When a young back yard 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.

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 reddish-brown clumps of chewed wood pushed out of the tree trunk. Other symptoms are depressed, dark bark, and oozing sap.

The larvae winter inside the trunk and resume tunneling the next spring. They remain inside the trunk throughout the second, and sometimes 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, applied June through August for other insect pests, will kill or deter borers 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 signs of infestations. You can try to exclude the beetles with wire mosquito screen or 1/4-inch mesh hardware cloth surrounding the lower 2 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 clumps of chewed wood and tunneling in September, and again in late May. If you see 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 the University of Maine Cooperative Extension Apple Pest Management Web site at http://pmo.umext.maine.edu/apple/, or call Glen Koehler at 581-3882.