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Foreign grain beetles in homes

Jeffrey Hahn and Stephen Kells

Foreign grain beetles, Ahasverus advena, can be common insects in homes in Minnesota in August and September, particularly in newly constructed buildings.



Figure 1. Foreign grain beetles

This is a small, flattened insect, about 1/12 inch long and reddish-brown in color (Fig 1). When viewed under magnification, it can be identified by two peg-like projections behind its head (Fig. 2). These small beetles are strong fliers and are sometimes confused with fruit flies or gnats. However, foreign grain beetles have a hard shell compared to flies and lack conspicuous, clear wings when at rest (they are folded under their wing covers).

Sometimes people mistake foreign grain beetles for fleas or lice but neither of these biting insects have wings and can fly. Lice are also flattened but are soft-bodied. Fleas are flattened from side to side and are strong jumpers.



Figure 2. Foreign grain beetle close-up. Note the small peg-like projections behind the head

Foreign grain beetles live in damp areas where fungus grows, requiring a relative humidity above 65% in order to reproduce. They feed on a variety of stored products that are damp and moldy. Foreign grain beetles can complete their life cycle completely in as little as 30 days. Because of this, they are able to produce large numbers of individuals in a short amount of time. Foreign grain beetle numbers drop dramatically by October. Once the humidity falls below 60%, they die and disappear on their own.


Fortunately when foreign grain beetles are found in homes, they are just nuisances. These beetles do not bite and are harmless to people. They do not infest furniture, clothing or other property. Foreign grain beetles are rarely found in dried food products in homes preferring grain that is old and moldy.

Most concerns come from people living in new homes. Beetles infesting new houses are usually found in wall voids feeding on the fungi that grow on moist wood and drywall. Most new homes have excess water in the wood and drywall, 50 gallons of water will be removed from the house as it dries out in the first year. Presence of foreign grain beetles does not mean that the mold present is harmful to humans. If populations are high, they exit around baseboards, wall sockets, and ceiling fixtures and move into the living quarters of the home. It is also possible for foreign grain beetles to infest older homes when moisture problems exist that encourages fungal growth.


The best short-term control of foreign grain beetles is to remove them physically with a vacuum cleaner. Remember they are not harmful and are just temporary nuisances. Insecticides are rarely warranted because foreign grain beetles do not reproduce outside of the walls and are short-lived. They are only a problem for about a month before going away on their own for the season.

In the long-term, foreign grain beetles associated with new construction are typically a problem for only one to two years. The wood will eventually dry out so that within a few years it will no longer support fungal growth and foreign grain beetles. You can promote drying in areas:

If an infestation is found in an older home, it is important to correct moisture problems contributing to the foreign grain beetle numbers.

Also, clean up any spilled grain or food product that may be present.

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