Two Dendroctonus spp. are of particular significance in Colorado, the mountain pine beetle (Dendroctonus ponderosae) and spruce beetle (Dendroctonus rufipennis), both of which are native to North America. All Dendroctonus spp. have a rounded tip at the end of the abdomen, which distinguishes these beetles from Ips spp., who often have overlapping hosts.
Mountain pine beetle
The mountain pine beetle is the most destructive bark beetle in the western United States. Adults are dark brown or black with cylindrical, 4-7.5 mm (1/6-1/3 inch) long bodies and clubbed antennae. Newly emerged adults are lighter in color with yellow or light brown bodies. The larvae are legless white grubs with a tan head capsule. Pupae are white and have soft bodies with visibly developing legs, wings, eyes, and antennae. Ponderosa (Pinus ponderosa), lodgepole (Pinus contorta), and limber pines (Pinus flexilis) are preferred hosts, but mountain pine beetle can attack all pines in Colorado when the pest population is booming. During such periods, mountain pine beetles have also been recorded attacking Englemann (Picea engelmannii) and Colorado blue spruce (Picea pungens). These periodic outbreaks can result in millions of tree deaths in forests and urban landscapes.
The spruce beetle is typically associated with Engelmann spruce (Picea engelmannii) and Colorado blue spruce (Picea pungens). Adults are 6 mm (1/4 inch) long and have black bodies. Some individuals have red wing covers (elytra) and are similar in appearance to the Douglas-fir beetle (Dendroctonus pseudotsugae). The larvae are white grubs with a brown head capsule, and pupae are white and soft with visibly developing legs, wings, eyes, and antennae.
- The mountain pine beetle and spruce beetle are two important pests of Colorado forests. Both are in the same genus (Dendroctonus) but have some notable morphological and life history differences.
- Effective monitoring involves inspecting trees for external indicators of an infestation, such as pitch tubes or the presence of small, round exit holes. Removing bark will help expose the frass-filled galleries indicative of Dendroctonus spp. The use of aerial detection maps can also be helpful in monitoring regional pest populations.
- Preventative and cultural controls, such as maintaining tree health and treating infested logs, are highly recommended management approaches that can help maintain low populations of both pest species.
Lateral view of spruce beetle. Note the black wing covers and clubbed antennae. Image credit: Steven Valley, Oregon Department of Agriculture, Bugwood.org
Dorsal view of spruce beetle. Note the red wing covers (elytra) and clubbed antennae. Image credit: Steven Valley, Oregon Department of Agriculture, Bugwood.org
Two mountain pine beetles constructing a gallery in ponderosa pine. All Dendroctonus spp. produce galleries with abundant frass. Image credit: William M. Ciesla, Forest Health Management International, Bugwood.org
Larvae of mountain pine beetle in lodgepole pine (Pinus contorta). Note the brown head capsule. Image credit: G.D. Amman, USDA Forest Service, Bugwood.org
Pitch tube caused by mountain pine beetle attack. Image credit: USDA Forest Service – Region 2 – Rocky Mountain Region , USDA Forest Service, Bugwood.org
Ponderosa pine with bark removed, exposing galleries produced by mountain pine beetle. Image credit: Ronald F. Billings, Texas A&M Forest Service , Bugwood.org
Frass and sawdust on the outer surface of tree bark excavated by spruce beetle. Image credit: Edward H. Holsten, USDA Forest Service, Bugwood.org
Exit holes produced by Dendroctonus spp. Image credit: Ronald F. Billings, Texas A&M Forest Service, Bugwood.org
Life history and habits
When populations are low, the mountain pine beetle and spruce beetle attack larger trees or stressed trees. During outbreaks, trees of all sizes can be attacked and killed in large numbers.
Mountain pine beetle
This species overwinters as a larva in the bark of a host tree, and development primarily occurs in pines. In Colorado, the entire life cycle is completed in one year. Peak flights tend to occur in July through August, but smaller numbers can emerge earlier or later in the summer. To successfully infest a tree, mountain pine beetle must overwhelm the tree’s defense system, which involves the production of sap and terpenoids. Hundreds of beetles can attack and overwhelm a single tree, at which point the beetles emit a chemical signal that attracts other individuals to the vulnerable host. Eventually, a maximum pest density is reached in the original host tree, and the beetles begin emitting additional chemical signals that cause other beetles to attack neighboring trees. This behavior causes patches of tree mortality in a region.
Epidemic cycles of mountain pine beetle tend to occur every 10 to 30 years, with the most recent outbreak killing millions of acres of pines from British Columbia to Colorado. When populations are low, mountain pine beetle targets stressed trees due to injury, unfavorable environmental conditions, overcrowding, root disease, or old age. When populations are high, healthy trees of all sizes are susceptible to attack. However, mature trees with a diameter greater than 20 cm (8 inches) are most susceptible and usually die when infested. Females tend to prefer large diameter trees when available. Mountain pine beetle can also introduce blue stain fungi to the vascular system of the host tree, which disrupts water transport from the roots and greatly increases the probability of tree death.
Individual trees or groups of trees can be killed by spruce beetle, depending on the pest population. Outbreaks of spruce beetle result in losses of large trees in spruce-dominated stands, which reduces overall tree density and cover. Such outbreaks can also alter the diversity of affected areas. As large trees are infested, aggregating pheromones attract more beetles who infest nearby trees of any size. Outbreaks can continue for several years until most large spruce trees in the region are killed.
Typically, spruce beetles attack larger trees and trees with root disease, but all sizes are susceptible during outbreaks. Attacks are initiated by adult females and begin in the lower trunk, gradually spreading to the middle and upper trunk as more beetles arrive and the infestation progresses. Most eggs are laid shortly after snow melt during June and July. In Colorado, overlapping generations are produced and the entire life cycle takes two to three years to complete, depending on elevation.
The egg galleries are oriented vertically and often filled with pitch, though spruce beetle attacks often do not result in the formation of pitch tubes. Young larvae feed in groups perpendicular to the egg galleries and eventually begin forming individual tunnels once partially grown. These tunnels can intersect, and when fully mature, the larvae form pupation chambers beneath the bark. Adults emerge in summer and must survive a second winter before they can reproduce. Adults can overwinter in pupation chambers or in new chambers formed in the lower trunk of infested trees.
Both species mine inside the bark of twigs, branches, or trunks. The formation of pitch tubes is a defense mechanism observed on healthy trees infested with mountain pine beetle. However, stressed trees may fail to form pitch altogether. Pitch tubes formed by boring of mountain pine beetle tend to be cream or red and about 13-25 mm (1/2-1inch) in diameter and are randomly scattered along the trunk. Both species create galleries by excavating dust, which is often present on bark and around the tree base. The galleries produced by Dendroctonus spp. tend to be packed with frass, unlike the clean galleries produced by Ips spp.
Mountain pine beetle
The egg galleries of mountain pine beetle run longitudinally up the tree trunk, terminating with a slight “J” shaped curve on the lower portion, and are usually straight on pines with two or three needles per bundle, or broad and curved on pines with five needles per bundle. Galleries produced by larvae tend to extend outward from egg galleries and end in oval shaped pupal cells that have no frass. When adults emerge, they leave exit holes on the bark surface.
The egg galleries produced by spruce beetle are also filled with frass and oriented longitudinally with hooks at the ends. They tend to be 6 to 30 cm (2-12 inches) in length. Trees infested with spruce beetle will have pale yellow or green needles that can drop during high winds. However, the needles will rarely turn rust colored, and tend to drop when they are still green the summer following an infestation. As needles drop, the twigs in upper crowns become exposed and produce a red or gray hue when seen from a distance. Resin may also be observed streaming along the main trunk of recently attacked trees.
For both of these species, aerial detection maps can be a useful monitoring tool for tracking regional pest populations. When inspecting trees, the presence of boring dust on the trunk or around the tree base indicates that an infestation of Dendroctonus spp. may be present. Heavy woodpecker foraging can strip portions of bark from infested trees as they search for larvae. Although not always present on infested trees, the occurrence of pitch tubes on the trunk is another external indicator of an infestation.
Typically, outbreaks can be prevented with practices that promote vigorous growth and tree health since beetles tend to attack stressed trees. In addition, maintaining age and species diversity across a landscape is recommended. Thinning practices can help reduce tree mortality due to mountain pine beetle, even during an outbreak. This is especially important in mature forests that have not been burned over many decades given their high density. In this instance, thinning will help prevent beetle outbreaks as well as catastrophic wildfires. It is recommended to consult a professional forester prior to thinning. For spruce beetle prevention, removing downed spruce trees before colonization or beetle emergence can help protect nearby trees by preventing local buildup of spruce beetle populations.
Many natural enemies exist for both species, such as woodpeckers and other predatory insects. However, they do not significantly reduce populations during outbreaks.
Logs infested with either species can be treated to kill developing beetles before they emerge and spread to new hosts. Burning will kill larvae under the bark, and intense solar radiation can desiccate the cambium and raise the temperature to lethal levels of over 43°C (110 °F). For controlling spruce beetle populations, infested trees can be felled, stacked in full sun, and covered with clear plastic. Bark can also be removed from logs to kill larvae developing within galleries under the tree bark, or logs can be buried under at least 21 cm (8 inches) of soil. However, these approaches may be impractical since both are very time consuming and labor intensive.
Mountain pine beetle
The application of the semiochemical verbenone to individual trees can help provide short-term protection of some pine species against mountain pine beetle. However, the chemical is not 100% effective and must be reapplied annually until the beetle population decreases below threatening levels. It is worth noting that area-wide applications of verbenone have shown inconsistent and unreliable protection against mountain pine beetle.
In some instances, chemical control can help protect high-value trees before they are infested. Insecticides for protection against mountain pine beetle involved injectable and trunk-applied formulations. Proper timing is important for injectable insecticides since they must be adequately distributed within the tree before beetle flight.
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