Aphid Alert 2002, No. 5, August 2

Aphid Situation in Week Ending August 2

Flight activity of aphid vectors of potato viruses increased dramatically during the week ending 29 July.

Aphid populations are greatly influenced by weather as well as local cropping practices. The relatively cooler weather with temperatures below 90 °F (32 C) experienced the past week in the Valley tends to favor green peach aphids and other aphid species. Maturation and harvest of small grains in the Valley has also begun, consequently, the flight activity of aphids associated with small grains have intensified. Among small grain aphids, bird cherry-oat aphid, Rhopalosiphum padi, capture (one of the most important vectors of PVY in our region) almost doubled during the week ending 29 July.

In addition, flight activity of turnip aphid, Lipaphis erysimi, showed a dramatic increase this week with 59% (2,170 individuals) of total capture comprised by this species alone. In contrast, the total 2001 seasonal capture of turnip aphid equaled 58 aphids. Turnip aphid is associated with Cruciferous crops and weeds. The two common Cruciferous species in the region are canola and wild mustard. A survey of wild mustards revealed heavy infestations of turnip aphids across the region. One of the medium sized plant (approximately 40 cm in height) contained a colony of 4,514 winged and 16,392 wingless turnip aphids. While an inefficient vector of PVY, the abundance of turnip aphids combined with the increased flight activity of small grain aphids indicates substantial pressure of PVY transmission for 2002.

WARNING: Soybean aphid appears to be a vector of PVY based on preliminary results conducted this spring in our laboratory. Initial attempts using individual aphids or small groups of aphids (10-20) to transmit PVY gave no transmission. In this study we used a mass inoculation technique where hundreds of winged aphids were caged with a PVY infected potato plant in the center with presumed healthy plants surrounding the test plant. In a replicated trial we had a total of 10 out of 32 test plants test positive this week for PVY by ELISA. Unfortunately, our control cages where there was no inoculum provided had 3 of 32 plants test positive for PVY. We will repeat this study shortly using virus free mini tubers as our test plants. These preliminary results seem sufficiently strong to warn growers that soybean aphid is likely an inefficient vector of PVY. Soybean aphid entered Minnesota in 2000 and spread throughout the state last summer. There are soybean producers in the Red River Valley that are treating for soybean aphid this week as they have reached the threshold of 250 aphids per plant. Note: One 100-acre soybean field with a plant population of 175,000 plants per acre could produce 4.4 BILLION soybean aphids! For information on soybean aphid itself go to: <www.soybeans.umn.edu > and follow the links to soybean aphid. MDA also has a web site where they post a weekly statewide pest report <http://www.mda.state.mn.us/pestsurvey/PestReports/PestReport.html>. For potato producers who surrounded seed fields with soybean you should treat your soybean crop border with insecticide to prevent soybean aphids from building up in the crop border. Keep in mind the larger threat is from soybean aphid and other vectors in the general vicinity.

Reducing transmission of PVY

Reducing the transmission of PVY in seed potatoes is challenging because PVY is transmitted in a non-persistent manner. Acquisition and inoculation of PVY requires only seconds and aphids rapidly lose their virus charge after brief feeding on healthy plants and must again feed on a PVY infected plant to continue to transmit PVY. Although the green peach aphid is an efficient PVY vector, other non-colonizing aphids such as those found on sunflower, canola, and small grains are far more abundant and play a significant role in the spread of PVY in our region. Insecticides are ineffective in controlling PVY transmission because they are unable to kill transient non-colonizing aphids rapidly enough to prevent transmission of PVY. In contrast to insecticides, mineral oils target the transmission process itself. We recommend using a 2-4% mineral oil spray (Aphoil) with the volume of at least 20 gpa to treat the entire fields as the best means of protecting your seed potato crop from PVY spread.

Flight activity of green peach aphid, Myzus persicae, the most efficient vector of PLRV increased 23-fold during this week. In contrast to the seasonal total of 22 green peach aphid captured in 2001, to date 16 green peach aphids have been captured in 2002. We observed the beginning of the colonization of potato fields by green peach aphid across the region. During the colonization flight aphids tend to use visual cues, such as those provided by the contrast between cultivated soil and crop margins to locate landing sites. Aphid landing rates along field edges are higher than landing rates in the center of a potato field. Therefore, during the early stages of colonization treating field edges of approximately 60 feet (20 m) with aphicides can provide effective control of green peach aphid. However, it is important to emphasize that edges are created where any contrasting dark-green interfaces are present. Potato fields that have suffered heavy water damage in the recent storms and contain numerous drown out areas will not benefit from edge treatment (Figure 1). The window of opportunity for edge treatment is also limited to the early stages of aphid colonization, thus it is imperative to scout the fields before employment of edge treatment is made.

Weather Conditions and Late Blight

Potato late blight status reports

Subscriber Alert

This is the fifth 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 2

table showing Minnesota aphid capture data for the week ending August 2, 2002

North Dakota locations: mean aphid captures per trap during the week ending August 2

table showing North Dakota aphid capture data for the week ending August 2, 2002

Manitoba locations: mean aphid captures per trap during week ending August 2 (see Manitoba Agriculture and Food Website)

table showing Manitoba aphid capture data for the week ending August 2, 2002

Wisconsin, South Dakota and Nebraska locations: mean aphid captures per trap during the week ending August 2

table showing Wisconsin, South Dakota, and Nebraska aphid capture data for the week ending August 2, 2002

Figure: Trap locations in the Aphid Alert network in 2002

illustrated map of the upper midwest showing 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.

graph showing green peach aphid trap captures for 1992-1994 and 1998-2001, plus the week ending August 2, 2002

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.

graph showing bird cherry-oat aphid trap captures for 1992-1994 and 1998-2001, plus the week ending August 2, 2002