lasting. They may also cause increases in nontarget pest populations (755, 9/2). 
Control costs vary considerably, depending on the insect and its habits, the methods 
and materials used, and the size and accessibility of infested stands. It may cost 
only several dollars per hectare for infested stands treated with aerial application of 
insecticides or it may cost several dollars per tree for individual treatments. 
Regardless of the control method employed, the benefits from its use should usually 
outweigh costs. Different methods of control involve different hazards; care must 
be taken in the use of any of these control methods. Occasionally, control costs may 
be retrieved through the salvage and sale of infested timber. 
Silvicultural Control 
Insect population density is influenced by the composition and condition of forest 
stands. Because of this, it is sometimes possible to create unfavorable conditions 
through the application of carefully designed cultural or management practices. 
Efforts directed toward this end are commonly called silvicultural control. 
Possibilities of silvicultural control occur during the establishment and through- 
out the lives of stands by properly timed planting, by selecting the more resistant 
tree species for planting, by putting them on sites best suited for them, or by 
controlling their composition and density. Stand conditions may be created or 
modified as needed by thinnings or cuttings. Pure stands may be broken up into 
mixtures of age classes in small units, with no two contiguous units of the same age 
class. Mixed stands may be broken up by cutting in small groups to maintain and 
promote diversification in species composition and density. Overmature trees may 
be removed from stands and the stands harvested as soon as they mature. High-risk 
trees may be removed in sanitation-salvage cuttings (659, /05/). Further discus- 
sions of silvicultural control are available (85, 457, 674, 1000, 1139, 1198). 
Physical and Mechanical Methods of Control 
Many forest, shade tree, and wood products insect pests are amenable to control 
by physical or mechanical methods. The method chosen is dictated by such factors 
as the habits and behavior of the insect pest involved, the location and value of the 
trees or products attacked, and cost. 
Trap trees and trap logs are sometimes effective in the control of bark beetles 
(903). Bands of tanglefoot or similar materials may be placed around the trunks of 
shade trees to prevent larvae or wingless female adults of several species, such as 
the fall cankerworm, from ascending the trees to feed or oviposit. Young, newly 
planted shade trees may be protected from borers of various kinds by enclosing the 
trunks in wrapping paper. Bagworms on ornamental trees may be controlled by 
handpicking. Subterranean termites can be kept out of structures by capping 
foundations with a 10-cm layer of high-grade concrete or with properly applied 
metal shields, by keeping untreated wood from contact with the ground, by using 
solid foundation masonry, and by providing good ventilation between the ground 
and timbers (/2). Similar treatments prevent damage by the old house borer (870). 
Logging and milling infested timber and destroying the slabs that contain the 
broods are often effective in preventing bark beetle outbreaks. Such methods may 
be feasible only where the trees are of merchantable size, are accessible, and when 
they can be harvested promptly. Bark beetle broods in or under the bark may also be 
killed by storing infested logs in water, by peeling infested logs and burning the 
