Home & Garden

Gardening: Beetle love--getting to know them

gardening by Mary Robson, Area Horticulture Agent, Washington State University
Garden fever begins to strike just after Valentine's Day, and gardeners wonder if the ground is too wet for digging. If you step out and sink in up to your boot tops, return to the house and settle down with a garden catalog for a few weeks.
   Considered seriously, there are simple checks and tests to determine if the vegetable or flower garden is too soggy to dig. If the garden is clay, or heavy soil, digging too soon can damage the soil structure. Stiff, intractable clods can result, and these remain difficult to manage for the rest of the growing season (and beyond). Don't dig too soon on clay ground.
   To check your soil for "friability" (the term for ready-to-work), pick up about a half-handful and squeeze it hard. Hold on to the squeezed bit and poke it with the fingers of your other hand. How the soil responds to the poking indicates whether it is workable. It's ready to work if, when you poke it, the soil wad breaks apart gently without resistance. It should fall into "crumbs" in your hand (this will happen more often with sandy or loamy soils). It's not ready to work if it stays in a stiff mass when poked. If the soil wad retains the outlines of your palm print and won't crumble, wait a few weeks and test it again.
   What if, when the soil's ready, the shovel turns over a throw of earth and the tossed ground reveals...a scuttling black beetle? If it's over an inch long, black and shiny and easy to see, this earth-dweller is probably a predatory ground beetle.
   Being easy to notice doesn't do this group of beetles much good. This is the critter that local entomologists refer to as the "EEEK, squish!" beetle, because it's so often jumped on by heavy boots the moment it's seen. People assume the worst of beetles, thinking parhaps that all of the Coleoptera move down gardens like Japanese beetles do in eastern U.S. gardens. But spare these!
   Predacious ground beetles are carabids, belonging to a large family of beneficial beetles called the Carabidae. The ground beetles most likely to be seen in the maritime Northwest are shiny black or brownish black, around an inch long plus or minus. They have noticeable antennae with the head narrower than the thorax (the section behind the head). These beetles, which look alarming, are garden beneficials.
   They live under logs, under rocks, or in soil crevices during the day. The life span of predacious ground beetles can be up to three years. They lay eggs which go through a complete metamorphosis, moving from egg through larval and pupal stages before emerging from the soil as adult beetles. Both the larval (immature) and adult stages are beneficial predators. Eggs mature into adults in about a year. Dwelling in the ground, they move about at night munching insects, many of them pests, such as cutworms and house fly maggots.
   The boat-backed ground beetle feeds on snails and slugs, which brings up interesting mental visions of a ground beetle with a wriggling slug gumming up its jaws. We aren't likely to see this happening, since the beetles eat at night. They hide during the day and we'll be most likely to see them during digging. They are attracted to light and could be watched after dark with a flashlight. Research studies have shown them eating a varied diet including cranefly larvae, weevils, aphids, spiders, and their own kin. On balance, they eat a lot more damaging insects than they do beneficials. They reduce damaging insect populations.
   Being very careful with the use of broad-spectrum insecticides is essential to maintaining a healthy ground beetle population. They can be killed by chemicals like acephate (sold as Orthene) and diazinon. Use these insecticides only if absolutely necessary, and read the label carefully before using.
   Don't drench the soil with broad-spectrum insecticides if you want to keep the ground beetles alive. (A broad-spectrum insecticide is one that will kill insects of several different genera, one that isn't restricted to action against only one or a few pests.)
   Some beneficial insects can be purchased for gardens, but these valuable predatory ground beetles cannot. Get acquainted with these beetles when you see them, rather than shouting and stomping them into small particles.
   A ground beetle, viewed from a distance, is a good insect to introduce to children. Let them know it's not something to play with, or to interfere with, but to appreciate for its role in the garden. These are beneficial creatures with independent life-styles quietly going about occupying their earthy niches. Their presence is one sign of a lively and healthy garden ecology.

Visit WSU's Agriculture site on the World-Wide Web at http://www.cahe.wsu.edu.