Carpenter Bees

By Rich Kozlovich



Picture courtesy : Kansas State University Research and Extension

There has been a lot of carpenter bees flying around this year.  In some of the cases I could see why and act accordingly.  In other cases there was no observable evidence of nesting.  Let's talk about that.

The size of carpenter bees varies between 1/2 inch to 1 inch.  They resemble bumble bees, but normally larger with the posterior being black and shiny, with yellow "hair" on the thorax.  Bumble bees are the opposite.  Hair on the posterior and a black thorax.  The male has a yellow face. The females face is black and play an important role in plant pollination. 

Now that we've gotten that out of the way let's deal with the problems they cause homeowners. 

Carpenter bees can drill an almost perfectly round hole about 3/8  inch in diameter, usually on the face of boards - behind and in front - of decks, fences, fascia boards and any location where there's bear wood,    leaving sawdust below the drilling location.   Untreated nests can be used year after year.  

In the spring we see a lot of carpenter bees flying amount because they've emerged and want to mate.  Unlike other bee species the male/female population is 50/50, and the males have no stinger.  I've not been stung by them, nor do I know anyone who has.  Males are territorial and will confront you if you enter their territory through intimidation - and do an excellent job of it - remember the males can't sting. 

Females will drill out a few inches to as much as 18 inches and start laying eggs.  They fly back and forth with pollen attached to their legs which they leave in the egg chamber.  When there's enough they will form it into a round ball to leave as a food source for the larva when they hatch, seal the chamber and then repeat the process anywhere up to as much as 8 times.  The female carpenter bee can squeeze through incredibly tiny places to bore into untreated wood, even under siding. 

Normally they drill in and then make a right hand turn.  But they're just as capable of drilling to the left, and even up and down in porch pillars.  Not typical, but it's done causing a lot of damage, including woodpeckers pecking even larger holes in order to eat carpenter bees and their larvae, even if it's a old no longer used nest, just in case it's still active.

So what to do?

First and foremost we have to understand they're job is to help clean up the dead trees in the forest.  If you have untreated, unpainted wood on your house, even if it's untreated cedar clapboard, they view it as dead wood...not your house.  I see a lot of stained rough cut wood as siding on homes.  They immediately recognize that as 'dead' wood and attack, even on homes with heavy stain.

Corrective steps to prevent long term damage to a home involves painting wood surfaces.  Although they will attack painted wood, it's less frequent, and any bear wood treated with oil or polyurethane bases will help substantially.  Siding is a good thing, but not fool proof as I've stated. 

Once they've gotten started a pesticide treatment is necessary when they get started and possibly in the fall, then I recommend caulking the following spring.  Why spring.  Carpenter bees emerge from the nest in reverse order to when they were laid.  The last egg laid is the first egg to hatch, otherwise they would have to eat through the other unhatched chambers or chew through the chamber wall exposing the nest to the environment.   That means there can be an unhatched chamber left over for the spring, even if pesticides have been applied earlier in the year.  If nothing emerges in the spring then caulk and paint the surface.