Control white pine weevil in early spring
If you need to control white pine weevil, your best opportunity is early in spring.
April 13, 2018 - Author: Dave Smitley, Michigan State University, Department of Entomology
White pine weevil larvae kill the terminal leader and the top two to four years of growth on many varieties of spruce, as well as white and Scots pine trees. Damage from white pine weevil can be distinguished from other types of injury by the somewhat curled shape of the leader, sometimes referred to as a “shepherd’s crook” (see photo). The curling effect is caused by more weevil larvae feeding on one side of the leader. If you had weevil damage last year and need to control the weevils, do it early this spring.
The adult weevils overwinter on the ground, protected and insulated by the litter (fallen needles). Once they warm up, the weevils move up to the tops of the trees in late afternoon or early evening and feed on the terminal leader. Each time a female weevil makes a feeding wound on the terminal, she lays one to four eggs in the wound. Those eggs will hatch within a few weeks and the larvae chew their way through the bark. They will feed in the phloem under the bark for several weeks, pupate and then emerge as new adults around mid-summer.
If you need to control white pine weevil, your best opportunity is early in spring. Insecticide should be applied to the terminal leader once it begins to warm up, somewhere around 25 to 65 growing degree-days base 50. This is happening now around Lansing, Michigan, and will probably not happen until the end of April around Cadillac, Michigan.
If you can direct your spray to the leader (uppermost branch pushing upwards), and avoid spraying the rest of the tree, you will conserve beneficial species like predatory mites. It’s better to be a bit early than to be late with this application. Use a persistent product because the adult weevils don’t all warm up at the same rate. Weevil adults could feed on the terminals for three weeks or perhaps longer.
Damage from white pine weevil can be prevented by spraying the upper trunk and terminals of spruce and pine in early to mid-April and again two weeks later with products that contain bifenthrin, permethrin or cyfluthrin. Apply these products at rates given on the label for bark beetles or borers. An alternative treatment is basal application of imidacloprid (Bayer Tree and Shrub Insect Control) at the rate given for borers. To be most effective, apply applications in October, March or early April to allow time for tree roots to absorb the insecticide and move it up into the tree. A basal soil drench with imidacloprid at this time may be too late to protect against white pine weevil this year.