Summer 1999: Aphid trapping data.

Results reported in this issue of Aphid Alert are total captures of winged aphids and species identifications for the following locations and sampling interval end-dates. Data for missing locations (no data) will be incorporated into this week's issue of the newsletter once the samples have been received and sorted.

Figure 1 (below). Mean number of winged green peach aphid captured per trap across all locations reported, 1998 data (blue line) and 1999 data (maroon line). These data are also available in table format (Table 1).

Figure 2 (below). Mean number of winged aphids of species known to be capable of transmitting PVY (exclusive of green peach aphid) captured per trap across all locations reported, 1998 data (blue line) and 1999 data (maroon line). These data are also available in table format (Table 1). Summary of observations for week ending 18 August.

Captures of winged green peach aphid averaged 4.3 per trap at 12 locations during the week ending 18 August. Data from two sites, Linton, ND and Mandan, ND are not yet available. This is the highest rate of green peach aphid capture that we have experienced in the 5 years that we have operated traps in the Red River Valley, 1991-1993, and 1998-1999.

We are quite certain that most of the green peach aphids and the turnip aphids (the aphid species that has been the most abundant in our traps this summer) are coming from canola. Aphid numbers in canola fields are often extremely high. It may be that the insecticides used on canola have contributed to this problem by eliminating aphid natural enemies.

In seed potato fields, we have observed a very strong "edge effect" associated with this "invasion" of winged green peach aphids, i.e., we find greatest numbers of winged aphids on the outside rows (e.g., the first 20 meters) and few winged aphids on the interior of potato fields. As time progresses, we expect these invaders to disperse across the field depositing their progeny.

All the winged green peach aphids captured in traps of the Aphid Alert network are sent to Dr. Ben Lockhart, Department of Plant Pathology, to assay by PCR for the presence of leafroll (PLRV) and mosaic (PVY). To date (data available only for captures through 8 August), all have tested negative for both viruses. However, some current season infection has been observed by state seed potato inspectors. Most of the current season infection of PVY and PLRV observed in seed fields has been in the first 6-7 rows. Spraying the margins of potato fields with Monitor or Provado (Admire in Canada) to kill newly arrived winged green peach aphids and prevent establishment of their progeny could be effective at this time.

Many of the seed potato fields we have visited are decidely late in tuber-bulking. For growers in this situation, early vine-kill will not be an option. It may be advisable to harvest and treat the outside rows as a separate seed lot.

If you are monitoring your crop for aphids, keep in mind that aphid flight mostly occurs on warm, sunny days when wind speeds are very low (e.g., 1-2 mph). Therefore, not surprisingly, aphid flight activity is most likely in early morning or late afternoon. When the weather is cold, windy, or wet, expect little aphid flight activity. By timing your sampling to coincide with circumstances favorable for flight you will have the best chance of detecting invasion before the aphids have dispersed in your field.

Captures of other aphid species known to be potential vectors of PVY have declined for the second consective week. Numbers are now far lower (<4%) than on the same date last year. There have been few "cereal aphids" captured this year other than bird cherry-oat aphid. With the cereals ripening there are now few cereal aphids flying. This year, the most abundant, non-potato colonizing, potential vectors of PVY have been turnip aphid, suflower aphid and corn leaf aphid.