When gardeners harvest damaged tubers, they often mistakenly attribute the damage to beneficial earthworms. While some soil-dwelling creatures do consume potato tubers, it is crucial to distinguish between organisms that improve the garden and those that cause destructive feeding. Clarifying this distinction is the first step toward effective pest management.
Earthworms Are Not the Culprits
The common earthworm (Lumbricidae) is not a pest; it is a significant contributor to soil health and does not feed on healthy, living potato tubers. These segmented creatures primarily consume decomposing organic matter, such as dead leaves, plant residue, and manure. They break down this detritus to release nutrients back into the soil structure.
As earthworms move through the earth, they create extensive tunnel systems beneficial for the potato crop. This burrowing improves soil aeration, allowing oxygen to reach the roots and water to drain efficiently. The castings, or excrement, they leave behind are rich in nutrients, acting as a natural, slow-release fertilizer. Earthworms only consume potatoes once the tubers have begun to rot or decay, which is a sign of decomposition, not pest damage.
Identifying the True Potato Pests
The most widespread culprit responsible for subterranean potato damage is the wireworm, which is the larval stage of the click beetle (Elateridae family). These larvae are slender, tough, and cylindrical, typically yellowish or light brown, giving them a “wire-like” appearance. Fully grown wireworms can measure up to three-quarters of an inch in length and possess a hard, chitinous exoskeleton.
The wireworm life cycle is long, with the destructive larval stage often lasting between three and five years, meaning a persistent infestation can cause damage across multiple growing seasons. These larvae seek out underground food sources, boring directly into the potato tuber to feed. Other pests include microscopic plant-parasitic nematodes, such as root-knot nematodes (Meloidogyne species), which are thread-like roundworms that feed on the roots and tubers. Certain species of cutworms, the larvae of some moths, can also cause damage by chewing holes into the potato skin.
Visual Signs of Potato Damage
Identifying the specific pest requires examining the visual evidence left on the harvested potato tuber. Wireworm damage is characterized by small, deep, circular holes on the surface of the potato. These holes represent the entry point where the larva bored into the flesh to feed, sometimes leaving narrow, shallow tunnels inside the tuber. The potato may attempt to heal these wounds, resulting in scars that affect the tuber’s market quality.
Damage from root-knot nematodes appears as galls or raised, knot-like bumps on the tuber surface. These swellings are caused by the nematode feeding inside the tissue, stimulating an abnormal cellular response from the plant. Lesion nematodes (Pratylenchus species) cause distinct reddish-brown or black lesions on the tuber skin and can create brown spots just beneath the surface. Above-ground symptoms of severe nematode infestation include stunted plant growth and yellowing leaves, indicating a compromised root system.
Practical Strategies for Pest Management
Managing soil-dwelling pests like wireworms and nematodes requires an integrated approach focusing on cultural practices and biological controls. Crop rotation is an effective preventative measure, as wireworms are often associated with fields recently in grass, pasture, or small grains. Avoiding planting potatoes in these areas for two to four years can significantly reduce the larval population. Tillage can physically disrupt the wireworm life cycle, as deep plowing in late summer or fall exposes the larvae to predators and harsh environmental conditions.
Using trap crops can help assess and reduce wireworm populations before planting.
Trap Crops
Burying small pieces of carrots, potatoes, or corn seeds a few inches deep for a few days can lure wireworms, allowing them to be dug up and destroyed.
Biological and Cultural Controls
Biological control agents, such as entomopathogenic fungi like Metarhizium brunneum, are being developed to target wireworms. For nematode control, specific cover crops like mustard or buckwheat can be grown and incorporated into the soil. These crops possess biofumigant properties that suppress nematode populations. Chemical control options for home gardeners are limited, making preventative cultural practices the most reliable long-term strategy.