White Pine Weevil

WHITE PINE WEEVIL
Pissodes strobi (Peck)

Symptoms and Damage

The white pine weevil, which kills the top of conifers, is the most serious economic insect pest of white pine (see photo).

One of the first symptoms of attack in the spring is the presence of pitch flowing from the feeding punctures in the previous year's leading shoot. Beginning in late June the new growth on infested shoots starts to droop. Shortly thereafter the tops die and turn brown (image). Up to 2-3 years of top growth may be destroyed. The damage results in trees that are multi-topped, crooked, and of much lower value for sawtimber. Plantations, open-grown trees, and ornamentals are most heavily affected.

Hosts

Eastern white pine, Colorado blue spruce, Norway spruce, jack pine, Scotch pine, mugo pine, and native spruces.

Description and Life Cycle

The adults, which are small brownish weevils (snout beetles), become active, very early in the spring and move up on to the previous years' shoots to feed (image). They may frequently be found "nestled" around bud clusters at the tips of the shoots. The adults mate and lay eggs from early May through June in the bark of the previous year's leader. Upon hatching the young, legless grubs start tunneling downward under the bark. As they progress downward, the grubs gradually enlarge their tunnels until they eventually completely girdle the infested shoot and may kill the top 1 1/2 - 2 feet of the tree. Mature grubs, which are yellowish white, legless and about 7 mm long, bore slightly into the woody stem where they form fibrous chip cells in which to pupate. By mid-July some weevils have transformed to the adult stage and start to emerge from the shoots. The new adults feed for a time on new shoots and by October or November all have entered the ground litter at the base of trees to overwinter.

CONTROL MEASURES IN PLANTATIONS

Non-chemical: Preventive Pruning of Infested Leaders - At low infestation levels the prompt removal and destruction of infested leaders before the beetles emerge reduces the chance of weevil population buildup. Infested leaders should be pruned at a point just into healthy tissue below the tunneling grubs. Immediate destruction (burning is suggested) of cut leaders is necessary to destroy the larvae and pupae. Wilting of leaders may be detected from late June through July. Pruning should be completed by mid-July prior to adult emergence. Pruning before completion of shoot growth encourages secondary budding on the pruned leader and may result in faster terminal recovery. Early pruning may also prevent the grubs from penetrating the node of the first whorl of branches, thereby reducing the amount of injury. (See corrective pruning below as a followup).

More details on Pruning to Manage White Pine Weevil Damage

Cultural Methods - Refraining from planting white pine or spruce in areas of high weevil hazard are alternatives to be considered. Heavy clay soils and heavily sodded fields have high weevil hazard potential.

Partial shade will help prevent the leader from becoming an optimum egg laying site because of its effect in reducing bark thickness and bark temperatures. Overtopping hardwoods or other conifers may help protect young pine and spruce from weevil damage when the young trees are in greatest need of weevil protection. Density of the overtopping species should be managed to maintain about 45-50% shade until final release when the pines reach a height of 16 to 18 feet.

Closer spacing of trees in a plantation will stimulate height growth and natural pruning to permit faster recovery.

Chemical*: Spray the leaders in the spring before the buds open usually mid April in the Augusta area (before May 1 in most years). Make sure they are thoroughly covered. Most control operations will require 2-3 gallons of mixed spray per acre. Refer to the insecticide container label for mixing instructions. Spray all leaders each time an area is sprayed and whenever current damage approaches 10% of total number of trees. High value Christmas trees may need to be sprayed every year. Spacing and tree height will greatly influence the time necessary to spray each area. If spraying is done in the fall, add the recommended extender to the insecticide.

Diflubenzuron** or oxydemeton methyl** are currently registered for white pine weevil control in plantations, and require an applicator permit or license as noted below.

Diflubenzuron is an insecticide which is an insect growth regulator and acts by interference with deposition of insect chitin. Efficacy of diflubenzuron against the weevil is greatly enhanced by the addition of 6-7 oz. of dormant oil (without ethion) per gallon of spray mixture. Caution: dormant oil may cause a reduction of blue color when applied to blue spruce.

CONTROL MEASURES IN ORNAMENTALS

Non-chemical: Preventive pruning of infested leaders (see above) followed by corrective pruning described below, can provide good control and management of this pest around landscapes, especially in ornamentals that are regularly pruned or cut back or in a hedge.

Corrective Pruning - Injured tops should be pruned in such a manner that all but a single dominant shoot at the topmost healthy whorl are removed to encourage formation of a new leader. Corrective pruning may be postponed until the year following attack to make sure that at least one live lateral will survive possible ice-snow damage, or a possible second successive year of weevil attack.

Chemical*: On ornamental conifers around the yard homeowners can use chlorpyrifos or methoxychlor to control white pine weevil particularly when large numbers of ornamentals need to be maintained. Use three applications applied at 7-day intervals, starting before the buds open (mid-April in Augusta). Be sure to check the label for use instructions on ornamental trees. Spray the leaders at the recommended dilution rate.

*NOTE: These recommendations are not a substitute for pesticide labeling. Read the label before applying any pesticide. Pesticide recommendations are contingent on continued EPA and Maine Board of Pesticides Control registration and are subject to change.

**Diflubenzuron and oxydemeton methyl are restricted-use pesticides to be sold only by licensed dealerships to certified pesticide applicators.

Caution : For your own protection and that of the environment, apply the pesticide only in strict accordance with label directions and precautions.

MAINE DEPARTMENT OF CONSERVATION
Maine Forest Service - Forest Health and Monitoring Division
April 2003

More information on the White Pine Weevil
Return to Insects and Diseases Fact Sheets Index