| Summary
of observations for week ending 8 August. Captures of winged green peach aphids per trap increased sharply
during the week ending 8 August. Indeed, captures of green peach per trap this past
week were considerably higher than during any week of 1998. Captures of other aphid
species known to be potential vectors of PVY appear to have declined somewhat from levels
recorded during the week ending 3 August and are well belows population levels on the same
date last year.
During this past week, we have observed winged green
peach aphids to be much more abundant on plants near the margins of potato fields than in
the interior. This suggests that these aphids originated from outside the potato
field. It is known that green peach aphid uses the visual contrast between foliage
and bare soil to orient to potential host plants when terminating a "long
distance" flight. The biological benefit to aphids of producing winged
individuals is that this permits them to deposit their progeny in a new habitat where they
may be more likely prosper. Aphids are more physiologically attuned to develop
winged forms when their present host plant declines or otherwise becomes less suitable.
The winged aphids we observed in the potato canopy this past week were giving birth
to young. This suggests that they were recent arrivals rather than aphids about to
disperse. That is further evidence that they are indeed recent invaders rather than
aphids produced in the potato crop. The final evidence that these winged aphids are
invaders is that few if the immature aphids in the potato field are developing wing pads
(i.e., these are not destined to become winged adults).
This evidence all suggests that our traps are indeed
capturing winged aphids as they move into the field rather than locally produced aphids.
This is what we want to happen because it means that the results would be of
greater predicitive value in forecasting risk of virus spread. Unfortunately, the
aphid news is not good. We believe that the risk of virus spread is presently very
high and probably will continue to be very high for the next couple of weeks.
The results of PCR tests to determine if the green
peach aphids captured this week were carrying PLRV or PVY will not be reported on this web
page until Friday of next week, our webmaster is traveling. However, state seed
inspectors in both Minnesota and North Dakota are reporting "top infection",
i.e., current season spread, so clearly potato viruses are being spread. |
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