Corn rootworm
Diabrotica virgifera & D. barberi
Sweetcorn that leans and then goes down in a storm because the roots were eaten off. Rotation breaks it completely, which is why it barely troubles a garden.
How to know it
The adult western corn rootworm is a slim yellow-green beetle 6 mm long with three dark stripes down the back; the northern is plain pale green. They look very like a striped cucumber beetle, and behave much the same, feeding on silks and pollen. The larvae are slender white grubs with a brown head, in the soil among the corn roots.
What you'll see
- Corn plants leaning over and then curving back up, giving the row a goose-necked look.
- Whole plants going flat after wind or heavy rain.
- Roots chewed back to brown stubs when a plant is pulled up.
- Silks clipped off short by the adult beetles, so the ears pollinate badly and have gaps.
- Beetles feeding on the silks and on nearby squash flowers in late summer.
Where it comes from
One generation a year, and it depends entirely on corn following corn. Eggs are laid in the soil of a corn field in late summer and hatch the following June; the grubs must find corn roots within a few days or they starve. Adults emerge in midsummer, feed on silks and pollen, and lay again in the same ground.
A field-crop pest that reaches gardens only where sweetcorn is grown in exactly the same bed year after year. A single year of anything else in that bed starves the whole generation.
Dealing with it
- Rotate. Move the sweetcorn bed and the problem is over — this is one of the few pests a garden can eliminate outright.
- Earth the row up to encourage brace roots above the damage on a crop already affected.
- Stake or support leaning plants so they can finish the ears.
- Hand-pick the adults off the silks during pollination if they are clipping them badly.
- Water well; a plant with half its roots gone is very quick to drought-stress.
Keeping it out
- Never grow sweetcorn in the same bed two years running. This is the whole of it.
- Interplant beans and squash in the corn block, which breaks up the ground and confuses egg-laying.
- Clear corn stubble at the end of the season.
- Encourage ground beetles and rove beetles, which take eggs and small larvae.
- Do not treat preventively with anything; rotation is free and completely effective.
What it goes for
Sweetcorn and field corn; the adults also feed on cucurbit and other flowers.
Trouble it brings with it
Who hunts it for you
- Ground beetleCarabidae Fast, glossy night hunters that patrol the soil surface for slugs, cutworms, and root-fly larvae. Give them daytime cover — thick perennial edges, stepping stones, a log — and skip the tillage that wrecks their burrows. Grow to recruit it: Lavender,Mint,Thyme
- Rove beetleStaphylinidae Slim, quick beetles with short wing cases that hunt in soil and litter, taking root-fly eggs and larvae, fungus-gnat larvae, and mites. Mulch and undisturbed ground keep them resident year-round. Grow to recruit it: Mint,Thyme
See also
The Cottage Garden Companion — the gardener's bestiary at cottagegardencompanion.com