Why Are There Thin Wiry Holes Through My Potatoes?

Small, clean, perfectly round holes boring into potato tubers — sometimes with the hole going right through, sometimes ending in a small cavity — are the signature of wireworm damage. Unlike slug tunnels, which are ragged and wider, wireworm holes are narrow, smooth-sided, and often leave an orange-brown stain around the entry point. The wireworm itself — a hard, shiny, yellow-orange larva about 2–3 cm long — is the juvenile form of the click beetle and can live in the soil for up to five years before pupating, which is why wireworm problems can persist for several seasons once established.

Where wireworm comes from

Click beetles (Agriotes species) lay their eggs in grassland. When old pasture, lawn, or long-established grass is dug up and converted to vegetable growing, large populations of wireworm larvae are already living in the soil and immediately begin attacking the roots and tubers of crops planted there. Wireworm problems are therefore worst in the first three to five years after breaking new ground from grass. They are much less common in established vegetable plots that have been cultivated regularly for many years, because cultivation exposes the larvae to predators and desiccation.

The damage pattern

Wireworms bore into seed potatoes, young stems, and developing tubers. Attacks on seed potatoes and young plants can cause poor emergence and gaps in the row. Attacks on mature tubers create the characteristic thin tunnels, often filled with soil. A single wireworm can damage multiple tubers as it moves through the soil, and one infested potato at harvest may have entry and exit holes from more than one larva. The wound sites are also entry points for secondary bacterial rot, which can cause the area around a wireworm tunnel to become soft and brown.

Managing wireworm in your potato plot

Regular cultivation — digging, forking, and turning the soil — exposes wireworm larvae to birds and drying, reducing populations over time. Growing a catch crop of mustard as a green manure and incorporating it into the soil before planting is reported to reduce wireworm numbers. Trap crops work well: bury pieces of carrot or potato at 10 cm depth around the plot, marked with a stick, and dig them up after a week to find and destroy any wireworms feeding on them. Delaying planting until the soil is warm (wireworms are less active above 15°C) and harvesting early — before autumn dampness increases larval mobility — also reduces damage.

Long-term improvement

On newly broken ground with severe wireworm, avoid growing potatoes (and carrots) for the first two to three years after breaking from grass. Grow instead above-ground crops — brassicas, courgettes, climbing beans — while wireworm populations decline through regular cultivation. By years three to four, populations are usually low enough that potato growing produces an acceptable crop. Biological nematode treatments (Steinernema feltiae) applied to moist soil in spring can reduce wireworm numbers organically on a persistent problem plot.