What Insect Causes 1/4-Inch Diameter Holes in Maple Trees?

Borers are insects that cause damage in plants by "boring" holes into them. There are many different types of insects that bore into and out of trees, and still many more species within each type. Certain species of beetles, weevils, worms, caterpillars and others are all known to cause boring damage to trees. Metallic wood borers, longhorned beetles, carpenterworms and horntails are the specific insects likely to attack maples of all varieties.

Holes themselves are the most obvious sign of infestation by a borer insect. Longhorned beetles feed deeper into the tree wood than do metallic wood borers, so you are more likely to notice the latter bug on the exterior of the tree. Carpenterworm infestations are recognizable due to their tendency to produce sawdust as they feed on maple wood; they also pupate into large moths that closely resemble the familiar sphinx moth. Horntails are wasps, so you may see the adult, pupated form hovering near your maple.

Borer insects are almost always attracted to trees that are already in a declining state of health, so the first step to stopping borer insect infestations before they start is to keep the tree in good health. In particular, drought stress leaves trees highly susceptible to infestation, so a good irrigation schedule that provides your maple with all of its moisture needs can often prevent an infestation altogether.

If borers have already taken up residence in your maple, consider the most effective ways to remove them. Contact insecticides are effective, but only when applied to adult borer insects before they lay eggs; once the larvae develop inside the tree wood, they are protected from insecticide treatments. Insecticides containing permethrin or bifenthrin are frequently marketed for borer insect control. Also effective are "soil drench" systemic insecticides containing imidacloprid or a similar active ingredient. Note, however, that imidacloprid systemic insecticide application must be timed to coincide with egg hatch and that this type of insecticide is not effective against all borer species.