Dendroctonus pseudotsugae (Hopkins)
Forest Insect and Disease Identification and Management Training Manual, USDA, Forest Service, R-1, Timber, Coop. Forestry and Pest Management, Idaho Department of Lands, Bureau of Private Forestry - Insect and Disease Section, Montana Department of State Lands, Division of Forestry
Douglas-fir beetle is the most destructive bark beetle attacking Douglas-fir in the Northern Region. Outbreaks develop in host trees following stand disturbances such as windthrow, fire, drought, or severe defoliation. Stands with extensive amounts of root disease may also predispose trees to beetle attack. Epidemics, though usually short-lived, may devastate susceptible stands before subsiding.
Hosts: Douglas-fir. Western larch may occasionally be attacked, but successful brood development has only been recorded in downed trees.
Damage - Beetles are attracted to slash, stumps, windthrow, and trees weakened by fire, drought, defoliation or disease. Populations expand rapidly in such material and in subsequent generations beetles attack and kill surrounding green trees. Douglas-fir beetles are not, however, particularly aggressive beetles. As more of the susceptible hosts are killed, and attacking beetles are forced into increasingly-healthier trees, populations decline. In sub-outbreak populations, mortality is confined to individual trees or small groups. During outbreaks, groups of dead trees may total 100 or more and yearly mortality may extend into the millions of board feet.
Life History - Douglas-fir beetle has one generation each year. Overwintering takes place beneath the bark of the tree in which they developed and occurs mainly as adults. A small percentage may overwinter as larvae. Spring emergence of adult beetles varies with location and weather, but usually occurs from mid-April to early June. Beetles that have passed the winter as larvae complete their development in spring and early summer. Those emerge and attack host trees in mid-summer. In addition, a few adults which made initial attacks in the spring may re-emerge to make a second attack in mid- to late summer. This "second flight" (in some years nearly a continual flight) usually accounts for less than ten percent of the yearly total of attacked trees. Often, these later attacks fill in trees which were attacked during the initial spring flight. Broods require one year to complete their development-beetles emerging in spring are from the previous spring's brood and beetles flying later in the summer are typically from summer broods.
IdentificatIon - Evidence that a tree has been successfully attacked is usually the reddish-brown boring dust found in bark crevices on the lower portion of the tree's bole or on the ground at its base. Wind and rain may remove the dust, however, and since attacks are often high on the bole, careful inspection may be required to determine if beetles are present. An occasionally evident sign of infestation may be clear resin which has exuded from the upper level of attacks-typically 30 to 35 feet off the ground. These pitch streamers are often visible for a considerable distance. Streams of pitch lower on the bole may be evidence of unsuccessful attacks or other injury. As a rule, successful attacks can only be confirmed by removing sections of bark to reveal egg galleries, eggs, and/or developing brood.
Distinctive egg galleries are constructed beneath the bark by female beetles as they bore upward through the phloem. Galleries are parallel to wood grain and are commonly 8 to 10 inches in length; usually longer in downed logs. Eggs are laid in nitches, alternately along opposite sides of galleries. After hatching, larvae mine outward from, and perpendicular to, the egg gallery as they feed in the phloem.
When the larvae have completed their development, they construct pupal cells at the ends of their feeding galleries. Pupal cells may be well within the bark. Larvae are white, legless grubs; pupae white to cream-colored. Immature beetles are light brown, becoming dark brown to black, with reddish wing covers, as they mature. Older beetles may be totally black.
Several months after a tree has been attacked, its foliage begins to discolor. Needles first turn yellow, then orange, and finally a reddish brown. Discoloration rate varies with local conditions and individual trees. During dry years, trees fade more quickly-occasionally becoming yellowish-green to orange later the same year they are attacked. Typically, trees begin to fade the year following attack. Tree-to-tree fading also varies with resistance to the staining fungi introduced by the beetles.
According to Furniss and Orr (1978), resistance of live trees is the most important natural factor controlling Douglas-fir beetle populations. Trees sustaining physical damage, or ones stressed by drought, defoliation, or disease are most susceptible to beetle attack and the furtherance of an outbreak. By keeping stands in a vigorous condition and removing susceptible trees or downed material, managers can most benefit from this natural resistance factor.
Climate and weather also influence beetle populations. Extremely cold, dry winters would have a detrimental effect on overwintering broods. At the other extreme, droughty conditions stress host trees and favor population buildups.
Naturally occurring parasites and predators play a role in population reduction during non-outbreak conditions, but apparently are not important regulating factors when populations become abnormally high. The most important insect parasite is a Braconid wasp which parasitizes the beetle's larval stage. Predators include Dolicopdid flies, the larvae of which prey upon beetle larvae; and Clerid beetles which are predaceous on both the larval and adult stages. Woodpeckers feed on developing larvae higher on the tree bole, where bark is thinner; but their overall effect is probably minimal.
Furniss, M.M. 1959. Reducing Douglas-fir beetle damage-how it can be done. USDA For. Serv., lntermtn. For and Range Exp. Sta., Ogden, UT. Res. Note No. 70, 6 pp.
Furniss, M.M. and P.W. Orr. 1978. Douglas-fir beetle. USDA For. Serv., For. Insect and Disease Leaflet No. 5, 4 pp.
Fumiss, M.M. 1979. An annotated bibliography of the Douglas-fir beetle (Dendroctonus pseudotsugae Hopkins). USDA For. Serv., lntermtn. For. and Range Exp. Sta., Ogden, UT. Gen. Tech. Rpt. INT-48, 39 pp.
Furniss, M.M., R.L Livingston, and M.D. McGregor. 1981. Development of a stand susceptibility classification for Douglas-fir beetle. In: Hazard-rating systems in forest pest management: Symposium Proceedings, Athens, GA. 1980. Tech. Coordinators:
Heddon, R., S. Barras, and J.E. Koster. USDA For. Serv., Washington, D.C. GTR WO-27.
Furniss, M.M., M.D. McGregor, M.W. Foiles, and A.D. Partridge. 1979. Chronology and characteristics of a Douglas-fir beetle outbreak in northern Idaho. USDA For. Serv., lntermtn. For. and Range Exp. Sta., Ogden, UT. Gen. Tech. Rpt. INT-59, 19 pp.
Gibson, KE. and R.D. Oakes. 1991. Efficacy of Douglas-fir beetle tree baits in containing outbreak populations of Douglas-fir beetles in North Idaho. USDA For. Serv., North. Reg., Missoula, MT. Forest Pest Mgt. Rpt., 91-04, 8 pp.
Lejeune, R.R., LH. McMullen, and M.D. Atkins. 1961. The influence of logging on Douglas-fir beetle populations. The Forestry Chronicle: 37(4):308.
McGregor, M.D., M.M. Fumiss, R.D. Oakes, KE. Gibson, and H.E. Meyer. 1984. MCH Pheromone for preventing Douglas-fir beetle infestation in windthrown trees. J. Forestry, Vol, 82, No. 10, Oct. 1984, p 613-616.
Weatherby, J.C. and R.W. Thier. 1993. A preliminary validation of a Douglas-fir beetle hazard rating system, Mountain Home Ranger District, Boise National Forest, 1992. USDA For. Serv., lntermtn. Reg., Boise, ID. Forest Pest Mgt. Rpt., R4-93-05, 7 pp.
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