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FS ShieldForest Health Protection, Southern Region


Prionoxystus robiniae (Peck)

Importance. - In eastern and southern states, oaks-particularly red oaks-are the most heavily damaged. Other hosts are green ash, black locust, elm, maple, willow, cottonwood, and sometimes fruit trees and ornamental shrubs. The damage-wormholes-causes unsightly scars on ornamental trees and degrade, estimated at 15 percent of the value of rough sawn lumber.

Identifying the Insect. - Newly hatched larvae are 1/4 inch (6 mm) long and reddish pink. They gradually become greenish white and are 2 to 3 inches (50 to 75 mm) long at maturity. Brown pupal skins protruding from entrance holes are common in early summer. Adults are grayish, stout-bodied moths. The hindwing in the male has an orange spot.

Larva in a typical gallery in the wood

Larva in a typical gallery in the wood. (Click for detail. JPG 30K).

Identifying the Injury. - Earliest signs of attack are sap spots on the trunk. Later, frass is ejected from entrance holes. Burrows 2 inches (50 mm) in diameter under the bark, and galleries 1/2 inch (12 mm) in diameter and 5 to 8 inches (12 to 22 cm) long in the wood are typical. Galleries are open or loosely plugged with frass. Holes in lumber are dark stained.

Attacks on the trunk
Attacks on the trunk. (Click for detail. JPG 34K).

Biology. - Adult moths appear in April to June and deposit 400 to 800 eggs in bark crevices. Eggs hatch in 10 to 12 days, and young larvae tunnel into the bark and wood. Pupation occurs within the gallery during spring and lasts 3 weeks. A life cycle requires 1 to 2 years in the South, and 2 to 4 years in the North.

Control. - Management practices such as maintaining high tree vigor, removing brood trees, preventing bark injuries, and spraying the trunk or fumigating the galleries with insecticides help to minimize damage.