Cottonwood Twig Borer (Gypsonoma haimbachiana Kft.)

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

The cottonwood twig borer, one of the most destructive insects of young trees, occurs throughout the host species’ range from Ontario to the Gulf of Mexico and west to the Great Plains. Larval feeding in the terminal tissues prevents normal elongation and may kill the growing tip. The stunted terminals are rapidly overtopped by vigorous, undamaged laterals, resulting in a tree top with two to six forks. Later, one fork may assert dominance and become a new terminal, but a crook usually develops where the new terminal originates. Heavily damaged trees may be stag-headed bushes of little value.

Stunted terminals and short brown tubes of silk and borings near leaf bases indicate twig borer damage. Lateral branches overtop the terminal, which persists as a short stub in the forked top. Small, red swollen areas along leaf veins and midribs show where newly hatched larvae fed in the vein tissues before molting and entering the branch and terminal tips.

Adults moths are ash grey and have a wingspan of 1/2, inch (12 mm). The forewing has a dark grey base and a dark spot on its outer tip. Full-grown larvae are 1/2 inch ( 12 mm) long and pale in color with a brown head.

Moths emerge from cocoons in April or May. Females deposit eggs singly or in small groups on the upper surface of leaves along the midrib and veins. Newly hatched caterpillars feed in the midrib or vein until their first molt, then move to the base of the first developing leaf and tunnel into the tender shoot. When they are fully grown, they move down the trunk and spin thin cocoons in bark crevices or in litter under the trees.

Successive generations as many as five in Mississippi- develop through the summer; and with each generation the twig borer population increases. By September there may be 20 or more larvae of 4 different ages in a 15-inch (38-cm) cottonwood terminal. Winter is spent as tiny second-stage larvae in shelters of silk and trash in old entrance scars near branch ends, under corky ridges below leaf bases, or near leaf buds. In spring the small larvae migrate to the new shoots and complete their growth.

Natural controls of the twig borer include the potter wasp (Eumenes sp.), which preys on larvae, and other parasites and predators of the eggs, larvae, and pupae. These are inadequate for protecting nurseries and plantations from serious economic loss; therefore, chemical controls may be needed.

Twig-borer damage in 3-year-old tree.

Brown frass tubes at borer entrances.