Mountain pine beetle (Dendroctonus ponderosae)

Synonyms: Dendroctonus monticolae

Common Names: Black Hills beetle, Rocky Mountain pine beetle

Description:

Mountain pine beetle (Dendroctonus ponderosae) is a small, tree-killing bark beetle native to the pine forests of western North America [1]. It is not an introduced species, but it has begun spreading eastward into the jack pine of the boreal forest, which puts the pines of the Midwest and East at potential risk [3]. MISIN tracks it as a watch species because a beetle that can wipe out whole stands of pine in the West could threaten native and planted pines if it reaches this region [1][3].

Identification:

The mountain pine beetle is a tiny insect ? only about ? inch (6 mm) long ? with a hard, cylindrical, dark brown to black body [2]. The beetle itself is rarely seen, because it spends almost its whole life under the bark of pine trees [2]. Instead, you usually notice the signs it leaves on a tree: popcorn-like blobs of resin (pitch tubes) on the trunk where beetles bored in, reddish-brown boring dust in the bark cracks and at the base of the tree, and pine needles that slowly fade from green to yellow to red-brown over about a year [1][2].

The adults fly and attack new trees in mid-to-late summer, roughly July through August, so that is when fresh pitch tubes appear [2]. The faded, red-needled trees are easiest to see the following year, often in groups, since the beetles attack many nearby trees at once [1].

At-a-glance cues:

1. Pine trees with popcorn-like blobs of resin (pitch tubes) on the trunk [1][2]
2. Reddish-brown boring dust in bark cracks and piled at the base of the tree [2]
3. Needles fading from green to yellow to red-brown over about a year, often in groups of trees [1]
4. Blue-gray stained wood and winding galleries under the bark [2]

Hosts:

The mountain pine beetle lives in pine forests, where its "hosts" are the pine trees it bores into and kills [1]. In the West it attacks lodgepole, ponderosa, whitebark, limber, and bristlecone pines, among others [1]. It is native to western North America, from Mexico north to central British Columbia and east to the Black Hills [1].

It is not established in the Midwest or eastern United States. The concern is its recent spread eastward: it has begun reproducing in jack pine in the Canadian boreal forest for the first time, which is a bridge toward the jack, red, and white pines of the eastern half of the continent [3]. That eastward range expansion is why it is watched in this region [3].

Life Cycle:

The mountain pine beetle spreads in two ways: the adults fly to new trees, and people move it long distances inside infested logs and firewood [2]. So the way it could reach a new area is in cut wood ? which is why moving firewood is risky. In summer, adult beetles bore through the bark of a pine and give off a scent that calls in many more beetles, so they overwhelm one tree at a time in a coordinated "mass attack" [1].

The females lay eggs in tunnels (galleries) in the living inner bark, and the larvae feed there and spend the winter under the bark before emerging the next summer [2]. The beetles also carry a blue-stain fungus into the tree; together, the tunneling and the fungus cut off the tree's flow of water and food and kill it, usually within a year [1][2]. Because each attacked tree can release a new wave of beetles, populations can build into huge outbreaks [1].

Impact and Damage:

The mountain pine beetle is not harmful to people or pets ? the damage is to pine trees and forests. In a yard or woodlot it kills pines, and the standing dead trees become fire hazards and falling hazards near homes [1]. Healthy, well-watered, and uncrowded trees resist attack better than stressed or crowded ones [1].

At the forest scale, the beetle can kill pines across vast areas. The outbreak that ran from about 2000 to 2015 was the largest bark-beetle outbreak ever recorded, killing pines across tens of millions of acres of western North America [3]. Beyond the dead trees, these die-offs change wildlife habitat, increase wildfire fuel, and affect watersheds and the forest's role in storing carbon [3]. For the Midwest and East, the worry is that the beetle's eastward spread into jack pine could expose the region's native and planted pines to the same kind of damage [3].

Similar species:

The mountain pine beetle is very hard to tell from other bark beetles without an expert, since you usually see the signs on a tree rather than the beetle itself. The plants and signs are most often confused with the work of other pine beetles.

Red turpentine beetle (Dendroctonus valens) also makes pitch tubes on pines, but they are large and reddish and sit low on the trunk near the base, and this beetle usually does not kill the tree [1]. Ips engraver beetles (Ips species) attack the tops of pines and downed wood and carve Y- or H-shaped galleries under the bark, unlike the straight, vertical galleries of the mountain pine beetle [2]. Other native bark beetles in the genus Dendroctonus look similar and can only be separated by close examination of the beetle, its galleries, and the host tree [1].

Monitoring and rapid response:

Signs to watch for:

1. Popcorn-like blobs of resin (pitch tubes) on pine trunks, with fresh ones appearing in July?August [1][2]
2. Reddish-brown boring dust in bark cracks and at the base of the tree [2]
3. Pines fading from green to yellow to red-brown over about a year, often several trees together, with blue-gray stained wood under the bark [1]

What you can do:

1. Keep your pines healthy and not crowded ? thin dense stands and water during drought so the trees can resist attack [1][2].
2. Don't move firewood ? buy and burn it locally, because beetles and their larvae travel inside cut logs and firewood [2].
3. If a pine shows pitch tubes and fading needles, have it checked, then remove and properly dispose of (chip, debark, or burn) infested trees before the next summer's beetles emerge [1][2].
4. In the Midwest and East, watch for and report unexpected die-offs of pines (below), since this beetle is expanding eastward into jack pine [3].
5. For land managers, the best long-term defense is forest management that keeps stands vigorous ? thinning and mixing tree ages and species ? plus prompt removal and destruction of infested trees during an outbreak, and monitoring at the leading edge of the beetle's spread [1].

When to call a professional:

1. Several dying pines, a confirmed infestation, or large or high-value trees ? have a forester or certified arborist assess them [1].
2. Any pine die-off in the Midwest or East that might be this beetle, given its eastward spread.
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Colorado State Forest Service, Colorado State University. Mountain Pine Beetle. csfs.colostate.edu
2. Montana State University Extension. Mountain Pine Beetle. montana.edu/extension
3. Cullingham CI, et al. (2011). "Mountain pine beetle host-range expansion threatens the boreal forest." Molecular Ecology 20(10):2157?2171. pmc.ncbi.nlm.nih.gov/articles/PMC3116149

---