Aphid Alert 2003, No. 10, August 22
Potato insect update for the Northern Great Plains, week ending August 22
Total aphid captures were 30% higher during the past week than in the week before. The most abundant species were buckthorn aphid (representing 90% of the aphids caught in Manitoba) > turnip aphid > corn leaf aphid > and sunflower aphid (the latter three accounting for 82% of the aphids caught in Minnesota and North Dakota). While turnip aphid was still the species most abundantly represented in the Minnesota and North Dakota trap captures (39% of total aphids), its abundance appears to be on decline. Seventeen green peach aphids were caught compared to 19 the week before. Green peach aphid flight activity appeared to be increasing in Manitoba, but decreasing in Minnesota and North Dakota. Bird cherry-oat aphid numbers were higher than in the week before, much of the increase occurring in Manitoba. Thirty three soybean aphids were captured compared to only 4 in the previous week. Potato aphids appear to be declining and were only captured in Manitoba.
The cartoon (below) shows cumulative capture of winged green peach aphid as mean number per trap (Minnesota and North Dakota data only) for the years 1992-1994, and from 1998 to the present. The gold line represents cumulative mean captures per trap to date for 2003. In the years we have operated the Aphid Alert trapping network, there were two with exceptionally high green peach abundance (1998 and 1999) and two with very few green peach aphids (1993 and 2001). It is beginning to appear that 2003 will not be a year with exceptionally high green peach aphid numbers. That is the good news. The bad news is that there are more than enough vectors to spread both Potato leafroll virus and Potato virus Y (mosaic). We have seen some commercial potato fields with comparatively high populations of green peach aphids. These fields could produce a large number of winged adults in late season. This year, there appears to be a greater risk of mosaic spread than of leafroll virus because of the abundant inoculum present in the crop and because the former can be spread by other aphids in addition to green peach aphid. We expect that soybean aphid numbers will continue to increase and we now know that this species also is a PVY vector.
Note that the data is plotted in this cartoon on a logarithmic scale.
Minnesota-North Dakota aphid data, week ending Aug. 18
Manitoba aphid data, week ending Aug. 21
Additional information on the aphid situation in Manitoba can also be found at www.gov.mb.ca/agriculture/crops/insects/index.html, and the Manitoba Agriculture and Food potato hotline at 1-800-428-6866.
Twenty seven seed potato fields volunteered by Minnesota and North Dakota growers were used in this research. Each of the 27 potato fields was sampled for aphids in the first 18 meters at either end and in the field interior (24 hours prior to treatment, 72 hours post-treatment and 7-days post-treatment). A single aerial application of Monitor 4 was applied in an 18 m spray swath at each end of the field (where the crop and headland interface was most obvious and where we expected to find most green peach aphids). Each border area was sampled by selecting one leaf from 30 randomly selected plants in the 18 m border area for 60 leaves total. The field interior, defined as the area beginning at least 30 m from the field edges, was sampled by selecting one leaf from 40 randomly selected plants throughout the interior area. Aphid species, numbers and forms (winged or wingless) were recorded for all samples.
Pretreatment samples of green peach aphid in 27 seed potato fields indicate that of the 1,432 green peach aphids recorded, 91% were encountered in the border area (Figure 1). Samples collected 72 hours post-treatment found 265 green peach aphid in the border area and 111 in the interior of the field (Figure 1). Samples collected from 24 fields 7 days after application detected 475 green peach aphids in the border areas and 406 in the field interior (Figure 1).
Our results indicate that for at least the first application of Monitor 4, a border application would effectively control colonizing green peach aphids in seed potato fields and would benefit the grower through reduced application costs.
Late Blight Situation
A fifth consecutive week of hot, dry weather has produced abnormally dry soils and stressed nonirrigated crops throughout the Northern Great Plains. While such conditions might seem unfavorable for development of Potato Late Blight, new infections were reported last week in Pembina County, North Dakota. There were no new reports of Late Blight in Manitoba this week. Thus far, Minnesota and Wisconsin have escaped without any reported Late Blight this year. Wisconsin's Walt Stevenson notes that “If this holds true for the remainder of the season, 2003 will be the first year in the last decade with no late blight.” Still the advice of Philip Northover to Manitoba growers bears repeating here. “Scouting for Late Blight should be continued. If Late Blight is found, plants should be destroyed immediately to prevent damage to the rest of the field. If fields with late blight are near yield and quality objectives, consider top-killing immediately.” And again from Walt Stevenson, “As we enter the last month of canopy maintenance, growers should continue protective fungicide programs that will also manage early blight. Weather conditions can change rapidly so stay alert to changing conditions."
Potato late blight status reports
Guide to the Field Identification of Wingless Aphids on Potato
|Potato Aphid, Macrosiphum euphorbiae (Thomas)||Buckthorn Aphid, Aphis nasturii (Kaltenbach)||Foxglove Aphid, Aulacorthum solani (Kaltenbach)||Green Peach Aphid, Myzus persicae (Sulzer)||Cotton (=melon) aphid, Aphis gossypii (Kaltenbach)|
This is the tenth issue of Aphid Alert for the 2003 growing season. This newsletter is intended to alert seed potato producers in the Northern Great Plains to flight activity by aphid species known to be potential vectors of potato viruses. We report results weekly on the WWW, by e-mail to subscribers, and by surface mail to all Minnesota and North Dakota seed potato growers. The hard copy and e-mail versions of Aphid Alert report aphid capture data available as of the date they are mailed. The WWW version will be updated as additional data becomes available. To become an e-mail subscriber to Aphid Alert 2003, send us an e-mail message with the word "subscribe" in subject line. Note that current subscribers need not resubscribe. If you have no interest in receiving this newsletter by e-mail, please reply with the word "unsubscribe" in the subject line.