Foreign Grain Beetles May Appear In Mass Inside

Huge emergences of the foreign grain beetle can occur in homes and buildings in late summer. This 1/16-inch long reddish-brown beetle (Figure 1) is a fungus feeder. Moldy, out-of-condition grain or grain products are traditional sources. However, structural infestations develop in wall voids, crawl spaces, or attics where high humidity supports fungal growth. The insect normally does not develop when the relative humidity is below 65%.

Figure 1. Foreign grain beetle – note knob on edge of segment behind head (Photo: Lee Townsend, UK)

Figure 1. Foreign grain beetle – note knob on edge of segment behind head (Photo: Lee Townsend, UK)

Areas Attractive to Foreign Grain Beetles

Foreign grain beetle problems tend to appear in new homes (2 to 3 years old), following renovation of older homes, or in structures with moisture problems. In these situations, the beetles feed on the molds and fungi that grow on wood, lumber, plaster, or wallboards that have not dried completely. Damp wood or accumulations of sawdust behind walls can support molds that attract the beetles. Female foreign grain beetles lay their eggs on these surfaces.

The adults usually become a problem in late summer when they move out of wall voids and are attracted to windows and lights. Foreign grain beetles can also be associated with plumbing leaks, condensation problems, or poor ventilation.

Points to Consider

  • If new construction or remodeling are not potential sources for the infestation, check for and correct moisture problems that could be responsible for mold growth.
  • Check for moldy grain products that could be a source of the beetles. Discard any that are found.
  • Infestations will end spontaneously in a year or so as the structure seasons and mold growth ends. One option is to do nothing other than vacuum beetles as they appear. This includes using a crevice tool along baseboards, cracks, and crevices to remove beetles and larvae developing there.
  • Aerosol insecticides labeled for control of flying insects indoors can be used to kill active beetles. Those beetles hit by spray droplets are killed, but the products usually provide no residual control. This approach does not correct the conditions that cause the infestation.
  • Crack and crevice treatments with ant and cockroach insecticides labeled for indoor use can provide more extended reduction of infestations than flying insect sprays but do not correct the contributing conditions.
  • A pest control operator can provide treatments that are more thorough.


By Lee Townsend, Extension Entomologist



Posted in Household Pests
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