Hickory Bark Beetle Scolytus quadrispinosus Say

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Cucujiformia
Superfamily	Curculionoidea
Family	Curculionidae
Subfamily	Scolytinae
Tribe	Scolytini
Genus	Scolytus

Scientific Name

Scolytus quadrispinosus

Common Name

hickory bark beetle

Solomon, J. D.; Payne, J. A. A guide to the insect borers, pruners, and girdlers of pecan and hickory. Gen. Tech. Rep. SO-64. New Orleans, LA: USDA Forest Service. Southern Forest Experiment Station; 1986. 31 p.

Importance

The hickory bark beetle, Scolytus quadrispinosus Say, is a pest of pecan and hickory and reportedly butternut and black walnut (McDaniel 1933, Goeden and Norris 1964). However, it shows a strong preference for the hickories and is reported to be the most important pest of this group of trees (McDaniel 1933). It is found from Quebec southward throughout the eastern half of the United States to the Gulf Coast States and westward to Texas. Both adults and larvae produce galleries between the bark and wood of trunks and branches. When attacks are numerous, the galleries soon girdle the tree. Trees stressed or weakened from drought, fire, storm, disease, or other cause are most susceptible to attack. Vigorous trees are seldom attacked except when large beetle populations are produced from nearby brood material. Although heavy infestations usually kill the tree, light infestations may only girdle branches or a portion of the trunk, causing top dieback.

Description

The adult is a short, 4- to 5-mm long, stout, thickly cylindrical, black to reddish-brown, almost hairless beetle (Goeden and Norris 1964, McDaniel 1933). There is a short curved spine or hook on the front tibia. The venter of the male is deeply excavated; the third abdominal segment is armed with three spines, the fourth with one large median spine. The venter of the female is without spines. The egg is ellipsoidal, creme colored, and barely visible to the naked eye. The larva is short, curved or slightly C-shaped, legless, yellowish white, and 5 to 8 mm in length when mature. The body appears wrinkled. The pupa is compact, fragile, and white.

Evidence of Infestation

Damage occurs from feeding in terminal growth and by tunneling in trunks and large branches for breeding purposes (Blackman and Ellis 1915, Goeden and Norris 1965). Throughout the summer months, newly emerged adults feed on twigs in the tree crown. Their short food tunnels are mainly confined to axillary buds and leaf bases in the current year's growth and the junctions of current and 1-year-old growth. Heavy twig feeding may cause yellowing and premature dropping of leaves and broken twigs scattered throughout the crown, but this seldom seriously weakens the tree.

The most serious damage results from tunneling and reproducing in the trunks (fig. 33). In the fall and winter after initial attack, the presence of numerous round, 3-mm diameter entrance holes in the bark are often the only signs of attack. During winter and spring, woodpecker holes in the bark are good indicators of infestation; in spring, sparse or yellowed foliage are signs of beetle attack. Dead or dying trees with bark perforated by numerous 3-mm holes from which beetles have emerged also indicated the beetles' presence along with galleries beneath and within the bark of such trees. The inner bark and wood surfaces are engraved with peculiar centipede-shaped designs consisting of broad vertical galleries with narrow burrows radiating outward like centipede legs from either side of them. The foliage of heavily infested trees turns red within a few weeks after attack and finally turns brown as the tree succumbs.

Biology

The beetles overwinter as larvae in various developmental instars (Goeden and Norris 1964, 1965, McDaniel 1933). During spring the oldest larvae transform to pupae in elliptical chambers terminating each larval tunnel just beneath the bark surface. Beetles emerge from May through late August. The beetle population and seasonal activity reach a maximum during July and early August. The newly emerged beetles fly to the crowns of host trees and feed mainly in ternimal and twig growth for 10 to 15 days. Sexually mature beetles are then attracted to low-vigor trees where they bore into the trunks and branches for breeding purposes. Here the females excavate short (12 to 50-mm) vertical egg galleries between the bark and wood. Mating and egg laying continue throughout the summer, with each female depositing 20 to 60 eggs singly in small niches along either side of the egg gallery. Each egg is covered with a plug of macerated frass. The eggs hatch in 10 to 12 days. The larvae mine at right angles to the main gallery and parallel to each other, but as the larvae become larger their galleries diverge more and more, resembling an engraving of "centipede legs." Larval mines extend 76 mm or more away from the egg gallery, severing the trees' food and water conducting tissues. Mature larvae leave the cambium and bore into the outer bark where they construct pupation cells. There is one generation per year in the northern range and two per year in the southern distribution.

Control

Since hickory bark beetles rarely attack healthy trees, good cultural practices such as thinning, pruning, fertilization, and irrigation are important for promoting and maintaining good tree vigor (Goedon and Norris 1964, Hopkins 1912). The most effective means of controlling a hickory bark beetle infestation is by destroying trees in which larvae are overwintering. Infested trees should be cut and burned or submerged in water, have the bark peeled, or be sprayed with an insecticide before beetle emergence begins in May to June. To protect high-value trees, thoroughly spraying the trunks and large branches with an insecticide during early July will curtail most breeding attacks.