Adult cottonwood borer.
The adult beetles appear in midsummer. After feeding briefly on the tender bark of the terminals the adults descend to the bases of host trees where the female deposits her eggs in small pits gnawed in the bark. Eggs hatch in about three weeks. The larvae bore downward in the inner bark, entering a large root by fall. Larval feeding continues into the second year as the larvae bore into the inner bark and wood. The larvae transform into the non-feeding pupal stage and finally into an adult in the summer of the second year thus completing a two year life cycle. Adult beetles are 1¼ to 1½ inches long. They are black with lines of cream-colored scales forming irregular black patches.
The best control for the cottonwood borer is to maintain a vigorous, healthy stand. Slow growing, off-site plantings of host trees are the most severely damaged. While some systemic insecticides have shown promise, there is currently no registered chemical control method.
WHITE OAK BORER, Goes tigrinus (De Geer)
A recent survey of damage caused by various wood borers to three species in the white oak group revealed an estimated annual loss in the South exceeding 20 million dollars. One of the more important borers responsible for this damage is the white oak borer.
Adult white oak borer.
Usually the white oak borer attacks oaks one to eight inches in diameter. The damage, like that of other hardwood borers, is the result of larval feeding in the wood. Galleries up to one-half inch in diameter extend upward through the sapwood into the heartwood. The white oak borer takes three to four years to complete one generation. The mated adult female beetle lays her eggs singly in the inner bark through a small oval niche chewed through the outer bark. After about three weeks the eggs hatch and the larvae immediately bore into the sapwood. Later they bore upward into the heartwood. The boring frass ejected out of the entrance is evidence of an active infestation. Pupation occurs behind a plug of excelsior-like frass at the upper end of the gallery in the heartwood. In about three weeks, adults emerge by boring separate and perfectly round holes through the wood and bark. In the South, adults generally emerge in May and June and feed for a short time on oak leaves and the tender bark of twigs before the females lay their eggs.
Woodpeckers may destroy up to 25 percent of the larvae during the winter months, but this and the small toll taken by insect predators and parasites are not sufficient to keep the white oak borer population low enough to avoid serious economic loss.