Aphid Alert 2002, No. 7, August 16
Aphid Situation in Week Ending August 16
Aphid flight activity was lower across the Northern Great Plains during the week ending August 16th than in the week previous. Cumulative mean green peach aphid captures per trap remain in the low to moderate range throughout all regions in the Aphid Alert network. At the University of Minnesota Rosemount Agricultural Experiment Station, green peach aphid populations are highly resistant to organophosphate insecticides and have reached densities of hundreds per leaf in plots sprayed with azinphosmethyl (Guthion). Growers should be cautious in choosing insecticides to control this pest. The most effective foliar insecticides for green peach aphid control in our insecticide efficiacy trials have been methamidophos (Monitor), pymetrozine (Fulfill), thiamethoxam (Actara ) and imidacloprid (Provado). We have observed that green peach aphid colonies are becoming established in potatoes treated with systemic insecticides at planting. At this time, most green peach aphid are found on field margins. In one 160 acre field located near Hoople, ND, we found 97% of all colonizing green peach aphid within 60 ft (18 m) of the field margin and 80% of those within the first 20 ft. All growers, including those who used systemic insecticides at planting, should scout their fields regularly for green peach aphid. Border treatments, e.g., 1-2 passes of an airplane or equivalent ground spraying may be good insurance for seed potato growers even if green peach aphid is not detected. Seed potato fields that can be harvested should be killed as soon as possible to reduce the risk of virus spread. While green peach aphid pressure is low to moderate, PVY vectors are abundant this year. Bird cherry-oat aphid abundance has been greater than in any previous year of the operation of the Aphid Alert network. Greenbug is also more abundant than usual. Both of these cereal aphids are efficient vectors of PVY. Flight activity of these aphids peaks as small grains mature. The turnip aphid has been very abundant in 2002. This aphid is also a vector of PVY, although apparently not very efficient. With the maturation and harvest of small grains and canola we anticipate the abundance of cereal aphids and turnip aphid will decline sharply.
Weather Conditions and Late Blight
The Northern Great Plains region (Manitoba, Minnesota and North Dakota) continues to dodge the potato late blight bullet. That reflects well on the diligence of the region's potato growers in maintaining regular applications of protective fungicides. As the season progresses and evenings become cooler it is important that growers continue to scout their fields and make regular fungicide applications. Cool nights increase the probability of dew formation which provides favorable conditions for potato late blight development and spread if the inoculum is present. Weather conditions can change rapidly so the maintenance of protective fungicidal residues is imperative. Conditions have been highly favorable for late blight for much of the growing season and nearly all areas where potatoes are grown have cumulative Disease Severity Values (for the season) that exceed the alert threshold.
Potato late blight status reports
This is the seventh issue of Aphid Alert 2002. This newsletter is intended to alert seed potato producers in the Northern Great Plains to flight activity by aphid species that are 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 is updated as additional data becomes available. To become an e-mail subscriber, send us an e-mail message with the word "subscribe" in subject line. If you have no interest in receiving this newsletter by e-mail, please reply with the word "unsubscribe" in the subject line. Some e-mail subscribers may not wish to receive messages containing graphics. If so, reply with the the words "no graphics" in the subject line.
Minnesota locations: mean aphid captures per trap during the week ending August 16
North Dakota locations: mean aphid captures per trap during the week ending August 16
Manitoba locations: mean aphid captures per trap during week ending August 16 (see Manitoba Agriculture and Food Website)
Wisconsin, South Dakota and Nebraska locations: mean aphid captures per trap during the week ending August 16
Figure: Trap locations in the Aphid Alert network in 2002
Figure: Cumulative captures of green peach aphid (per trap), 1992-1994, and 1998-2001. Three distinctly different seasonal patterns of green peach aphid abundance have been observed. In 1998 and 1999 green peach aphid were abundant with total captures approximately an order of magnitude greater (10X) than that of 1992, 1993 and 1994, and two orders of magnitude greater (100X) than that of 1993 and 2001. For the Minnesota and North Dakota seed potato industry, low green peach aphid pressure in 1994 coincided with the end of a multi-year PVY epidemic and the low green peach aphid pressure of 2001 coincided with the end of a multi-year epidemic of PLRV. Green peach aphid flight activity through August 16 of 2002 remains moderate to low.
Figure: Cumulative captures of bird cherry-oat aphid (per trap), 1992-1994, and 1998-2001. This aphid comes off wheat and other cereals. The species is typically abundant in the Northern Great Plains. In our area, green peach aphid and bird cherry-oat aphid appear to be the two most important vectors of PVY. Lowest abundance of bird cherry-oat aphid during the years the Aphid Alert network has operated was in 1994, which coupled with low green peach aphid pressure, coincided with the end of a multi-year epidemic of PVY. Bird cherry-oat aphid flight activity has been higher in 2002 than in previous years of the Aphid Alert network.
Figure: Cumulative captures of bird cherry-oat aphid and green peach aphid (per trap), 1992-1994, and 1998-2001. These two aphids have been implicated as the primary vectors of PVY in the Northern Great Plains (see Aphid Alert 2002, no. 2). Other potential vectors of PVY, e.g., greenbug and turnip aphid are also abundant this year.
Report on progress of research on green peach aphid migration
University of Minnesota Ph.D. student Min Zhu will present a report on the progress of her Ph.D. thesis research at the upcoming 15th Conf. on Biometeorology/Aerobiology and 16th Congress of Biometeorology in Kansas City, MO, October 27-Nov.1, 2002. An extended abstract of her work is available on that organization's meeting web site (Acrobat Reader is required to view the file).