Fir Engraver Biological Control

Scolytus ventralis LeConte

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.

Origin: North America

Range in North America: Arizona, British Columbia, California, Colorado, Idaho, Montana, Nevada, New Mexico, Oregon, Utah, Washington, Wyoming (Stevens 1956).

Plant hosts and damage: Abies grandis, Abies concolor, Abies magnifica, and occasionally Pseudotsuga menziesii, Tsuga spp. and Picea spp. Adults and larvae mine the cambium. Repeated or heavy attacks can lead to tree death.

Natural Enemies: Few natural enemies are reported in the literature. These include (Table 16) clerid predators, insect parasitoids, woodpeckers, and a mite (Stevens 1956). Other parasitoids or pathogens probably await discovery or reporting.

Table 11. Invertebrate natural enemies of Scolytus ventralis (after Stevens 1956)

Pest Status: Of the 18 species of Scolytus in western North America, Scolytus ventralis is the only pest of major significance in the management of western forests (Furniss and Carolin 1977). The other Scolytus species are primarily secondary attackers of trees weakened or killed by other insect species or other causes (Furniss and Carolin 1977). This species is responsible for losses of 450 million board feet annually in California alone, and outbreaks cause greater losses.

This species is univoltine throughout much of its range, but semivoltine (requires 2 years to complete one generation) in northern latitudes (Furniss and Carolin 1977). Adults bore directly into the bark on the bole, from the base to the top of the tree. Eggs are laid in transverse galleries, which can deeply score the wood. Subsequent larval feeding causes mines in the cambium that are usually with the growing grain of the tree. Pupation takes place in the inner bark.

Scolytus ventralis primarily attacks firs. Populations of this species are widespread in western forests, and are found in healthy as well as weakened or dying trees. Minor or modest infestations in healthy trees are not invariably fatal, as these trees often survive and continue to grow. Heavy infestations can lead to tree death. Because of the sporadic character of outbreaks and the prevalence of healthy broods in living trees, direct control measures are generally impractical. On intensively used areas, prompt removal or treatment of infested trees may protect remaining trees from immediate risk. In such areas, however, the best practice is to prevent tree stress by minimizing soil compaction and other adverse conditions (Furniss and Carolin 1977). Trees infected with the root rot fungus Fomes annosus are at greater risk of beetle infestation (Cobb et al. 1974, see also Filip and Goheen 1982). Outbreaks often follow droughts (Furniss and Carolin 1977). In the forest, stand sanitation together with measures to promote tree vigor are the most practical means for minimizing losses to this species.

Biological Control: No biological control program has been mounted against this species. The few natural enemies known are general predators of many species of bark beetles. Of the braconid parasitoids known, C. brunneri is also known from other bark beetles.

Recommendations: Although this species of beetle is responsible for major losses in western forests, the potential for enhanced biological control appears somewhat limited. The primary factors predisposing trees to attack appear to be related to forest health and tree vigor, not lack of natural enemies of the beetle. Additional natural enemies of Scolytus spp. from other related environments (e.g. Europe) might be considered for importation. Although the principal niches for natural enemy impact (predators, parasitoids) are currently represented in North America, other species could conceivably increase natural mortality. Additional studies should be conducted on life tables for this pest species, with special reference to the role of natural enemies in population demography, particularly pathogens (which are not represented in the literature for this species), before future programs for enhancing biological control are undertaken.

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