Ground beetles, also known as carabid beetles or carabids (family Carabidae), represent a diverse and ecologically significant group of insects. With over 26,000 species identified worldwide and approximately 2,339 species in the United States, they are a common yet often overlooked component of various ecosystems, especially agricultural areas. These beetles are generally recognized as beneficial insects due to their predatory nature, impacting pest management by feeding on other insects and weed seeds. This article explores the dietary habits of ground beetles, shedding light on their role in controlling pests and the factors influencing their feeding preferences.
General Characteristics of Ground Beetles
Ground beetles exhibit a wide range of physical characteristics. Most adults are glossy and black, but some are iridescent, green, yellow, or orange. Their bodies are typically flattened, with grooves or rows of punctures running down the wing covers. They are usually longer than wide, possess long legs for their size, and are capable of running quickly. These beetles have threadlike, long antennae that do not come to a club or branch, five tarsal segments on each leg, and an expanded hind trochanter (hip joint), which looks a bit like a jelly bean, lying next to the femur of the hind leg.
The front wings of ground beetles are modified into distinctive protective shell-like coverings called elytra that protect the hind wings. While adult carabid beetles have wings, most rarely fly, and many ground beetles are completely flightless.
Habitat and Behavior
Ground beetles can be found in just about any habitat that has other small animals for them to eat. They are specialized for many modes of life in and on the ground, as well as on plants, under bark, or as "miners" spending most of their time digging underground. The majority of carabid species are nocturnal, preferring to hide under rocks, logs, or leaf-litter during daylight hours. They emerge from their hiding places at night to search for prey, sometimes climbing up into vegetation. Larvae do not come out on the surface but stay in dead leaves and the surface soil.
Ground Beetle Diet: Predators and Seed Feeders
Ground beetle larvae and adults are primarily predators, feeding on other small animals. However, some species also feed on weed seeds. The diet of ground beetles includes:
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- Insects and Invertebrates: Ground beetles are known to prey on a variety of insects, including asparagus beetles, cabbage worms, Colorado potato beetles, corn earworms, cutworms, slugs, aphids, ants, maggots, and small wasps. They also feed on other invertebrates, such as snails.
- Weed Seeds: Many ground beetle species consume weed seeds, contributing to weed control in agricultural settings. Some species have been shown to consume 90% of the seeds of certain weed species. Weed seeds consumed by ground beetles include common ragweed, common lambsquarters, and giant foxtail.
- Other Food Sources: Some ground beetles also eat earthworms, although their benefits as predators generally outweigh this. Gut content studies have found ragweed pollen, ants, and mites in some species.
Feeding Habits of Specific Ground Beetle Species
Different ground beetle species exhibit specific feeding habits and preferences. Here are some examples:
- Megacephala carolina (Pan-American big-headed tiger beetle, Carolina tiger beetle): These beetles are predatory and nocturnal.
- Cicindela punctulata (punctured tiger beetle, sidewalk tiger beetle): This species is a widely distributed upland beetle often found in agricultural settings.
- Harpalus pensylvanicus: This beetle has been observed feeding on seeds of wheat, Timothy grass, and prairie grass. Gut content studies have found ragweed pollen as well as ants and mites. Harpalus species have been shown to consume 90% of the seeds of certain weed species.
- Anisodactylus fervus, Anisodactylus merula, and Anisodactylus sp.: These beetles are omnivorous, feeding on seeds, caterpillars, and in the laboratory on mealworms. Seed predation by these beetles can reduce weed pressure from redroot pigweed (Amaranthus retroflexus) and lambsquarters (Chenopodium album).
- Poecilus chalcites: This species is a common predator on a number of crop pests, including western corn rootworm, corn earworm, black cutworm, and armyworm.
- Calosoma sayi: These relatively large beetles are often called "hunters" or "searchers" and famously prefer caterpillars.
- Amara cupreolata (Sun beetles): This species is common and can often be seen running about vigorously on bright sunny days.
- Chlaenius tomentosus: This species seems to prefer cultivated land, old fields, and vacant lots.
Seasonal Diet Shifts
Many ground beetle species exhibit seasonal diet shifts based on food availability. They may move back and forth between insect prey and weed seeds, allowing them to exploit different resources throughout the year. This allows the long-lived beetles to survive in farm environments when any single pest or weed is not available as food.
Factors Influencing Ground Beetle Diet
Several factors influence the diet of ground beetles, including:
- Food Availability: Ground beetles are opportunistic feeders, and their diet is influenced by the availability of different food sources.
- Habitat: The habitat in which ground beetles live can influence their diet. For example, ground beetles in agricultural settings are more likely to feed on insect pests and weed seeds.
- Species: Different ground beetle species have different feeding preferences and habits.
- Life Stage: Larvae and adults typically have similar feeding habits; however, larval diets may be more restricted due to a limited search range underground.
- Sex: Females tend to have a more varied diet than males, which has been linked to greater egg size and egg number.
Benefits of Ground Beetles in Pest Control
Carabid beetles play a major role in reducing weeds and insect pests in agricultural fields. Farmers have long been encouraged to view carabid beetles as naturally occurring pest control. Although it is difficult to say exactly how much pest control is provided by these beetles on individual farms, it is safe to say that they have a meaningful impact like other general predators such as ladybeetles and spiders.
The pest control benefits of ground beetles include:
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- Reducing Insect Pest Populations: Ground beetles prey on a variety of insect pests, helping to keep their populations in check.
- Controlling Weed Populations: Ground beetles consume weed seeds, reducing weed pressure in agricultural fields.
- Contributing to Natural Pest Suppression: Ground beetles are among the most tenacious generalist predators that contribute to natural pest suppression on farms.
Conservation Strategies for Ground Beetles
There are a number of strategies growers can employ that show promise for conserving beneficial ground beetles in crop fields. These include:
- Reduced Tillage: Ground beetles are generally vulnerable to deep or inversion tillage and benefit from reduced tillage regimes.
- Increasing Ground Cover: Increasing amounts of ground cover in crop fields benefit ground beetles, and cover crops can enhance beetle numbers.
- Creating Refuges: Good refuges for ground beetles can be created in the borders of the field by planting a diverse mix of annuals and perennials. A similar strategy is to create these "beetle banks" within the crop field itself.
- Delaying Fall Cover Crop Planting and Fall Tillage: This strategy is specifically beneficial for weed seed-eating ground beetles.
- Constructing areas of tall grass as a refuge for beetles: These are often called beetle banks that can enhance the hunting activity of ground beetles in nearby farm fields by providing stable and insulated overwintering habitat, as well as alternative prey to help augment ground beetle diets while pest prey are at low abundances.
- Living mulches: Living mulches, like red clover and white clover, planted between crop rows provide an ideal microclimate as well, and can attract huge numbers of ground beetles.
- Nectar-producing plants: Nectar-producing plants will bring in airborne predators forcing some pests to flee onto the surface of the ground, where they are then available for ground beetles to devour.
- The use of compost around plants: The use of compost around plants has been shown to increase the abundance of carabids compared to bare soil, probably because the mulch maintains a cool moist climate and provides ground beetles with prey that feed on decaying plant matter.
- Less intensive nature of organic farming: The less intensive nature of organic farming in general is also positive for ground beetles, with organic farms having higher activity and biodiversity than conventionally managed fieldsâparticularly schemes with less intensive spraying and other chemical inputs.
Negative Impacts of Pesticides on Ground Beetles
Due to their slow reproductive rate, pesticides can cause growing-season-long reductions in ground beetle populations. It has been suggested that the decline of three carabid species, including Pterostichus melanarius, a ground beetle that consumes a particularly wide assortment of agricultural pests, was caused by an increase in insecticide use. Broad-spectrum insecticides have been shown to have devastating effects on carabid beetles at normal application rates. As a consequence, pest species normally controlled by carabid beetles can increase after application of some insecticides. One way to mitigate the effects of insecticides is to establish beetle banks, which have been shown to provide a site for colonizing beetle populations while crops are treated with pesticides.
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