The discovery of damaged potatoes riddled with tunnels and holes is frustrating for any grower. The term “potato worms” generally describes the larvae of several insect species that bore into developing tubers. These soil-dwelling pests reduce the marketability and yield of the harvest. Understanding which specific insect is causing the damage is the first step toward implementing an effective control strategy. This guide outlines the differences between the common pests and provides steps for both prevention and removal.
Identifying the Potato Pests
The two most common pests responsible for damage attributed to “potato worms” are wireworms and the larvae of the potato tuber moth. Differentiating them involves examining the pest’s appearance and the specific nature of the damage. Wireworms are the larvae of click beetles, characterized by their slender, cylindrical, and hard-bodied appearance, often yellowish or brownish. They can live in the soil for two to five years, reaching up to an inch in length.
Wireworm damage typically presents as clean, narrow, circular holes that penetrate deep into the tuber. These pests bore perpendicular or diagonal tunnels up to a half-inch deep, making the potato unmarketable. In contrast, potato tuber moth larvae are smaller, softer-bodied caterpillars that are whitish, pinkish, or greenish.
Tuber moth larvae generally tunnel just beneath the surface of the potato, creating dark, slender tunnels often associated with the tuber eyes. A defining characteristic of their feeding is the presence of frass, or dark excrement, filling the tunnels. This damage appears “dirty” compared to the clean holes left by wireworms. Moths lay their eggs on exposed tubers or soil cracks, so infestation is often concentrated near the surface. Proper identification is the foundation for choosing the most appropriate management technique.
Preventative Cultural Practices
Altering the growing environment to make it less hospitable to pests is the most effective long-term defense. Crop rotation is a fundamental practice, particularly for wireworms, which spend multiple years in the soil. Avoiding planting potatoes in fields recently used for grass, pasture, or hay helps break the wireworm’s extended life cycle. Planting non-host crops like brown mustard or buckwheat can suppress wireworm populations by disrupting their food source.
Soil management is another preventative measure, especially for potato tuber moths. Moths lay eggs on tubers exposed by cracks in the soil surface, common in dry or clay-rich soils. Regular and thorough hilling, or mounding soil around the plants, ensures developing tubers remain deeply covered and protected from egg-laying adults. Using sprinkler irrigation instead of furrow irrigation helps prevent deep soil cracking that provides access points for the moths.
Sanitation practices during and after the growing season are also important for reducing pest pressure. Promptly destroying cull piles (heaps of discarded, damaged potatoes) eliminates breeding sites for the potato tuber moth. Volunteer potato plants, which grow from tubers left in the ground, can harbor pests and should be removed. Harvesting the crop as soon as it matures reduces the window for late-season infestations, since wireworm activity peaks in mid-to-late summer.
Non-Chemical Control Strategies
When pest populations are present, several non-chemical methods can reduce their numbers before considering chemical treatments. Physical bait trapping is an effective way to monitor and manage wireworm populations before planting. Cut pieces of potato or carrot can be buried a few inches deep and marked with a stick. After several days, the bait is dug up, and any wireworms that have congregated are collected and destroyed.
For potato tuber moths, pheromone traps monitor the adult male moth population. These traps use a synthetic scent to lure males, helping to estimate population density and inform the timing of other control measures. Introducing biological control agents can also provide sustained suppression of soil-dwelling larvae. Beneficial nematodes, which are microscopic roundworms, seek out and parasitize wireworm and tuber moth larvae without harming beneficial insects.
Another biological approach uses products based on the bacterium Bacillus thuringiensis (Bt). Specific Bt formulations are effective against the larval stage of the potato tuber moth. They must be applied when larvae are actively feeding on the foliage or before they bore into the tubers. In fields with high wireworm pressure, planting a trap crop of wheat or corn a few weeks before the potatoes can lure the pests away. This sacrificial crop is then monitored and destroyed before the potatoes are planted.
Chemical Control Options
Chemical control should be viewed as a last resort and is most effective when applied preventatively before planting. For wireworms, chemical treatments are primarily soil-applied insecticides used at planting to protect the developing seed piece and young tubers. Once wireworm larvae have bored into the potato, treatments are generally ineffective because the pests are protected within the tuber.
Managing potato tuber moths with insecticides requires precise application timing, often targeting the adult moth or young larvae before they enter the tubers. Products containing active ingredients like spinosad or certain pyrethroids may be used. Application should occur in the evening when the adult moths are most active. Growers must use products specifically labeled for potato pests and follow all label instructions regarding application rates and pre-harvest intervals.