Foreign Grain Beetle a.k.a. New House Bug

The foreign grain beetle is found infesting brand-new homes in the late summer-early fall. First only a few beetles are seen, but when hundreds emerge, homeowners panic because their new house–which they think should be free from bugs and pests–is infested. They may even call a pest management professional to control the infestation. This beetle infestation is often misidentified–which is a costly mistake because treatments in new homes are generally ineffective and not recommended.

The foreign grain beetle is about 1/8" long and is reddish-brown. If you put this beetle under a microscope or hand lens, you might notice this beetle has clubbed antennae and noticeable bumps on the front corners of the thorax. These rounded projections are the primary features that identify this species.

These beetles have been nicknamed "plaster beetles" by home builders, because they appeared inside rooms of new buildings that had just been plastered. The foreign grain beetle is more properly known as a fungus beetle because it is attracted to and feeds on fungi and molds. Foreign grain beetles are often found in damp stored grain, spilled grain, and musty cereal products in grain mills and food processing plants.

These beetles get started during the construction of a new home when rain and moisture fall onto exposed wooden beams, studs and plywood before the roof and walls are constructed. If enough moisture accumulates, fungi starts to grow on the wood. Adult foreign grain beetles are attracted to the mold and begin laying eggs on the surface of the wood. The infestation, which is probably nearly unnoticeable because of the tiny size of these insects, continues even after drywall and plaster is added to the walls. Eventually larvae go through several stages and emerge as adult beetles from behind baseboards, electrical outlets and light fixtures. They are often attracted to light and, being strong fliers, will spread to all rooms of the house.

In older homes, foreign grain beetles can be found in damp basements, around rotting windowsills, under moldy wallpaper and on moist surfaces near leaky faucets. If foreign grain beetles are found in older homes, there may be moisture problems that are contributing to the infestation.

Our experience is infestations in older homes are not very common and infestations are nearly always found in new houses, often in rural settings. Most of the time, the infestation will only occur for a year or so at the most because the wood will dry out and become unsuitable as a breeding site. The best method of treatment is to vacuum and wait patiently because the infestation should go away by itself. Because insecticide applications will not treat the source of the infestation, treatments are not likely to be effective and not recommended.

This article was written by Dr. Barb Ogg, PhD, Extension Educator Emeritus and it appeared in the NEBLINE Newsletter. The information was updated November 2015 by Soni Cochran, Extension Associate.

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