update prepared by Dr. Ian MacRae, UMN Extension Entomologist, NWROC-Crookston
Many field projects are underway and we're scouting small grain and soybean fields to stay on top of what is happening with aphid populations in these crops. Following are comments based on what our field visits are revealing in northwest Minnesota.
Small Grain Aphids
There have been reports of wheat fields in the area with aphid populations reaching or exceeding treatment thresholds. Remember the treatment threshold for aphids in small grains is 8 out of 10 stems with one or more aphids (this will average out to at least 12-15 aphids per stem).
If fields are already at or above treatment thresholds, aphids have been there for a while already. In small grains, yield loss from aphids happens around 300 cumulative aphid days (an aphid on a plant for one day is one cumulative aphid day, 2 on a plant for 2 days is 4 cumulative aphid days, etc). So a field at or above threshold has already accumulated some aphid days. Waiting an extended period of time to tank mix an insecticide application with your fungicide treatments might lead to a yield loss from aphid feeding - depends on how long aphids have been in the field and how many cumulative aphids days have already accumulated.
To scout for aphids in small grains, pull 100 plants from across the field, if 80% or more have one or more aphids, you're above treatment threshold. Pretty straightforward.
There are a number of fields around with soybean aphid populations, some developed early (I suspect from successful overwintering) and some recently infested. All of the fields I have seen are well below treatment threshold but it's obviously time to start scouting for our favorite soybean insect again!
Soybeans tolerate higher populations of aphids than do small grains and we don't see a yield loss until approx. 4500 to 6000 cumulative aphids days. Currently the heaviest populations I've seen are around 50-80 aphids per plant as an average across the field in the Crookston area. As with any summer, this will likely change (maybe up, maybe down - ya just gotta watch 'em!)
We strongly recommend waiting for treatment until you reach the treatment threshold of an average of 250 aphids per plant with 80% or more of the plants in the field having aphids. Spraying early will remove natural enemies that can curb soybean aphid populations and prevent later immigrants from colonizing fields. In our experience, those fields treated early, below the treatment threshold have a much higher probability of requiring second applications to control aphid populations building later in the season.
Scouting for soybean aphids is easy, prior to flowering look at the top trifoliate, after flowering, look anywhere on the plant. Get to know roughly what 250 aphids looks like before you estimate (recalibrating your estimates every once in a while to keep accurate). And it's good practice for golf, keep your head down!
Cheers, keep scouting,