Scolytidae Biological Control

From: Bellows, Thomas S. ,Carol Meisenbacher, and Richard C. Reardon, 1998, Biological Control of Arthropod Forest Pests of the Western United States: A Review and Recommendations, USDA, FS, FHTET-96-21.

The Scolytidae comprise the bark beetles and their relatives, and are the most destructive insects in western coniferous forests. The principal pest species are in three genera, Dendroctonus, Ips, and Scolytus. These beetles kill or destroy billions of board feet of sawtimber annually (USDA Forest Service 1958). Four species of Dendroctonus cause most of this damage.

Over the western region, most tree killing is dispersed among the forests and is the result of normal or endemic infestations that are continuously present in mature forests. This normal mortality is widely scattered, is usually greatest in undeveloped areas, and occurs largely to trees that are predisposed to beetle attack. Factors that predispose trees to attack include being overmature, water stressed, or previously exposed to attack by other pests. In this context, beetle attack serves as natural thinning of these trees.

Under conditions favorable for the beetles, epidemics occasionally develop and spread over large areas, killing vast amounts of timber (Furniss and Carolin 1977), including trees that are healthy and normally would not be subject to attack. Many beetles also have associated fungi that stain the infected lumber, further reducing its value. These epidemics can set the stage for devastating fires. The end result is highly disruptive to orderly management of these forest areas.

For bark beetle attacks on healthy trees to be successful, the beetles must be present in sufficient numbers to overcome the natural defenses of the tree. Beetles use pheromones to communicate within and among populations, and beetle aggregations resulting from these pheromones serve to concentrate attacks on particular trees. A number of natural enemies also cue to these pheromones, using them as kairomones to locate their prey or hosts. As the pheromones can be quite specific among different bark beetle species, the responses of many of the predators and parasitoids to these pheromones can also be rather specific.

Most of the bark beetles currently in western North America are native, and have a native natural enemy fauna. Focus of biological control efforts will be most productive in working towards integrating knowledge of the biology and ecology of these species into an overall management program against the bark beetles. Thinning of potentially susceptible stands, harvesting susceptible trees, and using trap or bait trees to attract and then kill beetles (by insecticide or other means) (e.g., Payne and Billings 1988), may be beneficial in preventing or limiting the extent of outbreaks. Naturally occurring biological control is probably most valuable in limiting population densities in endemic situations, and this natural control is likely the principal use of biological control of these species for the present. Should a nonnative bark beetle establish in western North America, however, the importation of suitable exotic natural enemies should be pursued. Introducing natural enemies against an adventive bark beetle pest has shown promise in Australia against Ips grandicollis (Eichoff) (Berisford and Dahlsten 1989).

For native beetles, examination of exotic natural enemies may offer opportunities to increase natural mortality (Mills 1983, Mills and Kruger 1988), although in most cases these native beetles are already attacked by a widely diverse upper trophic level fauna. Biological control through introductions of exotic natural enemies against native bark beetles has not yet met with much success (Clausen 1978, Miller et al. 1987, Moser 1989, Van Driesche et al. 1996), but further work in this area may be warranted. At least one species of predaceous beetle, Rhizophagus grandis Gyllenhal from Europe, has shown attraction to pheromones from North American Dendroctonus spp. in laboratory trials (Miller et al. 1987).

A relatively undeveloped area is the potential for the use of nematodes, either inundatively or innoculatively, into populations of bark beetles. A large number of nematode associates of bark beetles have been described (Massey 1974), many of which are known or presumed to be parasitic on their hosts. Parasitism may lead to reduced fertility, sterility, or death of infected individuals. Nematodes are omitted from many of the lists of natural enemies in the discussions which follow except where their life histories or associations with their host beetles are known to be parasitis.

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