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The white pine weevil, native to North America, is a very serious pest of white pines in the forest and can also stunt and disfigure trees grown for ornamental purposes. Host Trees: The trees which bear the most serious attacks are white pine, Norway spruce and jack pine. Other trees which are commonly attacked are pitch pine, Japanese red pine, western white pine, limber pine, foxtail pine and red spruce. Scotch pine, western yellow pine, mugho pine and black spruce sustain occasional attacks. The conifers which are rarely attacked include red or Norway pine, Himalayan blue pine, white spruce and Douglas fir. Life History: The adult white pine weevils are reddish-brown snout beetles about a quarter-inch long which are marked irregularly with patches of brown and white scales. The adults overwinter in litter on the ground and resume activity in April. The weevils prefer small trees 3 to 15 feet in height and seldom attack trees growing in the shade of other trees. The weevils feed on the bark tissue of the terminal shoots. They then chew small pits in the leader and lay their eggs there in May. The eggs hatch in 7 to 10 days and the legless grubs feed on the inner bark and tissues that produce tree growth. When several larvae are feeding, the shoot is soon girdled and dies. The grubs mature and pupate inside the leaders. Adult beetles emerge from late June to early September. Since spring egg-laying lasts more than a month, one shoot can contain larvae in various stages of growth. There is only one generation a year. After emergence, the beetles spread to new areas by flight.
Damage: The first sign of attack ranges from small, glistening droplets to resin oozing from tiny holes in the leader. This is caused by adult weevils that are feeding before egg-laying. As the terminal is girdled, the new shoot of the current year's growth withers and the tip bends over and turns brown. This stage of damage usually becomes noticeable about mid-June. Examination of the dead shoots will show the white larvae or pupae beneath the bark or in the wood and pitch. That year's growth is always killed, but two or three years of growth is commonly killed. The result is forked and crooked trees. Control: Preventive Pruning of Infested Leaders: At low infestation levels, prompt removal and disposal of infested leaders before the weevils emerge reduces the chance of population buildup. Prune infested leaders at a point below the tunneling grubs, at least including some green bark at the base of injury. Immediately burn, chip, or deeply bury cut leaders to destroy the larvae and pupae. Wilting leaders may be detected in June and July, but be sure to prune by mid-July to prevent emergence of adults. Pruning infested leaders early in the season before completion of new shoot growth promotes establishment of a new dominant branch and correction of the stem form. Early pruning also prevents the grubs from penetrating the node of the first whorl of branches, reducing the amount of weevil-induced damage. Cultural Control Methods: One alternative for preventive pruning is to avoid planting white pine or spruce in areas of high weevil hazard. Heavy clay soils and densely sodded fields might increase the chance of weevil attack as well. Partial shade (45-50%) helps protect the leaders from weevil attack by encouraging less preferred bark thickness and bark temperature. White pines can be planted beneath hardwoods or other conifer species. These trees then can be removed when the young white pine trees are 16 to 18 feet high. However, the disadvantage of partial shade--reduced rate of growth--must be kept in mind. Placing tanglefoot or other sticky substances on the wrapped leader, or wrapping tape with the sticky side out around the leader, may also prevent infestation. Never place sticky materials directly on the tree, as injury may result. Some homeowners report success in controlling larvae by pinching terminal leaders. Because larvae feed just beneath the thin bark, it is possible to squish the larvae within the stems before they severely damage the shoot. This approach is best used in early June on short trees with shoots that show the resin from adult feeding and oviposition. Chemical Control: If it is necessary to use chemicals against this insect, it is best to use materials with moderate residual activity or systemics which are absorbed by the plant. Call the URI Gardening Hotline or check your gardening center for currently registered insecticides. To spray effectively, thoroughly spray the top half of leaders before the buds open in the spring, before May 1 in most years. The leaders and especially the buds must be sprayed to the point of runoff. An extended sprayer rod that reaches leaders of taller trees helps speed up treatment. Corrective Pruning: Corrective pruning of injured tops should remove all but a single shoot (one of the smallest) at the topmost healthy whorl. This promotes healing, resumption of vertical growth and straightening of stem form. Corrective pruning may be postponed until the year after weevil injury to ensure that at least one lateral branch survives ice and snow damage or repeated weevil attacks the following year.
Adapted
from the University of Connecticut Integrated Pest Management Program
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