White Marked Spider Beetle: Ptinus sexpunctatus

Subject: strange beetle
Location: southeastern Idaho
March 28, 2015 8:06 pm
I keep finding these beetles in my bathroom, and I’ve never seen them before.
Signature: mrs. Payne

White Marked Spider Beetles

Dear Mrs. Payne,
These are Longhorned Borer Beetles in the family Cerambycidae, and they resemble the Ivory Marked Beetle, Eburia quadrigeminata, but they seem very small and Idaho is considerably west of their range as listed on BugGuide, so normally we would discount that as a possibility but for one bit of information posted on BugGuide.  According to BugGuide:  “Notorious for emerging from furniture after as many as 10-40 yrs.  Delayed emergence of E. quadrigeminata was discovered from a birch bookcase 40 years old (Jaques 1918).”  Larvae from this family are wood borers, often remaining in the larval stage feeding for several years.  If infested lumber is milled and turned into paneling or furniture, it is possible that the larvae might survive, and individuals in that situation may emerge many years later and they are often considerably smaller than individuals that develop in nature.  According to BugGuide:  “hosts include a wide variety of hardwoods (oak, ash, hickory, locust, chestnut, maple, elm, beech, cherry); larvae bore in heartwood.”  It is possible that you bought a piece of furniture made from one of those trees that was milled in the normal range of the Ivory Spotted Beetle, and that could explain its presence in Idaho.  That is speculation on our part and the beetles you found might actually be a local species, but at this time, we have not been able to find a likely candidate.  We will seek a second opinion on this from Eric Eaton and our readers might also be able to provide some other information.

Whitemarked Spider Beetle

Eric Eaton Responds
Way too tiny for Ivory-marked Longhorn, but I see the resemblance otherwise.
These are spider beetles, family Ptinidae.  Probably the White-marked Spider Beetle, Ptinus fur.  Here’s the Bugguide page:
I rarely see these, but they are well-known “stored product pests.”
author, Kaufman Field Guide to Insects of North America

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