Cottonwood Borer
Plectrodera scalator(Fab.)

Morris, R.C.; Filer, T.H.; Solomon, J.D.; McCracken, Francis I.; Overgaard, N.A.; Weiss, M.J. 1975. Insects and Diseases of Cottonwood. Gen. Tech. Rep. SO-8. New Orleans, LA: U.S. Dept of Agriculture, Forest Service, Southern Forest Experiment Station. 41 p.

The cottonwood borer is a pest throughout the Southeastern States. It attacks the root collar and main roots. Young trees may be girdled or so structurally weakened that breakage occurs. Severe damage has been observed in young natural stands growing on sandy soils along the Mississippi River.

Light brown, fibrous frass is sometimes ejected from bark openings at or slightly above the ground line and accumulates in piles at the base of the tree. But since most attacks occur at or below the ground line and most larvae tunnel downward, infestations often go unnoticed unless breakage occurs. However, when the soil is removed from the root collar and shallow roots, wounds filled with protrudirrg frass can be found in the bark. Also, because of their large size and conspicuous color, the adult beetles are easily spotted while feeding and ovipositing during the summer.

Lateral root with larva and gallery of cottonwood borer. Photo by James Solomon, USDA Forest Service

The adult is a large, robust, long-horn beetle ranging from 1 to 1 1/2 inches (25 to 38 mm) in length and 3/8 to 1/2 inch (10 to 12 mm) in width. The body is black with white crossstripes. A strong spine is located on each side. The antennae are about as long as the body in the female and slightly longer in the male. The eggs are elliptical, white, and about 1/8 inch (3 mm) long. The larvae are legless, elongate, moderately robust, and yellowish white; they reach a maximum length of 1 1/4 to 1 1/2 inches (32 to 33 mm).

Adult beetles emerge during late spring and summer and begin feeding on the bark of tender cottonwood shoots. To oviposit, the female digs away the soil at the base of the tree to a depth of 3/8 inch (10 mm) or more, cuts a niche in the bark, and deposits one or more eggs. Upon hatching, the larvae mine downward in the inner bark, later tunneling into the wood. Taproots of small trees may be completely hollowed. In large trees, some larvae excavate irregular cavities and others produce long tunnels. Portions of the mines or galleries may be packed with excelsior-like frass. Pupation occurs within the gallery. The new adult chews through the pupal chamber and digs its way to the soil surface to escape. A life cycle is completed in 2 years.

Adult cottonwood borer Photo by James Solomon, USDA Forest Service

Since most grubs are below the ground line, they are well protected from both predators and parasites. Woodpeckers capture a few larvae exposed above the ground line. Although a fungus disease has been found, it does not appear to be common. Extended flooding will kill many larvae. Damage can usually be kept to a minimum by planting on good sites and utilizing cultural practices that maintain a vigorous, healthy stand.