What are they?
Two soil-dwelling beetle families are primarily 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 345 species of carabids and 980 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 generally black or brown in colour. The matt black devil’s coach horse (Staphylinus olens) is one of the larger rove beetles and is often found 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 and mostly black but some are brown or green. 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 mark it out as an active predator.