Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. Insects and Diseases of Cottonwood. New Orleans, LA. U.S. Department of Agriculture, Forest Service; Southern Forest Experiment Station; State and Private Forestry Southeastern Area. Gen. Tech. Rep. SO-8. 1975. 41 p.
The poplar borer is a serious pest of cottonwood and other poplars throughout the United States and Canada. It attacks the trunks of trees 3 years and older. Clusters of larvae tunneling close together may riddle portions of the trunk. Woodpecker excavations and decay fungi further weaken damaged stems. Badly infested trees may be so structurally weakened that wind breakage occurs. The value of infested trees sawn for lumber may be greatly reduced.
The most conspicuous early sign of attack is the appearance of sap spots on the trunk. Later, oozing sap mixed with fine frass is extruded through small openings in the bark. Although attacks may occur singly, they are typically clustered. After the bark is mined by a cluster of larvae, it begins to split or break irregularly as radial growth progresses. As the larvae grow, the frass becomes fibrous and excelsior-like. Coarse frass is usually conspicuous in large quantities at gallery entrances, lodged in bark crevices, and in piles around the base of the tree. Woodpeckers frequently excavate several holes in the wood and remove much of the loose bark in the vicinity of a larval cluster. Scars resulting from overgrown attacks remain for several years.
The adult is a long-horn beetle, elongate, moderately robust, and ranges from 7/8 to 1 1/8 inches (22 to 30 mm) in length. The body is grayish blue and heavily stippled with fine brown dots and yellowish spots. The antennae are about as long as the body. The eggs are slender, creamy white, and about l/8 inch (3 mm) long. The larvae are legless, elongate, cylindrical, yellowish white, and 1 1/8 to 1 3/8 inches (30 to 35 mm) long. The pupae are yellowish white.
Adult beetles appear during late spring and early summer. After feeding on the tender shoots of young cottonwood they mate and begin laying eggs in niches cut in the bark. Eggs hatch in 2 to 3 weeks, and the larvae begin mining beneath the bark. Later, they tunnel into the sapwood and heartwood and produce extensive galleries. The larvae overwinter behind frass plugs within the galleries. The pupal stage lasts 2 to 3 weeks. The new adults exit through the gallery entrances. Two years are required for the life cycle.
Parasites, predators, and disease help keep infestations in check. Considerable natural mortality also occurs among eggs and early instars because of heavy sap flow, which is enhanced by high soil moisture and tree vigor. Woodpeckers destroy many grubs in some stands and are probably the most important natural enemy once the larvae have established galleries in the wood. Brood trees, scarred by repeated attacks and harboring heavy populations of borers, should be removed to prevent or reduce spread to uninfested trees.
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