Friday, July 31, 2020

Trap Catches Identified to July 31, 2020.

More sites captured aphids this week than last (12 of 19 reporting) but overall numbers are down a bit.  The season's vector numbers are low to this point and so the PVY Vector Risk Index remains low.  The numbers of sites reporting aphids though does mean we have winged aphids flying in the area.  

So, while aphid vector numbers may be low, they are well-distributed.  And considering the very low threshold for aphids in seed potatoes, scouting remains important.

Aphid Fact of the Week - Potato Virus Y can be acquired or transmitted by aphids vectors in less than a  minute, there is data to indicate some strains are more transmissible than others.  Virus particles are picked up on the mouth parts of winged aphid vectors when they probe potatoes to ascertain if the plant is a suitable host.  These aphids will eventually fly to and probe other potato plants, transmitting the virus particles to the new plant.  Because aphids will continue to probe plants as they cross a field, efficiently moving PVY inoculum around fields.  

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 July 24, 2020.

Cumulative PVY Vector Risk Index for 2019 (for comparison)

Aphid Species Capture and PVY Vector Risk Index for the week ending July 31, 2020

Cumulative Aphid Species Capture and PVY Vector Risk Index to July 31, 2020.