Fir Engraver Beetle

The fir engraver beetle (FEB) is a common bark beetle in western coniferous forests. The beetle primarily attacks fir trees. In Nevada, the primary host tree species are white fir and red fir. Other species occasionally attacked are subalpine fir, Douglas-fir, mountain hemlock and Engelman spruce.

The FEB is normally present in forests at low (endemic) levels, and kills trees weakened by disease, old age, competition or other influences. The beetle often breeds in freshly windthrown trees and recent logging slash.

Its population is controlled by the available food source (weak trees), and several parasitic wasps, mites, nematodes and woodpeckers.

When food supplies increase substantially, the FEB population can erupt to epidemic levels. Droughts, insect defoliation, diseases, and other factors can cause large numbers of susceptible trees leading to epidemics. Normally effective natural controls are ineffective in controlling these outbreaks. The epidemics persist until the population of susceptible trees declines.

Biology

The shiny, black beetle, about 4mm long (Fig. 1), typically emerges from infested trees from June through September. Temperature probably controls when adults emerge, leading to earlier emergence during warmer springs and at lower elevations. The adults then fly to and attack randomly selected trees. Most attacks occur between July and August, but can extend into September. FEB prefer trees larger than 4″ in diameter. The female beetle bores through the trees bark and is followed by a male which she mates with. The beetle emits a chemical scent which attracts other beetles to the tree for a mass attack.

The female bores a horizontal gallery between the outer wood and inner bark, slightly scoring the wood. Hence the name “engraver” beetle. Up to 300 eggs are laid in niches along the gallery or tunnel. The eggs hatch into white larvae with brown heads within two weeks. These larvae then feed on the inner bark, forming parallel tunnels at right angles to the adults egg gallery. The gallery pattern is very distinctive, and combined with host tree species and shiny black appearance of the adult beetle, gives positive identification. The larval feeding on the important conductive tissue under the bark is what causes the trees death. The beetle will spend the winter under the bark as a larvae or adult, and emerge to attack new trees the following spring or summer. The life cycle generally takes one year to complete. Two years are required at high elevations, and during warmer years, a partial second generation may form.

Evidence of Attack

Often beetles will attack the tree crown, killing individual branches. The appearance of yellowed or red branches within a green tree, called flagging, is an easily seen sign of attack. More intense attacks along the tree trunk cause top-killing or death of the entire tree. Clear streams of pitch flowing from entrance holes down the trunk may be present. Unlike other conifers, firs do not produce the globules of pitch, pitch tubes. Reddish brown or white boring dust in bark crevices or cob webs along the trunk is usually present. Bark may be stripped off as a result of woodpeckers feeding on the beetle larvae.

The best way to protect trees from FEB attack is to keep them healthy and vigorous. Thinning crowded trees on your property to give adequate growing space is important. Tree spacing should follow the diameter plus ½ guide. That is, the distance, in feet, between adjacent trees should be equal to 1½ times the tree diameter in inches. Retain upright, undamaged, full crowned trees. Favor the more drought tolerant pine trees over firs for keeping. The best time to thin is during late fall and winter.

Infested trees should be cut and removed by early May, before the adult beetles emerge. If infested trees are kept on site after cutting, they should be burned, chipped or sprayed insecticide as directed below. If kept for firewood, the tree should be cut up, piled in direct sunlight and covered with thick (= 3 mil), clear plastic, making sure all edges are buried. The heat buildup under the plastic should bake the beetles. Keep covered for three months if weather is sunny, longer during cloudy periods. Peeling the bark off firewood is another effective, although difficult, control method.

Individual, high value trees can be protected by spraying the trunk with a pesticide prior to the beetle attacks (by early April). Be sure the pesticide is registered for use on bark beetles on fir trees, and apply it only as directed on the label. Contact the Nevada Division of Agriculture for a list of pesticides registered for use on feb. read, understand, and follow instructions on pesticide labels.

Authored by: John Christopherson, Nevada Division of Forestry.