Healthy trees have a number of defenses against insect damage. However, trees that have been “stressed” can be vulnerable to damage by insects, especially those that bore into the wood. The sapwood is the softest and most nutritious part of the wood, and is the preferred feeding site of the flat-headed borers of the family, Buprestidae. A recently introduced invasive species of flat headed borer, the emerald ash borer is causing widespread damage to ash trees in the US and Canada.
One of the most common bruprestid pests is the Flat-Headed Appletree borer, Chrysobothris femorata. These borers infest a wide range of tree species. Ted McRae at Beetles in the Bush has discussed some interesting aspects of the taxonomy of this species. Chrysobothris femorata is a cluster of species with similar features that make positive identification difficult. A scientific revision of the species group was made in 2007 by one of my former colleagues, Stan Welso along with Gary Manley.
One of the large maple trees near my house was hit by lighting which “stressed” the tree and opened it to borer damage. The weakened tree posed a danger of falling on the house and had to be removed. The stress had opened the tree to colonization by multiple species of insects included carpenter ants and the flat-headed borer (possibly the appletree borer) pictured below.
The flat headed borers have a thorax that is compressed top to bottom and gives the appearance of a “flat head”. The head is tucked underneath the thorax. Sharp, hard mandibles for chewing through the wood protrude from the head. The flat headed borers pack their frass (excrement) into the gallery behind them as they chew their way through the wood. These larva can cause a lot of damage while feeding undetected under the bark of the tree.