Beetles are found in almost any habitat occupied by insects and feed on a variety of plant and animal materials. Many are predatory; some are scavengers; many are plant feeders (phytophagous); others feed on fungi; and a few are parasitic on other organisms. Beetles may live beneath the ground, in water, or as commensals in the nests of social insects such as ants and termites. Plant-feeding species may eat foliage, bore in wood or fruit, and attack roots or blossoms; any part of a plant may be a food source for some type of beetle. Many beetles eat stored plant or animal products, including various types of foods and clothing.

Adult carabids are nocturnal hunting predators whose prey ranges from miscellaneous small insects to fairly large caterpillars and land snails. Many small carabids live in moss on tree trunks and feed on small insects hidden in the moss. The predatory larvae remain more hidden than the adults. Both larval and adult dytiscids (diving beetles) feed on various water insects, small frogs, and small fish. Gyrinids (whirligig beetles) feed both on water insects and on other insects that fall into the water. Staphylinids (rove beetles) are usually predatory, both as larvae and as adults. Hydrophilids are water scavenger beetles and eat various dead organisms and live algae in fresh water. Silphids, which dig under small dead animals so that they settle into the ground, lay their eggs on the carrion, on which the larvae feed. Dermestids feed on dead animal skins, dead insects, birds’ nests, other animal nests, and accumulations of debris. Ptinidae (spider beetles) feed on dead insects and animal skins, as do certain Anobiidae (e.g., the cigarette beetle, which also feeds on tobacco and other dried products). Heterocerids and histerids prey on fly larvae or those of beetles living in excrement or in carrion. Some Elateridae (click beetles) are predatory as larvae, feeding on the larvae of wood-boring insects; others feed on roots of various crops. Most larvae of Cantharidae (soldier beetles), which prey on worms and larvae of other insects, occur under bark, in rotting wood, or in soil; the adults are usually found on flowers. The lampyrids (fireflies), which are often luminous both as larvae and as adults, are primarily predators upon snails. Clerid larvae are largely predators of wood-boring beetle larvae, although some of the adults are flower feeders.

Among the lamellicorns, the true scarabs (Scarabaeinae) and several related groups (e.g., Aphodiinae, Geotrupinae) are dung feeders. Some make balls of animal excrement (e.g., of cattle) and roll them to protected spots for burying. Eggs are laid on the buried balls, and the larvae feed on the excrement.

The Melolonthinae (June beetles, chafers) are phytophagous, the larvae usually feeding on roots of grasses or other plants and the adults feeding on leaves. The larvae of Rutelinae (shining leaf chafers), which have similar habits, sometimes feed in humus or in rotten wood. The adults of Cetoniinae feed on pollen and tree juices; the larvae, often called white grubs, feed on organic matter in the soil and may damage plant roots. The Dynastinae (rhinoceros, Hercules, and elephant beetles) feed in rotting wood, decaying vegetation, or humus. Buprestidae bore into living and dead trees, generally feeding on the cambium layer. Some of the small buprestids are leaf miners; that is, they feed on leaf cells between the upper and lower surfaces of a leaf. Moderate numbers of larvae and adults of Passalidae (bess beetles), which feed on rotting logs, generally are found together; apparently the adults protect and feed their young, an unusual habit among beetles. The lucanids feed in logs and stumps as larvae; the adults often feed on juices from damaged trees, particularly at openings of larval tunnels of Cerambycidae (long-horned beetles).

Tenebrionidae (darkling beetles) live in various habitats as scavengers or predators. Those in rotten wood and under bark may be predatory on other larvae; those in damp places may feed on rodent excrement, on other wastes, or on fungi; and those in stored products feed on grain, meal, and other staples. Lagriinae (lagriids) and Alleculinae (comb-clawed beetles) feed in rotten logs. Melandryidae (false darkling beetles) usually feed on fungi or in old wood. Pythids usually are scavengers in burrows of other beetles, including weevils. Ripiphoridae (wedge-shaped beetles), which usually are parasites in wasps’ nests and undergo hypermetamorphosis, share several characteristics with another group of insects, the Strepsiptera, which are mostly parasitic in the bodies of wasps, leafhoppers, grasshoppers, and other insects. Some larvae of Mordellidae (tumbling flower beetles) may live in dead or dying deciduous wood or attack the heartwood of weak trees; others may be found in pith or herbaceous weeds. The adults frequent flowers and are good fliers. Meloidae (blister beetles) undergo hypermetamorphosis and usually live in bee nests, feeding on eggs and stored food. The larvae of Pyrochroidae (fire-coloured beetles) live under bark and in rotting wood. Most cerambycid larvae are wood borers; a few live in large herbs or in cones, and many feed in roots. The adults, which chew new bark or other plant parts, at times cut rings in branches, killing them so that the larvae can bore in. Some chrysomelid larvae eat leaves; many others are root feeders. Members of one group of chrysomelid larvae (Sagrinae) bore in stems of leguminous vines, causing gall-like swellings; others (Donaciinae) feed on and in the stems of freshwater plants, often below the water level, or hide (Cassidinae) in unopened parts of monocot plants such as palms, grasses, bamboos, and gingers; still others (Galerucinae) may feed on both roots and leaves or act as leaf miners. Bruchinae live primarily in the seeds of legumes, hence their common name, seed beetles (or bean weevils).

Weevils also are diverse in habits. Many larvae bore into solid wood of living or dead trees and stumps; some feed on or in roots or in stems of semiwoody plants; and others feed in seeds, pods, grain, meal, fruits, nuts, and other parts of plants; some are hidden inside the plants. Most adults are able to fly; nonflying forms, however, are more abundant on small oceanic islands than are flying forms. Some species, which develop in rotting stumps, soil, or palm trunks, make a rough cocoon from frass. Brenthids develop in dead trunks and stumps; anthribids live in various parts of plants, often in seeds; and scolytids feed largely under the bark of living trees, although some bore in other parts of plants, seeds, cones, and needles.

Special ecological relationships

Certain beetles have special ecological relationships. Beetles that live in nests of ants and in nests of termites have become modified in form because certain structures that are no longer needed have degenerated (e.g., wings, wing covers); some have almost lost the power of locomotion. Some species have evolved glands that produce secretions attractive to the host ants or termites.

An association of a different type involving beetles in the mountains of New Guinea is known; called epizoic symbiosis, the association occurs on the backs of large leaf-feeding weevils found on Nothofagus and other trees in the moss forests. Various kinds of algae, fungi, lichens, liverworts, mosses, and diatoms develop on the backs of the weevils. Among them live protozoans, rotifers, nematodes, phytophagous mites, and parasitic mites. The phytophagous mites, known only from this association, feed primarily in the fungal growth on the backs of the beetles. Plant spores, which may be carried from one weevil to another either by the mites during mating of the weevils or by air dispersal, are trapped with a sticky fluid that may be produced by the weevils for this purpose.

Ambrosia beetles (Scolytinae and Platypodinae) associate with fungi in the host tree. Certain adult scolytids and platypodids have specialized structures called mycetangia, which are used to carry the fungi when the beetles seek out new host trees.