Aphid Alert 2000, No. 8, July 30

Update: Summary of virus vector situation through 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.

Fig. 1. Captures of winged green peach aphid per trap in 1992-1994 & 1998-2000.

Data is presented for Minnesota and North Dakota locations only. Data for 30 July, 2000 does not include Crookston, Hollandale, Thief River Falls or Morris. Green peach aphid flight activity is increasing sharply and growers should scout fields frequently, pay particular attention to field margins, especially along fallow borders.

Fig. 2. Captures of potential PVY vectors other than green peach aphid in 1992-1994 & 1998-2000..

Results are very location specific. In locations with much wild mustard or canola, turnip aphid tends to predominate, in locations near corn, corn leaf aphid tends to predominate. In most locations, captures (all species combined) are still below 50 per trap per week (= high risk threshold).

Fig. 3. Captures of turnip aphid in 1992-1994 & 1998-2000.

We are finding this species to be abundant on wild mustard throughout the REd River Valley. In locations where crops have been lost to standing water there is often much wild mustard producing large numbers of winged turnip aphids.

Fig. 4. Captures of sunflower aphid in 1992-1994 & 1998-2000.

Captures of sunflower aphid are just starting to increase. Numbers at most locations are still very low. This aphid was very abundant in 1998, but its numbers were low in all other years that we have monitored aphid flight activity. Sunflower aphid is a very inefficient vector of PVY.

Fig. 5. Captures of small grain aphids per trap in 1992-1994 & 1998-2000.

Aphids associated with small grains, e.g., bird-cherry oat aphid, English grain aphid and greenbug, were implicated as key vector species in a previous PVY epidemic in the Red River Valley. Numbers of cereal aphids have been relatively low in the past three years and most small grains are now mature and support few aphids.

Fig. 6. Captures of other PVY vectors per trap in 1992-1994 & 1998-2000.

This figure includes all potential PVY vectors not represented in Figs. 1-5. The predominant species vary with location but include corn leaf aphid, thistle aphid and potato aphid. Species abundance varies greatly with location.