Conifer bark and twig weevils, pine weevils—Pissodes spp.
At least ten Pissodes species of conifer twig weevils, or bark weevils, chew and feed on conifers in California. Hosts differ by the species of weevil and include Douglas-fir, pine, spruce, and true fir.
Monterey pine weevil, Pissodes radiata, and certain other Pissodes spp. as larvae mostly feed beneath bark of the root crown and basal trunk of conifers that are already injured, severely stressed, or dying from other causes. Species such as white pine weevil, P. strobi, that feeds on young pine and spruce mainly bore in and kill shoot terminals of otherwise healthy hosts.
Adults and mature larvae of Pissodes species are 1/6 to 1/4 inch long. Adults have an elongated head and mouthparts and distinct, bent antennae. Adults can fly, unlike those of certain other weevils.
Adult white pine weevils are blackish with reddish-brown and white blotches and spots on the wing covers (elytra). Adults of some other Pissodes species are mostly brownish overall, have differently colored markings (e.g., yellow), or both. The oblong eggs are pale yellowish and occur under bark.
The larvae are white to pale yellowish with a brown head capsule and occur under bark. When disturbed or exposed they commonly have a C-shaped posture.
Note there are numerous potential causes of bark cankers and dying conifer shoots. See the table of symptom comparison in Pest Notes: Pitch Moths to help discriminate the likely cause(s) of damage to pines. To confirm that larvae of white pine weevil or other Pissodes spp. are the cause, cut or peel off bark from affected shoots and look for the presence of the insects, their tunnels, or the pupal chambers covered in wood fiber that persist after the adults emerge. Feeding by woodpeckers and their flaking off of bark can indicate that boring insects are present.
Weevils develop through four life stages: adult, egg, larva, and pupa.
In about April and May, adult white pine weevils puncture feed on terminal growth of the preceding year. In May and June, the females lay eggs in these terminals. The developing larvae feed beneath bark and the twig-boring species girdle and kill the main terminal (leader), but not until the new growth of the current year has elongated considerably. Small areas of oozing pitch may be visible on infested terminals. By late summer, the infested shoots wilt, then needles turn reddish and may drop prematurely. From August through fall, new adults emerge, chew and feed on shoots, then overwinter in organic litter on the ground. Some portion of the population may overwinter as mature larvae beneath bark, then in spring pupate and emerge as adults.
There is one generation per year. Multiple life stages can be present at the same time during much of the year.
Adults of Pissodes species chew foliage of small shoots, generally causing only minor damage. Larvae of white pine weevil feed and chew in the terminal shoots of pine and spruce that are about 6 to 30 years of age. These larvae bore in inner bark, vascular tissue, and the centers of small branches, stems, or treetops, killing the most-recent 2 years of terminal growth and stunting plant growth. Side shoots become new main terminals, resulting in severely distorted, bushy plants of reduced aesthetic quality.
Some other twig-boring Pissodes kill only the current year's terminal growth, so the plant is less severely affected. Species that primarily feed under trunk bark are generally not the primary cause of tree damage; trees stressed or injured from other causes become attractive to the egg-laying adult females, which oviposit in bark near the soil line. The emerging larvae chew and bore in the root crown and trunk, causing unsightly cankers on bark.
Provide valued plants with good growing conditions and proper cultural care. When Pissodes infest the lower trunk, identify and if feasible remedy whatever is the primary cause of unhealthy conifers that caused trees to become attractive to bark weevils.
To help control white pine weevil and other shoot-boring Pissodes, prune off and dispose of infested terminals before the weevils mature and emerge as adults. In plantations of pine and spruce, such as Christmas tree farms, spacing trees more closely during planting can reduce the proportion of terminals that become infested. Insecticide application is generally not recommended for these pests in landscapes.
See White Pine Weevil from Oregon State University for more information and photographs.
Adapted from Pests of Landscape Trees and Shrubs: An Integrated Pest Management Guide, University of California Statewide Integrated Pest Management Program (UC IPM).
Terminal killed by boring larvae of white pine weevil.
Emergence holes of adult twig weevils.
Adult white pine weevil.
Larva of white pine weevil and its tunneling damage, exposed.
Pissodes cocoons under bark, exposed.