Trap Catches to July 23, 2021.
Aphid vector catch numbers jumped for the third week in a row. Total trap catches totaled just over 800 aphids, about 25% of which (195 aphids) were species that can vector PVY. These high capture numbers drove the PVY Vector Risk Index up again this week (by about 65%). Most of the vectors captured (148 aphids) were English Grain aphids - more than twice the number of last week's catch of this species. So the movement of grain aphids to potatoes continues to increase. As small grains continue to mature and harvest begins, the number of grain aphids should decrease, but other vector species may rise. Soybean aphid captures doubled over last week, Corn Leaf and Black Bean aphids increased, and we recorded our first Damson-Hop aphid of the summer.
There were large captures of non-vectors this week, showing overall aphid activity was up. What was interesting about the non-vector species is that the capture was comprised of less than 6 species, which included Spotted Alfalfa aphid (which has been reported from areas in southern MN and ND.
With the increases in PVY vectors, continued scouting and use of crop oils is recommended, insecticides which stop feeding/probing may also prove necessary. Although with the drought conditions and temperatures in several seed growing locations, care should be taken with oils to monitor for phytotoxicity.
We will keep counting. 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 23, 2021
Cumulative PVY Vector Risk Index to July 24, 2020 (for comparison)
Aphid Species Capture and PVY Vector Risk Index for the week ending July 23, 2021
Cumulative Aphid Species Capture and PVY Vector Risk Index to July 23, 2021
