Pine root collar weevil
Owners and managers of pine plantations, even Christmas tree plantations, should be aware of a potential pest, pine root collar weevil. They're hard to notice until tree mortality starts showing up. The usual targets are pines in widely spaced plantations ( like 20' x 20'), plantations with more than one species of pine growing in them and any type of site with Scots pines planted on them. Damage occurs to planted pines, not naturally- seeded pines, because seedlings are often planted too deep, giving the trees a larger root collar area for the insect to attack.
Injury to the pines is caused by weevil larvae feeding in the inner bark and wood below ground in the root collar and between the major roots. The soil and bark near the root collar becomes blackened and soaked with pine pitch. Larvae can be found in tunnels in the bark and pitchy soil. They're easier to find if you use a large knife to pry and slice into this mess. Trees are weakened and girdled at the ground level and may fall over or die or both. The canopy fades to pale green to yellow then red.
These are hearty bugs; their life cycle lasts two years and some live three years. Eggs are laid any time during the growing season on the root collar or in the soil adjacent to the root collar. Larvae (whitish grubs with brown heads) burrow into the inner bark and feed until the weather turns cold and then they become inactive. In the spring, they resume their feeding. Pupation occurs in chip cocoons made up of sawdust-like frass. Adults emerge in thirty to forty days and feed on pine shoots until fall then overwinter in the duff or bark crevices. Adult weevils are not strong fliers and frequently move only a short distance to attack a new host tree. Sometimes they remain on the same tree. Each female lays ten to seventy eggs during the next two growing seasons.
Weevils are sensitive to light and temperature; they like it dark, cool and humid. Management recommendations therefore try to create conditions that thwart their environmental needs.
1. When planting trees, minimize the size of the root collar area by matching the dirt line at the root collar with the soil level in the planting hole. 2. Allow more light and heat to reach the root collar area by pruning away the lower whorls of branches. You only need to prune up three feet. 3. Create a drier and hotter root collar area by raking away the duff and scraping away surface soil (down one or two inches) in a circle about one foot around the base of the tree. 4. Avoid mulching as this creates a dark, cool and humid environment. 5. Avoid planting mixed pine species because the risk of damage is greater if you do so. Scots pine is most susceptible and white pine is least susceptible. 6. Plant at closer densities so that crown closure occurs as soon as possible in the plantation. 7. As a last resort, use an insecticide to kill adult weevils in the soil and root collar area. The threshold for this treatment is finding fifty to seventy percent of the trees with one or more weevil larvae.
Cultural control methods do work. In two jack pine plantations, that had twenty by twenty spacing of the trees, pine root collar weevil damage incidence dropped thirty percent in one year after doing basal pruning.