When I opened up a little-used storage closet upstairs, I found hundreds of tiny, black insects that look like beetles. There isn’t any food stored there and no dead animal. Where did they come from?

W. W., Hampton, NH

Ahasverus advena – Foreign Grain Beetle

I think your pests might be fungus beetles. They would be feeding on a thin layer of mold growing on walls or items in that closet. Check for a musty smell in the closet. The real question is why is that closet damp enough to grow mold? You should check for a roof leak above or a plumbing leak in the wall.

If It’s Damp and Moldy, They Will Come

In homes, fungus beetles are found around plumbing leaks or in areas that have had other moisture problems. Roof leaks wetting ceilings are a common cause of fungus beetles. Other places to find fungus beetles are in moldy stored foods, moldy plant materials, damp construction materials, around leaky windows or skylights, rodent nests in wall voids, hoarded rodent food in hidden voids, and sometimes in ant or termite nests.

There are actually several tiny (1/8 inch), dark beetles that collectively are called “fungus beetles,” or sometimes plaster beetles or mold beetles. They are easily confused with the tiny beetles that infest stored foods. One common beetle, the foreign grain beetle, belongs in both categories. Despite its name, it is most often found in nonfood sites that have mold. When it is found in stored foods, it is because it is feeding on mold growing on the grain, not on the food itself (see The Foreign Grain Beetle is Actually a Mold Pest).

Fungus beetles are mainly a temporary nuisance pest unless you have a situation with a chronic moisture problem. They tend to be first noticed and are more of a problem during warm, humid summer weather. They don’t do any damage; any damage to infested items is from the mold and moisture rather than from the beetles.

New Construction is Often a Source of Fungus Beetles

Fungus beetles are common in new construction (and after renovations), usually feeding on mold growing in wall voids. New construction introduces a lot of moisture into a home in “green” lumber, wet wallboard, plaster, damp wallpaper, etc. When walls are enclosed, moisture is sealed into the voids. Drying of building materials can take a while and during that time, mold can grow and become infested by fungus beetles (see Renovation Brings Fungus Beetle Problem).

Drying Out the Site Eliminates Fungus Beetles

Fortunately, fungus beetles can be one of the easiest pests to control. For small infestations, if you simply dry out the site where the mold is growing to kill the mold, the beetles will either die or move on. Drying the site might mean repairing roof or plumbing problems, or correcting drainage or condensation problems. In some cases, simply removing the moldy material (for example, wet cardboard boxes) will get rid of the pests.

The drying process (especially in new construction) can be speeded up with fans and dehumidifiers, by running the furnace or air conditioner, installing vents, and by opening up voids to let them dry. In your case, empty out that closet, leave the door open, and place a large fan in the doorway. It may take several weeks of drying before the infestation runs its course. In large infestations or where drying doesn’t seem to be working, contact an exterminator to treat the infested area with an insecticide.

Photo Credit: This image is created by user Dick Belgers at waarneming.nl, a source of nature observations in the Netherlands. [CC BY 3.0], via Wikimedia Commons