Insect ID: Dark-headed, pale-bodied larva-thing — what is it?

WW abdomen  WW head  WW soil

Brought to the Diagnostic Clinic for identification by Diane B. in Friday Harbor:

    Recently while pulling up some kohlrabi, I also pulled up these little crawly things.  They had pale bodies, dark heads, and little legs near the heads.  They looked like some kind of larvae.  When I shook them off the kohlrabi, they quickly burrowed right back into the soil.  What are these things?

Diane, what you’ve got there are:

wireworm wsu entomology

Wireworms.

Wireworms are the larvae of the click beetle, those dark streamlined beetles that click when they right themselves after landing on their backs.  The worms are 1/4″-3/4″ long, can be a yellowish-white to a coppery color, and have three pairs of small legs behind their heads.

Wireworms in the soil.

Biology & Habitat.  There are several different species of wireworm, but the basics are generally the same: While they prefer grassland pasture and sod, and are a serious pest to cereal crops, they really don’t seem to be all that picky, and may be perfectly happy with your vegetables, tubers and berries — and Diane’s kohlrabi.  They feed on the germinating seeds, the developing root systems, and young seedlings, which may explain why that entire bed of potatoes or carrots that you had such hopes for never came up.  (The adult beetles, however, are not a threat to crops.)

Oh, and here’s the fun part:  The larvae live for about 5 years in the soil, making them a difficult pest to get rid of, as there may be several generations of ’em in the soil all at once.

Adult wireworm, a.k.a
“click beetle.”

Adult beetles lay eggs in the soil near a food source, the eggs hatching in about a week.  The larvae — a.k.a. wireworms — move up, down and through the soil depending on food availability and temperature, 45 to 70 degrees being their ideal range.  In late summer, mature larvae will pupate in the soil, and the adult beetles emerge in spring.

How can I tell if I have wireworms?

What to look for in your plants:

  • Hollowed out seeds and dead seedlings
  • Stems that are shredded, but not cut off
  • Plants that are wilted and discolored, but still attached to the root
  • Plants where the central leaves are dead, but the outer leaves are still green
  • Bare patches; thin stands or no stand

To search for wireworms:

The easiest way is to dig in the dirt, although that may not give you an accurate idea of how many wireworms you have, or whether or not you even have them.

So, WSU has devised a simple bait trap, made from coarse whole wheat flour, corn and some knee-high hosiery.  Pour about 1/2 cup of the flour and corn into the hose (or any fine-mesh nylon bag), and place your home-made bait bag in a hole about six inches deep, in the area where you suspect wireworms.  Cover the bait bag with a piece of plastic, such as a trash bag, and cover the plastic with dirt.  Check the trap in one week.  According to WSU, an average of 2-4 wireworms per trap indicates a risk of infestation.

Wireworm bait trap

Management.  You’re really gonna love us now:  There are few wireworm management tools available to the home gardener; most wireworm insecticides are made for agricultural use only.  Which means we must rely on prevention and monitoring.

Most home gardeners encounter wireworm problems after planting a garden plot in an area that used to be grassland pasture or sod — the favorite habitats of wireworms.  When we remove such habitats and replace them with gardens, the hungry larvae turn to those newly germinated seeds of ours:  a regular smorgasbord.

So, if you’re planning a new garden, or adding to your existing one, don’t plant it in an area that had been turf or pasture, and keep that ground fallow during the growing season.  Likewise, if you find a wireworm problem in one of your beds, pull out any attractive plants and let it to go fallow, too.  (Some farmers and gardeners also try “trap cropping” — planting grains or tubers as bait to draw the larvae away from more valuable crops, although WSU has no solid documentation of the efficacy of this method.)

In addition, tilling helps expose tasty wireworm larvae to hungry predators and can damage vulnerable pupae, so careful monthly tilling, particularly in fall and early spring, can help reduce your wireworm population.

For more information, visit our sources: WSU Extension — Whatcom County Agriculture; the WSU Wireworm Project; AgWeb; the WSU Department of Entomology; and WSU Extension’s Wireworm Scouting Fact Sheet.

And if you’re a San Juan County gardener in need of help with a plant or insect identification and diagnosis, see here.

Photo credits:
Top row: 1&2-PNW Handbooks; 3-AgWeb
Center: WSU Dept. of Entomology
Right: top-AgWeb; bottom-WSU Extension, Whatcom County
Bait trap photo: WSU Extension

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