Lesser Grain Borer & Larger Grain Borer

Damage

Damaged kernels are the ones most frequently attacked by these beetles, however, the grain borers are capable of invading undamaged grain. Lesser grain borers also infest corn, wheat, barley, rice, and has been known to attack wood and books. Infestation in flour and dried cassave also have been encountered.

Biology and life cycle

The female lays from 200 to 500 eggs in her lifetime and either drops them loosely into the grain or lays each individually in cracks in grain. Higher egg production has been observed at higher temperatures. The larvae are white with a dark head and prominent legs. The larvae usually bore into grain kernels, taking advantage of any cracks in a kernel’s exterior shell. Larvae also can feed on and survive on the dust created by the adult beetles’ feeding. Development from egg to adult can be quite rapid at high temperature and high humidity, taking just 21 to 28 days. Pupation takes place within the kernel and exit holes are evident after the adult emerges. Grain kernels are often completely hollowed out with a thin shell left behind. Infested grain also contains quite a bit of grain dust created by adult beetle feeding activity.

Adult lesser grain borers are quite long-lived and are strong fliers. The adults are often not that evident when inspecting infested grain as they eat into the inside if grain kernels where they are not easily visible. Tiny parasitic wasps of the family Pteromalidae are natural enemies of this beetle and could be found in storage bins where these beetles are found.

Characteristics

The lesser grain beetle, Rhyzopertha dominica, is a very small (1/8″, 3 mm) cylindrically shaped beetle that is dark brown in color and somewhat shinny in appearance. The head is tucked underneath the prothorax and so is not visible from above. The beetle belongs to the beetle family Bostrichidae which include the false powderpost beetles. Most species of this family infest wood, bamboo, and similar cellulose materials and these powderpost species could possibly be found in a home or warehouse.