Much of the natural history information of the spruce bark beetle was 'borrowed' from:
Forest Insect & Disease Leaflet 127. 1999. Holsten, E.H., R.W. Thier, A.S. Munson, and K.E. Gibson. USDA, Forest Service.
Some natural history of the spruce bark beetle
The spruce bark beetle, Dendroctonus rufipennis, is one of the most important natural disturbance agents in mature white spruce forests in the boreal region. The range of the spruce bark beetle extends from Alaska, eastward to far eastern Canada. White spruce, Lutz spruce, and sitka spruce and the primary hosts for the bark beetle. The adult beetle is quite small, about 1/4" (6mm) long by 1/8" (3mm) diameter. Bark beetles may complete their life cycles in 1 year in warm years and on warm sites and up to 3 years on cooler sites.
In general the 2-year life cycle progresses like this. Emergence of adult beetles occurs in the spring when temperatures reach 60 F or greater. Adult beetles will search out new host trees immediately after emergence. Female beetles will bore through the bark and create a 3-12 inch long egg gallery in the phloem tissue. Eggs are laid on the side of the gallery and are packed with frass and boring dust as the female moves through. Evidence of egg galleries can be found on the xylem tissue as well and can persist for many years after tree death. Eggs hatch by August and larvae bore outward from the maternal egg gallery. The larvae will feed as a group until the 3rd or 4th instar stages. It is at this time when they construct individual feeding galleries. Pupation lasts for about 10-15 days and occurs about one year after initial beetle attack. During the second year, the pupae may overwinter in the host tree, however, the majority emerge as adults and move to the base of the tree and overwinter there. This reduces predation by woodpeckers and mortality by extreme cold temperatures. 2 years after the initial attack adults emerge and attack new host material.
Beetles kill their host trees by cutting off the transport of water and minerals from the roots to the crown. Many hundreds of beetles in one tree will effectively girdle the tree. During the second year of attack a tree will begin to show signs of beetle infestation. Needles will turn an orange-ish-red color and drop after one or two more seasons.
Evidence of Past Beetle Outbreaks
Spruce bark beetles kill mature overstory trees and generally leave small diameter (> 12-18cm) understory individuals. Since little or no mineral soil is exposed in mature spruce stands there is little in the way of regeneration by seed. However, recruitment and recovery of forest stands after beetle outbreaks is accomplished by the suppressed survivors. With the removal of the overstory competition, these trees experience a dramatic growth release. The tree in the last link was nearly 90 years old when released into the canopy in the mid-1880's. It continued to grow for the next 110 years until it was killed by beetles in 1995. Sampling cross-sections of beetle-killed trees is not the norm. We usually extract increment cores from living trees to find these growth releases. Click the next two links to see high resolution images of increment cores. Core1. Core2. Both of these trees show dramatic and sustained growth releases around the mid-1880's to the early 1890's. This release signature is seen in many forest stands on the Kenai Peninsula. However, the 1880's beetle outbreak was not as extensive as the current infestation. Forest stands that have been hit by beetles are often very open in the years after an outbreak. This stand was hit by beetles in the early 1970's. As you may imagine there is a great deal of woody material on the ground in these stands.
