Darkling Beetle

Darkling beetle adults are from 0.13 to 0.25 inch (3.5 to 6 mm) long and vary from black or blue-black to rusty brown. They may be hidden by dust or a thin veneer of soil. Do not confuse darkling beetles (Tenebrionidae) with predaceous ground beetles (Carabidae), which prey on various soil dwelling pests. Darkling beetles generally have clubbed antennae whereas predatory ground beetles do not.

Larvae are cylindrical, soil-inhabiting, wormlike insects that resemble wireworms, the larvae of the click beetle. Darkling beetle larvae are light yellow to dark brown and range from 0.03 to 0.33 inch (0.8 to 8 mm) in length. They are often referred to as false wireworms.

Development from egg to adult can take up to 50 days during summer. Eggs hatch in 3 to 6 days, followed by about five larval instars. The pupal stage lasts 8 days or longer. Beetles are most numerous in spring and early summer, often found under clods or organic debris during daylight hours and occasionally running along the ground.

Damage is similar to that caused by cutworms: seedlings are chewed off at the base and foliage may be chewed as well. Damage usually begins at field edges, as beetles tend to come in from weedy areas, alfalfa, or cover crops. Feeding occurs primarily in the evening and at night. Both larvae and adults can cause damage. After the plants reach a height of 5 to 6 inches, darkling beetles are usually not a problem.

Start inspecting plants for darkling beetle damage when the crop emerges. Inspect on and under the soil line for adult darkling beetles to ensure that a different pest, such as cutworm, did not cause the damage. Apply an insecticide if darkling beetles are causing a reduction in stand of the young plants. Since infestations are frequently spotty, damage and insecticide treatments may be confined to field margins or specific portions of the field.