Q: In the large oak tree in my backyard there’s a hole about 6 inches in diameter where it looks like a branch was cut off many years ago. The hole is about 15 feet up the tree, and a lot of bees go in and out of it every day. Does that mean there’s a beehive inside the tree and, if so, will bees will do damage to my tree?

A: It’s very common to see beehives in large oak trees in the Conejo Valley. In fact, it’s unusual when a large oak tree doesn’t have at least one hive of honeybees in its inner cavities from time to time. The way the cavities are used as beehives are formed is an interesting process in our oaks.

In a previous article, I wrote that some oak trees live for 500 years or more. Every geriatric oak tree I’ve seen has internal decay to some extent. Oak trees have the ability to live with internal decay for decades and probably even centuries. Usually the decay affects only the center portion of the main stem and branches, called the heartwood.

Over many years of decay, heartwood will degrade to the point where the material that’s left literally disintegrates, creating a hollow area. Sometimes just a branch or two of an oak tree will be hollow, but I’ve seen instances where most of the main stem of an oak is hollowed out, creating a massive cavern inside.

When a branch of an oak tree breaks off or when it’s cut off, a decayed opening in the wood is often exposed. That provides honeybees an entrance into the hollow part of the tree.

Generally this entrance leads to a small cavity and is confined to a relatively small area of the tree, so the bee activity is limited. Sometimes, however, the bees find an entrance hole that leads to a large cavity extending deep into the bowels of the tree.

Large hives may contain many thousands of honeybees and have large galleries just packed with honeycomb. If you look up at the hive opening in the middle of a warm day and it looks like there’s a constant flight of bees in and out, it’s a good bet there’s a large hive inside that tree.

As far as potential damage goes, I can’t think of anything a hive of honeybees could do to an oak tree that would damage it. Honeybees don’t bore into the wood like some kinds of insects do, and they don’t eat the wood. When they discover a cavity in the tree, they simply think it’s a pretty good place to take up housekeeping.

Beehives multiply by swarming, which is what honeybees do in order to divide a large colony into two groups of bees. A swarm of about half the original number of bees in a hive will move to a tree branch or to the eave of a house somewhere close to the old hive and just hang out in a big glob.

Scout bees fly around looking for a cool place to live, and once they find a nice cavity in an oak tree, they come back and tell the swarm all about it. The bees then take off as a large buzzing cloud and head over to their new digs to start building honeycomb.

If your big old oak tree could talk, I think it would say that honeybees make real good houseguests and can stay as long as they want. So unless the bees exhibit aggressive tendencies or are in some way bothersome, there’s no need to worry about them living in your tree.

David D. Mortimer is a certified arborist with more than 30 years’ experience in the tree care industry. E-mail questions to dmortimer@theacorn.com.