Beetles, belonging to the order Coleoptera, represent one of the most diverse groups of insects on Earth. Found in nearly every habitat occupied by insects, they exhibit an astonishing array of feeding habits. From voracious predators to dedicated scavengers and specialized plant feeders, beetles have adapted to exploit a wide variety of food sources. This article delves into the fascinating dietary habits of beetles, exploring the diverse feeding strategies employed by different species and their ecological significance.
General Feeding Habits of Beetles
Beetles possess chewing mouthparts, both as adults and larvae, equipped with strong, triangular jaws capable of exerting significant biting and grinding force. This allows them to process a wide range of food items. Beetles are far from picky eaters. As well as scavenging in leaf litter, beetles eat plants from the roots to the shoots. They also tackle mammal dung, carrion, other insects and even each other. Beetles graze mould in compost bins and are among the most important composting recyclers when it comes to dead wood and fungal decay. Some steal food from spiders’ webs. A few species invade our homes to infest our stored food and eat our carpets.
Beetles took advantage of the explosion in plant diversity with the evolution of flowering plants 120-100 million years ago, and now are the second most speciose plant-feeding insects after moth caterpillars. Many of the over 135,000 specialist plant-feeders in the important beetle lineage Phytophaga are extremely host-specific. They will feed only on a single plant species - and often on just one part of that plant. This does mean that some beetles have become serious garden or crop pests. Notorious examples include the lily beetle, Lilioceris lilii; the Colorado beetle, Leptinotarsa decemlineata, which devours potatoes; and the western corn root-worm, Diabrotica virgifera, which attacks maize.
Predatory Beetles
Many beetle species are active predators, both in their larval and adult stages. Their prey ranges from small insects to larger caterpillars and land snails.
Carabid Beetles (Ground Beetles): Ground beetle is a catch-all name given to insects in the Carabidae family of beetles of the order Coleoptera. Also known as carabids, ground beetles comprise one of the largest insect families, with approximately 40,000 species worldwide and 2,339 species in the United States. Ground beetles are known for their long legs and powerful mandibles which enable them to be voracious predators, important for the biological control of insect pests on farms. The adult beetles hunt primarily on the soil surface, but will occasionally climb into the foliage in search of food. In addition to the adults being beneficial predators, the burrowing larvae of these beetles seek out and feed on pests in the soil. Many ground beetle species have broad feeding habits, eating not only other insects but also seeds of plants (including weeds).
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- Bembidion: This group is the largest genus and has several species in North America. They are fairly recognizable due to their small size (<8.5 mm) and rapid movement. Additionally, they are often good fliers, which is beneficial for hunting and escaping. Bembidion include both day-active and nocturnal species; most are water-loving. Various species of Bembidion are active throughout the summer. Several studies have found them to be excellent predators of the small eggs and pupae of pest insects, particularly of flies. However, while these ground beetles are small enough to make good use of such tiny prey items, they are also very vulnerable to being eaten by other, larger, ground beetles, as well as other ground-dwelling predators such as spiders.
- Pterostichus: This is one of the most common ground beetles in agricultural fields in much of North America. Chances are you have seen members of this group many times as they run across the ground in search of prey items. The group is distributed across North America and found in a wide range of habitats including grasslands, open meadows, forests, agricultural fields, in urban areas, etc. They are medium-to large sized ground beetles (~ 12-28 mm). Many members of this group are potential biological control agents of slugs and other pests. Additionally, some studies have shown that they can climb the lower levels of plants to attack foliar pests such as aphids directly. As mentioned earlier, large Pterostichus ground beetles often feed on smaller ground beetle species which can harm natural pest control. Pterostichus species are typically active during the night, though day active species do occur. Pterostichus species tend to emerge as adults in early summer and are active through the fall before overwintering.
- Carabus: These are among the largest of ground beetles, ranging in size from 10-50 mm. Similar to Pterostichus, many North American species have dark coloration with some species displaying iridescent and brighter coloration. Additionally, many have distinctive dimples on their wing covers. Due to their size they have been studied as control agents of particularly large pests such as slugs and caterpillars. Because they are so large, Carabus might not directly compete with smaller ground beetles for food. Carabus are typically active during the night.
Diving Beetles (Dytiscidae): These beetles live in water and prey on small frogs and fish. Both larval and adult dytiscids feed on various water insects, small frogs, and small fish.
Rove Beetles (Staphylinidae): Rove beetles are usually predatory, both as larvae and as adults.
Soldier Beetles (Cantharidae): Most larvae of Cantharidae prey on worms and larvae of other insects and occur under bark, in rotting wood, or in soil. The adults are usually found on flowers.
Fireflies (Lampyridae): Fireflies, often luminous as larvae and adults, primarily prey upon snails.
Clerid Beetles: Clerid larvae are largely predators of wood-boring beetle larvae, although some of the adults are flower feeders.
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Heterocerids and Histerids: These beetles prey on fly larvae or those of beetles living in excrement or carrion.
Some Click Beetles (Elateridae): Some click beetle larvae are predatory, feeding on the larvae of wood-boring insects.
Scavenging Beetles
Many beetles play a vital role as scavengers, feeding on dead organic matter, carrion, and waste products.
- Water Scavenger Beetles (Hydrophilidae): These beetles eat various dead organisms and live algae in fresh water.
- Silphids: Silphids dig under small dead animals, lay their eggs on the carrion, and the larvae feed on it.
- Dermestids: Dermestids feed on dead animal skins, dead insects, birds’ nests, other animal nests, and accumulations of debris.
- Spider Beetles (Ptinidae) and Some Anobiidae: These beetles feed on dead insects and animal skins. The cigarette beetle (Anobiidae) also feeds on tobacco and other dried products.
- Tenebrionidae (Darkling Beetles): 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): These beetles feed in rotten logs.
- Melandryidae (False Darkling Beetles): These beetles usually feed on fungi or in old wood.
- Pythids: These beetles are usually scavengers in burrows of other beetles, including weevils.
Plant-Feeding Beetles (Phytophagous)
A significant number of beetle species are phytophagous, feeding on various parts of plants. 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.
- June Beetles, Chafers (Melolonthinae): The larvae usually feed on roots of grasses or other plants, and the adults feed on leaves. The name ‘May beetles’ applies to beetles in the genus Phyllophaga. They’re sometimes called June bugs because they tend to be more active and visible during that month of the year. May beetles are native to North America. Adult June bugs feed primarily on tree leaves. They eat the leaf at the margins but may sometimes eat all the way to the midvein. Common host trees they target include ash, oak, crab apple, and other trees with broad leaves. Farmers are often puzzled by the damage because the insects are not present during the day. The larvae of the June bugs live underground, feeding on plant roots.
- Shining Leaf Chafers (Rutelinae): The larvae sometimes feed in humus or in rotten wood.
- Cetoniinae: The adults feed on pollen and tree juices; the larvae, often called white grubs, feed on organic matter in the soil and may damage plant roots.
- Rhinoceros, Hercules, and Elephant Beetles (Dynastinae): These beetles feed in rotting wood, decaying vegetation, or humus.
- Buprestidae: These beetles bore into living and dead trees, generally feeding on the cambium layer. Some of the small buprestids are leaf miners, feeding on leaf cells between the upper and lower surfaces of a leaf.
- Passalidae (Bess Beetles): Moderate numbers of larvae and adults feed on rotting logs.
- Lucanids: The larvae feed in logs and stumps; the adults often feed on juices from damaged trees, particularly at openings of larval tunnels of Cerambycidae (long-horned beetles).
- Cerambycidae (Long-Horned Beetles): Most cerambycid larvae are wood borers; a few live in large herbs or in cones, and many feed in roots. The adults chew new bark or other plant parts and at times cut rings in branches, killing them so that the larvae can bore in.
- Chrysomelidae: Some chrysomelid larvae eat leaves; many others are root feeders. Members of one group (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 (Seed Beetles or Bean Weevils): These beetles live primarily in the seeds of legumes.
- Weevils: Weevils 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.
- Brenthids: These beetles develop in dead trunks and stumps.
- Anthribids: These beetles live in various parts of plants, often in seeds.
- Scolytids: These beetles feed largely under the bark of living trees, although some bore in other parts of plants, seeds, cones, and needles.
Dung-Feeding Beetles
- Dung Beetles (Scarabaeinae, Aphodiinae, Geotrupinae): These beetles feed on animal excrement. Some make balls of animal excrement and roll them to protected spots for burying. Eggs are laid on the buried balls, and the larvae feed on the excrement. Most dung beetles use the manure of herbivores, which do not digest their food very well. Their dung contains half-digested grass and a smelly liquid. It is this liquid that the adult beetles feed on. Some of them have specialized mouthparts designed to suck out this nutritious soup, which is full of microorganisms that the beetles can digest. Scientists group dung beetles by the way the beetles make a living: rollers, tunnelers, and dwellers. Rollers form a bit of dung into a ball, roll it away, and bury it. The balls they make are either used by the female to lay her eggs in (called a brood ball) or as food for the adults to eat. Tunnelers land on a manure pat and simply dig down into the pat, burying a portion of the dung. Dwellers are on top of the world-or at least the dung heap. The female lays her eggs on top of manure piles, and the entire development from egg to adult takes place inside the dung pat.
Seed Predator Beetles
- Harpalus: This is another large group of middle to large sized beetles (5.8-25.5 mm). While widely distributed, they mostly occur in open, dry, sandy areas. As adults they are mainly seed predators, munching away at the myriad weed seeds deposited in the soil. However, some species can make competent predators of pests, particularly in their larval stage, so this group is broadly useful. Harpalus are typically active during the night.
- Amara: This group of ground beetles are generally medium sized (3.9-14.3mm) and many species have a metallic sheen. Many species in this group can be recognized by their pill shape. Amara have primarily been shown to be weed seed predators and have been associated with general seed rain suppression. Amara as a group includes both day active and nocturnal species, and are often found in abundance on farms in early summer.
Special Ecological Relationships
Some beetles have developed unique ecological relationships, including:
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- Beetles in Ant and Termite Nests: These beetles have become modified in form, with certain structures degenerating (e.g., wings, wing covers). Some have evolved glands that produce secretions attractive to the host ants or termites.
- Epizoic Symbiosis: This association occurs on the backs of large leaf-feeding weevils in the mountains of New Guinea, where various kinds of algae, fungi, lichens, liverworts, mosses, and diatoms develop.
- Ambrosia Beetles (Scolytinae and Platypodinae): These beetles associate with fungi in the host tree, carrying the fungi in specialized structures called mycetangia.