Aphid Alert 1999, No. 5, July 13

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. Missing locations will be incorporated in this issue of the newsletter when available.

Discussion of results through week ending 13 July

Aphid 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.