Controlling Mountain Pine Beetles in Colorado’s Pine Forests

Category: Forest, Plants, Trees
Last Updated: 31 Mar 2023
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Mountain pine beetles (MPB) are the most important insect pest of Colorado's pine forests. MPB often kill large numbers of trees annually during outbreaks. Trees that are not growing vigorously due to old age, crowding, poor growing conditions, drought, fire or mechanical damage, root disease and other causes are most likely to be attacked. For a long-term remedy, thin susceptible stands. Leave well-spaced, healthy trees. For short-term controls, spray, cover, burn or peel attacked trees to kill the beetles. Preventive sprays can protect green, unattacked trees. [pic] |[pic] | |Figure 1: Adult Dendroctonus (left) versus Ips (right). Note gradually curved wing of | |Dendroctonus. Actual size of Dendroctonus from 1/8 to 1/3 inch, Ips 1/3 to 1/4 inch. | Mountain pine beetle (MPB), Dendroctonus ponderosae, is native to the forests of western North America. Periodic outbreaks of the insect, previously called the Black Hills beetle or Rocky Mountain pine beetle, can result in losses of millions of trees.

Outbreaks develop irrespective of property lines, being equally evident in wilderness areas, mountain subdivisions and back yards. Even windbreak or landscape pines many miles from the mountains can succumb to beetles imported in infested firewood. Mountain pine beetles develop in pines, particularly ponderosa, lodgepole, Scotch and limber pine. Bristlecone and pinyon pine are less commonly attacked. During early stages of an outbreak, attacks are limited largely to trees under stress from injury, poor site conditions, fire damage, overcrowding, root disease or old age.

However, as beetle populations increase, MPB attacks may involve most large trees in the outbreak area. A related insect, the Douglas-fir beetle (D. pseudotsugae), occasionally damages Douglas-fir. Most often, outbreaks are associated with previous injury by fire or western spruce budworm. (See fact sheet 5. 543, Western Spruce Budworms). Spruce beetle (D. rufipennis) is a pest of Engelmann and Colorado blue spruce in Colorado. Injured pines also can be attacked by the red turpentine beetle (D. valens).

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Mountain pine beetles and related bark beetles in the genus Dendroctonus can be distinguished from other large bark beetles in pines by the shape of the hind wing cover (Figure 1, top). In side view, it is gradually curved. The wing cover of Ips or engraver beetles, another common group of bark beetles attacking conifers, is sharply spined (Figure 1, bottom). Signs and Symptoms of MPB Attack Popcorn-shaped masses of resin, called "pitch tubes," on the trunk where beetle tunneling begins. Pitch tubes may be brown, pink or white (Figures 2 and 6).

Boring dust in bark crevices and on the ground immediately adjacent to the tree base. Evidence of woodpecker feeding on trunk. Patches of bark are removed and bark flakes lie on the ground or snow below tree. Foliage turning yellowish to reddish throughout the entire tree crown. This usually occurs eight to 10 months after a successful MPB attack. Presence of live MPB (eggs, larvae, pupae and/or adults) as well as galleries under bark. This is the most certain indicator of infestation. A hatchet for removal of bark is needed to check trees correctly (Figures 3, 5 and 8). Bluestained sapwood (Figure 9).

Check at more than one point around the tree's circumference. |[pic] | |Figure 10: Large, uninfested pine being preventively sprayed. This | |protects high-value trees and should be done annually between April 1 | |and July 1. | Natural controls of mountain pine beetle include woodpeckers and insects such as clerid beetles that feed on adults and larvae under the bark. However, during outbreaks these natural controls often fail to prevent additional attacks. Extreme cold temperatures also can reduce MPB populations.

For winter mortality to be a significant factor, a severe freeze is necessary while the insect is in its most vulnerable stage; i. e. , in the fall before the larvae have metabolized glycerols, or in late spring when the insect is molting into the pupal stage. For freezing temperatures to affect a large number of larvae during the middle of winter, temperatures of at least 30 degrees below zero (Fahrenheit) must be sustained for at least five days. Logs infested with MPB can be treated in various ways to kill developing beetles before they emerge as adults in summer.

One very effective way to kill larvae developing under the bark (though very labor intensive) is by peeling away the bark, either by hand or mechanically; this exposes the larvae to unfavorable conditions -- the larvae will dehydrate, starve and eventually die. Logs my also be burned or scorched in a pile -- preferably when there is snow on the ground (contact your local forester for assistance). They can also be buried under at least eight inches of soil, or chipped. Following beetle emergence, wood can be used without threat to other trees.

Chemical control options for MPB larvae have been greatly limited in recent years. At present, there are no labeled pesticides for use on MPB. Solar treatments may be appropriate in some areas of Colorado to reduce beetle populations in infested trees. For the treatment to be effective, the temperature under the bark much reach 110 degrees Fahrenheit or more. Such treatments can be performed with or without plastic. This method is also labor intensive; contact your local forester for more details on solar treatments. Prevention An important method of prevention involves forest management.

In general, MPB prefers forests that are old and dense. Managing the forest by creating diversity in age and structure with result in a healthy forest that will be more resilient and, thus, less vulnerable to MPB. Most mature Colorado forests have about twice as many trees per acre as those forests which are more resistent to MPB. Contact your local forester for more information on forest management practices. Certain formulations of carbaryl (Sevin and others) permethrin (Astro, Dragnet and others), and bifenthrin (Onyx) are registered for use to prevent attacks on individual trees.

These sprays are applied to living green trees in early summer to kill or deter attacking beetles. This preventive spray is generally quite effective through one MPB flight (one year). |[pic] | |Figure 11: The appearance of a forest thinned to help prevent MPB. This| |can also improve mountain views and reduce fire hazard. | During epidemic conditions, the pressure from beetle populations may result in less satisfactory results due to several factors:

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Controlling Mountain Pine Beetles in Colorado’s Pine Forests. (2016, Dec 16). Retrieved from

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