Q. I have a windbreak of white pine trees that seem to be dying from the top down. I really value these trees and do not want to lose them. Do you have any suggestions for saving these trees?
A. Judging from your description and the digital photos you sent, it appears the white pine weevil (Pissodes strobi) is responsible for the damage to the tops of your trees. It is not likely to kill trees, but it can destroy their natural shape. Although white pine weevils feed primarily on last year's growth, their feeding can kill the top two or three whirls of branches. In addition to white pines, this pest damages the tops of other pine species, spruce and Douglas fir.
This insect is a 1/4-inch long reddish-brown beetle with white spots on the ends of its wing covers and an elongated snout. Weevils overwinter as adults in leaf litter and other debris on the ground. As the weather warms in spring, they migrate to the terminal leaders (topmost growth) of susceptible trees where they mate. Adult females bore out a circular cavity and deposit anywhere from one to five eggs. Hundreds of eggs can be laid in a single leader.
Once they hatch, the larvae bore into the leader to feed. Their feeding cuts off the flow of water and nutrients through the stem, which causes it to wilt and die. New growth usually sprouts below the dead leader, but the damage destroys the shape of the tree. The larvae pupate and hatch out as adults by late summer. They drop to the ground to overwinter, and the cycle starts over again.
One nonchemical control is to prune out the infested leaders to remove the larvae from the trees. This will reduce the number of overwintering adults and help protect your trees from further damage. It is best done as soon as the leader droops, often in early June. Prune the infested stem just below where the bark is discolored. Discard it by sealing it in a trash bag or burning it. There should be a flush of new growth at the base of the dead terminal.
Select the most upright shoot of this new growth and try to tie it into an upright position with wide, nonbinding masking tape or the plastic tape used to stake tall flowers. Remove all ties after one growing season. Prune out the rest of the new growth at that whirl to preserve the shape of the tree.
Home gardeners can use a systemic insecticide containing imidacloprid, including Bayer Advanced Tree and Shrub Insect Control II, Monterey Once-a-Year Insect Control, and Bonide Annual Tree & Shrub Insect Control. Applications should be made in fall to allow enough time for the product to be absorbed by the roots and taken up to the terminal of the tree. It is important to rake back any mulch or accumulation of pine needles so you apply the product to bare soil within 18-24 inches of the trunk. It is also important that the soil is moist but not saturated at the time of application. Water it according to label directions after application and replace the mulch.
Other chemical control measures are directed toward adult white pine weevils as they begin feeding and laying eggs in early spring. Resin oozes from the punctures they make in the terminal leaders. Scout for this symptom from mid-March to April, using binoculars to examine the tops of tall trees. Adult controls should be applied to the terminals when you start seeing resin droplets. There is no need to spray the rest of the tree.
Unfortunately, there are no insecticides available to home gardeners to control white pine weevil adults. A licensed pesticide applicator can spray your trees for you. Because your trees are large, it is probably a good idea to hire someone with the training and equipment to spray them safely and thoroughly.
First Published: January 29, 2011, 5:00 a.m.
