Fir Engraver

From: Furniss, R. L. and Carolin, V.M., Western forest insects. Misc. Publ. 1339. Washington, DC: U.S. Department of Agriculture, Forest Service: November 1977. 383 p.

The fir engraver, Scolytus ventralis LeConte (Stevens 1956, Struble 1957b), is a major pest of true firs in western forests. It attacks trees from pole-size to full maturity, causing a yearly kill estimated to be 450 million fbm in California alone. Periodic epidemics cause even greater mortality. Such outbreaks often occur during and following periods of drought. Trees infected with Fomes annosus, a root-rot fungus, are especially subject to attack (Cobb et al. 1974). Trees defoliated by the Douglas fir tussock moth also are likely to be attacked by S. ventralis. It also breeds in slash and windthrown trees.

Attacks are made on the bole from the base to the top where S. praeceps, S. robustus, S.obelus, and other bark beetles take over. On the lower bole, Tetropium abietis is a frequent associate. Trees may be killed outright, may be top-killed, or may survive repeated attacks for many years. Broods often develop and emerge without destroying enough of the cambium to kill the tree. The patch of dead cambium heals over and leaves a brown pitch pocket in the wood to mark the place of injury. Some wood sections have shown as many as seven attacks is roughened patches of bark or scattered dead branches girdled at the base by the egg galleries. The beetles transmit a brown-staining fungus, Trichosporium symbioticum Wright, that apparently is essential for successful brood development.

The adults average about 4.0 mm long and are among the largest species of Scolytus. The second abdominal segment of the male has a sharp, median tubercle on its underside; that of the female has only a faint tubercle, or none at all. The egg galleries are excavated in the inner bark and cut transversely across the grain of the wood, which they score rather deeply for a distance of 5 to 15 cm (2 to 6 in) on both sides of a central entrance chamber.

Deep scoring of the wood differentiates their attacks from those of Pseudohylesinus and Dryocoetes. Eggs are laid in niches along both sides of these galleries and the larvae, on hatching, work up and down the bole, extending their individual larval mines for a distance of 13 to 18 cm (5 to 7 in). Pupation occurs in the inner bark at the end of the larval mines, and the new adults bore directly to the surface of the bark when ready to emerge. These beetles have one generation annually in much of their range, but in colder portions may require 2 years to complete the life cycle. Because of the sporadic character of outbreaks and the prevalence of healthy broods in living trees, direct control measures are generally impractical. On intensive use areas, prompt removal or treatment of infested trees may help protect remaining trees for a time, but the principal effort should be on measures to minimize soil compaction, excess exposure, and other adversities to tree vigor. In the forest, stand sanitation and improvement measures to keep the trees in healthy, vigorous condition are the most practical means for minimizing losses caused by the fir engraver.

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