Branch Pruner Psyrassa unicolor (Randall)

Taxonomy

Domain	Eukarya
Kingdom	Animalia
Phylum	Arthropoda
Subphylum	Hexapoda
Class	Insecta
Subclass	Pterygota
Infraclass	Neoptera
Order	Coleoptera
Suborder	Polyphaga
Infraorder	Cucujiformia
Superfamily	Chrysomeloidea
Family	Cerambycidae
Subfamily	Cerambycinae
Tribe	Elaphidiini
Genus	Psyrassa

Scientific Name

Psyrassa unicolor

Common Name

Branch pruner

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 branch pruner, Psyrassa unicolor (Randall), attacks pecan and hickory, and to a lesser extent the oaks and a few other species throughout the Eastern United States from Minnesota south to Alabama and west to Texas (Linsley 1963). The larvae tunneling in the stems cut off or prune large branches. Pruned branches, due to their size and weight, usually drop to the ground; they seldom hand loosely from partially severed branches as seen with the twig girdler and the twig pruner. Although individually pruned branches are generally larger than those girdled by the twig girdler or the twig pruner, the number of pruned branches is usually less. Although individual shade and ornamental trees and occasionally orchard trees may be seriously pruned, entire stands or groves seldom sustain economic damage.

Description

The narrow, elongate adults are colored light to reddish brown, have short and inconspicuous pubescence, and are coarsely punctured (fig. 18) (Linsley 1963). The antennae are about as long as the body in the female and slightly longer in the male. In length, females range from 9 to 13 mm and males 7 to 11 mm. In width, females average 2.2 mm and males 1.8 mm. The larvae are slender, elongate, cylindrical, and whitish with dark-brown mandibles; they have yellowish thoracic shields, very short rudimentary thoracic prolegs, and measure about 12 to 18 mm long at maturity.

Evidence of Infestation

Branches are pruned and fall to the ground during spring as opposed to summer, fall, and winter for those cut off by the twig pruners and twig girdler. Pruned branches are often much larger than those cut off by other pruners and girdlers, ranging from 10 to 50 mm in diameter and 0.6 to 3.6 m long. Pruned branches generally fall free to the ground, seldom hanging from the tree by the severed end as is true for the other pruners and girdlers. The cut made by the branch pruner is similar to that of the twig pruner in that it is made from the inside by the larva, which chews a uniformly smooth, circular cut in the wood, leaving only the bark intact (fig. 19). It differs from the twig pruner in that the larval hole is not at the center of the twig, but instead is near one side, usually just below the bark near a small side-twig, and the hole is often plugged with frass. Although the larva usually tunnels in the pruned branch, it will sometimes tunnel in the pruned stub still on the tree.

Biology

Adults emerge from late April to early June. Eggs are deposited on small twigs that arise from a larger branch 10 to 50 mm in diameter. The young larva tunnels down the center of the twig toward its base, but it does not hollow out the small twig as completely as Elaphidionoides spp. Upon reaching that larger branch, it bores into the branch and begins to girdle it. The girdle is completed during late winter and spring when the larva makes a smooth, uniform concentric circular cut, often completely severing the wood, but leaving the bark intact (fig. 19). Severed branches may break at any time, but breakage occurs mostly during spring windstorms. If breakage does not occur immediately, the larva usually tunnels into the severed portion of the branch just beneath the bark near the junction of the small twig. Here it tunnels toward the center of the stem for 15 to 30 mm then back toward the surface, and finally tunnels distally just under the bark for 30 to 80 mm. A small number of larvae, however, tunnel basally from the point of girdle. Larvae are sometimes dislodged at the time of the break and fall to the ground and die. Some retreat into the small twig before the branch breaks. Pupation takes place within the gallery just beneath the bark. The adult chews an irregularly shaped hole through the bark to emerge. Although the life history is little known, a generation apparently requires 1 to 2 years.

Control

Severed branches on the ground under trees in orchards and ornamental plantings should be picked up in the spring and destroyed before the adults emerge in late spring and early summer. To be most effective, the pick-up and destroy practice should be done for the entire orchard, woodlot, or neighborhood. Direct control in natural forest stands in rarely needed. Two icheumonid parasites of P. unicolor, Labena grallator Say and Agronocryptus discoidaloides Viereck, help to reduce infestations (Linsley 1963).