Beetles

The vast number and variety of beetles have inevitably had an important impact on the human populations that share environments with these insects. Beetles, like some other insects, pose a threat to agriculture, feeding on crops and wood, both harvested and stored. For example, the dermestid beetles of the family Dermestidae are widely distributed and feed on cereal products, grains, stored food, rugs and carpets, upholstery, and fur coats. Although the adults of some species may be destructive, usually it is beetle larvae that do the most damage.

The grain weevil and the rice weevil are particularly destructive, having evolved a snout that can penetrate food plants and also bore holes to deposit eggs. The boll weevil (Anthonomus grandis) is a major cotton crop pest in North America. The boll weevil deposits up to 300 eggs at a time in cotton buds or fruit. The larvae live within the cotton boll, destroying the seeds and the surrounding fibers. The Colorado potato beetle (Leptinotarsa decemlineata) attacks the leaves of potato plants; it became a major pest in the United States during the late nineteenth century.

Beetles, like other insects, also eat a variety of plants that are not of agricultural value, but may be of aesthetic value to humans. For example, two forms of ladybird beetles, the Mexican bean beetle and the squash beetle, are voracious garden pests; and some blister beetles commonly parasitize eggs or larvae of bees.

Not all beetle activity is destructive, however. Beetles, along with other insects, also help to pollinate flowers, which then produce fruits and seeds. Beetles crawling over flowers brush up against pollen-bearing organs, and carry the pollen dust to another flower of the same species.

In addition, some beetles keep gardens from being overrun by plant pests. For example, most species of ladybird beetles feed as adults and larvae on aphids, scale insects, mites, and other insect pests. These highly predatory beetles are an important factor in keeping populations of plant-feeding pests such as leaf beetle larvae and other insects from reaching plague levels. And the larvae of wedge-shaped beetles are parasitic on cockroaches.

An Australian ladybird beetle, the vedalia beetle (Rodolia cardinalis), is used throughout the world to control crop pests, such as the coconut scale, sugarcane mealy bug, potato aphid, and fir aphid. In addition, certain pollen or sap beetles of the family Nitidulidae prey on the eggs, nymphs, and adult stage of a variety of whitefly and aphid species, among other insects.