Hickory Spiral Borer

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Elateriformia
Superfamily	Buprestoidea
Family	Buprestidae
Subfamily	Agrilinae
Tribe	Agrilini
Genus	Agrilus

Scientific Name

Agrilus torquatus

Scientific Name Synonyms

Agrilus arcuatus torquatus

Common Name

Solomon, J.D. 1995. Guide to Insect Borers in North American Broadleaf Trees and Shrubs. Agriculture Handbook 706. Washington, DC. United States Department of Agriculture, Forest Service. 735 p.

Hosts

Pecan, hickory. Hickories seem to be preferred, but pecan is readily attacked.

Range

Throughout the eastern half of the United States (Beal and Massey 1942, Brooks 1926).

Description

Adult

Moderately elongate beetle, rather robust, strongly shining, and moderately flattened (Brooks 1926, Fisher 1928). Females uniformly brownish copper; males have reddish copper pronotum with greenish or bluish sides and black elytra with violet tinge. Males 8 mm long and females about 10 mm long.

Egg

Flat and disklike, 0.8 to 1.1 mm in diameter, and glued firmly to smooth bark of twigs. Egg resembles shield of small scale insects. Initially, eggs smooth and pale yellowish green, but before hatching become slightly wrinkled and almost black.

Larva

Slender, flat, and legless, with mature specimens reaching 15 to 20 mm long and 2 mm wide. Yellowish white except for dark brown or black mouth parts and anal forceps.

Biology

Adults emerge late April to late June, depending on location, and feed on foliage, chewing elongate notches and slits in the edges of the leaves (Beal and others 1952, Brooks 1926). Females begin oviposition 10 to 14 days after emergence. Eggs are deposited singly on the bark surface of terminals or laterial twigs, usually near the base of small shoots of current season's growth and are covered with a transparent secretion that glues them to the bark. Females lay 2 to 55 eggs each over 6 to 8 weeks. Eggs hatch in 3 to 4 weeks, and the larvae chew through the bottoms of the chorions and directly into twigs, where they make elongate threadlike burrows under the bark. Late in autumn, they begin spiral burrows that sever the twigs by winter or spring. Mining larvae pack their galleries behind with fine frass. During the second summer, they mine downward under the bark along the stem for 20 to 60 cm, leaving shallow (but relatively wide) burrows packed with brown frass. During late fall, they change course abruptly and cut thin, symmetrical rings around the stem. After the first circuit, they bore spirally inward in the same plane, encircling the stem until reaching its center. Larvae then turn and mine upward under the bark for 25 to 76 mm. Here, they form crescent-shaped pupal chambers, with ends that extend to the bark and whose bottoms curve toward the stem center. They plug both ends of pupal chambers with frass, then pupate during May and June for about 3 weeks. A generation requires 2 years.

Injury and Damage

Larvae sever branches and terminals during late winter and spring (Brooks 1926, USDA FS 1985). The portion above the girdle usually (but not always) dies in spring before foliage appears, the injury becoming apparent as the rest of the tree puts forth leaves. Most severed branches break and fall to the ground just before and soon after budbreak in spring. The borer severs stems 8 to 40 mm in diameter and 0.5 to 2.5 m long--many are larger than those girdled by Oncideres twig girdlers and Anelaphus twig pruners but slightly smaller than those pruned by Psyrassa branch pruners. Examination and removal of bark from infested branches will reveal the frass-packed galleries and sometimes the long white larvae. The long winding gallery beneath the bark abruptly spirals inward to sever the branch. The spiral cut made by a larva is characteristic; it is a winding concentric cut from the inner bark to the heart of the branch or stem. The coils of the thin burrow join and completely sever the wood, except for the bark and sometimes a few slender fibers at the center. Later examination will reveal curved pupal cells in the wood and D-shaped emergence holes in the bark. A major portion of the terminals of young trees is sometimes severed. Serious damage to large trees results in reduced nut production, ragged appearance, and asymmetry. Repeated attacks on young trees may cause stunted, misshapen, crooked, and forked stems. In one study in North Carolina, 7% of 500 young hickories examined had part or all of their main stems killed by this borer (Beal and Massey 1942). Although individual trees may be seriously damaged, entire stands, groves, nurseries, and other plantings are seldom badly harmed. Serious damage is most likely to occur in plantings adjacent to or close to forested tracts containing many heavily infested hickories.

Control

Five insect parasites--Labena apicalis Cresson, L. grallator (Say), Monogonogastra agrili (Ashmead), Tetrastichus rugglesi Rohwer, and Zatropis sp. near nigroaeneus (Ashmead)--help reduce populations (Brooks 1926, Burks 1979, Carlson 1979). To collect and destroy the larvae, young trees in heavily infested nurseries and orchards should be pruned to remove dead branches and terminals as soon as leaves develop in spring (Beal and Massey 1942, Brooks 1926). Special care should be taken to cut off the small dead twigs that have been severed by the first winter larvae. To be sure of getting the tunneling borers, such twigs should be clipped several centimeters below the dead part. Also, all fallen severed branches and terminals should be picked up and destroyed before adults begin to emerge. It had been suggested that old hickories growing near nurseries should be removed (Beal and Massey 1942).

Gallery