Since early in this century, considerable emphasis has been placed on importing 
and augmenting natural enemies of many species of introduced pests (2/8, 6/3). In 
many cases, the dispersal of these imported enemies has been accelerated by the 
transfer of colonies to distant infested areas (299, 34/, 704). The increase in 
numbers after successful establishment of several species has also been helped by 
the use of large numbers of field-collected or laboratory-reared individuals (647, 
928). 
The presence and abundance of natural enemies is taken into consideration in 
deciding whether to suppress an outbreak and in planning how to accomplish it. For 
example, the sudden appearance of a virus disease among the damaging insects in 
heavily infested stands usually portends the imminent collapse of the outbreak. 
Therefore, when evidence of this disease is encountered during surveys, it is often 
decided to withhold other methods of control, particularly the application of 
insecticide. Observation of abnormally high percentages of natural control provided 
by insect parasites and predators leads to a similar decision. In other situations 
where insecticidal control is deemed necessary, it is often possible to limit applica- 
tion to designated portions of infested areas because of the abundance of natural 
enemies in other portions (673). 
Knowledge of the population dynamics of a pest species provides opportunities 
for various other means of preventing or reducing losses, thereby eliminating the 
need for direct, suppressive measures to control outbreaks. The genetic basis for 
insect behavior is being explored to suggest and explain responses to pest manage- 
ment practices (428). Depending on the situation, outbreaks of certain leaf-feeding 
insects may be prevented or minimized by modifying the composition and density 
of susceptible stands (3/3, 75/). It is often possible to prevent or suppress out- 
breaks of some bark beetles by thinning the stand, removing infested slash, or 
salvaging infested trees before the emergence of beetle broods from the bark 
(1195). The utilization of mature trees growing on poor sites before the trees begin 
to deteriorate and become attractive to borers, such as the hemlock or bronze birch 
borers, is a recommended control practice. The avoidance of unfavorable sites in 
planting programs is recommended for preventing damage by certain species (5/5, 
548). Adherence to good construction methods when building structures reduces 
the need for chemical control of termites later (JO50). The destruction of elm 
material suitable for breeding by the smaller European elm bark beetle reduces the 
need for the use of insecticides in Dutch elm disease control (/287). 
Some of the new approaches to biological control, many of which are yet to be 
perfected, may be expected to provide additional opportunities for IPM in the 
coming years. These include the combined use of insect growth regulators or 
microbial insecticides with pheromone traps to rid the sprayed zone of remaining 
male insects. Application of these new-generation insecticides can be followed by 
the liberation of massive numbers of parasites, some species of which are known 
not to overwinter in the region. Comprehensive treatments of insect-pest manage- 
ment have been published (3/3, 664, 907, 1008, 1195). 
19 
