INSECT KNIOWIKS OF WKSTKUN KOUKStS 



123 



The fir engraver {Scolytus centralis Lee.) is found attacking 

 the balsam firs in all the Western States and in British Columbia, 

 and at times has been exceedingly destructive to white fir stands in 

 California and Oregon. The adult is a short, black bark beetle 

 about one-eighth inch in length, without a prominent spine on the 

 ventral declivity. The egg galleries are excavated .in the inner bark 

 and cut transversely across the grain of the wood, which they score 

 rather deeply for a 

 distance of from 2 

 to 6 inches on both 

 sides of a central 

 entrance chamber. 



Eggs are laid in 

 niches along both 

 sides of these galler- 

 ies and the larvae, 

 on hatching, work 

 up and d o w n the 

 bole (fig. 61), ex- 

 tending their indi- 

 vidual larval mines 

 for a distance of 5 

 to 7 inches. A brown 

 stain of the cambium 

 caused by a fungus 

 is always found in 

 the area in which the 

 larvae feed. Pupa- 

 tion occurs in the in- 

 ner bark at the end 

 of the larval mines, 

 and the new adults 

 bora directly to the 

 surface of the bark 

 when ready to 

 emerge. Frequently 

 green trees are at- 

 tacked and new 

 broods develop and emerge without destroying enough of the cam- 

 bium to cause the death of the tree. The patch of dead cambium 

 heals over and leaves only a brown pitch pocket in the wood to mark 

 where the injury occurred. Some wood sections have shown as many 

 as seven such attacks during the life of the tree, which indicates that 

 a certain amount of activity by this beetle is constantly going on in 

 the forest. Trees are attacked during the summer months, and the 

 eggs hatch and larvae develop before winter. The winter usually is 

 passed in the larval stage, and the new broods emerge the following- 

 year. There is normally but one generation of these beetles annually. 

 Because of the sporadic character of outbreaks and the possible pres- 

 ence of healthy broods in living trees no methods of control appear 

 practical. 



Other species of Scolytus w^hich may be found in western firs 

 include S. subscaber Lee, a large species wdiich makes E-shaped 



Figure 61. — Egg galleries and larval mines of the fir en- 

 graver {ti'colytus ventralis). 



