Two beetle families are largely ground dwelling and predatory and should be considered gardener’s friends: ground beetles (carabids) and rove beetles (staphylinids). Adults and larvae of most species in these families eat insects, slugs and other invertebrate animals. There are about 350 species of carabid and 1000 species of staphylinid beetles in Britain
Rove beetles have elongate bodies with short wing-cases that leave most of the abdominal segments uncovered. They are 2-30mm (up to 1¼in) long and most are black or brown in colour, but some have red wing-cases and markings. The matt black devil’s coach horse (Ocypus olens) is Britain's largest rove beetle and is often found in gardens under logs or pots. A smaller species, Atheta coriaria, is bred and supplied as a biological control for fungus gnat larvae in glasshouses.
Ground beetles are 2-25mm (up to 1in) long many are black but some are brown, green or blue, often with a metallic sheen. Most feed at ground level, but some will climb plants to feed on aphids and small caterpillars. One common garden species, the violet ground beetle (Carabus violaceus), has metallic-purple edges to the wing-cases and thorax. Being one of the bigger carabids, 20-25mm long, it will feed on larger garden pests, such as slugs, leatherjackets and cutworms. Its long legs and powerful jaws which mark it out as an active predator.