Potato tuber moth (PTM) or tuberworm (PTW) has appeared and spread in the US in the past century. The moth consists of several species. Phthorinaea operculella, the most common, is widely distributed in the world, found in North Africa, and parts of Asia, Europe, the Americas, and Oceania. It is the most widely distributed potato insect in the world. It is usually found in warm climates for overwinter survival and considered a subtropical pest. The moth or worm is considered the most serious pest of potato in tropical and subtropical regions. Recently PTM/PTW has been found in traps in northern latitudes. Whether this northern migration is due to global warming, mutation, etc. is not known. Since it is not a good flier, its migration is attributed primarily due to movement of tubers carrying the pest into storage facilities further north.
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| Potato tuber moth adult | Potato tuber worm larva |
Description
PTM/PTW has four stages: adult, egg, larva (damaging), pupa. Adults have a narrow, silver-gray body with grayish-brown wings patterned with small dark spots (pictured). The body length is around a third of an inch and a wing span of an inch (2.54 cm). It is mostly nocturnal and attracted to light. They are poor fliers. Eggs are oval, smooth and yellow, laid alone or in clusters on leaves or near eyes on infested tubers. Larvae, caterpillar-like (PTW, worm), are gray, cream or pale green with a dark brown head about half to three-quarter inch long in the final instar (pictured). Pupae are yellow or rust colored; pupation occurs among dead leaves or debris, in soil, or on stored tubers.
Life Cycle
Generation time is 17 to 125 days depending on temperature, commonly one month. Adult = up to 10 days; egg = 2 to 6 days; larva = 13 to 33 days; pupa = 6 to 29 days. Several generation may form per year. Life cycle can continue in storage on tubers.
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| Potato tuber worm fraas (Alvarez, U Idaho) | Potato tuber worm tunnels |
Damage
PTM attacks solanaceous crops with potato being favored. Foliar injury is due to the larvae (tuberworm) mining into leaflets, causing them to form transparent blisters, then move into stem tissue causing death. Tubers are marred when larvae reach tubers by two major means. Upon hatching from eggs laid on leaves, larvae can drop to the ground and burrow through cracks in the soil to a tuber, entering it through the eye. This is common after vine desiccation. Another common way is that the female PTM lays its eggs directly on exposed tubers at or near the eye. When the larvae hatch, they just enter the tuber through the eye making a slender tunnel along the surface or deep into the tuber (pictured). A tunnel can be detected by mounds of worm excrement (frass) appearing black at the entrance (pictured). Tunnels do not heal and are entryways for diseases most notably soft rot and dry rot.
Monitoring
The PTM phermone (male-attracting stimulant) can be used to bait pan-water traps for attracting and monitoring adult male PTM presence. Trapping will give an indication of their presence, population size and distribution, and timing for chemical management. Pan-water traps are recommended due to ease in cleaning between readings. In general, it is suggested to place four traps in each quadrant of a circle, about 50 ft from periphery. Trays should be checked twice per week. Note that economic thresholds have not been determined for potato damage and marketable yield loss. Although there no threshold determined, a reported guide is 15 to 20 PTMs per trap per night would trigger a spray recommendation. Another guide is if the average PTM/trap/night is 10 during the season to that point then field should be treated. These, however, still may be over- or under-estimates.
Field Management
1. Reject seed lots from fields or storage that had been infested with PTM or PTW, or tubers that are infested.
2. Avoid letting tubers be exposed outside of hill or be shallow, less than two inches covered by soil. Keep potato plants well hilled with tubers adequately covered, deeper planting depth and broader hills.
3. Avoid late-season washed out areas as this will expose tubers above ground and PTMs can lay their eggs on the exposed tubers.
4. Avoid deep soil cracks (2 inches and greater) using cultural practices such as irrigation. This will inhibit female PTM from laying eggs on the tubers and wandering PTW from getting through soil to the tubers.
5. Irrigate slightly after vine desiccation to avoid soil cracking and harvest tubers as soon as skin sets.
6. Do not leave potato tubers in containers in the field overnight; likewise do not dig and leave tubers atop of ground to be picked up the next day and stored. At night the female PTM is most active laying eggs.
7. Do not use vines as a cover for tubers as the PTW will move from them to tubers as the vines wilt.
8. Bury or feed to cattle cull piles or in some way destroy them.
9. Eliminate volunteer potato plants.
Chemical Management
Insecticides used against leafhoppers and aphids are adequate against PTM during the early part of the season but best applications are toward the end of the season when vines are dying and tubers are maturing.
1. Current products active against PTM (adult) and PTW (larvae) even when PTWs are in leaf tissue are Furadan, Lannate, Monitor, and Penncap M. (source: Alan Schreiber, Eltopia, WA)
2. Current products active against PTM and PTW but not when PTWs are in leaf tissue are Asana, Baythroid, Guthion, Imidan, and Leverage. (source: Alan Schreiber, Eltopia, WA)
3. For home gardens, apply Sevin to foliage for general insect control. Treat 10 to three days before harvesting. Bt may also be used if applied at the right timing to kill PTW but note has no effect on PTM.
There are no products registered for storage control of PTM/PTW.
Predation
The only natural predators identified are two wasp species that attack PTW.
Storage Management
PTM/PTW damage is year long as the PTM will continue to breed in storage and lay eggs which will hatch into PTW. The length of the life cycle will depend on the storage temperature.
1. Sanitize storage facility (walls, floors, ceiling).
2. Treat facility with malathione, if PTM or PTW was detected the previous year.
3. Keep storage temperatures below 52 deg F.
4. Screen storage area from the outside to keep out PTMs.
5. If potato sacks, crates or other containers are used, they should be new or thoroughly sanitized.
Varietal Resistance
Varieties that are thought as resistant are those that set tubers deeper in the ground. But, if there are deep cracks through which the PTM or PTW can travel to tubers, the tubers will infested; likewise, they will infest tubers on the ground. Therefore, there is no real resistance.
References
- Insects and Related Pests. 2006 In Integrated Pest Management for Potatoes in the Western United States, 2nd Edition, Ed. L.L. Strand, Publ. Univ Calif Agric and Nat Resources, Pub. 3316.
- Foot, M. 1998. Potato Tuber Moth Life Cycle. HortFact (on-line), The Horticulture and Food Research Inst of New Zealand.
- Sorensen, K.A. 1994. Potato Tuberworm. Vegetable Insect Pest Management: Insect Note, North Carolina State Univ. (on-line publication)
- Spring, A. and E. Day. 2006. Potato Tuberworm. Insect Identification Lab. Virginia Tech. (on-line publication)