Aphid Alert 2002, No. 6, August 9
Aphid Situation in Week Ending August 9
Flight activity of aphid vectors of potato viruses remained high during the week ending 9 August.
Although bird cherry-oat aphid, Rhopalosiphum padi captures declined this week by 24%, turnip aphid, Lipaphis erysimi, flight activity remained strong. Turnip aphid flight activity increased 1.5-fold compared to the previous week and comprised 67% of the total aphid capture. Current season spread of PVY has been reported on some potato fields. Plants with current season infection will serve as secondary inoculum sources that can further increase inoculum pressure for the region. Consequently, the risk of PVY transmission remains high for the Red River Valley.
Flight activity of green peach aphid, Myzus persicae, also increased 11% this week to 0.38 aphids per trap. It is important to note that the capture of 1 aphid per trap in a week corresponds to approximately 1 aphid per plant per day. Therefore, intensive scouting of seed potato fields for green peach aphids is extremely important and initiate treatment immediately when the threshold of 3 green peach aphids per 100 leaves has been reached.
The ripening and harvest of small grains are also associated with immigration of aster leafhoppers, Macrosteles quadrilineatus, into potato fields. Aster leafhoppers do not reproduce on potato so only the adult stage is found on potato. There is no evidence for aster leafhoppers causing direct feeding damage and hopperburn on potato but they can be important in transmitting the pathogen (not a virus) of the disease known as purple top or haywire. Purple top is especially important in processing potatoes because it is implicated as a possible cause for darkened chips. Aster leafhopper has the typical wedge-shaped body of leafhoppers but the body of aster leafhoppers is dull green, compared to the bright lime green color of potato leafhoppers, with two rows of three black spots on the top of the head. However, when disturbed, the large numbers of highly mobile aster leafhopper adults can create the illusion of heavy potato leafhopper invasion. In order to avoid unnecessarily treating aster leafhoppers sample 35 mid-plant leaves in each of the four locations throughout the field to determine if potato leafhopper immatures (nymphs) are present. Potato leafhopper nymphs are wingless, rod-shaped, 1-2 mm in length, with lime green colored bodies. The nymphs run characteristically sideways when disturbed. The threshold density to initiate treatment for controlling potato leafhoppers is one nymph per 10 leaves. Fortunately, most insecticides provide good control of potato leafhoppers but chemistries and rates needs to be chosen with caution to avoid flaring aphids. Our results suggest that effective control of potato leafhoppers can be achieved without flaring of aphids with the use of low rates of Asana (1.2 fluid oz/acre) and Dimethoate (4 fluid oz/acre) applied at threshold densities.
Weather Conditions and Late Blight
Potato late blight status reports
This is the sixth 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 9
North Dakota locations: mean aphid captures per trap during the week ending August 9
Manitoba locations: mean aphid captures per trap during week ending August 9 (see Manitoba Agriculture and Food Website)
Wisconsin, South Dakota and Nebraska locations: mean aphid captures per trap during the week ending August 9
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.
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.