Green peach aphids per trap declined from 4.30 per trap last week to 3.19 per trap this week.  Captures of other potential PVY vectors declined slightly, from 8.82 per trap last week to 8.50 this week. 

All winged green peach aphids captured in traps of the Aphid Alert network to date (14 June through 18 August) have tested negative for potato leafroll virus and potato virus Y. Last week, 230 green peach aphids were submitted for testing (Table 1). These results are markedly different from those of last summer, when the first PLRV-positive green peach aphids were collected 14 July. In August of 1998, 59% of the winged green peach aphids captured tested positive for PLRV and in September, 86%.    Results of the PCR testing of this weeks captures will be reported here by Wednesday of 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 indicated in previous updates, we believe canola is the principal source of these winged aphids. Most canola is now mature and much is harvested so that source of green peach aphid is essentially finished. Also, with the recent wet weather, we are seeing a very high level of fungal mycoses (disease) among the remaining green peach aphids on canola. Aphid-infecting fungi are likely to be much less effective in suppressing green peach aphid on potato because of the heavy use of fungicides to control potato late blight.

We are now observing development of winged aphids in potato fields (i.e., the immature stages are developing winged pads). 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.

Phil Glogoza, Extension Entomologist at NDSU, has pointed out to us there was relatively little use of insecticides on canola in North Dakota this year. Therefore, our previous suggestion that insecticide use on canola could have contributed to this year's green peach aphid buildup was probably unfounded. Phil observes that the southward expansion of canola production in recent years may simply be providing a very suitable host for green peach aphid arriving from southern overwintering sites (green peach aphid is not known to overwinter outdoors in Minnesota or North Dakota).