Evergreen icons of America's cool northern forests, spruce trees are valuable as commercial sources of lumber. Their silvery blue to deep green needles, invigorating fragrance and graceful conical form also make spruces valuable landscape trees. Like many conifers, however, spruces attract a variety of damaging insects. Learning what insects are likely to affect the spruces in your area and how to recognize their presence can help you keep your spruces at their best.
Spruce beetles (Dendroctonus rufipennis) are the largest insect threat to spruce trees in the United States, say Forest Service entomologist E. H. Holsten and his colleagues. These beetles have killed spruces from Arizona to Alaska. The Ips or "engraver" beetle family contains several spruce-infecting species--11 in Colorado alone. They damage newly transplanted or stressed trees. Spruce bud scale (Physokermes piceae) insects usually target Norway spruce. Western spruce budworms (Choristoneura occidentalis) defoliate trees in every state west of the Rocky Mountains except Nevada and California.
Signs of Infestation
As bark-borers, both spruce and Ips beetles create piles of yellow or reddish dust that settles at the base or in the bark crevices of infected trees. The beetle larvae that mature in the trees exit as adults, leaving small holes in the bark. Spruce bud scale insects release a sticky substance called honeydew. Black mold often feeds on the honeydew. Budworm larvae spin white webs on new spruce needles as they eat.
The beetles tunnel beneath inner bark into the sapwood that carries nutrients to the spruce's needles. Their larvae extend the tunnels. Trees die back above any trunk-encircling tunnels. Scale sucks fluid from needles, damaging or killing branches. The weakened plants grow more slowly, and are more susceptible to continuing scale attacks and beetle infestations. Heavy budworm infestations consume new needles, depriving trees of food and stunting their growth.
Insecticides with carbaryl, permethrin or bifenthrin will protect your uninfected spruces from spruce or Ips beetle attacks. No insecticides treat infected trees. Another defensive measure is to cut, strip and chip infested trees near healthy ones, and remove the bark and wood chips to a location at least a mile away.
Scale larvae are vulnerable to insecticides and, when overwintering on the needle undersides, to spring applications of horticultural soaps and oils. The oils and soaps suffocate them. Frost, other insects and birds may kill enough budworms to stop an infestation, say Colorado State University Extension's retired entomologist D.A. Leatherman and colleagues. Applications of Bt or carbaryl-based insecticides no more than three weeks after new needles emerge in spring are also useful.
Applying pesticides to prevent or kill the insects dangerous to spruce trees also puts beneficial insects, wildlife, pets, other plants and even people at risk. Always follow the manufacturer's instructions on recommended concentrations and application methods for your pesticides.