Hickory Bark Beetle

Hickory Bark Beetle - Scolytus quadrispinosus (Say)

J.D. Solomon - Research Entomologist (retired), Southern Forest
Experiment Station, Southern Hardwoods Laboratory, Stoneville, Mississippi

[From: Solomon, J.D. 1995. Guide to insect borers of North American broadleaf trees and shrubs. Agric. Handbk. 706. Washington, D.C.: U.S. Department of Agriculture, Forest Service. 735 p.]

Hosts. - Hickory, pecan, butternut. Prefers hickories, with shagbark and bitternut hickories listed most often in the literature (Goeden and Norris 1964, Hopkins 1912, McDaniel 1936). Pecan and butternut are less susceptible.

Range. - Quebec south throughout the eastern half of the United States to the Gulf Coast and westward to Wisconsin and Texas (USDA FS 1985, Wood 1982).

Adult. - Short, stout, thickly cylindrical, black to reddish brown, almost hairless bark beetle, 2.9 to 5 mm long (Goeden and Norris 1964, McDaniel 1936, USDA FS 1985, Wood 1982). Front tibiae have a short curved spine or hook. In males, venter deeply excavated; third abdominal segment armed with three spines, fourth with one large median spine. In females, venter less deeply excavated and without spines.

Larva. - Short, curved or slightly C-shaped, legless, yellowish white, wrinkled, and 5 to 8 mm long when mature.

Biology. - This bark beetle overwinters in various larval stages (Goeden and Norris 1964, 1965; McDaniel 1936). During spring, the oldest larvae transform to pupae in elliptical chambers, terminating each larval tunnel just beneath the bark surface. Beetles begin to emerge in May and continue through late August. Beetle populations and seasonal activity reach maximum during July and early August. Newly emerged beetles fly to the crowns of host trees where they feed mainly in terminal and twig growth for 10 to 15 days. Sexually mature beetles are then attracted to low-vigor trees, where they bore into the bark of trunks and branches. Here, females excavate 12- to 50-mm-long longitudinal upright egg galleries (without nuptial chambers) between the bark and wood. Mating on the bark surface and egg laying within the gallery continue throughout summer, with each female depositing 20 to 60 eggs singly in small niches along either side of the egg gallery. They cover the eggs with a plug of macerated frass. Eggs hatch in 10 to 12 days. Larvae mine outward and parallel to each other at first, but as they become larger, they diverge until the complex of mines resembles an engraving of centipede legs. Larval mines may extend 75 mm or more outward from egg galleries, thus severing the food and water conducting tissues of the tree. Mature larvae leave the cambium and bore into the outer bark where they pupate in cells. This beetle has one generation per year in its northern range and two in its southern range.

Injury and damage. - Reportedly, this bark beetle is the most destructive insect of hickory (Wood 1982). Adults feed in terminal growth, and larvae tunnel in trunks and large branches and cause greatest damage (Goeden and Norris 1965). Newly-emerged adults feed on twigs in the tree crown throughout summer. They make short food tunnels, about body length or less, in twigs, mainly at the base of axillary buds and leaf of current year and 1-year-old growth. Heavy twig feeding may cause yellowing and premature dropping of leaves and broken twigs scattered over the crown but rarely seriously weakens the tree. In fall and winter after initial attack, numerous round entrance holes 3 mm in diameter in the bark are often the only outward sign of attack. During winter and spring, woodpecker holes in the bark are good indicators of bark beetle infestation. In spring, sparse or yellowed foliage also indicates attack. Removing the perforated bark reveals the engraved peculiar designs of centipede-like galleries. When attacks are numerous, the galleries can girdle the tree. The foliage of heavily infested trees turns yellow and then red within a few weeks of attack, and finally brown as the tree succumbs. Trees stressed and weak from drought, fire, storm damage, disease, or other causes are most susceptible. Healthy trees are seldom attacked except when beetle populations are high. Although large infestations usually kill a tree, light ones may only girdle branches or a part of the trunk, causing top dieback.

Control. - Five species of hymeopterous parasites—Coeloides scolytivorus (Cresson), Ecphylus hickoriaeRohwer, Heterospilus scolytidae (Ashmead), Spathius trifasciatus Riley, and Trigonura ulmi Burks—help to reduce populations (Krombein and other 1979). Hickory bark beetles rarely attack healthy trees, so good cultural practices such as thinning, pruning, fertilization, and irrigation are important for orchard, ornamental, and other valuable trees (Goeden and Norris 1964, Hopkins 1912). The most effective control is to destroy trees harboring overwintering larvae during winter and spring. Infested trees can be cut and burned or submerged in water. An alternative is to peel the bark or spray with an insecticide before emergence begins in May or June. To protect valuable trees during epidemics, trunks and large branches should be thoroughly sprayed with an insecticide during early July.

Goeden, Richard D., and Dale M. Norris. 1964 The hickory bark beetle in Wisconsin—its biology and control. For. Pest Leafl. 6. Madison, WI: Wisconsin Conservation Department. 4 p.

Goeden, Richard D., and Dale M. Norris. 1965. Some biological and ecological aspects of ovipositional attack in Carya spp. by Scolytus quadrispinosus (Coleoptera: Scolytidae). Annals of the Entomological Society of America. 58(6): 771-777.

Hopkins, A.D. 1912. The dying hickory trees—cause and remedy. Circ. 144. Washington, D.C.: U.S. Department of Entomology, Bureau of Entomology. 5 p.

Krombein, Karl V., Paul D. Hurd, Jr., David R. Smith, and B.D. Burks. 1979. Catalog of Hymenoptera in America north of Mexico: Vol. 1, Symphyta and Apocrita (Parasitica). Washington, D.C.: Smithsonian Institution Press. 1,198 p.

McDaniel, E.I. 1936. Wood borers attacking deciduous trees and shrubs. Spec. Bull. 238, rev. East Lansing, MI: Michigan Agricultural Experiment Station. 52 p.

U.S. Department of Agriculture, Forest Service. 1985. Insects of eastern forests. Misc. Publ. 1426. Washington, D.C. 608 p.

Wood, Stephen L. 1982. The bark and ambrosia beetles of North and Central America (Coleoptera: Scolytidae), a taxonomic monograph. Great Basin Naturalist. Memoir 6. 1,359 p.