Numerous species of nematodes may be found in the gut, the 
hemocoel, or in particular organs such as the Malpighian tubules 
of insects. Some of these kill the host when they emerge; some 
cause death by initiating the action of lethal microbial disease 
agents; and some cause injury but not death. Certain bark beetle- 
infesting species riddle the intestines and gonads of their hosts, 
block the ducts, and cause the ovaries to shrivel. This may lead to 
a marked reduction in egg production by infested female beetles 
and a sharp decline in beetle populations (508). Welch (753) 
reviewed the literature on the subject. 
APPLIED CONTROL 
Applied control consists of the regulation by man of insect 
activities, distribution, and abundance through the use of prac- 
tices designed to enhance, supplement, or serve as substitutes for 
natural control. The action taken may be either direct or indirect; 
i.e., 1t may be directed either against the insect itself to suppress 
currently destructive populations or toward the alteration of en- 
vironmental conditions in such a way that destructive populations 
cannot develop. Whatever the objective, the aim is to use the 
cheapest, safest, and most economical methods available. 
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 care- 
fully designed cultural or management practices. Efforts directed 
toward this end are commonly called silvicultural control. 
Possibilities of silvicultural control occur during the establish- 
ment and throughout the lives of stands, 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 thin- 
nings 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- 
Ae trees may be removed in sanitation-salvage cuttings (413, 
631). 
For further discussions of silvicultural control, see Keen (412), 
Graham and Knight (309), and Prebble (605). 
PHYSICAL AND MECHANICAL METHODS OF CONTROL 
Many forest, shade tree, plantation, and wood products insect 
pests are amenable to control by physical or mechanical methods. 
The method chosen is dictated by many factors, such as the habits 
and behavior of the insect pest involved, the location and value of 
the trees or products attacked, and cost. 
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