Roundheaded Apple Tree Borer

When a young backyard apple tree dies, the cause is often insect borers.  The most important insect borer attacking home apple trees in Maine is the roundheaded apple tree borer. Images: larvae and adult.

Adult roundhead apple tree borer beetles begin emerging around June 10; egg laying begins around June 20, peaks in the last two weeks of July, and continues through August.  During their first summer, the young larvae feed under the bark. Infestation may be evident as clumps of reddish-brown sawdust-like pushed out of the tunnel, sunken and dark bark, and sometimes oozing sap.

The larvae overwinter inside the trunk and resume tunneling the next spring. They remain inside the trunk throughout second, and sometimes a third or fourth, summer, before emerging as adults in the spring. The tunneling disrupts the tree’s ability to transport water and nutrients between leaves and roots. Borer-infested trees grow slowly and have sparse foliage. Small trees with multiple borer larvae can be so weakened that they break off in a stiff wind.

Insecticide sprays during the June – August for other insect pests kill or deter the female roundheaded apple tree borer beetles before they lay eggs.

Nonchemical preventive measures include removing infested mountain ash, crabapple, hawthorn, shadbush and cotoneaster within at least 100 feet of apple trees. Remove vegetation or winter “mouse” (vole) guards that shield the lower trunk from sunlight. Apply a 50:50 mixture of white latex (not acrylic) paint and water as a thick whitewash to the lower trunk. The whitewash deters egg laying, and makes it easier to see frass from infestations that do occur. You can try to exclude the beetles with wire mosquito screen or ¼ inch mesh hardware cloth surrounding the lower two feet of the trunk. The barrier must be loose around the trunk but sealed at the top with a cord and at ground level by mounded soil.

Check lower trunks for frass and tunneling in September, and again in late May. If you find signs of infestation, first try to dig out shallow larvae by removing decayed tissue with a sharp knife. If there is a tunnel, you may be able to locate and kill the larva with a stiff wire.

For more information on apple pests, visit University of Maine Cooperative Extension Apple Pest Management, or call Glen Koehler, Associate Scientist for Apple IPM and NAPIAP, at 581.3882 or e-mail glen.koehler@maine.edu.