White pine weevils
What are they?
The white pine weevil causes extensive damage to many types of evergreen trees. It is also known as the Engelmann spruce weevil and the Sitka spruce weevil.
Adult weevils are about 8 mm (.31 inches) long. These beetles are dark brown with white and tan patches on their backs. They have the characteristic and prominent hooked snout of the weevil.
White pine weevils produce one generation a year. The adults overwinter in the “duff” or debris beneath trees, and in the spring they climb up to the top of the leader (the vertical stem at the top of the trunk) to feed and mate. Weevils can also spread by flying on warm sunny days. Their eggs are laid mainly from late April to early June in small holes made in the bark of the previous year’s tree leader.
The eggs hatch after 10 to 14 days and the larvae will begin to feed as a group, forming a ring around the leader stem as they eat their way downwards, first in the inner bark and then between the wood and the bark. This feeding action cuts off the water flow, causing the deformity and death of the leader stem. Trees will lose up to two or three years of height growth in the five or six weeks of feeding.
Finally, the larvae will build pupal cells in the stem, where they remain inactive for five to six weeks. They emerge as adults from late July until early September. The adults will feed on nearby tree branches until it is time to move to the duff for over-wintering.
Should I be concerned?
This harmful pest attacks at least 20 different species of trees. In eastern Canada, it prefers the eastern white pine, the jack, Scotch and red pines, and the Norway spruce. In western Canada, it attacks mainly the Sitka, white, and Engelmann spruce. Ornamentals like the mugho pine and blue spruce are also susceptible.
Repeated infestations may stunt the growth of trees and also cause deformed or forked trunks, a serious problem for Christmas tree growers. Forked trunks are created as one or more side branches of the tree assume the function of the dead leader stem.
Trees growing in the open without the canopy cover provided by larger trees appear to suffer greater deformation of their trunks. Small trees will sometimes die as a result of an infestation. Also, damaged trees are more susceptible to disease, especially heartwood rot.
How do I know if I have a problem?
A shepherd’s crook in the new expanding leader is a classic sign that an evergreen tree is infested with the white pine weevil, although other pest attacks may look similar. The curling and death of the leader stem of the tree indicates the presence of larvae in that stem. The needles on the affected stem will turn yellowish-green, then reddish-brown, and will eventually fall off.
This damage generally appears in mid-June or early July. The first sign of attack, though, is the appearance of droplets of resin oozing from tiny feeding holes in the leader stem early in the spring.
How can I get rid of white pine weevils?
- Physical control
The white pine weevil prefers open-growing trees 1.5 to 8 m (6 to 25 feet) tall, in sunny locations. Changing the environment can help prevent or reduce infestations.
- Grow evergreen trees under a canopy of 50% shade to make the leader shoots less attractive to the white pine weevil. Use a canopy of hardwood trees or artificial screening to achieve shade cover.
- Choose well-drained sites for planting new trees, or improve drainage for existing trees. (It has been noted that imperfect drainage results in trees being more severely attacked by the weevil.)
- Space plantation pine trees close together, 2 × 2 m (6 × 6 feet) apart, to encourage height growth and natural thinning of weevil-attacked trees.
- Prune infested trees. This can be an effective control for ornamentals or small plantations if it is done as soon as possible after the first signs of infestation are noticed, usually when top wilting is first spotted. Prune close to the topmost unaffected whorl of branches. Immediately burn or destroy the pruned branches and leader stems to prevent weevil emergence.
- Band tree trunks and the base of leader stems with a sticky tape or a sticky substance applied to a band. Do not apply sticky substances directly to tree bark. Unfortunately, because adult weevils are capable of flying on warm, sunny days, using this method alone may not provide complete control.
At present the only product registered to control the white pine weevil is for use on Sitka spruce trees only, and must be applied by a certified professional. Also, the product cannot be applied in residential areas, including parks, schools, and playgrounds. It is a systemic insecticide and should be applied at the time of egg laying, generally early May.