Insect Profiles: White Pine Weevil (Pissodes strobi)
White pine weevil (Pissodes strobi) is an insect that attacks and deforms at least twenty different tree species. The insect is sometimes referred to as spruce weevil in the western United States. White pine weevil is indigenous to North America. Eastern white pine is the insect’s preferred host, though infestations have also been reported on various other pine and spruce trees. White pine weevil poses a serious threat to spruce and pine trees across the United States, and Canada.
Distribution & Habitat
White pine weevil infests pine and spruce trees across the United States, and Canada. It is especially common in the northeastern United States.
White pine weevil attacks at least twenty tree species, including eastern white pine, western white pine, Jack pine, Norway spruce, foxtail pine, Japanese pine, limber pine, and Scots pine. Eastern white pine is the most suitable host for insect development. Infestations also occur on Austrian pine, table mountain pine, mugo pine, Jeffrey pine, pitch pine, red pine, black spruce, blue spruce, red spruce, white spruce, and Douglas-fir, albeit with less frequency. White pine weevil is attracted to trees with vigorous leaders that are rooted in open environments. Pine and spruce trees become susceptible to infestation once they have exceeded a height of three feet. Host trees are generally four to twenty feet tall, and at least ½ an inch in diameter.
White pine weevil produces one generation per year. Adults overwinter in the debris that collects beneath host trees. In early spring, adult females emerge, and ascend the trunks of host trees. The females navigate close to the terminal bud cluster, and begin constructing small urn-shaped egg cavities in the bark. The females lay their eggs in these minute cavities. Eggs may be laid individually, or in clusters of two or more per cavity. On average, each female lays one to two hundred eggs. To conceal the eggs from predators, the females seal the enclosures with a dark brown cap composed of their excrement. Most adults mate in fall, and overwinter, but propagation may continue into spring. Often, two or three mating pairs will occupy a single leader. When this occurs, the leader may become laden with eggs.
The insect’s eggs are nearly half an inch in length. The eggs hatch in six to fourteen days, depending on the weather conditions. As the eggs hatch, they reveal masses of small grub-like larvae. The larvae are white, and legless. When the terminal leader is heavily infested, the larvae gather together and feed side by side, creating a ring around the stem. The larvae burrow into the leader, penetrating into the inner bark, where they feed for five to six weeks. Within the inner bark, the larvae pass through four instars. Once mature, they construct pupal cells in the pith and wood of the stem. These pupal cells are formed from small strands of wood lining. Once the pupal cells have been constructed, the larvae pupate within them for five to six weeks.
From late July to September, the adults chew small holes through the cocoon and bark, and emerge onto the host tree. Adults measure ¼ of an inch in diameter. They are mottled brown, and have two sets of wings. The front wings are covered with white and tan scales, which are arranged in large and small dots. A white patch is prominent at the apex of the front wings. This coloration enables the insect to blend in with host trees. On the insect’s head, small antennae arise from a snout-like beak. In fall, the young adults feed on the host tree’s buds and bark tissue. During this period, many of the adults mate. As the temperatures decline, the adults seek shelter in the litter beneath the host trees to hibernate. Until the temperatures become consistently cool, the adults may continue to feed at the base of the tree during the day. Adult white pine weevils generally live for one year. In cooler regions, they may live for two to three years.
Symptoms of Infestation
In spring, the first visible symptom of infestation is the glistening of resin exuded from the puncture wounds made by the adults on the previous year’s growth. The persistent feeding of the larvae girdles the stem, causing the new shoot to wilt, and the needles to turn reddish-brown. Wilting is most conspicuous in June across the southern part of the insect’s range. In the northern and western part of its range, wilting becomes most apparent in late summer or early fall. Severe infestations often result in extensive foliar dieback. One or more lateral branches may bend, and grow upwards, assuming the terminal role. This causes the tree to become crooked, and malformed. Circular holes around an inch in diameter can be observed in the bark once the adults have emerged.
Effects on Trees
White pine weevil reduces tree vigor. Tree mortality is rare, but can occur in small trees. Weevil attacks generally reduce tree height growth by 40% to 60%. Stem deformation is common, and may result in the formation of wood defects such as tree forks, compressed wood, and bark knots. The loss in tree vigor renders trees more susceptible to heart rot organisms like Phellinus pini Ames, a serious heart rot disease of older eastern white pines, and invasion from wood decay organisms.
Prevention & Management
- Weevil populations may be limited by the insect’s natural predators. These include a dipteran predator, Lonchaea corticis, and two hymenopteran parasites, Eurytoma pissodis, and Dolichotomitus terabrans nubilipennis. Birds, such as the white-breasted nuthatch, the downy woodbecker, chickadees, grosbeaks, and warblers feed on the larvae and pupae. Small rodents, such as voles, field mice, and shrews destroy adults by hibernating in the litter.
- Maintain tree vigor through sound cultural practices. Ensure that trees are sufficiently watered, especially during extended periods of drought. Apply a layer of organic mulch around the base of vulnerable trees to improve soil quality, moderate soil temperature, and retain soil moisture.
- Reduce the potential for infestation by planting susceptible trees in dry locations. Growing pine under a hardwood overstory will reduce the incidence of weevil attacks by slowing the growth of the leader, and reducing sun exposure. This will prevent trees from becoming too vigorous. While this can be detrimental to overall tree growth, it will minimize the dispersal of the adults in fall, and help to stave off the adult females.
- Prune infested terminals and branches before the adults emerge. Pruning cuts should be administered at the first sign of wilting. This will prevent growth loss, and reduce overwintering adult populations.
- When a terminal leader becomes infested, and must be removed, pruning of lateral branches and tree forks can assist the tree in establishing a healthy main stem. When the branches are removed correctly, a stiff rod or stake can be attached to the tree top. A strong lateral branch should be selected, and tied to the rod or stake. Remove competing branches to prevent multiple trunks from forming, and allow the lateral branch to assume the terminal role.
- Banding trees with a sticky substance may deter adults from ascending the trunk. Avoid placing banding materials directly on the bark.
- Chemical treatments can help limit weevil populations, though they are only effective against adults. Treatments can be performed in spring as the adults emerge from hibernation, or in fall as the young adults appear. Applications in spring should thoroughly cover the leader, and upper branches. Adults are especially vulnerable to chemical control in fall when they are feeding on new growth in the upper crown.
Photo courtesy of Maine Department of Agriculture, Conservation and Forestry.