6 CIRCULAR 8 6 4, U. S. DEPARTMENT OF AGRICULTURE 



galleries intermingle. About 55 percent of the attacking beetles 

 emerge, and some construct galleries in a second and possibly a third 

 tree before they die. 



The eggs hatch in about 7 days. The tiny larvae bore short tunnels 

 in the cambium at right angles to the main egg gallery, where they 

 feed on the proteins and simple sugars found therein. As they in- 

 crease in size, they tunnel outward from the cambium and soft inner 

 bark into the corky outer bark (fig. 5.) There they complete their 

 larval development, pass through the pupal stage, and finally emerge 

 as beetles to seek a new host tree. Practically every beetle makes its 

 own emergence hole: so the bark of an abandoned tree appears to 

 have been riddled with bird shot. 



Figure 5 — Mature larvae of the western pine beetle. These fat larvae in the 

 outer bark are nearly ready to enter the pupal stage. Usually before they reach 

 this size the foliage of the host tree has faded and the tree has been considered 

 dead. 



In killing a tree the beetles appear to be aided by a fungus. 

 Ceratostomella pini Munch, belonging to the group of blue-stain- 

 ing fungi. Spores of the fungus, carried into the tree by the attack- 

 ing beetles, germinate and the hyphae spread rapidly through the 

 conducting vessels and choke them, thus hastening the death of the 

 tree. This fungus gives the blue color to the sapwood of dead trees, 

 a condition that has no other effect on the technical characteristics 

 of the wood. 



Seasonal History 



In the pine stands of central Oregon, which represent the midrange 

 of the western pine beetle, eggs laid in trees attacked in June and 

 July develop into beetles by August and September. During Sep- 

 tember and October these new adults attack and kill other trees, in 

 which they and their progeny pass the winter in the egg. larval, or 

 beetle stage. Thus, two sets of trees are killed annually in this area. 

 In years favorable for beetle development, that is. years with a long, 

 warm fall season, a third set of trees may also be killed. This sequence 

 varies, of course, with the latitude and altitude of the forest and the 

 prevailing climatic conditions. Attacks and development occur ear- 

 lier and extend over a longer period in the southern forests, and later 

 and over a shorter period in the more northern forests. 



