Potato Tuberworm

The adult is a small moth with a wing expanse of 0.5 inch (1.2 cm). When at rest, the wings are held close to the body giving the moth a slender appearance. The general color is gray with darker gray-brown or black markings. The eggs are very small, oval, and range from white to yellowish. Full grown larvae are caterpillars that vary in color from whitish, dirty white to grayish, pink, or greenish when feeding in stems or leaves. They are 0.4 inch (1 cm) in length with a brown head and dark prothoracic shield.

The potato tuberworm is not a problem in the northern potato-producing areas of California. In other areas, it can be a minor to serious pest depending upon the area and year. Though severe damage to young plants is rare, high numbers of worms in very young plants may result in stand reduction or stunted plants as a result of leaf and stem mining. The typical damage results from larvae mining in the tubers. Small larvae usually enter the tuber at the eyes. Small deposits of frass can be seen in webbing around an eye where a larva has begun to tunnel. Frequently, the larvae feed just below the surface of the potato leaving a dark tunnel. Occasionally they bore deep into the tuber. In either case, the tunnel is filled with excrement and can be described as a dirty tunnel compared to the clean tunnels made by wireworms or other soil-inhabiting insects. Tubers that are exposed as a result of shallow setting or cracks in the soil are most frequently infested. The longer the tubers remain in the ground after vine kill, the more damage that can be expected. Tuberworms do not tunnel through stems and roots into the tubers.

Any practice that reduces the exposure of tubers to egg-laying female moths will reduce tuberworm damage. Sanitation is also important for preventing infestations. Moths can be monitored with pheromone traps to determine the need to treat.

Shallow setting varieties are generally more susceptible than varieties that set tubers deep. Prevention of soil cracking in the beds will reduce tuberworm damage. Cracking of the soil is less severe under sprinkler irrigation than with furrow irrigation. Thus, furrow-irrigated fields have a much greater potential to become infested than sprinkler-irrigated fields. Prompt, thorough harvest and sanitation are also essential. Destroy cull piles and volunteer potatoes. Piles of cull potatoes provide a year-round breeding site for tuberworm.Volunteer plants provide a site for early season increase of the tuberworm numbers.

Use cultural control and sprays of the Entrust formulation of spinosad are on an organically certified potatoes.

Watch for damage during routine field monitoring and record your results (example form—PDF).

Pheromone traps can detect potato tuberworm moth activity and aid in the timing of insecticide applications for control. One option is to use water pan traps fixed with a protective lid from which the pheromone-laden bait is suspended. Place the traps on the top of the bed, one trap in each corner of the field, but well into the field. Check and service the traps at least once a week; record sampling results (example form—PDF). Fill pans with soapy water; the soap helps break the surface tension of the water, which increases trap efficiency. Another option is to place the pheromone in a sticky trap. The advantages of sticky traps are that they do not dry out like water pan traps, but they can become dusty and no longer catch moths.

Treatment levels are not established for all production areas, but the following levels of moths per trap per night (M/T/N) are guidelines. Kern County: The threshold level is 15 to 20 M/T/N or a cumulative average of 10 M/T/N. Once a treatment has been made, base additional treatments on the 15 to 20 M/T/N threshold rather than the season average. If moths do not reach threshold levels before vine kill, do not treat. Insecticides applied at vine kill do not reduce tuberworm damage.

Insecticide treatments of pyrethroids, carbamates, or organophosphates are aimed primarily at killing adults. Applications are best if made in the evening when moths are active. These insecticides do not directly reduce larval populations of tuberworm, thus repeat applications are often needed. Three larvicides are also registered for use on potatoes. Indoxacarb is the most widely used because of its translaminar activity, which allows greater contact with larvae that are within the leaf. Novaluron and spinosad can also control larvae, but only if the worm feeds on the pesticide before entering the leaf, which can be difficult to achieve late in the season when eggs are typically on the undersides of leaves and applications are made at low volume by air.

When moths are present, tubers that are exposed either through soil cracks or erosion are likely to become infested even though insecticides are applied. This is especially true if tubers remain in the soil for a prolonged period after maturity.

• Monitoring potato tuberworm adults form (PDF)
• Surveying for foliage pests (PDF)