Discussion of results through week ending 13 JulAphid
numbers are increasing rapidly. Mean numbers of winged aphids captured in traps of the
aphid surveillance network have been consistently higher than on the same dates last year.
While numbers are still much lower than the peak densities that occurred in 1998, the
trends portend a potentially serious situation. As of this date, aphid populations are at
levels not reached until 1-2 weeks later in the year in 1998. Green peach aphids have been
caught consistently in the traps the past two weeks. None of the green peach aphids
captured prior to this past week have tested positive for mosaic (PVY) or potato leafroll
virus (PLRV). Results from the testing of this week's green peach aphid captures will be
reported early next week.
The great unknown in aphid trapping is whether the
traps (pan or suction) measure anything more than local (within field) movement of winged
aphids. We suspect they may not. Monitoring aphid flight will be most useful in terms of
"virus management" if the captures actually do represent long distance
(inter-field) dispersal. There are no established economic thresholds for vector numbers
that we can reference for management decisions. Experience last year showed that green
peach aphid captured before mid-Jul were usually not carrying virus. After mid-Jul, the
proportion of green peach aphids carrying virus was considerably higher. We suspect that
when total aphid captures approach 50 per trap or 1 green peach aphid per trap, the risk
of virus spread is high. Obviously, aphid numbers are only part of this equation; the
proportion of the aphid population that is capable of transmitting virus and the duration
of the crop's exposure are also key elements.
Relatively high numbers of aphids are being found
colonizing this year's potato crop. Most often, these are green peach aphid. The accepted
threshold for wingless green peach aphid in seed potatoes is 3-10 per 100 leaves. In table
stock, the threshold is 10X higher except in cultivars susceptible to "net
necrosis." It is important to maintain control when population densities are still
low, because it is virtually impossible to reestablish control after the population
reaches outbreak levels.
This year, green peach aphid has also been found
in relatively high numbers on a wide range of weed and crop hosts including canola,
sugarbeet, redroot pigweed and lambs quarters. Aphids moving to potato from these sources
are unlikely to be carrying virus. Green peach aphids moving from fields with a high
incidence of potato viruses are very likely to be carrying virus. Turnip aphid numbers are
generally declining. We believe these are coming to potato from canola.
 We are establishing 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 radcl001@umn.edu. In
the message, write "Aphid Alert subscribe."
ERRORS: If you find
errors, typographic or factual, we would appreciate having them called to our attention so
they can be corrected. Phone: Robert Suranyi (218) 773-3346 or Ted Radcliffe (612)
624-773 or send an e-mail to radcl001@umn.edu.
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