Soybean Aphid Numbers On The Rise In Iowa

Soybean Aphid Numbers On The Rise In Iowa

Soybean aphids are becoming more abundant on Iowa's 2014 soybean crop. Do you need to spray?

As the 2014 growing season is entering the first full week of August, soybean aphid infestations are being found in more fields in Iowa. The population numbers are rising in many of those fields. More and more farmers each day are wondering whether or not they will need to spray an insecticide. Some have already started spraying the soybean rows along field edges near woods, or in other areas within fields where they are finding "hot spots" of increased aphid populations.

SCOUT SOYBEANS NOW: The calendar has reached the week of August 4, and soybean aphids are showing up in more fields. Farmers are wondering if they need to spray insecticide. Scout your soybean fields now to make timely treatment decisions, says ISU's Erin Hodgson.

Since 2000, soybean aphid has been the primary soybean insect pest in Iowa, notes Erin Hodgson, Iowa State University Extension entomologist. "Infestations are sporadic and unpredictable, but this insect has the ability to cause significant yield loss during periods of optimal reproduction," she adds. "Several notable infestations have been reported, particularly in north-central Iowa, this week, and therefore scouting to determine population densities is strongly encouraged. Scout your fields now to make timely treatment decisions."

Bean fields need to be closely monitored during August
Fields that have a fairly uniform infestation with low densities (e.g., 50% of plants infested with an average of 40 aphids per plant) should be closely monitored in August, advises Hodgson. She offers the following information and guidelines to help you make informed aphid management decisions.

Biology: Soybean aphid is the only insect species in Iowa that will colonize on soybeans. After developing on their overwintering host, buckthorn, the winged adults will migrate to soybean fields and potentially produce 15 or more generations. Initial infestations in soybean fields are patchy and are located near field edges, but winged aphids can quickly disperse within and between fields. Long and short distance immigration is more likely after the soybeans bloom. Aphids prefer to feed on the undersides of leaves (Photo 1) and will colonize on the newest leaves. If a large colony develops and the leaves are crowded, soybean aphid will feed on stems.

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Photo 1. Turn over the soybean leaves to estimate soybean aphid density. Aphids gather on the underside of soybean leaves.

Management:  According to Hodgson's efficacy evaluations, most insecticide products labeled for soybean aphid control are effective (Fig. 1). Some foliar insecticides have a 60-day preharvest interval, so check the label and the calendar when making product selections. You may have to wait 60 days between the time you make the treatment and when you harvest.

Now, at this time in early August, product choice is not as critical as getting sufficient coverage of the insecticide spray on the soybean plants, says Hodgson. With any foliar application, you should strive for the highest knockdown possible to avoid resurgent aphid populations. Ideally, increasing the volume of the spray and the pressure will generate small droplets that should make contact with the aphids on the undersides of the soybean leaves. For ground applications, use 20 gallons of water per acre and 40 pounds of pressure per square inch.

Figure 1. Mean separation of treatments for cumulative aphid days + standard error of the mean at the Northeast Research Farm in 2013. For a full list of treatments and rates, go to www.ent.iastate.edu.

Means with a unique letter are significantly different at alpha = 0.10.

Scouting: With the potential of many overlapping generations in a field, you should scout weekly from plant emergence until seed set to assess population dynamics, says Hodgson. The economic threshold for soybean aphid is well established for the north-central region. Consider a foliar application when the average density exceeds 250 per plant. Populations should be increasing and most of the plants have to be infested (greater than 80%) in order to justify an application. This threshold is appropriate until plants reach mid-seed set (R5.5; see photo 2).

Alternatively, you can consider using a binomial sequential sampling plan, called Speed Scouting, to help make treatment decisions.

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Photo 2. Mid-seed set (R5.5) have seeds that are expanding in the pod. Photo by ISU Extension.

Spraying at full seed set (R6) or later has not produced a consistent yield benefit in Iowa, says Hodgson. In 2013, soybean aphid populations did not peak until late August and a yield response was not consistent in our efficacy evaluation (Fig. 2). Regardless of application timing, you should leave untreated check strips to assess if the treatment decision was profitable.

Figure 2. Mean separation of treatments for yield (bushels per acre) + standard error of the mean at the Northeast Research Farm in 2013. For a full list of treatments and rates, go to www.ent.iastate.edu.

Means with a unique letter are significantly different at alpha = 0.10.

Erin Hodgson is an associate professor of entomology with extension and research responsibilities; contact at [email protected] or by calling 515-294-2847.

TAGS: Extension
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