Saturday, August 10, 2019

Trap Catches Identified to August 09, 2019


Sorry this is late getting out this week, I was on a road trip collecting Colorado Potato Beetles for resistance bioassays and didn't get back till late last night.  If I'd known what was waiting in the trap samples, I might have stayed on the road...

Aphid numbers took a big jump this week, with some important vector species (including Green Peach Aphid) making appearances.  The cumulative seasonal catch increased by almost 50% with just last week's catch (our total vectors captured was 214 last week, in the past week, traps captured a total of 197 vector species across the region).  The Vector Risk numbers rose accordingly, doubling over last week.  Especially with the first seasonal appearance of Green Peach Aphid, the most efficient vector of PVY.  

Green Peach aphid was recovered at 4 locations in northwest/west central MN (Gully, Humboldt, Lake of the Woods and Sabin).  Potato aphid, another important vector species that colonizes potatoes, also saw increases and distribution. 

Most vector species were recovered in increased numbers at many locations, but the crown for numbers this week again goes to English Grain Aphid, which was recovered at most trap sites.  The other small grain aphids, Corn Leaf and Bird-Cherry Oat aphids, also increased in numbers and locations being trapped.  This increase in cereal aphids at this point in the season is due to the rapid senescence of grain fields and is an important factor in the late season transmission of PVY in seed potatoes in MN & ND.  Soybean aphids have started to be more widely recovered, but in limited numbers.  Thistle, Buckthorn and Cotton/Melon aphids also seem to be on the wing and turned up in numbers at several locations.

Bottom line - aphids are flying, the late summer dispersal events have started and scouting and weekly oil treatments are strongly recommended.  Some early fields may be nearing vine kill - this may well be a season where early vine kill may decrease PVY infection in seed potato fields.

Aphid Fact of the Week -  Maturation is occurring in crops other than cereals, and aphids will be sensing a decreasing food quality there as well.  Aphids in these crops will also eventually be developing a winged generation which will disperse to find better food sources.  Many of these species will be non-colonizing species in potato and will work their way through fields, probing and testing plants as potential hosts.  In this way, an increased movement of PVY can occur.  The later development of aphids in the northern plains is evident by the seasonal flight dynamics monitored by the Aphid Alert trapping network.  Our peak flights tend to be in late July to early August (a benefit of living in the home of the Polar Vortex?).  Weekly trap catches are available on the archived Aphid Alert Blogs, going back to 2012 on this site.    

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 09, 2019.

Cumulative PVY Vector Risk Index for 2018 (for comparison)

Aphid Species Capture and PVY Vector Risk Index for Aug 02-09, 2019

Cumulative Aphid Species Capture and PVY Vector Risk Index to Aug 02-09, 2019