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Foreign grain beetle

Quick facts

  • Foreign grain beetles can be common insects in homes in Minnesota in August and September, particularly in newly constructed buildings.
  • They live in damp areas where fungus grows.
  • They are just nuisances when found indoors.
  • The best control is to remove them physically.
  • Pesticides are rarely, if ever, required.
  • They are only a problem for about a month before going away on their own.

How to identify foreign grain beetles

Adult foreign grain beetle.
  • It is a small, flattened insect, about 1/12 inch long and reddish-brown in color.
  • Under magnification, it can be identified by two peg-like projections behind its head.
  • They are strong fliers and can be confused with fruit flies or gnats.
    • Foreign grain beetles have a hard shell compared to flies.
    • Wings fold under their wing covers when at rest making them hard to see.
  • They can be mistaken for fleas or lice but neither of these insects have wings.
    • Lice are also flattened but are soft-bodied.
    • Fleas are flattened from side to side and are strong jumpers.

Biology

Foreign grain beetles live in damp areas where fungus grows.

  • They need a relative humidity of more than 65% to reproduce.
  • They feed on a variety of stored products that are damp and moldy.
  • They can also feed on pure mold.

They produce large numbers of offspring in a short amount of time.

  • Foreign grain beetles can complete their life cycle in about 30 days.
  • Once the humidity falls below 60%, they die and disappear on their own.

They are commonly found in newly constructed homes.

  • They are most common in August and September.
  • Beetles infesting new houses are usually 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.
  • Large numbers of beetles exit around baseboards, wall sockets and ceiling fixtures and move into the living quarters of the home.
  • Foreign grain beetle numbers drop dramatically by October.

Foreign grain beetles do not cause damage

A group of foreign grain beetles.

Foreign grain beetles are a nuisance when found in homes.

  • They 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, unless they are old and moldy.
  • Most problems involve newly built homes. Infestations typically go away on their own after one or two years.
  • Presence of foreign grain beetles does not mean that the mold present is harmful to humans.
  • They can infest older homes when there are moisture problems that encourage fungal growth.

How to get rid of foreign grain beetles

The best short-term control is to remove them physically with a vacuum cleaner.

  • Remember they are not harmful and are just temporary nuisances.
  • Pesticides are rarely required because:
    • Foreign grain beetles do not reproduce outside of the walls.
    • They are short-lived.
  • They may cause a problem for about a month before going away on their own for the season.

In new constructions

Foreign grain beetles associated with new construction are typically a problem for only one to two years.

The wood eventually dries out and within a few years, it will no longer support fungal growth and foreign grain beetles. Some steps can be taken to speed up the drying process:

  • Increase the air exchange in the house by increasing the comfort setting on the air exchange.
  • Use a dehumidifier for up to a month.
  • Use ventilation in bathrooms while taking showers or baths.

In older homes

If an infestation is found in an older home, correct conditions that provide high moisture situations and help foreign grain beetles survive.

  • Make sure you install and use a vent fan while taking showers and baths.
  • If you have a crawlspace under your home, contact a pest management company to see if you have adequate ventilation and that this crawlspace is dry.
  • Check along the roofline for poor attic ventilation, trees overgrowing the roof, or leaf litter in the eaves and correct any of these conditions that are found.
  • Clean up any spilled grain or food product that may be present.

Jeffrey Hahn and Stephen Kells, Extension entomologists

Reviewed in 2020

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