From: Ostry, Michael E.; Wilson, Louis F.; McNabb, Harold S., Jr.; Moore, Lincoln M. 1988. A guide to insect, disease, and animal pests of poplars. Agric. Handb. 677. Washington, DC: U.S. Department of Agriculture. 118 p.

Importance-  Larvae bore into newly developed shoots, killing shoots and causing irregular growth. Heavily injured trees become deformed, bushy, and of little value. This pest is found throughout eastern North America but is more serious in the South where there are multiple generations.

Look For: Dead or dying new shoots. Small patches of brown, webbed insect excrement at basal end of new shoots. Shoots may be swollen and/or stunted. Stub of shoot tips overgrown by new growth forming a fork. Larvae up to one-half inch long located inside injured shoots.

Injury caused by cottonwood borer Photo by Whitney Cranshaw, Colorado State University

Biology- This insect has from one generation in the North to five in the South. The adult moths emerge from cocoons in spring; soon after the females lay eggs singly or in small clusters on the upper leaf surface. Young larvae feed on the leaves and veins until they molt, then they move to the base of developing leaves and tunnel into the tender shoots. Full-grown larvae move down the tree and spin cocoons in bark crevices or in the litter under the trees. In warmer areas the borer produces more generations, and larvae of the last generation overwinter in small silken shelters on the shoots. These larvae move to new shoots in spring and complete their growth.

Monitoring- Inspect new shoots at regular intervals from early spring until late summer for boring activity. When you discover borings, examine 20 to 30 whips in each clone in the nursery. If 20 percent of the stock in a clone is infested, apply treatment. In plantations treat when 25 percent of new shoots become infested.

Control:

• If practical, clip off and destroy infested shoots.
• Apply an insecticide recommended for boring insects, and time spray to kill larvae before they bore into shoots.
• Plant resistant clones of Populus.

For Additional Information- 

Morris, Robert C. 1967. Biology of Gypsonoma haimbachiana (Lepidoptera: Olethreutidae), a twig borer of eastern cottonwood. Annals of the Entomological Society of America. 60: 423-427.