Poplar Borer
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Hosts
Poplars. Quaking aspen is favored in the northern range (Peterson 1947), whereas eastern cottonwood is the major host in its southern range (Morris 1963). Balsam poplar is moderately attacked (Drouin and Wong 1975). Other poplar species and cultivars are attacked to varying degrees (Peterson 1947).
Range
A serious pest of poplars throughout most of the United States and Canada; restricted in distribution and abundance in its western range (Morris and others 1975).
Description
Adult
Moderately robust, elongate, longhorn beetle ranging from 20 to 30 mm long (Morris and others 1975). Females slightly more robust and stouter than males. Body color grayish blue and heavily striped with fine brown dots that overlay a faint yellow pattern. Antennae about as long as body.
Egg
Slender, creamy white, and about 3 to 4 mm long (Peterson 1947).
Larva
Legless, elongate, cylindrical, yellowish white, and 30 to 38 mm long.
Pupa
Yellow white; 20 to 35 mm long.
Biology
Adults emerge from infested trees in May and June in the South and as late as July and August in the North (Hofer 1920, Morris and others 1975) and feed on foliage and bark of tender shoots of host trees. About a week after emergence, mating begins. Oviposition starts a few days later with single eggs (occasionally two or three) being deposited in niches cut into the bark of the trunk. After the eggs have incubated about 2 weeks, the eggs hatch, and larvae begin feeding on the bark tissue and later move into the sapwood. After overwintering behind frass plugs in the galleries, the larvae resume activity in spring, enlarging and extending their tunnels (Drouin and Wong 1975). The life cycle varies from 2 years in Mississippi (Morris and others 1975) to 3 to 5 years in the borer's northern range (Drouin and Wong 1975, Peterson 1947). Pupation occurs from April to July, depending on location, and lasts 2 to 3 weeks. The adults emerge from the entrance sites. Although trees as small as 4 cm in diameter may be attacked, most infestations occur in stems 8 cm in diameter and larger. Open-grown trees and those growing in sparse understocked stands with unshaded or partially shaded trunks are most attractive to females for oviposition (Graham and Mason 1958).
Injury and Damage
Wet spots with oozing sap mixed with fine frass on the trunk are the earliest signs of the presence of poplar borers. Although attacks may occur singly, they are generally clustered, especially in the southern range. Frass becomes fibrous and excelsior-like as the larvae develop, and large amounts of course frass may be seen at tunnel entrances, in bark crevices, and in piles around the base of trees. Feeding by a cluster of larvae results in the bark splitting and breaking away irregularly. Completed galleries 10 to 20 mm in diameter typically extend obliquely upward into the sapwood for 5 to 8 cm, then straight upward for another 10 to 18 cm, for a total length of 15 to 25 cm. However, galleries are often crooked because they intersect or avoid galleries of other larvae (Peterson 1947). Tree injury may be characterized by woodpecker excavations in the vicinity of larval feeding. Scar tissue may be evident for many years when injured areas are overgrown (Morris and others 1975). Small trees are occasionally killed by larvae girdling beneath their bark. Large trees are seldom killed outright, but clusters of larvae tunneling close together may riddle portions of trunks, seriously weakening them structurally. Woodpecker excavations and decay fungi further weaken trees so that serious wind breakage occurs. Surveys of quaking aspen showed that 64% of the trees in southern Michigan and 53% of the trees at five locations in British Columbia had been attacked by poplar borers (Contrell 1962, Graham and Mason 1958). In a survey of 10 commercial cottonwood plantations in the lower Mississippi River Valley, infestation rates ranged from 2 to 45% and averaged 20% (Nebeker and others 1985). In timber stands, the value of infested trees sawn for lumber may be greatly reduced by defect and degrade (Morris and others 1975). Moreover, the egg niches provide favorable infection courts for the hypoxylon canker disease.
Control
Natural mortality of the poplar borer is frequently high. Two species of hymenopterous parasites--Dolichomomitus irritator (Fabricius) and Habrolepoidea depressa Girault--and one dipterous parasite Ptilodexia canescens (Walker) have been recorded (Arnaud 1978, Carlson 1979, Gordh 1979). In Colorado, egg and larval parasites, predators, and fungus disease account for annual mortalities of 30 to 38% (Hofer 1920). In Mississippi, up to 65% of the larvae in young plantations are excavated by woodpeckers. Studies in Canada revealed that natural controls--including egg infertility, climate, excessive sap flow, and six species of parasites, predators, and disease--destroyed 81% of the population of this borer (Peterson 1947). Ten other species of parasites have been reported in western Canada (Drouin and Wong 1975). Cultural practices such as avoiding poor planting sites and maintaining healthy trees can markedly reduce incidence of the poplar borer. Borer infestations vary directly with stem diameter and inversely with stocking (Ewan 1960). But in aspen stands, periodic removal of infested trees proved worse than no cutting because the reduction in stand density resulted in higher infestations (Peterson 1947). In managing aspen, therefore, the recommended practice is to maintain well-stocked stands and clearcut at maturity (Ewan 1960). Insecticides may be necessary for protecting trees in parks and urban areas. Systemic insecticides have shown some promise for controlling damage when regenerating balsam poplar in Canada (Drouin and Wong 1975).
Gallery
 Photo by James Solomon, USDA Forest Service, Bugwood.org |  Photo by James Solomon, USDA Forest Service, Bugwood.org |
References
Solomon, J.D. 1995. Guide to insect borers of North American broadleaf trees and shrubs. Argic. Handbk. 706. Washington, DC: U.S. Department of Agriculture, Forest Service. 735 p.
