Hickory Spiral Borer Agrilus arcuatus Say

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.; 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 spiral borer, Agrilus arcuatus Say, is primarily a pest of pecan and hickory but occasionally attacks other deciduous species throughout the eastern half of the United States (Beal and Massey 1942, Brooks 1926). Twigs, branches, and terminals up to 40 mm in diameter on trees of all sizes may be severed. Many of the severed branches break and drop to the ground. Serious damage to large trees results in reduced nut production, ragged appearance, and poor tree symmetry. Repeated attack on young reproduction may cause stunted, misshapened, crooked, and forked stems. Although individual trees may be seriously damaged, entire stands, groves, nurseries, and other plantings are seldom badly damaged. Serious damage is most likely to be found in plantings adjacent or close to forested tracts containing many heavily infested hickories.

Description

The adults are dark, slender buprestid beetles (Brooks 1926). The head and thorax of the male are greenish bronze, the wing covers are purplish black, and the underparts are brassy; the female is bronze in color throughout. The average length of males is 8 mm and that of females about 10 mm. Eggs are flat, disklike in shape, 0.8 to 1.1 mm in diameter, and glued firmly to the smooth bark of the twigs. They resemble the shield of a small scale insect. Initially the eggs are smooth and pale yellowish green, but before hatching they become slightly wrinkled and almost black. The larva is a slender, flat, legless grub, with full-grown specimens reaching 15 to 20 mm long and 2 mm wide (fig. 20). They are yellowish white except for dark brown or black mouth parts and tail forceps.

Evidence of Infestation

Branches and terminals are severed during the winter and spring (Baker 1972, Brooks 1926). The portion above the girdle usually dies in the spring before the foliage appears, the injury becoming apparent as the rest of the tree puts forth leaves. Severed branches may break and fall to the ground either before or after bud-break in the spring. Stems 8 to 40 mm in diameter and 0.5 to 2.5 m long may be severed; although many of these stems are larger than those girdled by twig girdlers and twig pruners, on the average they are slightly smaller than those pruned by the branch pruner. The spiral cut made by the larva is a characteristic winding, concentric cut from the inner bark to the heart of the branch or stem, the coils of the thin burrow joining and completely severing the wood except for the bark and sometimes a slender fiber of wood at the center (fig. 21).

Biology

Adults emerge from late April to late June, depending on location, and feed on the foliage, making elongate notches and slits in the edges of the leaves (Brooks 1926, Ruggles 1918). Females begin oviposition 10 to 14 days after emergence. A single egg is deposited on the bark surface of terminal or lateral twigs, usually near the base of a small shoot of the current season's growth, and is covered with a transparent secretion that glues the egg to the bark. Each female lays from 2 to 55 eggs over a period of 1.5 to 2 months. The eggs hatch in 3 to 4 weeks. The larva hatches by chewing its way through the bottom of the egg chorion and directly into the twig. In the twig it makes an elongate threadlike burrow under the bark. Late in autumn it begins a spiral burrow, partially severing the twig by spring. The mining larva packs the gallery behind itself with fine wood dust. During the second summer it mines basally under the bark along the stem for 20 to 60 cm leaving a shallow but relatively wide burrow packed with brown-colored frass. During late fall it changes its course abruptly and cuts a thin symmetrical ring around the stem. When the first circuit is completed, it bores spirally inwars, encircling the stem until the stem center is reached. The larva then turns upward toward the bark where it mines under the bark for 25 to 76 mm where it forms a cresent-shaped pupal chamber. The ends of the chamber extend to the bark, and the bottom curves toward the stem center. Both ends of the pupal chamber are plugged wtih frass. Pupation occurs during May and June and lasts about 3 weeks. The adult gnaws a D-shaped hole in the bark 25 to 76 mm above its spiral burrow to emerge from the pupal chamber. A generation requires 2 years.

Control

Young trees in heavily infested nurseries and orchards should be pruned to remove the killed branches and terminals as soon as leaves develop in the spring in order to collect and destroy the larvae (Beal and Massey 1942, Brooks 1926, Ruggles 1918). Special care should be taken to remove the small dead twigs that have been severed by the first winter larvae. Such twigs should be clipped several centimeters below the dead part in order to make sure of getting the borer. Also, any severed branches or terminals should be picked up and destroyed promptly before adult emergence begins. Three parasites: an ichneumonid, Labena apicalis Cress., a braconid Monogonogastra agrili Ashm., and pteromalid, Zatropus sp. (near nigroaeneus Ashm.), help reduce populations (Brooks 1926).