The Potato Tuberworm

Life Cycle

• Through the winter, potato tuberworm larvae lie dormant in the soil or in potato tubers. Pupation occurs in the spring, from which the small, grayish-brown adult moths emerge in just over a week. The moths mate within two days of their emergence, and each female lays between 150 and 200 eggs in its two- to three-week lifespan. Eggs are usually deposited on the underside of leaves, and hatch in as quickly as five days if the weather is warm. The larvae typically mine the plant’s lower leaves, but may also mine its stems and burrow into its tubers.

Crop Damage

• Potato tuberworms like to feed on the potato plant’s older leaves and do so by tunneling between the leaves’ two sides, causing the leaves to become blotchy, brown and brittle. Tuber-burrowing larvae almost always enter the tubers through their eyes, rather than through their stems or roots. The insects feed on the tuber just below its surface, leaving behind dark, excrement-filled tunnels. Because potato tuberworms create what’s known as “dirty tunnels,” according to the University of California’s Integrated Pest Management Program, their presence usually renders the affected potato crop inedible.

Pest Control

• Sanitation is key to preventing and managing potato tuberworm infestations. Harvested potatoes shouldn’t be left in the field overnight because the moths are nocturnal and lay their eggs at night. Similarly, piles of stored or culled potatoes are also possible sources of infestation. Keeping the soil moist between vine kill and harvest prevents it from drying out and cracking and thereby diminishes the pest’s access to tubers. Maintaining mounds of more than two inches of soil over tubers during the growing season significantly reduces tuber infestation, according to the Oregon State University Extension Service. It’s also important to harvest potatoes quickly once vines begin to die. When the plants’ foliage decays, tuber infestation increases.

Considerations

• Pesticides have been successfully used against potato tuberworms when applied from up to four weeks before to just after the vine kill, and before harvest. Applying pesticides on the plants’ vines does not diminish existing tuber damage or infestation, however. Further, exposed tubers are still likely to become infested when moths are present, even after the application of pesticides, according to the University of California's Integrated Pest Management Program. Certain pesticides also kill the tuberworm’s natural enemies, including two types of braconid wasps, which parasitize tuberworm larvae, according to the Virginia Cooperative Extension.