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