Large black beetles found indoors at this time of the year are often ground beetles that have wandered inside by accident. Ground beetles are harmless. They do not attack people, pets, structures or household contents. They are an annoyance because of their presence.
Ground beetles live outdoors on the ground under leaves, logs, stones and other debris where they are ecologically beneficial because they feed on insect larvae. They are mostly active at night and may be attracted to lights. They gain entrance into the house by crawling in through small cracks or openings, or through open doorways and windows.
Special controls for ground beetles are usually not necessary. Invaders inside should be vacuumed or swept up and discarded. Invasion of ground beetles can be reduced by eliminating entry points by caulking gaps and cracks in the foundation and siding, or repairing doors, windows and screens. In particularly persistent situations or in sensitive accounts you can treat outdoor hiding places and apply a residual barrier of residual insecticide on and around the foundation, door thresholds and likely points of entry.
Identification. There are over 3,000 species of ground beetles in North America, but they all share certain characteristics that distinguish them from all the other large black insects that may be crawling on the basement floor. First, ground beetles vary in length from teeny-tiny to almost 3 inches in length but all are elongate and somewhat flattened in shape and have thread-like antennae. Most are hard-shelled and dull black in color. A few are green, tan or brown. Click here for more information. Click on the Images tab near the top center of the page for 124 more pages of ground beetle photos. Finally, when an entomologist is uncertain if a dark-colored, hard-shelled beetle is a ground beetle or not, we look for the "bean-shaped trochanters" (a handy term guaranteed to enliven any dull conversation). The trochanter is the second segment of the insect leg and is usually very small and inconspicuous. However, on the ground beetles the trochanters of the hind legs are large and obvious and on the larger species, can be seen with the naked eye as in this photo.