Weevil alert: Check your spruce trees

Insect causes damage that leads to a tree's deformed growth

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— White pine weevil is the No. 1 pest of immature Colorado blue spruces in Steamboat Springs. Feeding by the developing insects causes the topmost terminal branch (leader) to suddenly wilt and die in early summer. Upper branches are sometimes also affected. This damage can result in deformed growth.

Immature larvae of the white pine weevil feed underneath the bark of the spruce leader, girdling the plant. When sufficient damage has been done, the top growth will wilt and curl, becoming completely dead in a few weeks. Only the top leader and upper branches are affected by the insect. Once the top leader is killed, some side branches will change their growth habit and begin to grow upward to take the place of the killed leader. This changes the form of the tree from its normal tapering growth to one that is bushy at the top.

The adult weevil overwinters under leaf litter and in other protected areas. After snow melts and temperatures warm, the females seek out spruce trees. They feed just inside the bark of main branches near the leader and insert eggs into the feeding cavities that are formed. Small points of oozing pitch on the main leader are indicators of this feeding and egg-laying activity.

Eggs hatch in one to two weeks and the young grubs tunnel downward underneath the bark. Damage increases as the insects grow and wilting starts to become noticeable in June and July.

When full-grown, the white pine weevil larvae tunnel deeper into the stem and form a cocoon made of wood chips in which they pupate. In about two weeks the adult beetles emerge through small holes they chew through the bark. The chip cocoons remain behind and are a useful means of diagnosing old white pine-weevil injury.

Small spruce trees in open locations are most susceptible to white pine weevil. If some shading can be provided until the trees become 12 to 15 feet high, most attacks can be avoided.

Insecticides applied in spring when adult weevils feed on trees and lay eggs can provide control. Standard bark beetle or wood-borer insecticides should provide control if used as labeled for the above insects. Timing of these treatments will vary by location and year but usually should be made mid-March to early-May. Only the upper areas of the tree need to be treated.

Mechanical removal of infested terminals while the insects are still present can also contribute to control. This is best done in June or July before adult emergence. Infested terminals should be destroyed since weevils can survive in pruned wood.

If top growth has been killed, proper training of a single side branch as a replacement leader can help to salvage the future appearance of the tree. This new leader should be annually protected with insecticides until the tree is no longer highly susceptible to attack.

Whitney Cranshaw is an author and CSU Entomologist who instructs local Master Gardeners through the Colorado State University Cooperative Extension office in Routt County. Questions? Call the Master Gardeners at 879-0825 or e-mail gardeners@co.routt.co.us.

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