What is causing holes and sawdust on ceiling rafter beams?

Termites or whatever it may be, I vote for a consultation with a pest control professional. Typically free, so just a matter of taking time to schedule.

https://www.consumer.ftc.gov/articles/0242-hiring-contractor

Powderpost beetles are a group made up of several species of wood-boring insects that get their name from their ability to turn the inside of a piece of wood into powder. Commonly confused with carpenter ants and termites, powderpost beetles are thought by some researchers to be the second leading cause of insect damage to homes in America. Specifics vary depending on the exact species, but all powderpost beetles have similar signs of infestation and ways to be prevented and treated.

Indicators of Powderpost Beetle Infestation vs. Carpenter Ant Infestation

In most cases, homeowners will see the damage caused by powderpost beetles rather than the beetles themselves, whereas carpenter ants will always be visible when an infestation is present. This is because the majority of the beetle’s lifetime is spent inside the wood. Eggs are laid on bare, unfinished wood, where they hatch into larvae and begin tunneling. Adult beetles are difficult to spot because of their small size and the fact that they only tend to live for a few weeks. Carpenter ants, however, tend to be very active and their activity can lead a trained professional to the location of a parent or satellite nest. As a homeowner, keep an eye out for the following:

• Powdery, sawdust like byproduct of the beetle larva’s or carpenter ant’s excavation of the wood, called frass. The texture of the frass can vary widely depending on the species of powderpost beetle and can be difficult to differentiate from the frass created by carpenter ants, so this is usually not enough to determine the pest creating it.
• Exit holes, made by either fully grown larva or adult beetles; vary in size depending on species from approximately 0.03 inches to 0.28 inches in diameter. The exit holes for a powderpost beetle tend to be much easier to identify than the entrance to a carpenter ant nest.
• Wood that has a thin shell of solid wood around a powdery mass interior is a tell-tale sign of powderpost beetles. Conversely, carpenter ant nests will be a series of smooth “galleries” and tunnels.

My first thought would be "Carpenter Bees" or bumble bees. They usually make nice round holes around a half an inch in diameter.

This looks like something happening with the nails used to attach the flooring to the beams. Evenly spaced, always over the beam. Could be a chemical reaction with the type of nails used that is destroying the wood and friction from walking on the floor is creating the black dust ( another red flag) you see which may be the wood or even the nail deteriorating.

What ever pest it is you need to get a professional ASAP! I would be really worried!

Michael,

This is a time you don't "ask a friend" but "ask a Professional!'

I agree with everyone who's thought is this is a man-made issue. How new is your home? Is it possible that the builder (or someone else?) pulled nails from these spots in the floor above and left a void which has now allowed the chip board floor to disintegrate in those specific areas? Difficult to photo, I know, but you can see what's going on better than Hometalkers can. I would watch this area for another month and see if there is any further deterioration - and if it is in a uniform pattern with what has already occurred. You can also test the area for extensive damage with a few taps of a hammer. If nothing happens, wood filler will plug the holes.

I would immediately spray the area with a bug spray and see what happens. If it continues in other areas then get someone immediately to remediate the problem.

When I see holes like that I think of carpenter bees. Is there a way they are getting into your cellar? They build nest in wood. I would spray them with bee killer deep into the hole.

Call a professional. Don't waste money guessing.

“Powderpost beetle” is a term used to describe several species of small (1/8-3/4 inch long) insects that reduce wood to a flour-like powder (Figure 1). The developing grub-like larvae inflict damage as they create narrow, meandering tunnels in wood as they feed. Tunneling and larval development take place entirely below the wood surface. Infestations typically are discovered after noticing powder, accompanied by small, round “shot holes” in the wood surface. These are exit holes where adult beetles have chewed out of the wood after completing their development. Newly emerged adults mate and lay eggs on or below the surface of bare, unfinished wood. The eggs hatch into tiny larvae that bore into the wood, emerging as adults one to five years later, usually during late winter, spring or summer depending upon species. Customers are more likely to see damage, rather than the beetles themselves, because the adults are cryptic and active mainly at night. Occasionally, the beetles may be found near damaged wood, or on windowsills since some are attracted to light.

So bugs are covered here. Carpenter ants don’t eat wood but push out sawdust as they make nests. Beetles eat wood and they track sawdust looking feces. Termites eat the wood and that’s not my go to bug here. That the holes are in the OSB subfloor and not the floor joist is odd. OSB is wood and glue designed to be insect resistant.

That said I’ll pose a possibility that came to mind. You may have a loose subfloor where two sheets of OSB meet over the joist. Possibly not glued well as it is a joint where two sheet ends meet and rub and move at the nail holes. Nails are typically placed on a slight angle at this point and could be creating sawdust. Picture is awful grainy so it’s an educated guess. If the floor squeaks there, it’s a likely cause.

Call a bug guy for inspection because if there are bugs there they’re likely elswhere.

As it turns out, an inspector told me today that the damage is not insect related at all but probably just the result of some drilling that took place during installation of the wood. I did end up signing up for a 5 year termite treatment and inspection plan costing about $3150; expensive but I think worth it for peace of mind. includes a ten thousand dollar warranty in the event of damage.

Hello, I also had termites in my ceilings and also in drywall. I have shared here how I got rid of them. Check my articles below.

1. Signs of Termites in Ceiling and How to Get Rid of Them?