carpenter bees
I, myself do not prefer the wd 40 method as it can leave a discoloration on the wood and, I've had bees that just avoided the opening until it seemed to dry. On a vertical 4x4, the wd just seemed to soak downward into the grain and disappear except for a place that paint doesn't want to adhere and the bee went sideways into the next board. The thing that makes the wd40 effective, though, is that the female drags some of it into the cells where she seals the young. Once she gets the first hole about a half inch across and a couple of inches deep, she right angles and goes as far as 3 or 4 feet into the wood. She then makes smaller burrows off that main tunnel. She lays eggs and places food in them and then seals them. If you get her before she goes too deep, you can just use any bee spray but if she's made that first couple inch hole, assume she's already started the egg laying tunnel and get something that she'll take in and poison the larvae. Here's a dust that I found but haven't had a reoccurance yet to test it on
http://www.bugsaway.com/woodbee.htm
We've had some in our home (built in 1848 with 10x10 hand adzed UNTREATED lumber that's exposed in several places along the foundation and eave areas) that we thought we'd killed only to have them start coming inside the house. We found out that we hadn't gotten all of the young and the things dug through the other side of the beam and into our dining room. Apparently, some of them also bored a new hole outside and left a nice entry tunnel for paperwasps the next year to get in and build an impressive structure in the crawlspace above our dining room. By the time we figured out where the paper wasps were coming from, the nest was almost 4 feet long. All because of one loose piece of soffit. If they weren't such a nuisance, their ingenuity and industriousness would be downright impressive.
Good luckwith your little "pets"