320 MISC. PUBLICATION 273, U. S. DEPT. OF AGRICULTURE 
ing the newly planted trees nonresistant to their attacks, they pro- 
ceeded to cause serious damage. 
The control of insects that feed on twigs and terminal shoots pre- 
sents many difficult problems, and under forest conditions little of a 
practical nature can be done to control them, after they have become 
established. Under management, however, much damage from this 
source can be avoided by growing trees in dense stands or by keeping 
the trees in as vigorous growing condition as possible. 
Under special conditions, where the value of the young trees justi- 
fies the expense, control can be accomplished by hand picking the 
FIGURE 11.—Group of young, thrifty ponderosa pines killed by scale insects. (Group 
killings of this type are most frequently caused by engraver beetles. ) 
infested shoots and either burning them or placing them in cages 
designed to retain the destructive species, but ailow the escape of its 
parasites. Recently one or two projects of this character have been 
carried out with very satisfactory results. In the case of the isolated 
Nebraska sand-hill plantations much good was accomplished by in- 
troducing the native parasites of the tip moths. 
TWIG BEETLES 
The bark and pith of the smaller twigs, and branches of various 
coniferous and broad-leaved forest trees, are frequently mined by 
the smaller species of bark beetles of the family Scolytidae. These 
small twig beetles are often very abundant in the branches and twigs 
of dead, dying, or recently felled trees and in the twigs of healthy 
trees in the vicinity of slashings. Usually they confine their attacks 
to the twigs of trees of various ages and are commonly referred to as 
“twig beetles.” 
The attack of the twig beetles on living trees (fig. 12) is indicated 
by a small pitch tube or the exudation of fine boring dust at the point 
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