Tiny insects tell stories

A variety of beetles with similar feeding behaviors, sometimes collectively
called "plaster beetles". Drawing from Mallis Handbook of Pest Control.
These are tiny beetles ranging in size from 1-3 mm.

Who doesn't love a story?  Humans are suckers for narratives, so it's natural we should be interested when insects have their own stories to tell.  Especially when they are tiny insects that come from, seemingly, out of nowhere.

Among the insects I've been seeing lately are a group of beetles historically called "plaster beetles" or "mold beetles".  Though no longer a very accurate moniker (since joint compound has mostly replaced plaster in construction), plaster beetles feed on the fungi and mold spores found on building materials (including plaster and gypsum) after being exposed to rain and humidity during construction. The most common plaster and mold beetles come from the families Lathridiidae and Cryptophagidae (the Latin translation of which means "hidden feeders"). According to Smith and Whitman in the NPMA Field Guide to Structural Pests, these beetles can also become serious pests in otherwise squeaky-clean pharmaceutical, food manufacturing, canning and bottling facilities.

In Frank Meek's chapter on occasional invaders in the Mallis Handbook of Pest Control, he notes that the life cycle of plaster beetles can be completed in as little as 13 to 28 days.  That's pretty fast for an insect, and it explains how hundreds or thousands of tiny beetles can "suddenly" appear, coming from the walls and flooring of new homes. These beetles can also show up in older homes when moisture and high humidity occur.

Tiny (0.75 mm) beetle larvae swept up from a wood floor with
moisture "issues". The likely diagnosis is some kind of plaster or mold
beetle. Within weeks adult beetles will be infesting the home and
puzzling everyone as to their identity. 

I have always wanted to see (but never have seen) the immature stages of these beetles feeding in walls on building materials; and I've never heard of anyone else having seen them either.  Our deductions about what plaster and mold beetles are doing indoors are based on these beetles' biology, what they are known to feed on, and accounts of when and how the adult beetles appear. 

I recently received a sample from a PMP that comes close to providing "smoking gun" evidence for how these beetles come to infest buildings.  The sample consisted of thousands of the tiniest larvae swept up from a wooden floor (see picture).  As expected, the floor reportedly had moisture issues. Somewhere, hidden under the glued-down flooring of that home,  I imagined millions of "plaster beetles" breeding while feeding on mold spores and fungi associated with the slowly rotting wood.

Most plaster beetle problems can be solved by eliminating the source of moisture (and patience).  In new homes that were exposed to the elements during construction, properly designed and installed HVAC systems should eventually drive down humidity in the walls and floors. It may take multiple months, but eventually these beetles will disappear as molds desiccate and go dormant.

Wooden floors glued to concrete slab foundations pose a different challenge, as my wife and I learned the hard way.  We paid a contractor who assured us he had successfully installed many wood floors as glue downs on concrete slab foundations (the most common house foundation in our area). A few months after installation we noticed cupping and buckling of the new floor.  A moisture meter showed high (up to 20%) moisture in parts of the installed flooring. We had no water leaks under the house, but natural moisture exists under any concrete slab.  We got off relatively easily by pulling up all warped sections of flooring, putting down a chemical moisture barrier (usually epoxy or urethane based), reinstalling the wood and having the whole floor hand-scraped to hide further imperfections. Over ten years later we've not had any more problems, but I learned my lesson.  There are plenty of contractors out there who do not plan for moisture, and hardwood floors will fail because of it.

An unidentified cossonine weevil (2 mm) collected from a home with
moisture problems.  The presence of similar-looking beetles
should alert PMPs to likely moisture issues.  Note the elongated
face typical of weevils.

While we didn't have insects associated with our wood flooring, some do. Besides plaster beetles, another insect that can occur in wood floors is a group called the "wood weevils".  This is a name for beetles in the weevil subfamily Cossoninae.  I find little written about these weevils in the scientific or pest control literature, but have seen evidence from several homes where these beetles are associated with moist wood. In nature these beetles are found in damp and rotting wood--presumably the same resource that draws these beetles indoors. Unlike termites, these wood weevils don't seem capable of feeding on dry, sound wood; so treatment consists of solving the moisture issue and replacing the damaged wood. These beetles are most likely to be confused with powderpost beetles, so look carefully before diagnosing.

The next time you are faced with tiny beetles in a home that do not seem to fit the description of stored product pests, consider plaster beetles and wood weevils. They may be trying to tell you a story about your customer's home.