Foreign Grain Beetle

Description of foreign grain beetles

An interesting, but unsettling thing happens to many occupants of brand new houses, especially when the houses are built in the summer and occupied in late summer or fall. New houses are frequently infested with foreign grain beetles, a common and often abundant "fungus" beetle found throughout the world.

In my opinion, this annoying little beetle should be renamed the "new house beetle" because I seldom receive samples or calls from people finding this insect in foreign grain. Calls and samples from new home owners, on the other hand, are quite common.

The foreign grain beetle is abundant during late summer and fall. The reddish-brown beetles are small, only 3/32 inch long which is slightly more than half the length of the familiar red flour beetle. The shape is similar to that of the flour beetle, but the most noticeable difference, in addition to the smaller size, is the presence of tiny knobs or bumps on the front corners of the thorax. A good magnifier is necessary to see this distinguishing character. The beetles are strong fliers and they are attracted to lights. 

Red flour beetle and foreign grain beetle next to a penny
 Red flour beetle (left) and a foreign grain beetle (right) for a size comparison.

Life cycle of foreign grain beetles

Foreign grain beetle adults and larvae feed on molds and fungi, and not on grain as the name would imply. However, they often infest damp or spoiled grain and are common in grain storage facilities, giving rise to their name. Almost any mold or fungus growth may support foreign grain beetles.

They are common "outdoor" insects and may enter homes in small numbers as "accidental invaders" through screens, cracks, and crevices, or around windows and doors. Normal exclusion techniques as for other accidental invaders can be used in this case. However, as indicated above, foreign grain beetles are most frequently discovered as an unwanted house-warming guest in newly-constructed homes. The beetles come from inside walls where molds are growing because of moisture that was sealed into the walls during construction. This moisture could come from wood left outdoors and exposed to rain before or during building, rain that blew into the house before it was "closed in," or moisture from drywall compound applied over sheetrock. Beetles emerge from the new house walls for a period of several weeks until the house completely dries out.

Management of foreign grain beetles

Foreign grain beetles are a nuisance and annoyance but do not harm anything within the home. They cannot bite or sting and do not attack plants, furniture or the house structure. Further, they do not infest stored products and thus, are not "pantry pests." Problems with this pest are usually self-limiting as the seasons change and as moisture trapped in new houses dries out naturally. If known moisture problems exist in an infested house (e.g., leaky pipes within a wall), these should be corrected. If the beetles are known to be originating outdoors, there may be some benefit to exclusion techniques such as a use of tight-fitting screens and doors and sealing or caulking cracks and crevices around windows, doors and screens to reduce the amount of entry. Beetles already inside the home can be vacuumed or swept up and discarded.

Do you live in Iowa and have an insect you would like identified?

The Iowa State University Plant & Insect Diagnostic Clinic will identify your insect, provide information on what it eats, life cycle, and if it is a pest the best ways to manage them.  Please see our website for current forms, fees, and instructions on preserving and mailing insects.   

Contact information for each states diagnostic laboratory for U.S. residents.  If you live outside of Iowa please do not submit a sample without contacting the Plant & Insect Diagnostic Clinic.

Last reviewed:
May 2024

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Yard and Garden, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on September 12, 2016. The information contained within may not be the most current and accurate depending on when it is accessed.