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PNW 594 • April 2007 Biology and Management of the Potato Tuberworm in the Pacific Northwest S.I. Rondon, S.J. DeBano, G.H. Clough, P.B. Hamm, A. Jensen, A. Schreiber, J.M. Alvarez, M. Thornton, J. Barbour, and M. Dogramaci he potato tuberworm, Phthorimaea These losses are of particular concern in the operculella (Zeller), is a pest of many Pacific Northwest, where large quantities of Tsolanaceous crops, including potatoes. potatoes are stored. Commonly found in tropical and subtropical The pest is difficult to control, and many regions throughout the world, potato tuberworm farmers in other parts of the world have relied (PTW) is one of the most important constraints extensively on the use of insecticides for PTW to potato production worldwide. Larvae of control. Protecting potatoes in an integrated this species mine leaves, stems, and petioles manner is essential to the production of a high- and excavate tunnels through potato tubers quality crop without PTW damage. (Figure 1). Adequate control of PTW is critical because The potato tuberworm in the larval infestation of tubers renders potatoes Pacific Northwest unmarketable. There is zero tolerance for the Although PTW was recorded in California as presence of tuberworm larvae in raw process- early as 1856, it was first reported in the Pacific ing product because they are classified as for- Northwest (in Washington) by Chittenden in eign material. Additional losses are likely from 1913. There was no further evidence of PTW infested tubers that rot in storage or from spread in the Pacific Northwest until 2000 and 2001, that may occur early in the storage season. when tubers suspected to have been damaged by PTW were found in Oregon. However, PTW was not a major concern for growers in the Columbia Basin potato production region until 2002, when a field with severe tuber damage was documented in northeastern Oregon. Silvia I. Rondon, Sandra J. DeBano, George H. Clough, Philip B. Hamm, and Mahmut Dogramaci, Hermiston Agricultural Research and Extension Center, Oregon State University; Andrew Jensen, Washington State Potato Com- mission; Alan Schreiber, Agriculture Development Group, Eltopia, WA; Juan M. Alvarez, University of Idaho, Aber- Figure 1. Potato tuberworm larva. Photo: University of deen; Michael Thornton and James Barbour, University of California Idaho, Parma. A Pacific Northwest Extension publication Oregon State University • University of Idaho • Washington State University In 2003, potatoes from several fields in the no damage has been reported in these areas. Columbia Basin of Oregon were rejected for In addition, PTW has been confirmed in at market due to PTW infestation, resulting in an least three counties in western Idaho (Canyon, economic loss of about $2 million. By 2004, Payette, and Elmore counties); however, only large numbers of the insect were confirmed by adults have been observed, and no foliar or pheromone trapping in Umatilla and Morrow tuber damage has been detected. counties (Oregon)—the southernmost region of the Columbia Basin potato production area— Distribution and hosts and in southeastern Washington. Potato tuberworm has been reported in tropi- Economic losses increased substantially in cal and subtropical areas in South, Central, and 2004 and 2005 due to increased PTW densi- North America; Africa; Australia; and Asia. In ties in areas already infested, range expansion, the United States, PTW has been reported in tuber damage, and the cost of control measures. California, Arizona, Maryland, Virginia, Colo- In those 2 years, the pest’s range expanded rado, North Carolina, South Carolina, New 140 miles north into Washington, including York, North Dakota, Oregon, Washington, and Benton, Franklin, Adams, Grant, and Lincoln Idaho. counties. In Oregon, PTW has spread beyond Besides potato, PTW has been reported to Umatilla and Morrow counties to western, infest other solanaceous plants such as tomato, central, and eastern Oregon potato produc- pepper, eggplant, tobacco, and nightshade. In tion areas, including Washington, Multnomah, the Pacific Northwest, PTW has been found Jefferson, Crook, Klamath, Union, Baker, infesting only potatoes. and Malheur counties (Figure 2), although Infestation status Established—consensus Established—survey Not established Being eradicated Eradicated Not found Figure 2. Reported distribution of the potato tuberworm in Oregon. Data retrieved from the National Agricultural Pest Information System on March 15, 2006. Identification and biology Larvae feed on leaves throughout the canopy PTW has four life stages: adult, egg, larva, but prefer the upper foliage. They mine the and pupa. Adults are small moths (approxi- leaves, leaving the epidermal areas on the upper 3 and lower leaf surface intact. Larval feeding mately ⁄8 inch long) with a wingspan of approx- 1 results in necrotic areas on leaves. Leaf damage imately ⁄2 inch. Forewings have dark spots (two to three dots on males and a characteristic is unremarkable and not always visible without “X” pattern on females) (Figure 3). Both pairs careful scouting. Larvae close to pupation drop of wings have fringed edges (Figure 4). from infested foliage to the ground and may Females lay their eggs on foliage, soil, plant burrow into tubers. Exposed tubers are most debris, or exposed tubers. Moths can crawl vulnerable to PTW damage (Figure 6). Larvae through soil cracks or burrow short distances can move via cracks in the soil. through loose soil to find tubers on which to Ultimately, larvae spin silk cocoons and deposit eggs. pupate on the soil surface or in debris under Eggs are less than 0.02 inch, spherical, trans- the plant. Pupae occasionally are found on the lucent, and ranging in color from white or yel- surface of tubers, most commonly associated lowish to light brown (Figure 5). Larvae usually with indentations on the tuber eyes, but they usually are not found inside tubers. PTW pupae are light brown with a brown head (Figure 1, 3 3 (approximately ⁄8 inch long) are smooth and page 1). Mature larvae (approximately ⁄8 inch long) may have a pink or greenish color. brownish (Figure 7) and often are enclosed in a covering of fine sediment. Figure 3. Potato tuberworm female (left) Figure 4. Both pairs of wings have fringed Figure 5. Potato tuber- and male (right). Photo: S.I. Rondon, edges. Photo: Oregon Department of worm eggs are small, Oregon State University Agriculture spherical, translucent, and range in color from white or yellowish to light brown. Photo: S. DeBano, Oregon State University Figure 6. Exposed tubers are predisposed to tuber- Figure 7. Tuberworm pupae are worm damage. Photo: S. DeBano, Oregon State smooth and brownish. Photo: University International Potato Center Number of PTW moths per trap Figure 8. Population dynamics of the potato tuberworm in northeastern Oregon, 2004–2006. Preliminary studies in Oregon indicate that Monitoring PTW adults potentially can emerge from soil Trapping at depths up to 4 inches. Once adults emerge, Researchers, crop consultants, and growers in mating occurs. Within a few hours, depending Oregon, Washington, and Idaho are monitoring on temperature, females seek a host on which to the presence of PTW. Growers in areas poten- lay eggs. tially impacted by this insect are encouraged to Studies in Maryland and Virginia indicated monitor insect numbers using pheromone traps that PTW can survive subfreezing temperatures. (Figure 9). Pheromones are concentrated quan- In the Columbia Basin of Oregon, trapping data tities of the female “scent.” Male moths are from spring 2004 to fall 2005 showed that PTW attracted by a pheromone-impregnated rubber males were present every week except one (in septum in a sticky liner inside the trap. mid-January), with the highest numbers per trap occurring in December (Figure 8). It is unknown where PTW overwinter in the lower Columbia Basin. PTW eggs, larvae, and pupae can potentially survive in or near cull piles, potato tubers left in the soil after harvest, or volunteer potatoes late in the season. PTW is known to survive as pupae in the soil. Trapping data suggest that PTW is capable of producing at least two or three generations per year in the Columbia Basin. Figure 9. Delta trap. Photo: S.I. Rondon, Oregon State University The recommendation is to place at least one Sample at least 10 plants per field or circle. trap per potato field, beginning after canopy Individual plants may be examined for the pres- closure. Since other insects, including other ence or absence of PTW damage, rather than Gelechiidae moths, can be trapped in the liners quantifying the amount of damage per plant. (Figure 10), change liners once a week for easy Nearly 55 percent of PTW mines are found viewing of moths. Lures usually should be in the upper third of the potato plant. Mines are changed monthly but may last longer, depend- not easily found, so look carefully. ing on environmental conditions. Cooler tem- peratures increase the longevity of lures. Region-wide trapping Treatment levels have not been established Since 2004, a pheromone trapping network for Oregon and Washington. California rec- has been deployed in the Columbia Basin of ommends a threshold of 15 to 20 moths per Oregon and Washington to assess and document trap per night. However, remember that PTW the distribution and spread of PTW. In 2005, populations vary greatly from field to field traps were also deployed in Idaho. For current and from area to area. Management recom- information, visit the following websites: mendations should be based on field-specific • Oregon (Hermiston Agricultural Research information. and Extension Center, Oregon State Univer- sity): http://oregonstate.edu/Dept/ Field scouting for foliar damage hermiston/TrapReports.php Most economic damage occurs when PTW • Washington (Washington State Potato Com- infests potato tubers, but heavy foliage infesta- mission): http://www.potatoes.com/research. tion may also cause yield losses. When signifi- cfm cant green foliage is available, larvae prefer to • Idaho feed in the foliage, rather than in tubers.

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