2009 Potato Insect Pest Survey for the Columbia Basin of Washington
Aphids, Potato Tuberworm, and Beet Leafhoppers
Washington State University Extension and the Washington State Potato Commission have joined efforts to conduct a regional potato insect pest monitoring program that targets green peach aphid, potato tuberworm, and beet leafhopper. Twenty-five potato fields across the Columbia Basin were surveyed weekly in 2009 to determine the size and whereabouts of these important potato insect pest populations. Results were communicated to potato growers on this webpage. These insect monitoring efforts present a regional snapshot of insect populations, and are not meant to replace insect monitoring activities by growers and fieldmen in individual fields. The survey also contributes to a better understanding of the movement and biology of important potato insect pests in the region.
2009 ANNUAL REPORT
North Basin: Six years (2004-2009) of beet leafhopper (BLH) trapping data from the North Columbia Basin allows comparisons of population size and trends among years. In general, BLH can be found in and around potato fields from mid-May to the end of the growing season (Figure 1).
BLH populations increase rapidly from late May through June, and the largest populations usually occur in late June and in early August. The BLH population in 2009 was about a week behind schedule, but increased very rapidly once it got started. The average BLH counts in 2009 were higher than the six-year average of BLH counts most weeks of the season. Large BLH populations were also recorded in 2005 and 2006. The average peak population for 2009 was 26 BLH per trap the week of July 6, and the most BLH trapped on any card in a week was 127 BLH in late August in a trap near Quincy. BLH populations in the Quincy-Ephrata area tended to be the largest, while populations east of Moses Lake tended to be the smallest.
South Basin: There are only two years (2008-2009) of data on BLH populations in the Southern Columbia Basin (Figure 2). The average BLH counts in 2009 were much higher than the counts in 2008 from late May to mid-August. In 2009, there was a rapid increase in the BLH population from late May to early June. The peak population in 2009 was 38.5 BLH per trap the week of June 8th. The population continued to be large until early July, and gradually decreased thereafter.
North vs. South: The peak BLH population was larger in the South Basin than in the North Basin, and occurred earlier in the season (Figure 3). But, middle and late season BLH populations in the North Basin were larger because their population decline was slower.
Recommendations: We encourage potato field managers to monitor BLH populations closely in May-June. Studies by Munyaneza et. al (2008) indicate that this is the time when potato plants are most susceptible to purple top, an important disease transmitted by BLH that can result in significant yield losses. Treatment thresholds for BLH have not been established, but it is known that the risk of infection increases as BLH populations become large. The publication, Integrated Pest Management Guidelines for Insects and Mites in Idaho, Oregon, and Washington Potatoes, suggests that an average catch rate of 40-100 BLH per week in May and/or June is large enough to cause concern. The same publication provides information about several foliar insecticide products that may be used to control BLH. It is important to note that insecticides applied at planting may not provide adequate protection against the disease, even though their labels may indicate activity against leafhoppers. Three of the fields we surveyed in 2009, which had received an application of aldicarb (Temik) at planting and no other insecticides until July, were widely affected by purple top disease (verified by leaf samples in September).
POTATO TUBERWORM MOTHS
North Basin: No potato tuberworm moths (PTM) were found in any of the traps located in the Northern Columbia Basin in 2009. PTM have been found in North Basin traps in previous years of the survey, but the catches have always been few and infrequent.
South Basin: The first PTM we caught in 2009 was in a trap located near the city of Pasco the week of June 12. After that, PTM were found in this trap almost every week until the field was harvested. PTM were found at only 2 of 11 locations before July and their numbers were low (1-2 PTM per trap). PTM began to show up at more locations in July; we found at least one PTM in 9 of 11 traps the week of July 31. The average number of PTM caught in traps tended to increase between mid-July and October (Figure 4). The most we found in a trap was 23 PTM the week of October 23 near Burbank, WA. This trap collected the most PTM over the course of the season, a total of 110 PTM. The trap nearest Pasco collected a total of 62 PTM for the season, the second highest total. A trap located at the most northern part of our South Basin Route collected the fewest PTM for the season, only 3.
Recommendations: We recommend that potato field managers in the South Basin pay close attention to PTM populations over the latter half of the growing season. They should deploy their own pheromone traps, because infestations of PTM are highly localized. Potato growers with fields near the cities of Pasco and Burbank or by the Columbia River (based on data from 2008) are the most likely to have a problem with PTM. Fields near potato storage buildings are also at greater risk. Pre-harvest control measures in September-October may be warranted in fields where numbers of PTM in pheromone traps are found to be increasing every week. Unfortunately, we do not have enough information to translate PTM trap counts into a risk assessment. It is clear, however, that more moths in traps equal more risk. The most PTM caught in WA survey traps in a week ranged 23-134 PTM per trap per week (2006-2009 data).
North Basin: The first aphids we found in the North Columbia Basin were a couple of winged green peach aphids on June 15, 2009. Aphid populations were small and scattered through June, but became more widespread in July. On July 13, we found colonizing aphids in 13 of 14 fields sampled. More than one-quarter of these fields had populations greater than 1 aphid per plant, which is the recommended action threshold to limit the spread of potato leafroll virus to long-season storage potatoes. We continued to find aphids in most potato fields from mid-July to early August, but their populations were usually small. Aphid populations were very small and scattered for the remainder of August, but began to build again in September as potato crops in the area matured and were harvested. The largest field population of aphids for the season was 500 aphids per plant on September 14. This field was harvested the following week.
South Basin: Aphids were found in three fields on the South Basin Route in the first week of sampling (early June), and two of these fields had more than 5 aphids per plant. They also had large numbers of Colorado potato beetle. None of these fields had received a systemic insecticide treatment at planting. Pike (2008) has shown that fields treated with an at-plant systemic insecticide have consistently lower numbers of aphids through mid-summer compared to fields treated only with foliar insecticides. We continued to find more aphids in more fields in June-July. On July 16, aphids were found in 8 of 10 fields sampled, and one-third of these fields had populations greater than 1 aphid per plant. Aphids were found in about 5 of 10 fields sampled from late July to early August, but the populations tended to be smaller. Aphid populations began to build again in mid-August. More than two-thirds of the potato fields sampled from mid-August through September were infested with aphids, with an average population of 4 aphids per plant.
North vs. South: Aphids were seen earlier in South Basin potato fields (Figure 5). Aphid populations also peaked earlier in the South Basin than in the North Basin, late June vs. late July. Both regions saw a second build-up in aphids towards the end of the growing season as potato crops matured and were harvested. Aphids were found in more fields every week in the South Basin vs. the North Basin, and aphid populations tended to be larger in the South.
2009 vs. 2004-2008: Aphid populations in long-season potato fields in the Columbia Basin from 2004 to 2009 are shown in the graph (Figure 6). These data indicate that aphid populations were largest in 2007 and 2008, and were smallest in 2009 and 2005.
Recommendations: We urge potato field managers to regularly monitor their fields for aphids once the aphids began to show up in their part of the Basin. Green peach aphid is the most efficient vector of potato leafroll virus (PLRV) which can cause substantial yield and tuber quality losses. PLRV causes net necrosis in some cultivars, an unacceptable defect in processing potatoes. Early recognition and control of aphids is considered the best tactic to limit the spread of PLRV. The current recommendations are to control aphids in short-season potatoes when field counts average 5 aphids per plant, and in long-season storage potatoes when field counts average 1 aphid per plant. Low tolerances have been established because even a low incidence of seedborne PLRV can spread rapidly if aphids go unchecked. However, many potato growers are questioning these recommendations, because there has been an extremely low incidence of PLRV in the Columbia Basin in recent years. The recommendations are also in question because some of the cultivars grown in the Columbia Basin do not develop severe net necrosis symptoms in tubers infected with PLRV. It is important to keep in mind, however, that aphids spread other viruses and can cause direct injury to plants when aphid densities are high. Field studies that aim to improve our understanding of the current threat posed by aphids and PLRV will be underway next year as part of the project “Area-wide Management of Potato Pests in the Pacific Northwest.”
The following factors contribute or may contribute to an aphid build-up in potatoes (from Dr. Keith Pike, WSU Entomologist):
- Fields not protected with an at-plant systemic insecticide.
- Mild winters that afford greater survival of green peach aphids on winter annuals and biennials.
- Green peach aphid infested radish or volunteer potatoes near new potato fields.
- Green peach aphids on undersides of vines and leaves when potatoes collapse, and treatment coverage is difficult.
- When insecticides are warranted late season but not used.
- When natural enemies of aphid are few in numbers.