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Among forest insects of the Rocky Mountain Region, bark beetles in the genus Ips are second only to Dendroctonus as destructive agents of pine. Ips beetles have the ability to attack and kill standing sapling- and pole-sized pines and the tops of larger pines. This can disrupt planned stocking levels and other stand prescription objectives and investments. Ips activity can accelerate and intensify mountain pine beetle (D. ponderosae) epidemics. Recreational and aesthetic values, particularly in high-use areas, can be adversely impacted by Ips activity, due to the appearance of dead trees and the creation of hazard trees. Ips outbreaks result from the creation of abundant breeding material. In managed stands this generally is from logging slash or TSI operations combined with drought or other stand stress factors. Slash management and treatment plans can prevent or reduce the loss of trees to Ips attack.
The Rocky Mountain Region has at least 13 species of Ips. I. hunteri, I. borealis and I. pilifrons attack spruce (Picea) species and are rarely reported to be significant pests. One or more of the remaining 10 Ips species attack all species of pine in the Region. These Ips species are mexicanus, latidens, woodi, pini, integer, plastigraphus, calligraphus, grandicollis, confusus and knausi. The pinyon engraver, I. confusus, is a common pest of pinyon pine in Colorado. The pine engraver, I. pini, is the most destructive and aggressive species in the region, attacking lodgepole, ponderosa, limber, and jack pines.
Life cycles and attack behavior of most Ips species are similar. Furniss and Carolin (1977) summarize available knowledge of Ips, presenting detailed life cycle information. Generally, Ips have two to five generations annually, depending on climate, elevation, and species. One generation annually is common in high elevations of the northern Rocky Mountains. Most species overwinter as adults under the bark of infested material or in forest litter and duff and emerge in April or May to seek and colonize green host material. Adult males initiate attack, constructing a nuptial chamber under the bark. Males attract females through the secretion of pheromones or semiochemicals in combination with host terpenes. From one to seven mated females then construct galleries radiating away from the nuptial chamber along which the eggs are deposited. The white, legless larvae develop and feed in the cambium layer along individual tunnels, generally created perpendicular to egg galleries. Pupation occurs at the end of these widening tunnels and resulting adults then bore out through the bark to begin a new generation. Dependent on temperature, a generation is completed in about 30 to 60 days. The flight or attack season of adult I. pini usually continues until September, although at high elevations in the north, flight may be confined to a period from June to August.
Ips prefer to attack fresh downed wood or logging slash (diameter > 5 cm or 2 in.) or parts of damaged and severely weakened trees. Examples of damaging and weakening agents are lightning, windthrow and other weather disturbances, disease, or attack by other bark beetles. Ips are generally considered secondary bark beetles, as contrasted with Dendroctonus beetles, which initiate attacks on and are primary killers of standing, healthy trees. Research studies indicate Ips adults prefer to land on horizontal logs as compared to Dendroctonus adults, which show a preference for vertical targets. Population build-up in and consequent depletion of slash or weakened trees can result in Ips attack on nearby healthy trees.
Ips, like other bark beetles, respond to a complex set of chemical signals (semiochemicals) to locate hosts, mates, and to regulate the numbers of beetles colonizing specific logs. Like other bark beetles, Ips transmit blue-staining fungi to the wood they infest and have a great variety of natural enemies whose role in population trend is poorly understood. These enemies include insect predators and parasites, mite predators, nematode parasites, fungi and other disease agents, and non-specific predators of insects such as woodpeckers and other birds. Competition between bark beetles can impact their populations, especially in spruce, where spruce beetle (Dendroctonus rufipennis) numbers can be significantly reduced by coincident Ips infestation. Also like other bark beetles, biotic and abiotic mortality factors are insufficient to prevent occasional population increase to very high levels. It is the availability of suitable breeding material that is the major factor allowing such increases. With the exception of manmade forests in Nebraska and a few other localities, severe tree killing rarely continues for more than one season.
In logging slash, down wood, and standing trees, Ips infestations are most easily detected by the presence of reddish-brown, dry boring dust. Pitch tubes are seldom present. If the bark is carefully removed, various stages of the insect may be present, as well as the distinctive gallery patterns. Ips galleries are characterized by a central nuptial chamber from which several egg galleries radiate in a Y-shaped or star-shaped pattern. These galleries are open and free of pitch or boring dust, distinguishing them from Dendroctonus galleries, which are filled with both. Adult Ips are small (3-6 mm in length), brown to black, cylindrical beetles. They are distinguished by the presence of a pronounced concavity (declivity) at the rear.
Needles on trees successfully attacked by Ips generally fade from green to yellow within 2 to 4 months and continue to fade to a dull red. Trees attacked late in the summer may not fade until the following spring. When small groups of sapling- or pole-sized pines fade, particularly in managed stands, it is likely the result of Ips. Faded trees rarely if ever contain living Ips, which have already developed and departed. In large diameter trees, Ips attacks are often limited to the upper crown and large branches. Other bark beetle species often attack the bole.
Ips bark beetles can be found wherever pines or spruces occur, including urban settings on exotic host trees.
The following stand conditions constitute a hazard of Ips population build-up and attacks on standing trees:
1. Stands stressed by drought, particularly in the spring. Low spring soil moisture has been correlated with severe Ips-caused tree mortality. Conversely, normal or above normal spring moisture is a key factor in trees' resistance to attack.
2. Managed stands following thinning, harvesting, or prescribed burning where an abundance of green slash has been created. Operations such as these are stress factors. When combined with abundant green slash, ideal conditions for Ips-caused losses have been created. If these scenarios are repeated in close proximity annually, hazard conditions can be greatly increased.
3. Natural disturbances such as windthrow and snow and ice damage. These disturbances can create an abundance of suitable Ips habitat.
4. High-density, sapling- and pole-sized stands. These conditions describe low-vigor, stagnant stands, which have low resistance to Ips attack. Dwarf mistletoe infection can contribute to tree susceptibility.
Should stand conditions fit one or more of any of the previously described categories, a potential for Ips infestation exists.
The following tactics can be applied to prevent or reduce potential losses to Ips:
1. Maintain or improve stand vigor through precommercial or commercial thinning, especially in very dense young stands and decadent old ones.
2. Monitor green slash and standing trees for evidence of Ips attack. Monitoring should be increased during periods of below normal precipitation, particularly during the April to July period. Areas to inspect should include those where recent or ongoing management activities, such as thinning, harvesting or prescribed burning, have occurred. Aerial surveys conducted by Forest Health Management staff can be used as a monitoring tool. Should abundant evidence of Ips activity be located, consultation with a pest management specialist should be undertaken.
3. When a threat of Ips is a concern, optimal time to conduct slash-creating activities is July through December. Slash created during this time period is likely to dry sufficiently by spring to be unsuitable Ips habitat. Creation of green slash between January and June should be minimized or prohibited. Slash created during this time period is excellent habitat for Ips.
4. When it is impractical to cease or minimize slash production during the January to June period, institute a slash treatment program. Material greater than 5 cm (ca. 2 in.) in diameter should be destroyed as soon as possible by burning, chipping, crushing, debarking, burying, or piling under a clear plastic tarp in a sunny location. Piling material without treatment, especially cull logs and larger diameter material, should be avoided. In some situations, a lop and scatter treatment to promote drying may be sufficient.
5. Avoid scheduling continuous annual management activities in adjacent areas. Where slash is created in adjacent areas for more than one year, the hazard of Ips population build-up exists. The hazard can be greatly reduced by separating activities by two miles or more. While no good evidence exists on how far beetles fly, two miles appears to be a sufficient barrier. Another tactic is to conduct activities in adjacent areas in alternating years.
6. In high-use recreation areas and urbanized settings, judicious removal of injured and diseased trees and prompt disposal of fresh slash and windthrow before it is attacked is especially important. Such settings often have trees of high value that are under stress from human activity, combining increased potential loss of value with risk of attack.
Determining the need to undertake suppression actions against Ips can be difficult for two reasons. First, there is no reliable method to determine if populations in slash will attack and kill standing trees. Second, infestations must be detected early enough to allow sufficient time to organize and conduct suppression actions. Often, Ips outbreaks are not detected until the appearance of faded trees. At this point, suppression tactics are ineffective. Prevention is the preferred tactic.
Should a decision be made to suppress Ips populations, the following tactics can be effective:
1. Infested slash (5 cm or 2 in. diameter or greater) can be piled and covered with clear plastic in a sunny location. The greenhouse effect created by the covering can raise phloem temperatures high enough to kill developing brood.
2. Infested slash can be destroyed by crushing, debarking, chipping, burning, and so forth, before the insects complete development and emerge.
3. Insecticides can effectively suppress Ips populations in slash. Forest Health Management staff should be consulted for additional information on insecticide suppression strategies.
4. Infested slash and debris can be removed from the site and hauled at least three miles from susceptible stands. Removal must be accomplished before beetle emergence.
5. Traps baited with pheromone have been successful in reducing I. pini populations in thinned stands when placed in a grid pattern of at least 5 traps per acre. There is no guarantee that residual populations of I. pini will not attack trees in the treatment area, however. This tactic is relatively expensive, environmentally safe, and is therefore best used in high-value, sensitive settings such as campgrounds. Forest Health Management staff should be consulted for additional information on trapping strategies.
Sartwell, C., Schmitz, R.F., and W.J. Buckhorn. 1971. Pine engraver, Ips pini, in the Western States. USDA For. Serv., Forest Pest Leaflet 122, 5 p. U.S. Government Printing Office.
Furniss, R.L. and V.M. Carolin. 1977. Western Forest Insects. USDA Forest Service Misc. Publ. 1339, 654 p. (see especially pp. 383-398). U.S. Government Printing Office.
Caring
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