Green peach aphids per trap declined from 3.19
per trap last week to 2.44 per trap this week. This is the first time this summer
that green peach aphid captures per trap were less than in the corresponding week of 1998.
Captures of other potential PVY vectors increased slightly, from 8.50 per trap last
week to 11.73 this week. Turnip aphid has consistently been the most abundant of the
potential PVY vectors. Turnip aphid is not as efficient a vector of PVY as is the
green peach aphid, but it is a capable PVY vector and, therefore, cause for some concern.
Like green peach aphid, we have found turnip aphid to be abundant this year on
canola.
All winged green peach aphids captured in traps of the
Aphid Alert network to this year (18 June through 25 August) have tested negative for
potato leafroll virus and potato virus Y. Last week, 272 green peach aphids were
tested and all were negative (Table 2).
Table 2.
Location |
End date for collection |
Number of green peach aphids tested |
Baker |
8/23 |
14 |
Cando |
8/20 |
2 |
Climax |
8/25 |
13 |
Gully |
8/23 |
5 |
Hoople |
8/20 |
65 |
Hollandale |
8/11 |
1 |
Hollandale |
8/18 |
1 |
Karlstad |
8/25 |
22 |
Little Falls |
8/17 |
2 |
Little Falls |
8/23 |
3 |
Linton |
8/15 |
10 |
Linton |
8/23 |
6 |
Mandan |
8/23 |
18 |
Morris |
8/23 |
9 |
Rolette |
8/20 |
7 |
Walhalla |
8/20 |
43 |
Walhalla |
8/12 |
51 |
Total |
|
272 |
These results have surprised us, but are encouraging.
These results suggest that, at most, a very small percentage of the winged green
peach aphids migrating to potato prior to 25 August were infected with PVY or PLRV.
Last summer, the first PLRV-positive green peach aphids were collected 14 July. In
August of 1998, 59% of the winged green peach aphids captured in our traps tested positive
for PLRV and in September, 86%. Results of the PCR testing of green peach
aphids captured this past week will be reported next week.
The absence of any positives for either PLRV or PVY in
the aphids collected this year is consistent with our interpretation that these are
migrants moving into the potato crop from hosts other than potato.
As noted last week, winged aphids are now being produced
in potato fields (i.e., the immature stages on potato are developing winged pads and will
be able to fly as adults). Aphids developing on potatoes are much more likely to acquire
and transmit potato viruses. Much current season transmission has already occurred. We
believe that to date this has mostly been secondary transmission, i.e., within field
transmission from plants grown from infected tubers.
We maintain a list of e-mail addresses to facilitate communication with
those of you interested in receiving updates about this research or any resulting
advisories sooner than this information can be delivered by mail or even before it is
posted on the WWW. If you wish your name added to receive such messages please send
us an e-mail at