Aphid Alert 2003, No. 3, July 3
Potato insect update for the Northern Great Plains, week ending July 3
During the past week, aphid flight activity was light throughout the Northern Great Plains. The species most commonly caught in our traps was bird cherry-oat aphid. Bird cherry-oat aphid is a pest of wheat and other small grain cereals. Flight activity of this species can be expected to increase as small grains ripen. Bird cherry-oat aphid does not colonize potato, but is an efficient vector of PVY. Turnip aphid was also commonly represented in the captures. Turnip aphid, like green peach aphid, reproduces abundantly on canola, but unlike green peach aphid, does not appear to be of importance in transmission of any potato virus. Potato aphids have been abundant colonizers of potato this spring. To date, we have found very few green peach aphids in the potato fields that we have scouted. We anticipate green peach aphid flight activity will remain light for the next 3-4 weeks and then peak in early August.
Adult potato leafhoppers are very abundant at present in potato fields throughout the region. Nymphs are now beginning to appear. Adult leafhoppers insert their eggs in leaf veins and petioles. The eggs take about 10 days to hatch. Potato leafhopper nymphs are found almost exclusively on the underside of the leaves and will be most abundant mid-plant. The first instar nymphs are very small, narrow and almost translucent. Older nymphs are a vivid lime green. Wing pads can be easily discerned by third instar. The insect has five nymphal instars. The economic threshold is very low, in the range of 10-15 leafhopper nymphs per 10 leaves (true leaves not leaflets). It is important to apply insecticidal controls before late instar nymphs are present. Plant damage occurs before the classic symptoms of "hopperburn" become evident. There is controversy about how damaging adult leafhoppers are. It is not common in Minnesota for potato leafhopper injury to occur before nymphs are present. In some cultivars insecticidal treatment may be justified if adult leafhoppers exceed 1 per sweep. Nymphs should be sampled by plucking leaves. Sweep nets do not work well for sampling nymphs because the insects are small, soft-bodied, and easily crushed. Many seed growers and some commercial growers use nicotinyl insecticides (Platinum or Admire) at planting. These treatments provide reasonably good early season potato leafhopper control. If potato leafhoppers require subsequent treatment in nicotinyl-treated potatoes a foliar spray of a different insecticidal class should be used. This is important because a second application of a nicotinyl insecticide could hasten development of insecticide resistance in Colorado potato beetle.
Potato leafhopper nymph and adult. Adult insect is about 1/8 inch long. Nymphs run sideways when threatened (e.g., if you place your finger beside them).
Manitoba Aphid Data, week ending July 3
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.
Minnesota Aphid Data, week ending July 1
Late Blight Warning
Minnesota has issued late blight alerts for Karlstad (Jul 3), Perham (Jun 26), Brooten (Jun 25), and Morris (Jun 23) North Dakota reports (Jul 2) that Sabin and Stephen, Minnesota, and Linton and Norwood, North Dakota, have reached cumulative late blight severity values of 15 or higher. Thunderstorms over much of the Northern Great Plains last night have made conditions generally favorable for blight. It is strongly recommended that you scout your fields on a regular basis and begin a regimented fungicidal spray program to prevent the development and possible spread of late blight in your area.
Potato late blight status reports
This is the third 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.