Trap Catches to August 06, 2021.

Aphid numbers were down for the second week in a row.  Traps last week recovered approximately 120 fewer aphids across the region as compared to the previous week.  The majority of this decline was in non-vector and small grain aphid species.   

Only about 1/3 the number of English Grain aphids were recovered in traps in the past week, which is reflective of the amount of grain harvested over the past week or two.  Conversely, Corn Leaf aphids became the most numerous vector species captured this trapping period.  Again, perhaps reflective of the maturing corn in the region.  

The changing status of crop hosts is starting to be reflected in different aphid species being recovered in the suction traps.  Sunflower and Thistle aphids were present in several locations last week.  And both Soybean and Buckthorn aphid numbers ticked up last week as well.

Soybeans in many locations are becoming increasingly drought stressed but we have yet to encounter the low night temperatures which are linked to Soybean aphids returning to buckthorn.  Their increase last week may be more influenced by the typical late July / early August dispersal event we usually see.  

Aptly coined the SDE (Sturgis Dispersal Event) by Bruce Potter, our IPM Specialist at the Southwest Research & Outreach Center in Lamberton, this dispersal event seems to historically co-occur with the Sturgis Motorcycle Rally (which starts Aug 06 this year). Bruce first developed this hypothesis in response to some surprising photomicrographs he was able to obtain and share a few years back.  The orange and black coloring, strangely emblematic, on that particular Soybean aphid's thorax is certainly atypical for the species...  😉

As mentioned last week, much of the peak aphid flight in Minnesota and North Dakota seem to occur in August.  So we may yet have a lot of aphids moving between fields.  As long as there's green plant material, aphids will probe and PVY can be transmitted.  Most of our PVY is transmitted later in the season, so continue appropriate disease management tactics.

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 06, 2021

Cumulative PVY Vector Risk Index to Aug 09, 2020 (for comparison)

Aphid Species Capture and PVY Vector Risk Index for the week ending Aug 06, 2021

Cumulative Aphid Species Capture and PVY Vector Risk Index to Aug 06, 2021