Problem type: Insect
Name of problem: White Pine Weevil
Plant name(s): Eastern white pine, Norway spruce; other pine and spruce species and occasionally Douglas-fir
Symptoms / Characteristics:
The top growth or leader of the host tree will droop and form the characteristic "shepherds crook" associated will twig weevil attacks. Larvae feed on the second or third year's growth, with the result that the current year's flush of new growth shows symptoms. Wilting of the new shoots and browning of needles show an early sign of attack. Top dieback can occur causing the tree to look crooked or straggly. Adult feeding on shoots and branches may cause excessive sap flow. This generally does not kill the branches.
Adults usually overwinter in debris on the ground, emerging in the spring, then crawling or flying to the leaders of suitable host trees to mate and lay eggs. Many pairs of weevils can gather on the same leader. The females excavate round holes in the bark, deposit their eggs and then plug the holes with bark shavings. Hundreds of eggs may be deposited within a single leader. Once the eggs hatch and the white legless larvae begin feeding inside the branch, the previous year's growth is killed and the new flush of growth begins drooping and is eventually killed, causing top dieback. Larvae develop into pupae inside chambers bored in the wood and then emerge as adult weevils by late summer. The adults are 6-7 mm long, reddish brown and covered with white patches, and like most weevils, have a long snout. The new adults feed on other hosts before preparing to overwinter.
Trees are rarely killed by weevil attacks, but growth can be greatly reduced, deformity can occur and infestations give other pathogens an entryway. As leaders are killed, multiple lateral branches will assume dominance, creating a very bushy, crooked or forked tree.
Control / Preventions:
Prune infested leaders as soon as drooping occurs by cutting just below discolored bark. Destroy all branches removed so that any weevils remaining in them will not cause further damage. Pruning of lateral branches and forks will be needed to help train a new leader to form a straight main stem.
Chemical insecticides are available to control adult weevils. This can be done either in the spring as adults emerge from hibernation, or in the fall after new adults emerge from branches. Look for sap flow on terminal branches and leaders, which indicates adult feeding. Spray applications should be concentrated on the leader and upper branches. Weevils are especially susceptible to sprays in the fall as they are feeding, rather than in spring when they are mating and laying eggs.
Systemic insecticides can also be applied to control larvae feeding inside the branch but are not as commonly used. Pruning, as well as applying chemicals often gives the best results.