (2006) found, the response of bark beetles to high concentrations of volatiles released from chips is more significant in the spring because a large segment of the beetle population is most active during this time.

If you plan on chipping, it would be best to avoid treatments in the spring. The best time to chip (or do any sort of management that releases pine volatiles for that matter, including pruning), would be in late summer (late July/early August) through early winter (December). When producing chips, avoid piling them and absolutely do not pile chips at the base of any residual trees.  Spread the chips out in the sun if you can. The quicker they dry out, the quicker the volatiles will dry out, and the less overall bark beetle risk there will be.

By Melissa Fischer, forest health specialist Washington State Department of Natural Resources, Northeast Region, melissa.fischer@dnr.wa.gov

References

Fettig, C.J., J.D. McMillin, J.A. Anhold, S.M. Hamud, R.R. Borys, C.P. Dabney, and S.J. Seybold. 2006. The effects of mechanical fuel reduction treatments on the activity of bark beetles (Coleoptera: Scolytidae) infesting ponderosa pine. Forest Ecology and Management. 230: 55-68.

Are Those “Stink Bugs” in Your Home?

Since September, I have been awakened from a dead sleep countless times by an obnoxious buzzing in my ear and, even more unsettling, the feeling that something is crawling on top of my head. Although groggy, I usually realize pretty quickly that it’s just a stink bug, grab it, and toss it across the room. Admittedly, this method of disposal did backfire once when the stink bug turned out to be a paper wasp; a regrettable mistake.

So what are these annoying insects? Why are they in our homes?

 Consperse stink bug
Figure 1. The Consperse stink bug (Euschistus conspersus) Photo: Mike Quinn, Bugguide.net

You may be surprised to find out that these insects are not actually stink bugs. Stink bugs are in the family Pentatomidae and have bodies shaped like shields (Figure 1). The insect in question is actually in the family Coreidae, or the leaf-footed bugs. The common name “leaf-footed” refers to the enlarged leaf-like structure on their hind legs (Figure 2). The genus and species of this particular leaf-footed bug is Leptoglossus occidentalis whose common name is the western conifer seed bug (WCSB).

Although you may be familiar with the WCSB because of its presence in your house, this insect is actually a forest dweller. The WCSB feeds on seeds, developing cones and the needles of pine trees, as well as Douglas-fir.

Western conifer seed bug
Figure 2. The Western conifer seed bug; notice the leaf-like structure on the hind legs. Photo: Bugguide.net

The WCSB has a single generation a year. Adults emerge in late May or early June. At this time, they feed on one-year-old cones and inflorescences (small flower clusters). Eggs are laid in rows on conifer needles and hatch within 10 days. The WCSB is a “true” bug in the order Hemiptera and does not go through complete metamorphosis as a fly or butterfly would. In other words, there is no larval stage. Instead, the young look very similar to the adults, only smaller, and are called nymphs rather than larvae (Figure 3).

The WCSB goes through five nymphal instars (stages) prior to the final adult stage. The first instar feeds on needles and cone scales while older nymphs will feed on developing seeds. The WCSB reaches adulthood around mid-August and will continue to feed on ripening seeds until early fall, after which time it will begin to seek overwintering sites. This is when you may begin seeing them in your home. The WCSB will overwinter under bark, in dead and dry logs, in bird and rodent nests, and of course, in buildings. With its flattened body shape, it can enter a house through most small openings. Once inside the house, they tend to become active and conspicuous when warm.

Nymphal stages and final adult form of the Western conifer seed bug
Figure 3. Two nymphal stages (on the left) and the final adult form (far right) of the Western conifer seed bug. Photo: Brandon Woo

Although annoying, the WCSB is not harmful to people. It does not bite, sting, eat wood, or breed indoors. It does emit an unpleasant odor when disturbed, but this is not harmful to anything but our noses. The tendency to think that WCSB is harmful may be due to a case of mistaken identity, as leaf-footed bugs have a similar appearance to assassin bugs (family Reduviidae), which do bite. There are morphological differences that distinguish WCSB from assassin bugs; the most obvious is the leaf-like structure found on the hind legs of the seed bug, which assassin bugs lack (Figure 4).

Assassin bug
Figure 4. Assassin bug Sinea diadema; notice the lack of the leaf-like structure on the hind legs, which the Western conifer seed bug possesses. Photo: Michael Hughes, Bugguide.net

The WCSB can be a pest to the forest industry, particularly in seed orchards, as it can result in a substantial loss of seed crop. However, in a general forested setting, there are typically plenty of seeds to go around and there is no lack of regeneration in our forests as a result of these bugs.

So, is there any way to control the WCSB? The best method of control is prevention via mechanical exclusion. This of course would involve filling in any cracks around the house where they may be getting in. Replacing loose screens, windows and doors; caulk gaps; and screen your fireplace, attic and wall vents, and other openings — basically, blocking all points of entry. Unfortunately, if your house is anything like mine, this is a near impossible endeavor. I opt for the vacuum cleaner and just suck them up.

By Melissa Fischer, forest health specialist, Washington State Department of Natural Resources, Northeast Region, melissa.fischer@dnr.wa.gov 

Blue Spruce Damage Indicates Elevated Risk of Douglas-fir Tussock Moth Outbreak

Douglas-fir tussock moth
Source: Forest Health Highlights in Washington, 2016, by DNR and U.S. Forest Service.

Recently I have begun to observe damage to ornamental blue spruce throughout the area of Colville, Washington. Upon closer inspection of these trees, I found that they are being defoliated by the Douglas-fir tussock moth. The Douglas-fir tussock moth is a native defoliator of Douglas-fir, true firs (such as grand fir) and spruce. For reasons unknown, a year or two prior to an outbreak of Douglas-fir tussock moth on forested land, we tend to see defoliation of ornamental trees such as blue spruce. Given the number and area of defoliated blue spruce I have been seeing, it is likely we will have an outbreak of the Douglas-fir tussock moth sometime within the next two years. Unfortunately, no relationship has been found between the location of the sentinel trees and the forested areas that will be defoliated in the future. In other words, we know that there will likely be an outbreak, but we do not know exactly where it will occur.

Outbreaks of Douglas-fir tussock moth are cyclical, typically occurring every 7 to 14 years. On average, outbreaks last 2 to 4 years. The last outbreak began in 2008 (Figure 1); therefore, we are due for another.

 

Lifecycle of the Douglas-fir tussock moth

The Douglas-fir tussock moth spends the winter months in the egg stage. Eggs are protected in gray, hairy masses that are approximately an inch in size. An egg mass can contain as many as 350 eggs. The eggs will hatch in late May or early June, depending upon temperatures.

The caterpillar (larvae) will be present from June through August. The caterpillars are quite hairy, with two long hairy tufts projecting from the head and the rear end. They also have four dense tufts of hair on their back, called tussocks, which are whitish in coloration with red tips.

The caterpillars pupate in July-August. Cocoons are grayish-brown, about one inch in size, and can be found on the foliage and trunk of trees as well as in the understory.

The pupae develop into moths and begin emerging in late July. They will continue to be active through November. The females are gray-brown, with large abdomens and are wingless. The males have gray-brown forewings and reddish-brown hind wings. They also have large, feathery antennae.

 

Damage: What to look for

The larvae feed on new needles in the upper crown first. These needles will turn an orangish-brown color (Figure 2). Overtime, the caterpillars will disperse to the lower crown and begin defoliating needles there. The larvae will feed on both new and old needles, sometimes completely defoliating the tree.

trees defoliated by the Douglas-fir tussock moth
Figure 2. Left: Ornamental blue spruce defoliated by the Douglas-fir tussock moth. Center: Ornamental blue spruce and white fir defoliated by the Douglas-fir tussock moth. Right: Close-up of blue spruce defoliation by the Douglas-fir tussock moth. Photos: Melissa Fischer/DNR.

In addition to defoliation, you may find the moth in one of its life stages, depending upon the time of year. Another thing to look for is silk and/or frass ( poop) on the branches (Figure 3).

Signs of Douglas-fir tussock moth
Figure 3. Left: Douglas-fir tussock moth caterpillars feeding on blue spruce foliage. Right: Silk and frass (poop) produced by the Douglas-fir tussock moth. Photos: Melissa Fischer/DNR.

Management

Defoliation by the Douglas-fir tussock moth can cause top and branch kill, which can lead to reduced vigor and growth loss. This can increase susceptibility to bark beetle attack or infection by diseases. Complete defoliation or several years in a row of defoliation can lead to mortality.

Douglas-fir tussock moth is usually controlled over time by natural enemies such as predators, parasites, viruses, cold temperatures, and/ or starvation (eating themselves out of house and home), but can also be managed through use of insecticide treatments.

Bacillus thuringiensis var. kurstaki (Bt) is an insecticide that is specific to Lepidoptera larvae. Its specificity is advantageous because it does not affect other insects, such as the natural enemies that help reduce populations. Because Bt is specific to the larval stage, it would have to be applied when the Douglas-fir tussock moth is in the caterpillar stage.

In forested settings, severe damage can be prevented through thinning. It would be best to remove host trees (Douglas-fir, grand fir, Engelmann spruce) and favor the retention of non-host trees (ponderosa pine, western larch, lodgepole pine) thereby reducing the amount of food available. Additionally, thinning breaks the crown continuity within a stand, so that when the caterpillars disperse, many will fall to the ground and dessicate or be eaten by passing birds.

If you think you may have a sentinel tree on your property, I would love to know about it! Feel free to contact me via Melissa.Fischer@dnr.wa.gov.

By Melissa Fischer, forest health specialist, Washington State Department of Natural Resources, Northeast Region, melissa.fischer@dnr.wa.gov 

Potential for Douglas-fir Beetle Outbreaks in Eastern Washington

Douglas fir killed by bark beetle
Beetle damage in stand of Douglas fir. Photo: Kenneth E. Gibson, USDA Forest Service, Bugwood.org

Several windthrow events in 2015 have paved the way for a potential outbreak of Douglas-fir beetle in areas of eastern Washington this spring (2017).

Douglas-fir beetle egg and larval tunnels
Characteristic pattern of Douglas-fir beetle egg and larval tunnels. Photo courtesy of Oregon Department of Forestry.

The Douglas-fir beetle is a bark beetle that normally breeds in felled, injured, windthrown or root-diseased Douglas-fir. It may also attack western larch, but can only produce brood in downed trees. At outbreak levels, this bark beetle can attack and kill large diameter, healthy Douglas-fir. Outbreaks tend to occur after extensive windthrow events such as was seen in 2015. Outbreaks may also occur after defoliation events, fire and extended drought. Dense stands surrounding areas where windthrow, defoliation, fire and drought events have occurred may be at high risk for an outbreak, particularly if those stands contain a 50 percent or more component of Douglas-fir that are greater than 120 years of age and larger than 14 inches DBH (DBH = diameter at breast height; diameter of a tree bole 4.5 feet from the base).

The Douglas-fir beetle has one generation a year. Brood that developed through 2016 will pupate and emerge as adults this spring. Once emerged, they will begin attacking standing trees surrounding the windthrow, as the windthow is no longer habitable for them.

What can be done?

The best approach to prevent an outbreak this spring is to salvage any large diameter Douglas-fir or western larch that were downed by the storms prior to the adult beetle flight, which should occur in April, depending on temperatures. Windthrown trees can also be burned or chipped on site if salvage is not an option. Time is running out; if you find you cannot take care of this material, the use of the anti-aggregate pheromone MCH is another option.

A pheromone is a chemical released by bark beetles that is used to affect the behavior of other beetles of the same species. Aggregating pheromones attract beetles, while anti-aggregates repel them. A bark beetle might use an anti-aggregate to prevent overcrowding within a tree. An anti-aggregate basically tells other beetles that there is no room for additional inhabitants in the tree.

MCH bubble capsule stapled to a tree
MCH bubble capsule stapled to a tree. Photo: US Forest Service.

The Douglas fir-beetle naturally produces an anti-aggregate to repel others. A synthetic version of this anti-aggregate, MCH, has been produced and is available for purchase through several online companies. MCH comes in a “bubble capsule” and can be used to protect individual live, high-value Douglas-fir or even an entire stand. For individual tree protection, two bubble capsules can be stapled on either side of a Douglas-fir bole at approximately 6-8 feet from the ground for a tree less than 24 inches DBH. Four bubble capsules should be used for Douglas-fir larger than 24 inches DBH. To protect a stand of Douglas-fir, 30 bubble caps per acre can be evenly placed through the stand.

MCH costs approximately $2.50 per capsule and should be hung prior to the beetle flight in April. It is advisable to contact your local forest health specialist if you are considering this method of management. Additional information about this method can be found in the free publication, “Using MCH to protect trees and stands from Douglas-fir beetle infestation,” published by the US Forest Service.

By Melissa Joy Fischer, forest health specialist, Washington State Department of Natural Resources, melissa.fischer@dnr.wa.gov