Predacious ground beetles, also referred to as carabids, belong to a large family of beneficial beetles called the Carabidae. There are several thousand species of these beetles in North America alone. The adult beetles range from 1/8-inch to 1╝ inches long. Variable in shape, they are usually elongate, heavy bodied, and slightly or distinctly tapered at the head end. While generally dark in color (dark brown to black), some beetles are an attractive purple or metallic green, or are multi-colored, as in the case of some tropical species.
Ground beetles are fast-moving insects, which generally have prominent, long legs and fairly threadlike antennae. Most species native to this area are found in the wetter regions (or around waterways) of the state. There they hide under logs, rocks, or in soil crevices during the day, because they are largely nocturnal insects. Adult carabids often are among the many insects that are attracted to lights at night.
Fig. 1. Carabid ground beetle larva.
Larval ground beetles are elongate and wormlike in appearance (Fig. 1); most live in burrows in the soil or in leaf litter or other debris. Both larval and adult ground beetles have powerful and prominent mandibles. This is an adaptation for their predacious life style. Most ground beetles in this area feed on a varied diet of insects and insectlike creatures, many of which are garden or house pests, such as cutworms or house fly maggots. One specific group of ground beetles feeds on snails and slugs. Some occasionally feed on earthworms, but their beneficial feeding habits, in general, far outweigh any detrimental effects they may have on local earthworm populations. A few species will even feed on pollen or seeds.
When carabids are disturbed they often demonstrate some form of defensive behavior. Many northwestern native species secrete chemicals that have a bad odor. This behavior is thought to be a deterrent to animals which might eat them. One group of ground beetles includes the famous bombardier beetles, a subject of many nature films. These beetles emit, from their anal glands, toxic chemicals that explode into puffs with an audible report. This action deters frogs or other vertebrates which try to sample these apparent "morsels."
Carabid ground beetles go through egg, larval, and pupal stages of development before reaching adulthood. Development from the egg to the adult stage generally takes about a year. The adults of some species lay their eggs in specially constructed mud or twig cells, while others lay their eggs in debris. Some adults may live 2 to 3 years.
Fig 2. European ground beetle adult.
European Ground Beetle (Carabus nemoralis). The adult beetle is about 3/4 -inch long, with wing covers that are violet, or black-tinged with violet, or greenish bronze in color (Fig. 2). Three rows of pits or dimples appear on the surface of the wing covers.
Green Pubescent Ground Beetle (Chlaenius spp.). This species is elongate-oval in shape and up to 5/8-inch long. The top surface is bright green, the underside is black, and the legs are brownish yellow. These beetles often dwell around the margins of rivers, streams, ponds, or lakes.
Fig 3. Common black ground beetle adult.
Common Black Ground Beetles (Pterostichus spp). These are elongate, shiny black beetles that have prominent, lengthwise grooves on the wing covers (Fig. 3). They are from 1/2- to 5/8-inch long.
Fig 4. Boat-backed beetle adult.
Boat-Backed Ground Beetles are distinctively different in appearance from most common ground beetles, in that the body region under the wing covers is oval, and purple or black in color, while the front region of this beetle is narrow and black (Fig. 4). Boat-backed beetles (Scaphinotus spp.) live in very moist areas and are noted for their habit of feeding on snails and slugs.
Fig. 5 and Fig. 6. Oregon tiger beetle adult and tiger beetle larva.
Tiger Beetles. While tiger beetles (Figs. 5 and 6) are technically members of a different family of ground-inhabiting beetles (Cicindelidae), they are closely related to the carabid group. The most noticeable difference between these two families is that the head of the tiger beetle adult, including the eyes, is almost always wider than the thorax behind. The carabid's head is almost always narrower than the thorax. Tiger beetles vary in color, and are often metallic, elongate (3/8- to 3/4-inch long), with prominent heads and extremely large, powerful mandibles. They are sun-loving and found in open, sandy or dry soils. Extremely fast, they prey on many small insects, both as adults and as larvae. The s-shaped larvae ambush their prey from vertical tubes in the soil. One of the more common tiger beetles is the Oregon tiger beetle (Cicindela oregona).
Fig. 7. Metallic green ground beetles. These were victims of a garden pesticide application. Such disasters are commonly noticed as groups of dead beetles scattered on sidewalks, or near gardens.
Avoid using insecticides when and where they are not truly needed. For example, do not spray the lawn simply because you have a chemical left over that you were using on shrubs. Better yet--mix only enough pesticides to cover the target plant or area where the plants are found. Avoid drift--deliberate or otherwise--that will contaminate large areas of the ground where these beneficial insects live. Use selective pesticides whenever possible. Many chemicals are applied for control of caterpillar pests. A bacterial pesticide called Bacillus thuringiensis (Dipel«, Thuricide«, and others) kills only caterpillar-type insects and will not harm beneficial insects such as carabids. Derivatives of this pesticide that are currently being used are selective against mosquitoes. Selective chemicals similar to these will go far in preserving beneficial species. Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock. It is a violation of the law to disregard label directions.
Use pesticides with care. Apply them only to plants, animals, or sites listed on the label. When mixing and applying pesticides, follow all label precautions to protect yourself and others around you. If pesticides are spilled on skin or clothing, remove clothing and wash skin thoroughly. Store pesticides in their original containers and keep them out of the reach of children, pets, and livestock. It is a violation of the law to disregard label directions.
For more information contact your local WSU Extension office.