Insect populations may reach levels in both natural and human-made forests where they are considered pests. High populations of cone and seed insects in a conifer seed orchard, mortality of seedlings resulting from feeding by weevils, defoliation of mature trees, and bark beetle outbreaks are all examples for which a management activity aimed at reducing the density of the insect population may be appropriate. These management activities all have a financial cost associated with them, which needs to be weighed against the anticipated benefit from the activity. With a few notable exceptions, the use of traditional insecticides to control forest insects is rarely practical, because of cost, or desirable because of impacts on nontarget organisms. Exceptions may be high-value urban trees and trees in seed orchards. In these situations, the area being targeted with the pesticide is limited. Biological pesticides are, however, very useful in limiting the impacts of defoliating Lepidoptera. Extracts from the bacterium Bacillus thuringiensis (Bt) are used in aerial applications where defoliators threaten to kill large tracts of trees. Extensive areas in North America are treated with Bt to reduce the damage from the gypsy moth and to slow its spread. Insect outbreaks may be a result of fire suppression that creates an overmature forest that is susceptible to these insects. Long-term management strategies should therefore address this issue rather than use short-term solutions such as insecticides.
In populations of native and introduced forest insects, especially defoliators, it has been possible to identify viruses that are specific to their host insect. Rearing and release of these viruses offer promise for pest control in some situations, especially for defoliating Lepidoptera and Hymenoptera. The nuclear polyhedrosis virus (NPV) of the European pine sawfly, Neodiprion sertifer, has been applied to thousands of hectares of forests in Europe over the past 35 years. An NPV that infects gypsy moth has also been used in efforts to slow the spread of this species in North America. The accidental introduction of an NPV in the 1930s, along with a parasite of the European spruce sawfly, Diprion hercyniae, into eastern Canada has reduced this species to very low densities.
Biological control has achieved considerable success in managing forest insect populations. It is especially useful when addressing problems caused by introduced insects. For example, as the European spruce beetle spread westward across Europe during the 20th century, outbreaks were most severe on the edge of its range, in part because of the lack of natural enemies in newly invaded areas. Mass releases of the predatory beetle Rhizophagus grandis had some success in limiting the impact of this species in many European countries. Similarly, in urban forests where many tree species are introduced, management of exotic pests is often achieved by introduction of predatory or parasitic insects. In California, eucalyptus are widely planted in urban landscapes, and in the latter part of the 20th century, many new pests were introduced. Prompt identification and monitoring of pest populations coupled with introduction of appropriate biological control agents have successfully reduced the adverse impacts of many of these introduced insects, especially the eucalyptus longhorned beetle, Phorocantha semipunctata.
Much scientific research has focused on the pheromone systems of forest insects, especially defoliators and bark beetles. Pheromones have been incorporated into management plans aimed at reducing the impacts of the target pest species. For example, mountain pine beetle pheromones are used to bait living lodgepole pines that are to be logged following infestation. This treatment attracts beetles into plots that will be harvested, and the beetles are removed with the logs. The treatment is aimed at reducing mortality in the unharvested areas. It prevents trees from dying at more scattered locations in these unharvested areas where most trees would not be harvested because of cost and accessibility. Other techniques for reducing the impacts of target pest species can be achieved in a number of ways. Pheromones can be used to monitor insect populations as a guide for when to apply pesticides or embark on an alternative management strategy. They are also used to detect invasive pest species in new areas. Throughout the western states of North America, pheromone traps are placed to detect gypsy moth so that early detection can be met with rapid response eradication programs. Similar use of pheromone traps and traps baited with other behaviorally active compounds occur at ports to detect the arrival of exotic organisms.
Attempts have been made to use pheromones to mass trap insects and thereby reduce their density to nonpestiferous levels. Because insect populations naturally decline at some point following an outbreak due to a range of factors, it is often difficult to determine whether such mass trapping efforts were in part responsible for a return of the insect population to endemic levels. In Scandinavia, mass trapping of the eight-toothed spruce bark beetle occurred during 1979 and 1980, and billions of insects were caught. The subsequent drop in population size of this insect may have been in part a result of this intensive mass trapping effort. Mass trapping may be useful locally where a small-scale disturbance may have resulted in elevated activity of a particular insect. This situation may occur when management activities have produced a large amount of host material. Pheromone traps may be deployed in this situation to reduce populations. Trap logs that have become infested with bark beetles are removed from the forest in an attempt to lower population levels. Pheromones may be used to bait the logs or trees to enhance the effectiveness of this technique.
Pheromones have also been used in larger doses to confuse or disrupt their natural function. Male moths following pheromone plumes to locate females for mating are not able to find the female if there is a large concentration of the pheromone present, such that the plume is effectively hidden. Western pine shoot borer, Eucosma sonomana, pheromone release of 10 to 20 g per hectare has been shown to reduce damage by over 65% in ponderosa pine plantations in the western United States.
Many forest management practices are available for use to reduce the impacts of insect populations. Removal of susceptible trees, thinning of the stand to increase tree vigor, prescribed burning to eliminate susceptible host material, and enhancement of natural enemy populations are all tools available for the management of forest insect populations. The thinning of lodgepole pine stands reduces tree mortality from mountain pine beetle. Prescribed fire following harvesting operations in ponderosa pine forests reduces populations of the California five-spined ips, Ips paraconfusus, that breeds in logging debris. This practice lowers the probability of these populations emerging and killing living trees in the area.
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