4i8 



Yearboo^ of Agriculture 1949 



saw-timber size 2,900 million board 

 feet of it was killed. 



IN ITS ADULT STAGE, the Engelmann 

 spruce beetle is a small, cylindrical, 

 hard-shelled beetle, about one-fourth 

 inch in length, about the size of an 

 ordinary housefly. When the adults 

 leave the dead trees and start to fly in 

 June and July, they are reddish brown 

 to black in color. They soon settle on 

 recently felled or standing green trees 

 and bore through the outer bark into 

 the living inner bark. This attack ex- 

 tends over most of the lower main stem 

 of the tree. 



The beetles work in pairs of male 

 and female, each pair raising separate 

 broods. The female makes the en- 

 trance, followed by the male, and bores 

 a tunnel between the bark and wood, 

 which usually extends in a vertical di- 

 rection and parallels the grain of the 

 wood. This tunnel is known as the egg 

 gallery. The eggs are laid in alternate 

 groups along the sides of the gallery, 

 and the galleries are packed with bor- 

 ing dust mixed with pitch. There are 

 usually 3 to 4 groups of eggs and a 

 total of about 125 eggs in each gallery. 

 On the average there are from 6 to 8 

 such egg galleries for each square foot 

 of bark. 



When the eggs hatch in 3 or 4 weeks, 

 the larvae feed on the succulent inner 

 bark and cut mines that run at right 

 angles to the egg gallery. This larval 

 feeding continues through the late 

 summer and fall. When winter arrives 

 they are still in the inner bark, where 

 they become dormant. 



The following spring the larvae re- 

 sume feeding. As summer advances 

 they become mature, transform to 

 pupae, and then into adult beetles. 

 This new adult stage is reached by mid- 

 summer, and the beetles first start feed- 

 ing on the inner bark of the tree in 

 which they were reared. By August or 

 September they appear to be mature. 

 Then some of the beetles emerge and 

 congregate under the bark around the 

 base of the tree. Others remain under 

 the bark where they developed. In both 



cases they rest quietly during the sec- 

 ond winter, and when warm weather 

 returns the following spring they are 

 ready to take flight and attack other 

 living trees. 



While the development of the insect 

 is going on, the trees that have been 

 attacked die, usually by the end of the 

 first season of the attack. Death of the 

 trees is caused by the girdling action of 

 the egg galleries and the larval mines 

 and by a blue-staining fungus that per- 

 meates the sapwood and stops sap con- 

 duction in the tree. This fungus is 

 carried by the beetles and is always 

 found in trees that are successfully at- 

 tacked by the insects. The foliage of 

 Engelmann spruce does not change 

 color until about a year after the trees 

 are attacked; then it fades to a yel- 

 lowish green and the needles drop 

 within a short period. 



IN NATURE many forces operate to 

 keep beetle populations at a low level 

 over long periods. There is always a 

 high mortality during the brood-devel- 

 opment period from eggs to new adults. 

 The number that reach the full-grown 

 larval stage has been found to average 

 from 215 to 360 to the square foot of 

 bark. By the time the new adult stage 

 is reached and the beetles emerge, this 

 average has been reduced to about 150 

 to the square foot. These averages vary 

 widely. The larger trees tend to pro- 

 duce heavier emergence than smaller 

 trees. Then, when the beetles take 

 flight, they are exposed to all sorts of 

 hazards from wind, weather, and birds. 



The factors that tend to limit beetle 

 populations are parasitic and preda- 

 tory insects that feed on the broods 

 while they are developing in the inner 

 bark; woodpeckers, which locate the 

 infested trees and chip off the outer 

 bark to feed on the broods that are 

 thus exposed; and good growth condi- 

 tions in the spruce stands where young 

 and vigorous trees predominate. Wood- 

 peckers are especially effective; when 

 they are abundant they destroy 45 to 

 98 percent of the brood. 



Among the conditions that favor 



