The most important bark beetle enemy of Douglas-fir (red fir) is the Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins. It is found throughout the host range of this tree in North America. At low population levels the beetle maintains itself in windthrown or injured trees, or in those infected by root disease. Under these conditions attacked trees may be scattered throughout a stand or watershed. When epidemics occur, tree mortality can be wide spread killing thousands of apparently healthy trees. When this happens, beetle killed trees are usually found in groups ranging fromiust a few trees upwards-to one hundred or more.
(right) Eggs are whitish, cylindrical, and deposited along edge of the egg gallery.
In Idaho, Douglas-fir beetle outbreaks are usually initiated by catastrophic events, such as blowdown, or winter breakage. Trees weakened by fire also contribute to increasing beetle populations. The downed or weakened trees caused by these events are very attractive to this beetle, which attacks and builds up large populations.
The following year, new generations emerge and attack susceptible standing trees in nearby stands. Once an outbreak has started, it normally lasts two to three years in that area, with the peak number of attacks occurring the year the beetles first emerge from the down material. If other factors such as defoliation caused by the feeding activity of larvae ofthe western spruce budworm or Douglas-fir tussock moth, drought, fire, or root disease have weakened additional trees, the beetle outbreak may be prolonged several years.
Damage in standing trees is greatest in dense stands of large, mature Douglas-fir, and where the percent of Douglas-fir is high.
Indicators of Attack
Reddish brown boring dust (frass) caught in bark crevices or accumulating in small piles on down logs, is the first evidence of attack by the Douglas-fir beetle. Wind and rain tend to remove the frass, especially from standing trees. For this reason great care needs to be taken when examining standing trees for evidence of attacks. When one infested tree is located it is very likely that there will be others in the area.
The foliage of attacked trees turns color, fading to yellow, then sorrel and finally reddish brown. Discoloration may become evident by fall but more commonly does not show up until winter or spring of the following year.
Resin sometimes exudes from attacks high up in the tree. This pitch will be seen streaming down the trunk for a few feet. The position of the pitching marks the upper limits of the successful attacks. Care needs to be taken to not confuse this pitch with that from broken branches or other wounds, or with the small globules of pitch that are very common on Douglas-fir bark.
The Douglas-fir beetle has four stages in its life cycle: egg, larva, pupa and adult. The eggs are oval, pearly white, and the size of a pinhead. The larva is a yellowish-white, legless grub with a brown head. It grows to approximately 1/4 inch.
The pupa is shiny white and resembles the adult but its wing covers are folded around its abdomen. A new (callow) adult is pale yellow, but turns dark reddish-brown before flying. The adults are cylindrical and approximately 3/16 inch long.
Life, History and Habits
The Douglas-fir beetle has one generation per year. Depending on area and weather the overwiritering adults emerge from April through June.
They fly to new host trees where fhey initiate attacks by boring through the bark to the surface of the wood. As they tunnel into the tree the beetles produce a chemical scent (pheromone) that attracts more beetles, overcoming any resistance the tree may have.
There they excavate tunnels or galleries approximately 8 to 12 inches long (Figure 1). These galleries run parallel with the grain of the wood and are packed with boring frass.
The eggs are laid in groups of 10 to 3 ) 6 in grooves that alternate from side to side along the gallery. The eggs hatch in I to 3 weeks depending on temperature.
The newly hatched larvae feed outward from egg gallery through the inner bark (phloem). When fully grown the larvae chew out pupal cells where the change to the adult stage takes place. Broods remain in the tee overwintering mainly as adults. The feeding activity of the beetle larvae gridles the tree leading to its eventual death.
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