Ecology and Behavior of Ground Beetles (Coleoptera: Carabidae)

1. What they did

Ground beetles (family Carabidae) are one of the largest beetle families, with more than 40,000 described species. They live in almost every habitat except deserts and are common in farm fields. The authors reviewed what had been learned about their ecology and behaviour since the last major overview of the group. They point out that most knowledge comes from temperate regions of the Northern Hemisphere, although the family is richest in the tropics, and they offer their examples as illustrations and starting points for work elsewhere.

The review covers evolution, study methods, life cycles, habitats, density and dispersal, daily and seasonal activity, feeding, reproduction, mortality and defences, population dynamics, assemblages and competition, and the beetles’ importance in farming and as environmental indicators.

2. What they found

Study methods

• Pitfall traps (containers sunk level with the ground) are the most used method, but catches reflect how active beetles are, not how many there are. The authors suggest pitfalls are fine for species presence and some population measures, but probably not for comparing species composition or diversity.
• Larvae are very hard to study, leaving a large gap in knowledge.

Life cycles and habitats

• Most ground beetles develop from egg to adult in under a year, breed once and die, but development can take up to four years in harsh climates. Adults of some species live up to four years in the wild, and one cave species up to 6.5 years.
• The larva is probably the stage that decides whether a species can live in a habitat, because larvae are weak, cannot travel far and must find food where their mother left them.
• Beetles find suitable habitat using internal clocks, a sun compass, and dark shapes on the horizon (silhouettes). Some riverside species follow the smell of algae, and one forest species is probably drawn to the smell of smoke from burnt areas. Food, temperature, moisture, competitors and season all shape where they live; for example, marked beetles moved from wheat into a weedy strip with better food much more often than the other way.
• True densities range from less than one to more than 1000 beetles per square metre, the highest at good overwintering sites.

Flight and dispersal

• Flight is costly, and flightlessness has evolved many times. In some species only some individuals have wings. In Canada, 60 to 70% of a spreading population at the edge of its range had wings, compared with only 20% in the source population in the city.

Activity

• More species are active at night than by day; in the United Kingdom, 60% are nocturnal and 20% diurnal. Night-active species tend to be larger and dull, day-active ones shiny and colourful.
• Activity times can change with habitat, season, temperature, light and humidity, and between individuals. One common species is nocturnal until August and then mainly diurnal. Low doses of insecticide can make beetles much more active.

Feeding

• Many find food by random search, some day-active species hunt by sight, and others follow chemical cues from aphids, springtails or snails. After finding prey, a beetle searches the area more intensely for a while before giving up.
• Many large species squirt digestive fluid onto prey and drink the result. Traces of a meal could be detected in the gut for up to 14 days.
• Most are generalists that eat live prey, carrion and plant material. A worldwide survey classed about three quarters of species as carnivores, but the authors think predation has been overestimated and plant and carrion feeding underestimated. Gut contents included aphids, spiders, caterpillars, mites, springtails, worms, snails and slugs, seeds, pollen and fungi; cannibalism has also been reported.
• Beetles can eat close to their own body mass in a day, yet in the wild they regularly go short of food and lay fewer eggs than they could.

Reproduction, mortality and defences

• Egg numbers range from five to ten per female in species that guard their eggs to several hundred in species that do not. Parental care goes no further than guarding eggs or storing seeds in the egg chamber for the larvae.
• Egg and larval deaths are probably more important than once thought: in one species 83% of eggs died in fresh leaf litter, and larval and pupal deaths together reached 96%.
• Hundreds of vertebrate species eat ground beetles. Keeping small mammals out of study plots increased beetle numbers and species.
• Defences include resting under stones, being active at night, running and hiding, escaping into water, playing dead, regurgitating, biting, making sounds by stridulation, warning colours, mimicry and, most effective of all, spraying chemicals from glands at the tip of the abdomen.

Populations and communities

• In The Netherlands, local populations of poorly dispersing species survived 40 to 50 years on average; one heathland species could not cross more than 200 m between fragments and needed 50 to 70 ha of habitat to persist. Since 1850, poorly dispersing species have declined there while good dispersers and farmland species have held steady or increased.
• Usually 10 to 40 species are active in a habitat at the same time. Disturbance removes the larger species.
• There is no convincing evidence that competition between ground beetles shapes their communities, although ants and ground beetles do seem to compete.

In farming

• Ground beetles are common in Northern Hemisphere farm fields. Together with spiders and rove beetles, they can significantly lower peak aphid numbers in English cereal fields, especially by feeding early in the season.

3. What they concluded

• Knowledge of ground beetles is biased toward north-temperate species, adults and farmland, and larvae remain poorly known.
• Feeding on both plants and animals, and scavenging, are probably more important than currently acknowledged.
• The most important causes of death are weather and other physical conditions, and predators; parasites and diseases can matter for some life stages.
• Competition occurs, but its importance in shaping communities is not proven. Studying “carabid communities” on their own is misleading, because ground beetles share their role as ground-hunting predators with spiders and ants.
• Their main value against pests is probably keeping pest numbers low between outbreaks; keeping good habitat (such as refuge strips) near crops helps.
• Their use as environmental or biodiversity indicators still needs testing against set criteria.

6. Other technologies, skills or innovations

• Methods reviewed for studying ground beetles: pitfall traps, marking and recapture, soil sampling and flooding, fenced pitfalls, quadrat and vacuum sampling, gut dissection, immunological (serological) tests and electrophoresis for diet, radioactive and isotope-labelled prey, video recording of walking, searching and feeding, and harmonic radar to follow movements within a habitat.

7. Implications for conservation (from the paper)

• Habitat fragmentation threatens poorly dispersing species: across most of Europe, suitable habitat patches change faster than these beetles can recolonize them.
• Ground beetles have been used to assess pollution, classify habitats for nature protection and judge soil nutrients in forestry, and may serve as biodiversity indicators once tested against agreed criteria.
• Field edges and habitat islands can act as refuges and recolonization points that keep these natural pest enemies close to crops.