Friday, August 11, 2017

Trap Catches Identified to August 11


Welcome to the Aphid Alert update for the week ending August 11.  This continues to develop into a very high aphid year.  Trap catches continue to be high and the PVY Vector Risk Index continues to rise.  We strongly recommend scouting and the use of crop oils.  Anti-feeding insecticides should be applied if required (see below) 

Trap catch was very high in several locations, with three sites topping 3 figures in vector counts (Ada at 217, Brooks at 173, and Humboldt at 205).  Green peach aphid (the most efficient vector of PVY) continues to be numerous this year with 28 being captured in the past week (12 from Humboldt).  The numbers of this vector are not only high but widespread across the region with few sites reporting no presence in trap catches. 

Cereal aphids are starting to move out of grains and are being recorded in traps at high number, especially prevalent this season are corn leaf aphids which are to this point are the most common vector species recovered from traps this year.  This week is no exception with many sites recovering relatively high numbers (e.g. 64 of which were recovered from the Ada location, 40 from Humboldt, and 56 from Brooks).  

Also numerous this week were soybean aphids, a startling 128 of which were recovered from the Humboldt trap location, indicating a major flight.  The current outbreak of soybean aphids in the Red River Valley of MN & ND poses a particular risk to seed potatoes.  These aphids will not colonize potatoes but in high population years will readily enter fields, probing to test if plants are suitable hosts.  Because potatoes aren't hosts for this species, they do not initiate feeding but move onto the next plant.  In so doing they will acquire and transmit any inoculum in the field.  This is problematic as they enter in high numbers, passing through the fields until reaching soybeans.

Given the high vector catch numbers, and the potential for more, the use of crop oils is strongly recommended.  Data shows that narrow range mineral oils (e.g. Aphoil, Stylet Oil) applied at least once per week can significantly decrease the spread of PVY (twice per week is even better).

Remember, keep scouting!

Scouting for aphids in potatoes:
·         Select leaves from the lower to mid canopy.
·         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 for Full Sized Version

Total 2016 PVY Vector Risk Index for Comparison
Weekly Trap Capture by Location

Cumulative Seasonal Capture & PVY Vector Risk Index by Location

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