Ground Beetles
Ground beetles (family Carabidae) are important predators found in most
agricultural and garden settings. They may be the most numerous predatory
insects in certain systems. Hundreds of species of carabids occur in the
Midwest. They vary in size from a less than 1/4" to over 1-1/2" long. The adults
of most species are dark brown or black, shiny, and somewhat flattened, with
slender legs for running. A few are an iridescent blue or green. They are
commonly found under leaves or debris, in cracks in the soil, or running along
the ground. Some species also climb into trees, shrubs, and crop plants looking
for prey. Adult ground beetles run quickly when disturbed, but they rarely fly.
Some species emit a strong smelling irritant when handled. Many are nocturnal
and some are attracted to lights at night. In general, arable land contains more
carabids than does land planted to permanent crops such as clover or alfalfa.
Most species are more common on clay than on sandy soils.
The adults are fierce predators that chew up their prey with their large,
sharp mouthparts. Caterpillars, grubs and adults of other beetles, fly maggots
and pupae, earthworms, and other small soil dwellers are common prey for many
ground beetles. They can consume their body weight in food daily. Eggs are
deposited either on objects above ground or in cavities made in the soil. One of
the better studied ground beetles in vegetable crops, Lebia grandis, lays
hundreds of eggs. Other species may lay only a few. The three larval instars
live in debris or in burrows in the soil. The larvae are usually also
predaceous, although in some species the adult and larval foods are very
different. A few species are not predaceous as larvae. For example, Lebia
grandis, a medium-sized reddish beetle with metallic green or blue
forewings, is an external parasite of Colorado potato beetle pupae as a larva.
There is usually one generation per year, but the larvae of some species may
require more than one year to complete development, and adults of larger species
can live 2 to 4 years. The majority of species overwinter as adults in the soil
or in sheltered sites.
Beetles in the genus Calosoma are called caterpillar hunters. They are
among the largest in the family and both the adults and larvae are very active
predators. Calosoma sycophanta, a large, bright metallic green beetle,
was imported from Europe to New England for the biological control of the gypsy
moth in 1905. The larva feeds day and night, consuming 50 caterpillars during
its two-week developmental period. The adult will eat several hundred
caterpillars during a life span of two to four years. There are also several
native species of Calosoma.
Although ground beetles have been captured in large numbers during insect
surveys, there is relatively little information on their actual impact on pest
populations. Since they are so numerous and do consume many times their weight
in prey (if available) they probably provide significant pest control in many
situations.
These important natural enemies can be conserved by avoiding disruptive
gardening practices, such as frequent tillage, and not using broad-spectrum soil
insecticides or fumigants. Their survival will be enhanced by providing refuges
and overwintering sites such as hedgerows or mulch for the adult beetles. This may be as simple as straw mulch, which provides
humid, sheltered hiding places for nocturnal predators.
– Susan Mahr, University of Wisconsin
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