Why Are My Sweet Potato Tubers Full of Holes and Tunnels?

Discovering that sweet potato tubers are riddled with small holes, tunnels or dark bore marks when you dig them at harvest is one of the most frustrating outcomes of a season's growing. The damage happens invisibly underground and is only revealed when you dig. Two main culprits cause this damage: wireworms (the larvae of click beetles) in temperate gardens, and sweet potato weevils in warmer regions. Both are difficult to eliminate once established, making prevention the most important strategy.

Wireworm identification and biology

Wireworms are the larvae of click beetles (Agriotes spp.). They are hard-bodied, shiny, golden-yellow to brown, cylindrical larvae 1–2.5 cm long — the diameter and colour of a piece of copper wire, which gives them their name. They live in the soil for three to five years before pupating, feeding on the roots, stems and storage organs of many vegetables. They are most abundant in soil recently converted from grassland or in weedy plots — the grass root system is their preferred food in the wild and their population builds under permanent pasture.

Identifying wireworm damage

Wireworm holes in sweet potato tubers are small (2–4 mm), round, and tunnel inward from the skin surface. The tunnels are narrow and may contain the wireworm itself or just dark discolouration. Multiple holes may be present across the tuber surface. Lightly damaged tubers can be used by cutting out the affected sections; heavily tunnelled tubers rot quickly and should be used immediately or discarded.

Sweet potato weevil

The sweet potato weevil (Cylas formicarius) is a significant pest in tropical and subtropical regions and warmer parts of the USA. Adults are distinctive — slender beetles with a blue-black body and red thorax. Larvae tunnel into stems, vines and tubers. Affected tubers develop a strong, unpleasant bitter flavour from the terpenes produced in response to infestation — making tubers inedible even when damage appears minimal. Strict quarantine regulations exist for sweet potato weevil in many countries.

Cultural management of wireworm

Avoid growing sweet potato in beds that were under grass or weeds within the last three to four years — wireworm populations are highest in these. Cultivate the soil repeatedly during the season — wireworms are exposed to bird predation when brought to the surface. Harvest promptly once tubers are mature — leaving tubers in the ground in cool autumn soil increases exposure time. Trap wireworms with pieces of potato or carrot buried 10 cm deep and marked with a stick — check every few days and destroy the wireworms found.