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. 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. Wind and rain tend to remove the frass, especially from standing trees.