Aphid Alert 2023 - Monitoring PVY Vectors in Minnesota and North Dakota

Saturday, July 23, 2022

Trap Catches to July 22, 2022.

Aphid vector captures are up this week over last week, but still behind last year's numbers. This may be due to the wetter spring, which probably gave a boost to the fungal pathogens that often control aphid populations early in the year. Last year, the dry conditions likely precluded the establishment of these natural controls in many areas. This week, with most traps reporting, 74 vector aphids were recovered. Still no Green Peach Aphids, but cereal aphids are getting more numerous and other aphid species are showing up as well. With the increasing numbers, there was an increase in our regional PVY Vector Risk Index as well.

Most numerous were non-vector aphid species, indicating that the late July movement of aphids that we encounter in this region is underway. Soybean aphid numbers are slightly increasing, with 9 sites reporting low numbers. Cotton/Melon and Buckthorn aphids are both well-distributed across the region, Potato aphids were recovered at 6 locations as were Buckthorn aphids.

The PVY Vector Risk Index, which reflects the risk of PVY transmission within fields based on the relative effectiveness or the aphid species' ability to transmit the virus, also increased this week. Overall, our numbers are below where they were this time last year, but higher than 2020 or 2019.

It appears that our aphid numbers are following the standard pattern for our region, with populations starting to grow in mid-late July and peaking through August (see graph below). If this is the case, we'll see increasing aphid populations over the next few weeks.

So... 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.