Poplar Twig Borer
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Hosts
Poplar. Eastern cottonwood is preferred (Hicks 1962, Morris and others 1975). The insect has been observed less frequently in swamp cottonwood and occasionally in hybrid poplars. It probably occurs in other poplar species.
Range
Eastern United States from Michigan to Massachusetts south to the Gulf Coast and west to Kansas, from Ontario and Quebec in southeastern Canada (Hicks 1962, Morris and others 1975).
Description
Adult
Slender, elongate longhorn beetle measures 10 to 14 mm long and 2.5 mm wide (Knull 1946). Antennae of male extend to apical fourth of elytra; female antennae slightly shorter. Coloration of head, prothorax, and ventral surfaces somewhat variable, ranging from yellow to black; legs yellow; and elytra usually black with outer margins yellow. Surfaces of elytra faintly ridged, with coarse punctures between ridges.
Egg
Creamy white, cylindrical, about 3 mm long and 0.5 mm in diameter.
Larva
Greatly elongate, slender; and nearly cylindrical except body segments, which gradually decrease in width toward rear. Body smooth, shiny, and pale yellow except for brown head, dark brown mandibles, and light brown thoracic shield. Mature larvae about 19 mm long.
Pupa
White and delicate and 8 to 12 mm long.
Biology
Adults emerge from April to June in the Deep South (Cook and Solomon 1976) and from mid-June through July in Michigan (Gosling and Gosling 1977). Adults feed extensively on the leaf midribs and large lateral veins after they emerge and mate. Females deposit eggs singly in niches chewed in the bark with their mandibles, typically selecting green shoots 5 to 13 mm in diameter. The niches remain visible for weeks after oviposition has ended. Eggs hatch in about 2 weeks, and larvae tunnel down the center of the stem, expelling little or no frass from the entrance holes. Larvae continue to feed into fall and overwinter in the shoots; feed briefly the following spring, and pupate within shoot galleries in March. Completed galleries range from 63 to 76 mm long and 3 to 5 mm in diameter. This twig borer completes a life cycle in 1 year (Cook and Solomon 1976, Morris and others 1975).
Injury and Damage
The beetles injure foliage by feeding on the leaf midribs and, primarily, lateral veins, indicating that females are ovipositing in nearby twigs (Morris and others 1975). Egg niches are most common on twigs 6 to 10 mm in diameter, but larvae may be found in stems up to 13 mm in diameter. Infested branches usually become enlarged and appear swollen or even gall-like and frequently become slightly crooked at the point of attack. Larvae of this borer do not make a series of holes at regular intervals through which to eject frass as do many other species of Oberea. Infested twigs and terminals may be stunted or die back and occasionally break, especially when worked by woodpeckers. Dissection of infested twigs reveals dark galleries, partially filled with frass and the yellowish, elongate larvae. Round, exit holes about 2.5 mm in diameter are left in the swollen portion of the stem, or slightly below, by the emerging beetles. Damage is most destructive when terminals are attacked, often producing crooked, deformed, or excessively branched trees. Infestations have been observed in nurseries, plantations, natural stands, and ornamentals, but damage has been greatest in young plantations, adversely affecting tree form in crop trees. Damage has been greatest in the lower Mississippi River Valley (Cook and Solomon 1976).
Control
Woodpeckers, especially the downy woodpecker, are among the most important natural controls (Cook and Solomon 1976). Local counts have estimated predation rates up to 60% during some years. Unidentified diseases kill sizeable numbers of larvae, especially during wet springs. Direct controls have rarely been needed.
Gallery
 Photo by Lawrence Abrahamson, State University of New York, 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.
