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Pest AlertsApril 27, 2001

Spruce beetles

Spruce beetles, although native to Minnesota, have been collected very few times. However, last year they were found killing mature, healthy-appearing white spruce in a number of locations in Cook County. It's likely that they can be found in other locations in northern Minnesota as well.

Major outbreaks of this bark beetle causing extensive spruce mortality have occurred in New England and eastern Canada and also from Alaska to Arizona.

Hosts include all native species of spruce but white spruce is probably the most important. The spruce beetle tends to prefer larger diameter trees (12" and up) but have also been found on smaller trees.


Signs of attack include red boring dust on the bark at the base of the tree, pitch tubes on bark of the main bole of the tree, exit holes, fading foliage, dead and dying trees. When fresh, pitch tubes are easily seen. As they age, they become flattened on the bark and take on a color similar to the bark and can be very difficult to see.

Adult bark beetles are 4-6 mm long and are two toned (reddish brown wing covers and black head and thorax) or all black.

The beetles build up on trees stressed by soil compaction, spruce budworm or root rot and in blowdown or in piles of logging slash.

bark beetle 

The distribution and importance of this bark beetle in Minnesota is not known. Please report infested trees and stands to your DNR Regional Forest Health Specialist.

Phone numbers are listed on the index page of this newsletter.


Larch beetle: Got bark?

Reports of tamarack trees with no bark were received this winter from locations in St Louis, Itasca, Pine and Aitkin Counties. Woodpeckers were stripping the bark off the main bole right down to the snow line! Bark was sometimes also removed from the larger branches.

Tamarack tree without bark

The woodpeckers were looking for larch beetle adults which overwinter under the bark of the tamarack (larch) trees. The beetles kill the trees and the woodpeckers are feasting on the beetles. Damage levels varied from scattered individual trees killed to 30-50% of the stands killed. In one case, a stand contained a large pocket of damage where 80-90% of the trees were dead. Most of the damaged trees observed this winter appear to have been attacked by the beetles last spring and summer. In some stands, trees killed over the past few years were obvious as well.

Small, 1/16 inch diameter holes in the bark, lots of dark brown boring dust and copious resin flow are indications of attack that can be observed at any time of year. In August and early September, about half of the infested trees turn yellow earlier than normal. And of course, in winter, woodpeckers may strip bark off looking for breakfast, lunch and dinner.

close up of Tamarcks with exit holes

Larch beetle is a native insect. Tamaracks and exotic larches are the only species attacked by the larch beetle. Larch beetle is usually considered a secondary pest attacking stressed and recently cut trees. Flooding, drought, defoliation and old age have been associated with larch beetle attack. However, larch beetle also appears to be able to develop widespread outbreaks and kill healthy trees as well.

moss cover Tamarack tree

Some of the damage seen this winter appears to be the result of beetle populations building up on logging slash. The beetles then attacked and killed live trees left for natural seeding or pockets of trees in surrounding stands. In other situations, there was no readily apparent cause for the buildup and attack. Old age is likely a contributing factor in some stands.

Larch beetle adults overwinter in attacked trees. Adults emerge in the spring, seek out and bore into suitable live trees or fresh logging slash. There they construct galleries and lay eggs. Larvae hatch from the eggs, feed on the phloem and eventually pupate and change into adults about 3/16 inches long. These new adults stay in the tree and overwinter. There is only one generation per year.

Tamarack tree without bark

We are interested in knowing how widespread this damage is. If your trees have larch beetle damage please send the location to your local Regional Forest Health Specialist.