Douglas-fir beetle, Dendroctonus pseudotsugae
Order: Coleoptera
Family: Curculionidae
Description
The Douglas-fir beetle (Dendroctonus pseudotsugae) is native to North America and the most important tree-killing bark beetle inhabiting the western United States. This pest is closely related to the mountain pine beetle (Dendroctonus ponderosae) and spruce beetle (Dendroctonus rufipennis), both of which are discussed in the webpage on Dendroctonus spp. At 4-6 mm (~1/4 inch) long these stout-bodies beetles are smaller than a grain of rice. As adults mature, they change from light brown to dark brown, and eventually black with red-brown wing covers (elytra), although in some variants the mature beetle is entirely black. The larvae of Douglas-fir beetle are white, legless, and C-shaped with an amber head capsule. Pupae are white or tan in color and may possess some adult features such as folded wings positioned beneath the abdomen. The eggs are white and very small.
Quick Facts
- The Douglas-fir beetle is an important pest of coniferous forests containing high densities of its host, Douglas-fir.
- Outbreaks of Douglas-fir beetle can kill large numbers of trees, which increase the risk of wildfires and poses a safety hazard in recreational areas.
- Some common feeding injuries produced by Douglas-fir beetle include the discoloration and loss of foliage, formation of pitch tubes, and the production of emergence holes.
- Management approaches can be costly and impractical. However, there are a variety of tactics that can help prevent outbreaks such as destroying infested trees, using an anti-aggregation pheromone, and applying insecticides insecticides.
Adult of Douglas-fir beetle. Note the clubbed antennae, dark body, and red-brown elytra. Image credit: Joseph Benzel, Screening Aids, USDA APHIS PPQ, Bugwood.org
Larvae of Douglas-fir beetle. Note the C-shaped body, amber head capsule, and parallel galleries. Image credit: Malcolm Furniss, Bugwood.org
Douglas-fir tree with frass and pitch tubes due to an infestation of Douglas-fir beetle. Image credit: Doug Page, USFS / BLM, Bugwood.org
Close up of a downed tree containing frass produced by Douglas-fir beetle. Image credit: Elizabeth Willhite, USDA Forest Service, Bugwood.org
Douglas-fir tree killed by infestation of Douglas-fir beetle. Fruiting bodies are evident on the bark due to fungal infection of C. volvatus. Image credit: Michael Freeman, University of Washington, Bugwood.org
Life history and habits
The Douglas-fir beetle has four life stages: egg, larvae, pupa, and adult. This species primarily overwinters as adults, but a proportion of larvae overwinter as well. The adults emerge the following spring in April and May when temperatures are consistently above 15.5°C (60°F), while overwintered larvae emerge during the summer in July or August. Along the Front Range of Colorado, peak flights occur in mid to late June as beetles seek out new host trees. When adults emerge earlier in the spring, a second wave of flights can be observed in late summer from June through August. Adults mate after boring an egg gallery in the wood of a suitable host tree. The female then deposits eggs singly in an alternating arrangement on the inner walls of the gallery, which is packed with frass and measures approximately 20-25.5 cm (8-10 inches) long. After one to three weeks, the eggs hatch and newly emerged larvae begin excavating side galleries parallel to one another and perpendicular to the egg gallery. The larvae of Douglas-fir beetle develop through three instars before constructing pupal chambers at the end of their respective gallery. Adults emerge by late summer, with one generation typically produced each year.
In addition to feeding and oviposition injuries, attacks by Douglas-fir beetles introduce a pathogenic fungus, Cryptoporous volvatus, to host trees. Typically, low populations of Douglas-fir beetle are maintained in trees stressed from root diseases or other factors. These beetle populations tend to be widely scattered and can kill small groups of trees. Research suggests communication via attraction and anti-aggregation pheromones are important mechanisms in overwhelming the defenses of healthy trees. Outbreaks of Douglas-fir beetle are more likely to occur after defoliation events, fires, or downed trees due to strong winds. Typically, outbreaks last two to four years, but can be prolonged under drought conditions. Trees with a large diameter are most susceptible to attack by Douglas-fir beetle. In Colorado, extensive tree mortality due to Douglas-fir beetle is commonly observed in the central and Northern Rockies.
This species typically attacks the underside of downed trees or trees damaged by environmental conditions such as wind, fire, lighting, or from biotic factors such as pathogen infections or activity of defoliating insects such as the western spruce budworm (Choristoneura freemani) or Douglas-fir tussock moth (Orgyia pseudotsugata). The destruction of large numbers of trees can increase the risk of wildfires, and standing dead trees pose a safety hazard in recreational areas. Research has indicated that stands with a density greater than 250 square feet basal area, species composition of greater than 50% Douglas-fir, average age greater than 120 years, and timber diameter greater than 35.6 (14 inches) are at the greatest risk of infestation. While large-diameter trees are more at risk than young trees, younger trees can also be killed during outbreaks in stands with high densities of Douglas-fir.
Injury
When attacked, healthy trees exude resin through resin canals, called pitch tubes, which repels or traps colonizing beetles. The trees can also induce necrosis of tissue surrounding egg and larval galleries. The fungus C. volvatus frequently produces fruiting bodies on the surface of bark one to two years after an attack. However, there are other means by which this fungus is transmitted to Douglas-fir trees, and the presence of fruiting bodies alone is not a reliable indicator of an beetle infestation.
Stands that contain groups of dead or dying Douglas-fir trees indicate that an infestation of Douglas-fir beetle may be present. About one year after an attack, infested trees may have foliage that fades from green to rust colored before falling off. Non-resinous boring dust is often present around the base of trees and in bark crevices. Adults produce small, round exit holes in bark when emerging from a host tree. Although not always present, streams of white or clear resin may be observed along the main trunk. The presence of larval and egg galleries may be observed when bark is removed. Heavy woodpecker activity is another indication of an infestation but may also result from infestations of other wood-boring insects.
Monitoring
Positioning pheromone traps near high-value stands of Douglas-fir is not recommended and has been shown to facilitate attacks. In addition to visual inspections for injuries on susceptible trees, activity of Douglas-fir beetle can be monitored through aerial detection maps.
Cultural control
Maintaining healthy trees can help prevent major outbreaks and can be accomplished by thinning overly dense stands. Trees infested with other insects or diseases, and debris from thinning or pruning should be removed before May in the year following an attack. The debris can be chipped to kill developing beetles or buried a minimum of 20 cm (8 inches) in the ground.
The use of trap trees involves exposing susceptible trees to recently emerged Douglas-fir beetles. The trap trees are then removed and destroyed during fall or winter. Solar treatments can be effective in stands infested with Douglas-fir beetle, which involves felling infested trees and exposing the cut logs to full sun before covering them with clear plastic wrap. For solar treatments to be effective, the temperature of stacked logs must reach a minimum of 43°C (110°F). This can be especially challenging since forests containing high densities of Douglas-fir are often shaded, moist, and cool.
The anti-aggregation pheromone methylcyclohexanone (MCH) can act as a repellent that disrupts the attraction mechanism of beetles and may help reduce localized attacks. In Colorado, packets containing MCH can be deployed on uninfested trees before Douglas-fir beetles begin emerging in April or May. However, it is recommended that community members coordinate with local foresters to assist with the spatial arrangement of MCH, which is subject to pesticide regulations and is only appropriate in designated areas.
Biological control
Natural enemies of Douglas-fir beetle include various species of parasitoid wasp, flies, and predatory beetles. Woodpeckers will often strip the bark from infested trees to feed on developing larvae. However, the activity of natural enemies does not prevent outbreaks, nor does it help suppress populations of Douglas-fir beetle during outbreaks.
Chemical control
Preventative insecticide sprays of carbaryl and pyrethroids are the primary chemical approach for managing Douglas-fir beetle. It is recommended that applications be made on high-value trees near homes, businesses, or recreation areas since overuse can lead to negative environmental impacts on local water supplies and wildlife.
Insecticides should be applied annually to living trees with green foliage before adults emerge in late spring, or in the fall if winter conditions prevent access to sites in early spring. It is important to thoroughly cover the entire tree using the proper dosage. Insecticides may not be effective if applied to trees already infested with Douglas-fir beetle, applied at improper dosage, or when significant rainfall or high air temperatures follow immediately after an application.
References
CSFS. 2016. Douglas-fir Beetle. Colorado State University – Colorado State Forest Service. Available https://csfs.colostate.edu/wp-content/uploads/2016/01/Douglas-Fir-Beetle-QuickGuide2016.pdf
Kegley, S. 2011. Douglas-fir Beetle Management. United States Department of Agriculture – United States Forest Service. Available https://www.fs.usda.gov/Internet/FSE_DOCUMENTS/stelprdb5187396.pdf
Negron, J., A. Lynch, W. Schaupp, and J. Mercado. 2014. Douglas-Fir Tussock Moth- and Douglas-Fir Beetle-Caused Mortality in a Ponderosa Pine/Douglas-Fir Forest in the Colorado Front Range, USA. Forests. 5: 3131-3146. Available https://mdpi-res.com/forests/forests-05-03131/article_deploy/forests-05-03131.pdf?version=1418382737
Progar, R., N. Sturdevant, and M. Rinella. 2010. Trapping Douglas-fir beetle (Dendroctonus pseudotsugae) with pheromone baited multiple-funnel traps does not reduce Douglas-fir (Pseudotsuga menziesii) mortality. The Pan-Pacific Entomologist. 86(4): 111-118. Available https://libkey.io/libraries/166/articles/48846673/full-text-file
USDA. 2017. Douglas-Fir Beetle: Dendroctonus pseudotsugae. United States Department of Agriculture – Forest Service. Available https://apps.fs.usda.gov/r6_decaid/views/douglas_fir_beetle.html