164 MISC. PUBLICATION 273, U.S. DEPT. OF AGRICULTURE 



short, nearly straight, longitudinal egg gallery in the inner bark, 

 slightly scoring the sapwood. The gallery is wider than the beetles 

 and is packed with boring dust through which the adults keep a 

 passageway open. Eggs are laid in masses in elongate cavities 

 which alternate from side to side of the gallery. At first the larvae 

 bore out en masse, transversely from the egg gallery, but later 

 make separate mines. The pupal cells are usually constructed in 

 the inner bark, being exposed when the bark is removed, but are 

 sometimes deeper in the bark and quite concealed in thick-bark 

 trees. 



Two years are required to complete a generation, from attack 

 to attack, in the main body of Engelmann spruce stands. At high 

 elevations 3 years may be required, and in the lower elevations one 

 generation may occur each year. The principal flight, attack, and 

 egg laying takes place when hibernated adults emerge after the 

 snow disappears late in June and in July. Eggs hatch and larvae 

 develop during the summer. The progeny pass the winter as half 

 to nearly full-grown larvae, and complete development to adults 

 by the following August. The new adults emerge and migrate to 

 the basal trunk and root collar of the host tree from August to 

 October ; there they bore beneath the bark and hibernate until the 

 ensuing June and July. Overwintering stages consist primarily 

 of hibernating adults of the previous seasonal attacks and half- to 

 three-fourths-grown larvae of the current seasonal attacks. 



The thin bark of Engelmann spruce and the characteristic lower 

 bole infestations offer good possibilities of control with insecticides 

 dissolved in fuel oil. One part of orthodichlorobenzene in six parts 

 of fuel oil has been most commonly used. Benzene hexachloride at 

 0.46 percent of the gamma isomer was 96 percent effective when 

 sprayed on standing trees. Adults are slightly more susceptible to 

 the lethal effects of these sprays than larvae. The best time for 

 control is in September and October, when all overwintering stages 

 may be killed. Outbreaks are difficult to detect, because the foliage 

 does not fade until 1 year after attack, and it turns pale green only 

 before dropping. There are no pitch tubes. First-year attacks can 

 be detected only by the presence of brown boring dust around the 

 base of trees. Woodpecker work also helps to identify the trees 

 after about October 1, when the larvae are large. 



The Alaska spruce beetle (Dendro clonus borealis Hopk.) attacks 

 white and Engelmann spruce in Alaska and northwestern Canada. 

 In appearance and habits it is very similar to the Engelmann 

 spruce beetle, to which it is closely related. 



The Sitka spruce beetle {Dendroctonus obesus (Mann.) ) is usu- 

 ally considered a secondary enemy of Sitka spruce, but at times 

 it becomes destructive and kills a considerable volume of spruce 

 along the coast of Oregon, Washington, British Columbia, and 

 Alaska. The most recent outbreak on Kosciusko Island, in Alaska, 

 killed an estimated 35*2 million board feet of Sitka spruce during 

 the few years prior to 1946. The adults are black and similar to 

 the Engelmann spruce beetle in size and shape. The pattern of 

 their work is also much the same, except that the larval mines 

 appear to be even more generally connected toward the egg gal- 



