Why Are There Holes and Tunnels in My Potatoes?
You lift a potato and find it riddled with tunnels, cavities, and ragged holes that go deep into the flesh. This is slug damage, and it is one of the most frustrating problems at harvest — especially because the tubers can look fine from the outside until you cut them open or find the entry points. Slugs are the principal pest responsible for internal potato damage, particularly the keeled slug (Milax species), which lives and feeds underground rather than at the surface and specifically targets tubers.
Which slugs cause the damage
The species responsible for most underground potato damage is the keeled slug, a small, dark, keel-shaped slug that lives in the soil rather than on the surface. Unlike the large grey slugs you see on the surface at night, keeled slugs are rarely seen because they spend almost all their lives underground. They bore clean, smooth-edged tunnels into tubers, eating the flesh from the inside. The damage is often not visible until you cut into the potato. A single keeled slug can create extensive tunnelling over weeks, and populations build up rapidly in damp, organic-matter-rich soils.
When the damage occurs
Slug tunnelling in potatoes is most severe from late August onwards as temperatures drop and soil moisture rises at the end of summer. The tubers become progressively more susceptible as they grow larger and their skin thickens, and slug activity peaks in the warm, damp conditions of autumn. Leaving maincrops in the ground too long — past mid-October in most UK gardens — significantly increases slug damage. Harvesting before the worst of autumn slug activity arrives is one of the most effective management strategies.
Managing slugs in the potato patch
Apply ferric phosphate organic slug pellets (approved for organic growing) around plants from mid-August onwards, focusing on the area immediately around the potato ridges where slugs are active underground. These are safe for wildlife, pets, and children. Traditional beer traps are effective for surface slugs but less so for the underground keeled slug. Encouraging natural predators — ground beetles, hedgehogs, frogs, slow worms — by maintaining habitat near the vegetable garden provides long-term pressure on slug populations. Reducing mulch and debris near the potato bed in late summer removes slug sheltering sites.