Small, dark holes appearing in sweet potatoes at harvest signal an infestation by underground pests that feed directly on the developing storage roots. Since the damage occurs beneath the soil surface, diagnosing and managing the problem can be challenging. Understanding the specific culprits and their life cycles is the first step toward protecting the crop.
Identifying the Primary Pests Creating the Holes
Most holes found in sweet potatoes are caused by two main groups of insects: the sweet potato weevil and wireworms. These pests create distinctly different types of damage, which helps growers determine which insect is present. The sweet potato weevil, often considered the most destructive pest, is a small, ant-like beetle with a long snout, characterized by a blue-black body and a reddish-orange midsection.
Adult weevils create small punctures on the root surface as they feed and lay eggs. The most severe damage comes from the larvae, which are legless, white grubs that tunnel extensively inside the root, riddling the tuber with cavities. This internal tunneling often leads to rot and causes the plant to produce a bitter-tasting compound, rendering the sweet potato inedible.
Wireworms, the larvae of click beetles, cause a different injury: cleaner, deeper, narrow bore holes, typically about one-eighth of an inch in diameter. These larvae are hard-bodied, cylindrical, and yellowish-orange. Unlike weevil larvae, wireworms do not tunnel throughout the entire root but chew small, circular holes and irregular surface gouges. Damage is frequently observed in fields that were previously grassy or newly tilled, as these conditions favor their lifecycle.
Understanding When and How the Damage Occurs
Damage to sweet potatoes is caused almost exclusively by the larval stage of these insects, which feed beneath the soil line. Sweet potato weevil adults lay eggs in tiny cavities chewed into the vines or the root, sealing the site with excrement. Once the legless larva hatches, it immediately begins boring and tunneling through the root tissue, causing cumulative damage seen at harvest.
The weevil life cycle is relatively short, taking about four to six weeks from egg to adult under ideal conditions. This means multiple generations can attack a single crop during the growing season. Damage is continuous and increases the longer the roots remain in the soil, and weevils can also continue their life cycle in harvested sweet potatoes left in storage.
In contrast, wireworm larvae can reside in the soil for multiple years before completing their development into click beetles. Adult click beetles lay eggs in the soil during the spring and summer, often near weeds or grasses. The resulting larvae feed on a wide variety of underground plant parts, including sweet potato roots. Damage can occur at any point during the root’s growth; early feeding results in healed, knot-like scars, while later feeding creates fresh, circular holes.
Effective Strategies for Prevention and Management
Managing these soil-dwelling pests requires a proactive approach focused on cultural controls, since traditional insecticide sprays often fail to reach the feeding larvae underground. Strict crop rotation is one of the most effective strategies for both pests, as it breaks the insect’s life cycle by removing their primary food source. For wireworms, avoid planting sweet potatoes after crops like corn or small grains, which are preferred egg-laying sites. Instead, rotating with a non-host plant like soybean or rice can significantly reduce the wireworm population.
Field sanitation is important for controlling the sweet potato weevil, which can overwinter in crop residue and volunteer plants. After harvest, all remaining stems, crowns, and small tubers should be removed from the field and destroyed, such as by burning or feeding them to livestock. This eliminates the insects’ overwintering and breeding sites, preventing a severe infestation the following season.
Hilling or mounding the soil around the base of the plants throughout the season is another physical control measure that protects the developing roots. This practice also prevents adult weevils from accessing the tubers through soil cracks to lay eggs. If a field has a persistent wireworm problem, deep plowing in the fall can expose the larvae to freezing temperatures and desiccation, reducing their numbers before the next planting season.
The use of beneficial nematodes, specifically species of Steinernema and Heterorhabditis, offers a non-chemical option for managing both weevils and wireworms. These microscopic worms are natural parasites that seek out and kill the larval stages of the pests in the soil. Promptly harvesting the crop once the roots reach maturity limits the exposure window to late-season pest attacks, minimizing cumulative damage.