Trap Catches Identified to August 28, 2020.
Another big aphid week leading to increasing late season PVY risk, more than doubling over last week's. Numbers were up over last week (a total of 70 more vector aphids from all sites over last week) and several sites saw a considerable jump in the cumulative seasonal number of vectors captured and their PVY Vector Risk Index jumped accordingly. Small grain aphids were up as well as damson-hop and cannabis aphids in a number of locations. Green peach aphids were recovered in more locations and at higher numbers at multiple locations.
The number of vectors recovered at the Staples I location was very high (92 aphids from 10 different vector species), captures at Ada and the Nebraska I sites were also much higher than last week (26 more at Ada and 23 more at Nebraska I). The Nebraska sites saw a flight of damson-hop and cannabis aphids aphids were also recovered from a number of locations in MN. Green peach aphids were recovered from the Crookston, Crystal, Nebraska I & 2, Sabin and Staples II sites.
This represents a considerable increase in late season risk of PVY transmission within seed fields. For those who have not yet vine killed, it is worth considering. Watch for green vines remaining in the field, from and aphid's point of view - if potatoes are green, by vectors they'll be seen! We've all long suspected that most of the within-field inoculum movement occurs in the late season. And the aphid dynamics indicated by the flight captures indicates why!
Aphid fact of the week - many aphid species don't overwinter on the same plants they feed on in the summer. Late in the season, cold nights and shorter days signal the movement back to the overwintering plant hosts. So late season flights may result in winged aphids (including vector species) entering potato fields where they may probe plants and spread virus.
If you have a field where Colorado Potato Beetle aren’t being controlled by insecticides, please let us know. We’d like to get samples of that population to test for insecticide resistance. Contact Ian MacRae at 218-280-9887.
As always, keep on scouting!
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 August 28, 2020.
Cumulative PVY Vector Risk Index to August 30, 2019 season (for comparison)