White Pine Weevil

White Pine Weevil (Pissodes strobi) is probably the most serious pest of white pine in the area. The attacks of this insect stunt or distort trees and can kill two or three year’s growth.

The white pine weevil is a small (3/16 to 1/4 inch), brownish beetle with a curved snout that is almost as long as the rest of the body. The wing covers are spotted with brown and white scales. White pine weevil eggs are small, pearly white and translucent.Mature larvae are yellowish-white, legless grubs with brown heads. They are 3/16 to 1/4 inch long. White pine weevil pupae are creamy white and 3/16 to 1/4 inch long. This insect is also known as the sitka spruce weevil and Englemann spruce weevil.

Rice weevils larval stage are legless, humpbacked, white to creamy white, with a small, tan head. Weevils in the pupa stage have snouts like the adults.

Life Cycle: Adults spend the winter in the leaf litter under or near host trees. On warm spring days they fly or crawl to the leaders of suitable hosts usually during the period from mid-March through April. Most feeding by adults is done within 25 cm of the terminal buds. From mid-April through early May, females mate and each deposits one to five eggs in feeding wounds . Hundreds of eggs may be deposited in one terminal leader.

The eggs hatch in about seven days. When the terminal is heavily infested larvae feed side by side in a ring encircling the stem. They feed downward on the inner bark of the leader. Larvae reach maturity in mid to late July and pupate in the infested terminal. The pupal chambers called “chip cocoons” are filled with shredded wood and can be found inside the terminal at this time. Adults emerge in 10 to 15 days through small holes at the base of the dead terminal of the host plant usually in late July and August.

During this time feeding by adults is not considered important since little is done before they enter the leaf litter to overwinter. The white pine weevil has one generation per year.

Habitat: White pine weevil spends the winter in the adult stage, under dropped needles and other sheltering debris, usually very close to previously infested trees. In spring, the adults crawl to trees, although some may fly when temperatures exceed 21.11 degrees C.

Damage:The first symptom evident from successful attack by this pest is glistening droplets of resin on terminal leaders of the host plant in late March and April. This is the result of punctures made by adults in the process of feeding and cutting egg-laying sites.

Injury to eastern white pine and some species of spruce is usually confined to the previous year’s terminal leader . Damage on Scots pine, Colorado blue and Serbian spruces often extends downward through two or three year’s growth. Infested trees are seldom killed.

Most damage is done by the larval stage. Larvae are found just under the bark of infested terminals from May through July. Larvae chew and burrow completely around the stem causing the current year’s growth to wilt, droop, and eventually die. One or more side branches (laterals) may then bend and grow upward to take over as the terminal leader. At this point the tree is now permanently crooked. For several years after successful attack by this pest, a few more laterals may grow as leaders. This condition may result in a forked tree.

Management: The standard approach to manage white pine weevil is to spray insecticides in spring so that they cover the terminal. This treatment is directed at the overwintered adults to kill them before they lay eggs. Current insecticides useful for this treatment would include products that contain bifenthrin (Talstar, Onyx), permethrin (Astro), or cyfluthrin (Tempo). Rates of use should be to the high-end of what is labeled, such as for bark beetles or borers.

Timing is very important but difficult to determine. Sprays should be applied shortly before adults begin to feed on the terminal and lay eggs. The adults will likely begin to renew activity and move to trees during sunny days in April, when temperatures exceed 70 degrees F. In the Midwest, adult activity is often coincident with forsythia bloom; this may not be an available indicator in Colorado where forsythia is more rarely planted. A second application, two weeks after the first, is recommended to maintain coverage of the terminal.

Other spray treatments may also help manage this insect during outbreaks. Treatment of the top of the tree during midsummer, after adults have emerged from the wilted terminals, can kill weevils as they feed in this site, reducing overall numbers that may cause problems in the subsequent year. Also, as many weevils crawl to the trees in spring, treatment of the lower trunk at the same time spring terminal applications are made may also help kill some of the migrating overwintered insects.

Soil drench/injection treatments of the systemic insecticide imidacloprid (Merit, Touchstone, etc.) can prevent white pine weevil injury. These applications should be made in fall to allow sufficient time for uptake of the insecticide to the terminal in spring. Use of soil-applied imidacloprid also requires that the soil be sufficiently watered for a couple of weeks, to allow initial uptake by roots.

Pruning out terminals that are currently infested by larvae can be used to reduce white pine weevil populations. However, the time for effective use of pruning is brief, limited to the late spring/early summer period between when terminals show evidence of infestation (wilting) and the insects emerge from the terminal, as evidenced by exit holes. The pruned area should be limited only to the part of the terminal that is infested, which often may not extend to the next set of branches. The pruned material should be removed from the site and disposed since weevils may continue to develop in prunings.

The primary injury by white pine weevil is esthetic, deforming the growth of the tree as co-dominant uninjured side branches later grow to form multiple new leaders. This can be prevented by forcing a single leader to be dominant, suppressing the others, which will allow the tree to ultimately restore nearly normal form. This can be achieved by selecting as the new leader the most vigorous side branch and pinching the terminal buds of the other side shoots.

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