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Foreign Grain Beetles in New Construction

Does your newly constructed home have some unexpected roommates? They might be foreign grain beetles.

As new homes are constructed across South Dakota, we continually receive questions about small, brown insects showing up in bathrooms and basements. These insects are the foreign grain beetle. Their name implies that they are a pest of grain; however, they actually feed on mold and fungi, which is usually associated with stored grain but can also be present on lumber. The foreign grain beetle samples we receive almost exclusively come from newly constructed homes rather than grain bins.


Elongate, reddish-brown beetle on a white background annotated with an orange circle around the right node on the thorax.
Figure 1. Foreign grain beetle. The orange circle highlights one of the characteristic nodes on the thorax (the segment behind the head). Courtesy: Amanda Bachmann

Foreign grain beetles look very similar to other common stored grain pests (elongate and reddish-brown), but they are about half the size at barely 1/8 of an inch long. Unfortunately, their most distinguishing characteristic, knobs on the front corners of their thorax, is only visible under magnification (Figure 1). Since they are small, they are sometimes confused with fruit flies, except that foreign grain beetles have a crunchy exterior.


The good news is that these beetle populations tend to disappear on their own. They can also be effectively managed by reducing humidity in the home. The adults that emerge from wall voids and other spaces inside new construction are generally unsuccessful at beginning another generation indoors. These adults will often die out on their own in a few weeks. Please note that foreign grain beetles are not a health threat to humans or pets, and they do not bite.

They can be managed like many of other fall invaders by vacuuming or sweeping up the adults and disposing of them. Because they need a relatively high humidity level (>65%) to survive, running exhaust fans in bathrooms and dehumidifiers in basements or other damp spaces is an effective way to manage them. As the air dries, the adult beetles will die and can be easily cleaned up. Foreign grain beetles can be a nuisance, but no insecticide application is necessary.

If you have an infestation of foreign grain beetles in an older home, it may be due to high humidity or spilled grain or food in the home. It’s important, not only for foreign grain beetles, but also other stored insect pests to always clean up any spills and keep stored foods (grains, nuts, pasta, birdseed and dog food) in airtight containers.

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