July 11, 2011                       Issue No. 10

SOYBEAN APHID

Soybean aphid populations have increased very quickly in some fields in NW and west central MN over the past week with some fields having already gone over threshold. In their natural obstinacy, however, soybean aphids have neglected to infest all fields in the region making it necessary to scout individual fields to determine if they're over threshold (or even if they have aphids!). I've seen fields well over threshold and a ½ mile down the road, nothing – so scouting still remains the best way to maximize an economic return on management this year. But, hey, if Ag was easy, everybody'd do it...
I've heard from several folks that this is the fastest they've ever seen soybean aphid populations increase. There are a couple of reasons I think it's happening that way this year.

I'm seeing very few predators in the fields I've been scouting this year, Consequently, there just isn't that much limiting aphid population growth when they get established in a field. This really exemplifies why spraying out predator with early, below threshold applications can exacerbate aphid populations (although as a demonstration, that point doesn't help much this year if you've got fields over threshold, but it's good to keep in mind for next year). As Bruce Potter noted in a conversation we had yesterday, wheat is late, there have been some aphid populations in that crop (not soybean aphids obviously...) and there's a fair number of predatory insects, like lady beetles and lacewings, hanging out in that crop. When wheat starts to mature, we can probably expect some movement of those predators into soybean.

Bruce also mentioned he'd just finished looking at this season's climate data and pointed out west central and NW MN are drier than normal. Another key point because dry conditions impact both aphid reproduction and the development of fungal diseases which also help keep lower population levels in check.

In any case, given we're still early in the game,you might want to give some thought to what treatments to use if you have fields over threshold. Some points to consider:

• there's still lots of plant growth left to come along (and those new leaves are going to be added quickly with upcoming higher temps and moisture)
• we're still likely to get immigration of new soybean aphids when the SDE begins (the Sturgis Dispersal Event!)
• fields that get treated now still have a long way to go before they're out of the aphid woods, so to speak...

With all this in mind, you may want to use the lower or mid rate of an insecticide that has a good performance profile and a bit of residual activity (although the residual will not cover the new growth). You're looking for >95% mortality with this insect to prevent a rebound in its population.

And there's a strong possibility that we may have to go back and treat these fields again later (remember, never turn your back on an aphid!), if we do, remember to switch up the modes of action, go with a different class of insecticide for the second application. We should start thinking about managing insecticide resistance for this insect before we have the problem.

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.

Soybean Aphids

There are a number of fields around with soybean aphid populations, some developed early (I suspect we had some 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 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, but, as with any summer, this will likely change (maybe up, maybe down - ya just gotta watch 'em!) 

We strongly recommend waiting 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). 

Growing Degree Days and Corn Maturity 

"Growing-degree days" (GDD) or heat units has been developed to more accurately rate corn maturity. It is based on the number of growing degree days between emergence date and physiologic maturity of a hybrid (Table 1). Growing degree days vary in North Dakota from 2400 GDDs in southeastern areas to 1900 GDDs in the northern areas (Table 2).

In calculating growing degree days (GDD), temperatures from a lower limit of 50F and an upper limit of 86F are accumulated for the growing season by applying the following formula to each day's maximum and minimum temperatures.

GDD=(Max. Temp. + Min. Temp.) - 50
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                2

Maximum temperatures higher than 86F are entered as 86 and temperatures below 50 F are entered as 50 in the formula. GDDs are accumulated from seedling emergence until physiological maturity. Kernel moisture content at physiological maturity generally averages about 34 percent. At physiological maturity a "black layer" will form under the outer layer of the kernel tip. When this forms it signals that kernel dry matter accumulation has reached the maximum level. Corn will not be hurt by frost after that point. Hybrids may vary up to 10 percent in kernel moisture at physiological maturity.

Table 1. Approximate GDD Heat Units and Relative Maturity for Corn in the Northern Growing Regions (North Dakota and Minnesota).

Departure from normal -95

2011 NDSU Research Extension Center Field Days Scheduled

A list of NDSU REC Field Days for 2011.

Risks of FHB (Scab) in Wheat and DON (Vomitoxin) in Barley

Updates on FHB on wheat and DON in Barley.

Wheat:   On July 6, the NDSU Small Grain Disease Forecasting site (www.ag.ndsu.nodak.edu/cropdisease) and the national FHB forecasting web site (www.wheatscab.psu.edu) indicated only a few areas of moderate risk of scab infection in susceptible to moderately susceptible wheat varieties that are flowering.  Changes in weather conditions, or spotty showers, will potentially change these risks in areas that do get some rains over the next week.  As stated before, continued vigilance for FHB risk will be warranted for some time, as growth stages of wheat across the state are quite variable.

Barley:  The barley DON prediction model, also found at www.ag.ndsu.nodak.edu/cropdisease/, indicates a few NDAWN locations with high risk of DON formation in barley, but most sites have low to moderate risk at this time.  Growers need to choose the NDAWN station nearest them to determine risk at any given time.  Many barley fields headed out over the 4^th of July weekend.   

Marcia McMullen - Plant Pathologist, Cereals

marcia.mcmullen@ndsu.edu

Wheat Midge Emergence has Started

Wheat midge flight is at about 10% in the area.  With wheat midge control, the stage of the wheat is critical.  Insecticide applications on wheat that is before heading, or in late flowering or beyond, will not be effective for wheat midge control.  Traps at the McLean (Max), Renville (Mohall), and Williams (Ray) sites had low numbers of wheat midge.  Traps at the McHenry (Velva) and Ward (Minot) sites had high numbers of wheat midge that were above the threshold of 3 midge per trap per day.  Insecticides containing chlorpyrifos (Lorsban, Warhawk, Yuma, and Cobalt) are most effective against wheat midge because they kill adults, eggs, and larvae.  In areas with high populations of wheat midge, the full labeled rate of insecticide is recommended.   Field specific scouting is the best way to determine your field’s risk for wheat midge infestations.  Trap catches and fall survey information may give a general idea of wheat midge threats in your area.  For more information on wheat midge visit:

www.ag.ndsu.nodak.edu/aginfo/entomology/entupdates/Wheat_Midge/owbm.htm

Leafy Spurge Beetle Collection Date

July 14, 10am ½ mile east of Pleasant lake Rest area.  If inclement weather on the 14^th, the collection will be on July 15^th.