Bark Beetles
FIELD GUIDE TO INSECTS AND DISEASES OF ARIZONA AND NEW MEXICO FORESTS
Spruce Beetle
Dendroctonus rufipennis (Kirby)

Hosts:  Engelmann spruce is principal host, blue spruce is an infrequent host

Figure 118. Figure 118. Egg and larval galleries of spruce beetle. Egg galleries (highlighted in black) are vertical. Larvae feed perpendicular to egg galleries and frequently their galleries intersect.

Symptoms/Signs:  External evidence on green infested trees consists of entrance holes in the bark and occasional pitch tubes. However, pitch tubes are often indicative of an unsuccessful attack. Red boring dust from entrance holes usually accumulates in bark crevices around the bases of infested trees. Woodpeckers frequently remove large sections of bark looking for larvae to feed on. About 1 year after attack, needles of infested trees usually turn a yellowish green and fall, though some may remain green until the second year. Needles do not turn a reddish color like those on bark beetle infested pines. Under the bark, egg galleries are vertical and slightly groove the xylem. Egg galleries range from about 6 to 30 cm long. Adult beetles are cylindrically shaped and 4 to 6 mm long. They are generally dark brown to black with reddish brown wing covers; however, older adults are usually entirely black. Larvae are creamy white, cylindrical, legless grubs. Larvae reach a length of about 6 mm. Pupae are creamy white and similar in size to the adult.

Figure 119. Figure 119. Fading Engelmann spruce mortality caused by spruce beetle in White Mountains, Arizona.

Biology:  Spruce beetles generally produce one generation in 2 years; however, generation time can take from 1 to 3 years. Adults emerge from May through July and begin attacking new host trees. Eggs hatch and larvae develop during the summer. Broods overwinter as larvae or callow adults and complete development to mature adults by the following August. Spruce beetle attacks standing trees, windthrown trees, or logging residuals. It prefers large diameter, green downed material to standing trees. Populations are affected by a number of factors including the number of susceptible hosts, natural enemies, and extreme cold.

Effects:  The direct effect of spruce beetle attack is tree mortality or in some cases strip kill, when only a vertical section of the bole is attacked. Strip attacked trees often die in subsequent years. Spruce beetle epidemics can have both immediate and long-term effects on spruce-fir forests. The immediate effects include changes in stand structure and composition. The longer-term effects come from successional changes that result from microclimate changes as well.

Similar Insects and Diseases:  The most common associates are in the genus Ips. In New Mexico, I. pilifrons sulcifrons is the most common, while in Arizona, I. pilifrons utahensis is found most often. Ips beetles frequently infest the upper (exposed) portion of downed material, while spruce beetle favors the shaded material. Ips egg galleries differ from those of spruce beetle by containing a nuptial chamber and by being free of frass.

References:  42, 87

Figure 120. Figure 120. Remnants of spruce beetle galleries on a downed log.
Figure 121. Figure 121. Flaking of bark caused by woodpeckers indicates spruce beetle larvae in the outer bark of Engelmann spruce.