Wireworm

• Planted seed potatoes with round tunnels bored through the flesh — damage visible at harvest or when cut for inspection
• Potato tubers with multiple entry holes, often 1/8–1/4 inch diameter, sometimes with secondary rot inside
• Seedling vegetables — corn, bean, carrot — failing to emerge or dying shortly after germination with stems tunneled at or below the soil line
• Firm, shiny, yellowish-brown worm-like larvae, 1/2 to 1 1/2 inches long, found in soil when digging
• Root crops (beets, parsnips, sweet potato) with small round entry holes and internal tunneling at harvest
• Patchy germination failure in a bed with a history of sod or lawn — wireworm populations are often highest in recently converted turf

Wireworms are the larvae of click beetles — the adults are harmless, but larvae live in soil for 2–6 years before pupating, feeding on seeds and roots the entire time. Adult click beetles lay eggs in grassy, moist soils in late spring and early summer. Wireworm populations tend to be highest in soils that were recently converted from sod or lawn, and in fields with cool, moist conditions. As soil warms above about 50°F in spring, wireworms move toward the surface to feed — this is when most crop damage occurs. They move back deeper in hot summer soil and again in fall before going dormant over winter.

1. 01Before planting in a new or recently converted bed, dig a 6-inch deep sample plot and count wireworms — more than one or two per square foot warrants extra precautions
2. 02In high-pressure beds, delay planting until soil has warmed thoroughly (above 55°F); wireworms near the surface cause the most damage in cool spring soils
3. 03Trap wireworms by burying cut potato pieces 2–4 inches deep, marked with a stake — check every 2–3 days and remove the bait with larvae; replace bait regularly
4. 04Avoid planting susceptible crops (potato, corn, root vegetables) in beds with known high wireworm pressure for at least one season; use the bed for leafy greens or alliums instead
5. 05Rotate crops and till beds in fall to expose pupating larvae and adults to birds and cold
6. 06Beneficial nematodes (Steinernema carpocapsae or Heterorhabditis bacteriophora) applied to moist soil can reduce wireworm populations over time, though results are variable

If you're finding wireworms in high numbers in a bed where you want to grow potatoes or root crops, an extension specialist can advise on whether soil fumigation or a labeled insecticide application is warranted for a severe infestation.

• Wireworms in the Garden— University of Minnesota Extension
• Wireworm Management— Utah State University Extension