| 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. |
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