Most corn fields in Iowa are in the pollination period now, or are past pollination. "We had very good weather during pollination and continue to have good weather for the later fields too, so kernel set should be quite high," says Mark Johnson, an Iowa State University Extension field agronomist who covers central Iowa.
"For about two weeks after fertilization of the ovules, the plant will abort back enough kernels to allow it to fill the remaining kernels," he explains. "If weather continues to be favorable, we should have very little abortion this year. Then we need good warm sunny days and cool nights to really fill the kernels. If we have hot nights, respiration will use up more products of photosynthesis and less will be deposited in the kernels. That is what happened in 1995. Things looked very good at the blister stage or R2, but then we had a few weeks of very warm nights and the state yield was well below the then record set in 1994."
What about soybeans? Most fields are well into the R2 to R3 growth stage of the soybean plant and some fields are even in R4. That growth stage, R4, begins a critical time for soybeans, says Johnson. The R4 to R5.5 growth stage is when the soybean plant is most sensitive to moisture stress.
There have been multiple reports of finding soybean aphids in some fields, but to date in central Iowa, none of those infestations have reached economic threshold, he notes. "The cooler, drier weather we have been experiencing can be ideal for soybean aphids, allowing populations to increase rapidly—so you should continue to scout your soybean fields and watch for this insect pest. Check for aphids on the youngest two or three trifoliate leaves and stems in the plant terminal. Scout 5 locations for each 20 acres."
When to spray insecticide for soybean aphid
Keep these two terms in mind—Economic Injury Level vs. Economic Threshold. What is the Economic Injury level for soybean aphid? That is the point at which economic damage can occur, and is approximately 650 aphids per plant. The Economic Threshold, however, is something different.
The Economic Threshold is the point set below the Economic Injury Level in order to prevent yield loss, says Johnson. The Economic Threshold is also sometimes called the "action threshold." You should consider applying a foliar insecticide if there are 250 per plant and populations are increasing on 80% of the plants, he says. This threshold allows a 5 to 7 day lead time in order to make timely treatments to protect yield.
Why not treat before aphids reach the threshold?
What is the harm of spraying an insecticide before an infestation reaches the threshold for field crop pests? "Your input costs may not be recovered and you will kill beneficial insects," says Johnson. "Keep in mind beneficial insects control aphids at low to medium reproductive rates. They also help control other soft-bodied herbivores, such as two-spotted spider mites and caterpillars."
"There are several beneficial insects in soybeans that are natural predators of aphids. Adult and larval lady beetles or ladybugs, damsel bug nymphs and adults, insidious flower bugs, and green lacewing larvae are beneficial insects. You don't want to kill these beneficials. Learn what these insects look like, and look for them in your fields while you scout."
KEEP SCOUTING: Aphids on a soybean leaf last week in northwest Iowa.
Paul Kassel, ISU Extension field agronomist at Spencer in northwest Iowa, says soybean aphids began showing up in some fields in his area a week ago. However, they are at very low levels. "The weather forecasts have our weather as relatively cool for the next week or so, and that may be ideal for aphid development," he says.
The area that Kassel covers had a wet spring and a lot of ponding occurred in poorly drained areas of fields. He says, "I'm still somewhat in awe of the size and the number of drowned out areas in fields. The towns of Royal to Webb to Ayrshire to Mallard—there are a lot of downed out spots in fields. In addition to that, there have been some major lakes in the highway 3 and highway 71 area in central Buena Vista County."
Crop development in 2014 appears to be about average
Some of these drowned out areas were just getting dried out last week. Other drowned out areas have a good stand of replanted soybeans, says Kassel. So far those replanted areas appear to be doing well. He adds, "Some of the dried out lakes are almost purple with new waterhemp plants, so farmers are encouraged to manage these areas to prevent a huge waterhemp seed bank for future years."
He notes that the Northwest, North Central and West Central crop reporting districts in Iowa are running a little behind in growing degree days or GDDs. "We would prefer to be ahead of schedule on GDDs this time of year. However, our crop development appears to be about average—with much of the corn crop in our area having pollinated around the July 22 to 24 time period."
Some upcoming meetings and field days at ISU
* Aug. 5-7 is the Resilient Agriculture Conference at Iowa State University in Ames. For more information click here.
* Aug. 14 is the Soybean Aphid Field Day at ISU's Field Extension Education Laboratory or FEEL Lab. It is located at the ISU Agronomy Research Farm, approximately 5 miles west of Ames. Visit this link for more information. This regional field day features key researchers and specialists participating in the North Central Soybean Research Program discussing ongoing research and recommendations for managing soybean aphid. There is no fee to attend but advance registration is requested to assist with facility and catering arrangements.