No. 8, 2000
30 July

The week ending 30 July saw a 2-fold increase in captures of winged aphids at most trap locations in the Aphid Alert network.   Two locations, Crookston and Thief River Falls, had very high aphid numbers.   The traps in these locations are adjacent to research plots where the aphid pressure has been deliberately enhanced so this data is not included in figures 1-6 below.

Green peach aphid captures per trap last week were comparable to the same date in 1999, which unfortunately was a particularly bad year.   Numbers of green peach aphid on canola do not seem as high as in 1999, perhaps because most canola was planted early and is now mature.  We have found very high densities of green peach aphid and turnip aphid (another PVY vector) on wild mustard.   There is a considerable wild mustard present in the Valley, particularly where crops were lost to surface flooding.  Many of the aphids on wild mustard are now winged and can be expected to move.  Captures of potential PVY vectors other than green peach aphid remain relatively low at most locations.  We are not collecting many aphids from small grains.  At some locations there we captured large numbers corn leaf aphids.   Thistle aphids and sunflower aphids are locally abundant.

Growers should monitor potato fields frequently for aphids.  Initial invasion tends to be heavily concentrated on the margins of the field, particularly if the field interfaces with fallow land.  In some instances, this may permit growers to target application of pesticides on the margins of the field.   This can reduce control costs and will be less disruptive to natural enemies.  For detailed information on sampling for aphids please see last week's Aphid Alert newsletter.

Aphids are very weak flyers and are capable of controlled flight only on days with essentially no wind.  While aphids can be passively transported long distances they do not move very far on their own, perhaps only a few hundred yards.  Essentially, potato is the only possible source of potato viruses.  If the aphids arriving in a clean potato field are capable of transmitting virus there is little that can be done to prevent that from happening.  Insecticides can provide some protection against the spread of potato leafroll, but are of no benefit in preventing spread of PVY.  The present situation is much worse than usual because there is much inoculum present, both in commercial fields and in some seed fields.  Because of this it is important that aphids not be permitted to increase on your crop.  For some discussion of insecticides for aphid control on potato please see last week's Aphid Alert newsletter.