Dendroctonus spp.
Order: Coleoptera
Family: Curculionidae
Description
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. This species is native to western regions of North America containing pine forests and poses a threat to ornamental trees, Christmas tree plantings, and coniferous forests. 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 eggs of mountain pine beetle are white and very tiny, and 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 nearby stands of Englemann (Picea engelmannii) and Colorado blue spruce (Picea pungens), although brood typically do not complete development in these alternate hosts. These periodic outbreaks can result in millions of tree deaths in forests and urban landscapes.
Spruce beetle
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.
Quick Facts
- 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
All beetles develop through four life stages: egg, larva, pupa, and adult. 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. The mountain pine beetle spends most of its life beneath the bark of host trees, except for periods during which the beetles emerge from the natal tree and fly in search of a new host. The beetles can also remain in their natal host where they extend pre-existing egg galleries and lay eggs. Newly hatched larvae construct galleries perpendicular to the egg gallery. After developing through four instars, mature larvae excavate an oval pupation cell. When development is complete, adults begin feeding on fungal spores and tree tissues within feeding chambers. Sometimes adults can be found in groups when the feeding chambers merge. Multiple beetles can emerge from a single exit hole. Peak flights tend to occur in July through August, but smaller numbers of beetles can emerge earlier or later in the summer.
When initiating an attack, male and female beetles emit aggregating pheromones that attract hundreds of additional beetles. Even a healthy tree can become overwhelmed when enough beetles are recruited. Once the tree’s defense system is weakened, the beetles continue emitting aggregating pheromones that further attract beetles to the weakened host. Once a maximum pest density is reached in the overwhelmed host, male beetles begin emitting anti-aggregation pheromones that cause incoming beetles to attack neighboring trees. This behavior often results in patches of tree mortality in a region. After colonizing a tree, the beetles mate under bark and females construct vertically oriented egg galleries that are packed with boring dust. At lower elevations, the entire life cycle is completed in one year. Populations at higher elevations may require two years to complete development.
The mountain pine beetle has a specialized mouth structure called a mycangia that carries the spores of two fungal species, Ophiostoma montium (Ascomycota: Ophiostomataceae) and Grosmannia clavigera (Ascomycota: Ophiostomataceae). Attacks of mountain pine beetle introduce these fungi to the host tree, which are commonly referred to as “bluestaining” fungi since they disrupt the tree’s vascular system and cause the sapwood to develop a blue tint. Infections of these fungi greatly increase the probability of tree death.
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. Trees with a diameter greater than 20 cm (8 inches) are most susceptible and usually die when infested. Outbreaks of mountain pine beetle are most likely to occur in stands containing high densities of older (>80 years old) lodgepole pines with a diameter greater than 20 cm (8 inches), and mid-sized 20-30 cm (8-12 inch) diameter ponderosa pine. However, outbreaks can also occur at higher elevations in stands of whitebark (Pinus albicaulis) and limber (Pinus flexilis) pine.
Spruce beetle
Adults can overwinter in pupation chambers or in new chambers formed in the lower trunk of infested trees. The following summer, adults emerge and must survive a second winter before they can reproduce. Adults excavate vertically oriented egg galleries which are often filled with pitch. However spruce beetle attacks do not always 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. In Colorado, overlapping generations are produced and the entire life cycle takes two to three years to complete, depending on elevation.
Typically, spruce beetles attack recently fallen trees, trees with a weakened defense system, or trees with root diseases. However, 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 in June and July. Individual trees or groups of trees can be killed by spruce beetle, depending on the pest population. Females tend to attack susceptible trees first, and only begin initiating attacks on healthy trees once the preferred resource is depleted. As with mountain pine beetle, the spruce beetle overwhelms a healthy host by emitting aggregation pheromones that recruit more beetles to the host. While high densities of spruce beetle also trigger the emission of anti-aggregation pheromones, their response tends to be less pronounced than that of the mountain pine beetle.
Unlike the mountain pine beetle, the spruce beetle does not have a mycangia. However, the spruce beetle can still transmit several species of blue-staining fungi such as Ophiostoma spp., Leptographium spp. (Ascomycota: Ophiostomataceae), and Ceratosystis spp. (Ascomycota: Ceratocystidaceae), with Leptgraphium abietinum being the most common species isolated from spruce beetle populations in Utah and Colorado.
Outbreaks of spruce beetle result in losses of large trees in spruce-dominated stands, which reduces overall tree density and cover. Historically, most outbreaks of spruce beetle have been associated with freshly downed hosts due to timber harvesting, wind, landslides, and avalanches. However, such events do not necessarily mean that an outbreak will occur. Interestingly, laboratory research has shown that beetles from epidemic populations respond more strongly to alpha-pinene, which is a terpene associated with healthy trees. This suggests that spruce beetles are more likely to attack healthy, well defended trees during outbreaks, and that the increase in attacks on healthy trees is not due to the depletion of weakened hosts. Such outbreaks can continue for several years until most large spruce trees in the region are killed, which can alter the diversity of affected areas.
Injury
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
In many cases, mountain pine beetle infestations begin on the lower 4.6 m (15 feet) of a tree trunk. However, on sugar pines (Pinus lambertiana) the crown is usually attacked first. Outbreaks can result in mass-attacks in which a single generation of beetles can cause girdling of the phloem. At low beetle densities, partial attacks are usually observed around the trunk in strips. These infestations do not usually kill trees until many years later as subsequent beetle generations colonize the same tree. As females bore into a host, the tree responds by producing resin which appears as randomly scattered cream or red colored pitch tubes that measure about 0.6-1.3 cm (1/4-1/2 inch) in diameter. In addition to pitch tubes, a dry red-brown boring dust is present in bark crevices on the trunk and around the base of the tree. When the tree successfully repels the mountain pine beetle, pitch tubes are usually larger with a diameter of 1.9-2.5 cm (3/4-1 inch), more widely scattered, and lighter in color. Stressed trees may fail to form pitch tubes entirely, although boring dust will still be present.
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. During dry periods, infestations of mountain pine beetle can cause discoloration of foliage from green to yellow, to red, and finally rusty brown in as little as several weeks. In lodgepole pine, discoloration of foliage tends to begin in the lower crown and progress upward. Other hosts, such as large-diameter sugar pines, tend to show initial signs of discoloration in the upper crown.
Spruce beetle
The egg galleries produced by spruce beetle are also filled with frass and oriented longitudinally with hooks at the ends. They tend to be 6-30 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.
Monitoring
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.
Prevention
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 since stands with high densities of host trees are more at-risk. When done methodically, thinning practices in at-risk stands of lodgepole and ponderosa pine can help reduce tree mortality prior to outbreaks of mountain pine beetle by altering wind patterns and microclimates within stands, allowing aggregation pheromones to dissipate, and increasing the health of remaining trees by reducing competition for nutrients and water. This can be especially important in mature forests that have not been burned over many decades. In this instance, thinning will also help prevent catastrophic wildfires. It is recommended to consult a professional forester prior to thinning, and it should be noted that such strategies have not been successfully developed for stands containing high densities of hosts other than lodgepole and ponderosa pine.
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.
Biological control
Natural enemies of both the mountain pine and spruce beetles include certain species of predatory flies, beetles, parasitoid wasps, nematodes, and woodpeckers. These natural enemies can limit populations of mountain pine beetle in individual trees but have not been shown to significantly reduce populations during outbreaks. Some of these natural enemies also help maintain epidemic population levels of spruce beetle, especially parasitoid wasps, which have been shown to kill high percentages of spruce beetles in the Rocky Mountains. Furthermore, woodpeckers have been shown to cause up to 98% spruce beetle mortality, and the red squirrel can consume substantial quantities of larvae when other food sources are scarce. Together, these vertebrate predators also play an important role in suppressing endemic populations of spruce beetle.
Cultural control
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.
The aggregation pheromone produced by mountain pine beetle can be used as a bait on trap trees. These trees are designated for removal once colonized by the pest. In addition, applications of the anti-aggregation pheromone verbenone to individual high-value 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. Prior to the use of pheromones, it is recommended that individuals contact forest health professionals since misuse can result in unintended tree mortality.
The use of felled or standing trap trees can also be used in spruce beetle management. Baits consist of a lure consisting of formulations of various chemical attractants. For felled trees, green trees with a diameter at breast height larger than 40 cm (~16 inches) are recommended. These trees are felled and ideally left in shaded areas with limbs intact to prevent desiccation before treatment. This approach is most effective when trap trees are felled in the spring before adults emerge and begin searching for new hosts. It is estimated that these trap trees can effectively attract spruce beetles in a 0.4 km (1/4 mile) radius, and felled trap trees often attract up to 10x more beetles than standing trees. It is important to note that trap trees should not be located near uninfested stands or near stands susceptible to a spillover effect. For this reason, it is recommended that trap trees be felled in sites containing many non-host species or in host-dominated areas with trees that have a diameter at breast height smaller than 20.3 cm (8 inches).
Another approach to trap trees for spruce beetle management involves baiting standing trees with the chemical attractant. This approach is most effective when standing trap trees are baited within 30-60 m (98-197 feet) of an active spruce beetle infestation. Under this approach, cluster baiting is recommended, which consists of baiting trees spaced 15-30 m (49-98 ft) apart in a triangular pattern within the pockets of a spruce beetle infestation.
Chemical control
Preventative insecticides can be applied to high-value trees in campgrounds, around homes, and production zones near increasing or epidemic populations of either pest. Insecticides for protection against mountain pine beetle involved injectable and trunk-applied formulations and can be applied to stands with conditions that facilitate outbreaks. Chemical control of the spruce beetle involves preventatively spraying the trunk of susceptible hosts before beetle flights.
Failed applications are often the result of inadequate coverage, improper mixing, improper storage conditions, improper timing, or a combination of these factors. To help address these potential obstacles, limbs that obstruct the lower portion of the tree trunk can be removed. However, this can be very challenging on large trees, as incomplete coverage on the upper portions of the trunk leaves these areas susceptible to attack. To ensure proper use of pesticides, it is recommended that concerned individuals contact pest control or forest health professionals prior to making any treatments. Such applications can be expensive when done over large areas and are typically not a recommended management approach for public use.
References
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