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



hole and harden on the bark in various forms of pitch or resin 

 tubes. Some species construct ventilation tunnels at intervals along 

 the egg galleries. These are perpendicular to the egg galleries and 

 extend through the bark to the surface or may end before the sur- 

 face is reached. Boring dust is pushed out of those that are open 

 at the surface of the bark, and they all are probably used as turn- 

 ing niches, as well as for ventilation of egg tunnels. Later, as the 

 mines progress, these are sometimes plugged with boring dust. 



The eggs, which are very small, are oval, round, or slightly 

 elongate, and clear or chalky white. They are deposited in small 

 cup-shaped cavities along the sides of the egg galleries. Usually a 

 single egg is placed in each cavity or egg niche, which is closed 

 with a plug of boring dust in such a way that the smooth cylindri- 

 cal egg gallery is but little altered. Some species cut larger cavities 

 or egg pockets and deposit from two to eight eggs in each. Others 

 cut an elongated groove on one or both sides of the egg gallery 

 and deposit the eggs in layers or rows. 



The larvae, or grubs, are thick-bodied, always legless, cylindrical 

 and curved, white or cream colored, with a distinct head and 

 prominent dark-colored mandibles. At first the larvae and their 

 mines are very small, but both increase in size as feeding pro- 

 gresses. The larval mines start away from the egg gallery more or 

 less at right angles and may continue nearly straight or turn and 

 run parallel to the egg tunnel. They are always packed with excre- 

 ment and boring dust. 



Transformation to the pupal stage takes place at the end of the 

 larval mine in a specially constructed pupal cell. The pupae are 

 soft, white, and unprotected. The antennae, mandibles, legs, and 

 wing pads are clearly visible, and hairs and spines are often pres- 

 ent on the various regions of the body. Gradually the pupae 

 darken, turning light yellow and then brown, as the adult form is 

 reached. 



The adults, after a short hardening period, emerge and fly to 

 attack new host trees, or congregate in cavities under the bark of 

 the old host tree, or drop to the ground to hibernate. Some adults 

 do a certain amount of feeding under the bark before emerging, 

 and food tunnels made in this way are quite distinct in character 

 from the regular egg galleries. Others upon emerging feed upon 

 twigs or buds of other trees before again attacking the bark of a 

 new host. 



Normal or endemic infestations of bark beetles are present in 

 practically all mature forests (151), causing an annual loss of a 

 fraction of 1 percent of the timber on the area. Under conditions 

 favorable to the insects, serious epidemics develop from these nor- 

 mal infestations in a very few years. Such outbreaks may be of 

 short duration, or they may continue for many years, destroying 

 large volumes of merchantable timber over extensive acreages. 



If bark-beetle attacks are to be successful, the attacking insects 

 must be present in sufficient numbers to overcome the resistance of 

 the tree. Dead and dying trees offer little resistance to attack and 

 for this reason they are usually chosen by the secondary species 



