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Soybean aphid populations are reaching treatable levels (>250 aphids per plant) in scattered fields throughout Minnesota this week. With populations increasing dramatically in some areas, we’re also hearing increasingly of insecticide applications (both above threshold and below threshold). Aphid populations can vary widely among fields so despite the push from pesticide marketers and applicators towards spraying everything as soon as possible, its time to take stock of what aphids are doing in your fields. I know scouting can be a 4-letter word in some circles but if you haven’t been scouting fields already, it would be wise to check them now.

Each year soybean aphids present a different pattern in the state. Low captures of soybean aphid flights in suction traps last fall plus a colder winter and delayed soybean planting had some people predicting a low aphid year. However, aphids have a way of making fools out of all of us at some point. This year soybean aphids are on track to trash some widely held beliefs: the every-other-year hypothesis, the predictive ability of the highly touted suction traps, and presumed inability to over winter in NW Minnesota. What’s happening in your fields? We’d like to hear from you on the soybean aphid web site: www.soybeans.umn.edu .

Here’s a snapshot of soybean aphid reports from across Minnesota:

SE – Some fields are reaching treatable levels, especially where drier weather has occurred and soybeans are under slight moisture stress reports Fritz Breitenbach. In field trials near Rochester, unprotected soybean was averaging near 350 aphids per plant with Cruiser Max treated plots at about 170 per plant. Cruiser Max has long since worn off so expect rapid increase in aphid numbers. Fields SW of Rochester were variable with one over threshold and several in the 100-150 range, and 100% incidence. In contrast a sentinel plot for soybean rust near Goodhue had 3% of the plants with aphids, according to Lisa Behnken. Lady beetles and syrphids are becoming visible but doubling times seem faster than the last few outbreaks.

SW – Like 2006 a lot of fields in MN River Valley are at or near threshold with a separate pocket near Mountain Lake says Bruce Potter, IPM specialist. For example, many river bottom fields near Wegdahl are being treated reports Michael Lund, Pioneer. Most of bigger fields outside of the valley don’t have that many aphids yet. Bruce observed a drop in numbers at Lamberton with recent heavy rains. With some dealers pushing glyphosate tank mixes at below threshold levels, Bruce encourages growers to watch fields closely. There’s a lot of summer left for re-invasion. A novel twist this summer is pockets of intense populations within fields that leave growers scratching their heads over whether or not to treat immediately.

SC – Mark Bernard, Agro Economics, reports fairly light populations in open country so far towards Waldorf, Waseca and New Richland with higher populations near Mankato and the MN River valley. This pattern echoed by Dave Pfarr, Pioneer, who says soybean aphid populations are higher in the usual trouble spots, typically smaller fields with nearby woodland (e.g., river bottom fields) with larger fields in open country less infiltrated. 

EC & C – In drier pockets, aphid populations are building rapidly. At Rosemount, the first field reached treatable levels Monday with most fields expected to reach threshold levels in the next 3 to 10 days. Heavy rainfall last week knocked aphid populations down slightly, but only in the paths of heaviest rain (>1.6”). At the doubling rates we've been seeing, the benefits didn't last long. A slightly different picture is emerging north of the Twin Cities where Krishona Martinson, regional extension educator, reports spraying is just beginning in the 9 counties north of the metro area. Insecticide treatments are just beginning west of the Twin Cities according to Curt Burns and Darrol Ike, crop consultants. Darrol says alates are common and they don’t seem to be leaving since beans are healthy with minimal stress. Populations are increasing faster east of Hutchinson (Winthrop, Waconia, Winsted areas) and slower west of Hutchinson where drought stress is more intense. In these areas two-spotted spider mites are also appearing in fields. He notes that spider mites are building in the more-intensely stressed portions of fields so watch what you’re applying. Pyrethoid applications (Warrior, Proaxis, Asana, Mustang Max, Baythroid etc.) may flare mites so watch closely in the first couple of weeks afterwards.

WC – Some premature spraying began last week with Doug Holen reporting populations building quickly in traditional hot spots. Fields approaching 150-200 aphids per plant are beginning to be treated near Elbow Lake. General absence of predators widely noted by agricultural professionals scouting aphids. This is the 6th consecutive year with treatable populations and it looks like a classic pattern developing with a lot of fields requiring treatment within the next three weeks. The only wrinkle is a zone of hail damage where re-development of the soybean canopy may lead to higher populations later in the season and potential issues with pre-harvest intervals of some insecticides.

NW – Small fields with adjacent wooded areas getting close to treatable levels, according to Phil Glogoza, with close to 75-80% of plants with colonies of 10-40 aphids per plant. Larger fields without nearby buckthorn have highly variable aphid densities within the Red River Valley. Most fields averaging 10 to 30 aphids per plant with alates very common. Hot spots in fields commonly composed of nymphs developing wings. Things could be changing quickly in the next week. Discouraging use of insecticides tank mixed with glyphosate. Farther north, Ian MacRae sees a lot of field to field variability with populations building steadily. Near perfect temperatures for reproduction have combined with drought conditions (less fungal activity) to favor rapid population increase. Nearly all fields at NWROC in Crookston were at or above economic threshold of 250/plant early this week and have been treated. Populations showed up early this year, indicating a local source of infestation. Winter mortality wasn't that much of an issue. Cold spring presumably delayed egg hatch with earlier spring moisture given way to drier weather as aphid eggs hatched. Lady bug populations have been slow to build... just starting to see lady bug adults and nymphs.

How fast are soybean aphid populations increasing?

While inquiring about the soybean aphid situation, several people commented on either the relative absence of natural enemies or that these natural enemies (lady beetles, syrphids) were just showing up. Insect natural enemies, rainfall, and disease cause mortality and slow the rate of population increase. Typically, doubling times across the Midwest average about 4-5 days but during outbreaks maybe reduced to 2-3 days. Many field reports this year indicate faster doubling times than usual from across Minnesota. To illustrate this point, consider aphid dynamics at Rosemount where the majority of fields had doubling times less than 2 days in July, average = 1.64 days July 2 – 15, 1.50 days from July 15-24. Such low doubling times reflect optimal temperatures for reproduction, limited natural enemy impact, and ongoing immigration.

Fig. 1.  Doubling Times for Soybean Aphid in Various Fields at UMORE Park, Rosemount MN.   

What about soybeans after peas? 

Do recent heavy rains eliminate the need for an insecticide application? 

 What about fungal outbreaks?

Why do we encourage scouting?

There are a few lessons to be learned from the Rosemount fields. Overall a lot of biological momentum is behind this population. However, populations are currently quite variable with field averages ranging from 4.5 to 138.8 to 560 per plant…so there’s no simple decision regarding all fields.  At least two of the 9 fields we’re monitoring won’t reach threshold within 10 days barring massive colonization.  Spraying them now could prove counter-productive since immigrants would find the fields cleared of natural enemies.

Once a field is sprayed, don’t walk away from fields assuming a single spray will provide protection for the rest of the summer.  There’s a lot of time left for re-colonization and aphid populations are reaching a point in the season where long-distance dispersal is more likely.  Insecticide residues last less than a week, regardless of insurance or assurance program claims of 30 or more days.  Check fields roughly two weeks after spraying to see if colonization and resurgence of the population has taken place.  Aphids arriving in the field will find a field cleared of predators by the insecticide.  With fields treated this early, re-sprays are more likely.

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