Friday, August 27, 2021


Trap Catches to August 27, 2021.

The numbers of aphid captures per trap continues to decrease this week, but vector numbers overall remained similar to last week.  There was only a slight increase in the PVY Vector Risk Index, putting us well behind last year at this time.  In 2020, we were recovering large numbers of late-season aphids that vector PVY and Vector Risk Index numbers at Aug 28 last year were roughly twice what they are now.  The high temperatures early in the summer likely accelerated the development, and subsequent decrease in host quality of other aphid plant hosts.  This stimulated an earlier dispersal of aphids, including those that vector PVY.  However, those earlier moving populations means there are fewer vectors flying now, we already seem to be past peak vector numbers.

There were still numbers of certain aphid species moving.  Soybean aphids were recovered from several trap locations and there are still Corn Leaf aphids being recovered in many locations.  Non-vectoring species of aphids ticked up a small amount this week.

The high temperatures and lack of precipitation have meant that in some locations tuber set and size are less than desired.  As noted, while numbers are decreasing, there are still aphids present that can vector PVY.  If delaying vine kill to allow more time for bulking, keep aware of aphid numbers.  

 Keep scouting, we'll keep counting...

Scouting for aphids in potatoes:

- Select leaves from the lower to mid canopy. Start at the edge of the field.
- Lower, older leaves will have more established colonies and aphids prefer the balance of nutrients found here; aphids are rarely found on leaves in the upper canopy.
- Avoid leaves on the ground or in contact with the soil.
- In seed potatoes there is only a threshold for PLRV (10 aphids/100 leaves), reactive application of insecticides an effective control for PVY.
- The use of feeding suppressing insecticides, such as pymetrozine (Fulfill®) or flonicamid (Beleaf®) and refined crop oils, such as Aphoil and JMS Stylet Oil, at or prior to field colonization by aphids may reduce the transmission of PVY within fields. Some other insecticides, such as clothianidin (Belay®), imidacloprid (Admire Pro® or Provado®), and spirotetramat (Movento®), have also been demonstrated to reduce the transmission of PVY.
- In table stock potatoes, a treatment threshold of 30 aphids /100 leaves should deter yield loss due to aphid feeding.

The PVY Risk Index Index
Not all species of aphid are equally efficient at transmitting PVY, some are better than others (green peach being the most efficient vector of PVY).  So, the total number of aphids in a trap don't necessarily reflect just how much vector pressure there is at that location.  The PVY Vector Risk Index compares aphid numbers, incorporating their relative vector efficiency compared to the Queen of PVY vectors (green peach aphid!).  Using averaged reference comparisons from the literature, we multiply the number of each aphid species captured by its efficiency compared to Green Peach Aphid to more accurately depict risk posed by the species being trapped.  We then sum the totals.  The PVY-VRI values are presented on the tables below but also on maps comparing current cumulative risk to the total risk from the sample sites of last year (to compare with your local winter grow out results).

Click on any image below for full-scale version.

Cumulative PVY Vector Risk Index to Aug 27, 2027

Cumulative PVY Vector Risk Index to Aug 28, 2020 (please note different scale) 

Aphid Species Capture and PVY Vector Risk Index for the week ending Aug 27, 2021

Cumulative Aphid Species Capture and PVY Vector Risk Index to Aug 27, 2021