INSECT ENEMIES OF WESTERN FORESTS 97 



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 surface 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 turning 

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

 progresses, these are sometimes plugged with boring dust. 



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

 gate, 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 cylindrical 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 elon- 

 gated 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 promi- 

 nent dark-colored mandibles. At first the larvae and their mines are 

 very small, but both increase in size as feeding progresses. 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 excrement 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 present 

 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 faeding 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 (80)^ causing an annual loss of a frac- 

 tion of 1 percent of the timber on the area. Under conditions favor- 

 able to the insects, serious epidemics develop from these normal 

 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 that 

 are not capable of coping with a vigorous pitch flow. Light attacks 

 by primary species on living trees often fail because the flow of 

 pitch is so copious that the attacking beetles are overcome or driven 



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