Ground Beetles “Don’t Get No Respect”

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Common black ground beetle (Pterostichus melanarius).

Some insects really don’t get the respect they deserve and one of those is the ground beetle. This terrestrial beetle is very common, usually black with metallic overtones, but sometimes brown, metallic green or other colors. Although it is probably the most effective predatory insect found in home gardens (one adult can eat up to three times its weight in slugs per day!), it never seems to appear in texts about “beneficial insects”. It just doesn’t seem to have the sex appeal of ladybugs or lacewings.

In fact, the average gardener seems to think ground beetles are harmful, just on the basis that it is an insect and crawls on the ground and therefore just must be bad. An awful lot of them end their life crushed under a shoe or shovel handle… and that’s unfortunate!

A Effective Predator

Usually you run into ground beetles when you move a rock, dig out a plant or cultivate the soil, as they are primarily nocturnal, hiding during the day. They move very quickly when disturbed, seeking a new hideout. Although some species are able to fly, others rarely do and many, in fact, have sealed wing covers and can’t fly at all.

What they do do is eat other insects and invertebrates. They are in fact formidable predators of mollusks and insects. Their range includes maggots and maggot pupae, aphids (up to 50 a day!), caterpillars, slugs, snails, other beetles, grubs, cutworms, and much more. They’ve been extensively studied for their role in keeping down insect attacks in orchards, but they are just as effective in vegetable gardens and flowerbeds. True enough, they do sometimes also prey on beneficial creatures (earthworms, ladybugs, etc.) and are even cannibalistic, but on the whole, they are much more beneficial than harmful.

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Ground beetle larva.

Note also that their larvae too are predators… and just as hungry as their parents.

Although seen on the ground during the day, at night they don’t hesitate to climb into foliage and shrubs in search of their prey, sometimes picking a plant free of aphids in a single night. Take that, ladybugs!

Most species are generalists, going after pretty much any organism of an appropriate size, but some are specialists. Caterpillar hunters (Calosoma), for example, are well-known for their habit of consuming huge quantities of social caterpillars, like tent caterpillars and gypsy moth larvae. Caterpillar hunters are just the insect you’ll want to have around those years when a caterpillar infestation is making your gardening life hell.

Life Cycle

Most species overwinter in the soil, in garden waste, and in mulch. Some species lay their eggs in spring, others in fall. In general, the larvae take a whole summer to mature, not becoming adults until the following spring. They are long-lived for insects: 2 to 4 years, depending on species.

There are over 40,000 species of ground beetle throughout the world, with some 2,000 in North America alone. The species most often seen in gardens is the common black ground beetle (Pterostichus melanarius), accidentally imported from Europe in the 1920s and now well established in disturbed soils (read “gardens”) throughout North America.

How to Encourage Them

They are easy insects to encourage. In fact, many precepts of laidback gardening are exactly what they need. Just maintain a good, thick mulch in your gardens, avoid cultivating the soil as much as possible, and use any pesticides with extreme care. Plus avoid fall cleanup: it was never necessary in the first place and deprives ground beetles of their winter home, since so many of them overwinter in “plant waste”.

Yet another proof that gardeners have much to gain to by learning to apply benign neglect and by minimizing disruptions to their own environment with archaic gardening methods.

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