Q: I have an oak tree in my yard that has two branches about a foot in diameter growing out from the main trunk near the base. About a year ago, I noticed something that looks like coffee grounds at the base of one of the branches after it fell on my house. Inside the branch, it was hollow about 2 feet up. Now, the other branch is dead. It has a large hole in the bottom. The same coffee grounds are coming out of the bottom of it. What is eating my tree?
A: The ” coffee grounds” are probably droppings from carpenter ants or from big cockroaches (or both). They didn’t cause the limb to be hollow, that happened a long time ago. Hollow trunks and limbs are caused by water dripping into the center of the tree part through a crack. The crack could be where a big branch is pulling away from the trunk high above your hear. It could be an opening where a stub was left on a limb when a smaller branch broke off.
In any case, as water drips into the heartwood, decay begins. The decay gradually spreads downward. Eventually a huge, seemingly vigorous, tree will be nothing but a shell of healthy bark surrounding an empty space where the heartwood once was. You won’t realize the situation until the tree (or limb) falls.
Carpenter ants and cockroaches find hollow trees to be excellent spots in which to set up housekeeping. Their droppings accumulate at the bottom of the hollow but they didn’t cause the damage. You really should hire an arborist (look in the Yellow Pages) to assess the tree’s health and long-term prospects.