Keeping an apple tree healthy and full of vigour -- and away from mechanical wounds to the bark -- is the primary way to protect it against infestation from an insect that regularly causes bark to peel away from the tree. The flatheaded apple tree borer can prove lethal to this ornamental, shade and fruit specimen.
Flatheaded Apple Tree Borer
Gardeners who notice bark falling away from an apple tree should be on the lookout for the flatheaded apple tree borer (Chrysobothris femorata). This insect feeds just under the bark, creating a series of tunnels that cut off the supply of water and nutrition to the tissue of the tree, causing a dieback of twigs and branches and potentially death. The adult apple tree borer is approximately 3/4 inch long with a metallic brown or green colour and a flat head that gives the pest its name.
The adult female lays eggs that she deposits just under the bark or in crevices along the trunk and large branches. Upon hatching, the larvae are a pale green colour and legless, with a length of just over 1 inch. These larvae burrow deeper into the sapwood of the tree, overwintering until the spring when they begin to chew their way back out through the bark and emerge as adults, ready to start the cycle again. As they depart the bark, they leave a large, oval hole.
Adult flatheaded apple tree borers are present on the tree from May through November, and spend the summer feeding on the leaves, sometimes seriously defoliating the apple tree. The pest is most attracted to apple trees that have already suffered some sort of bark damage; this makes it easier for the hatched larvae to enter the bark. Newly transplanted and already-stressed trees, lacking in vigour, are also more susceptible to an attack from large numbers of borers.
The borer causes liquid to ooze from the bark surface and the bark itself begins to take on a darker, shinier appearance. Younger apple trees with thin bark will see the bark begin to sink or appear depressed, with the bark splitting and falling off the tree. As the bark sloughs away, the interior holes caused by the borer will be more apparent and the heads of the insects themselves may be seen.