Soybean Aphids Still Simmering … Some Fields at Economic Threshold

Eileen Cullen, Extension Entomologist

Soybean aphids have simmered at low densities throughout the 2009 soybean growing season in Wisconsin and the North Central region. This week, some fields in western and southern Wisconsin are being reported at economic threshold of 250 aphids/plant.

The map (Figure 1) from ipmPIPE http://sba.ipmpipe.org shows aphid densities in Wisconsin sentinel fields through early August. These field reports are collected by UW Extension County Agriculture Agents and the WI DATCP Pest Survey from commercial soybean fields in Wisconsin to give you an overview of statewide aphid activity.  Additionally, I have received calls this week from the field. Thank you to Dave West, CCA; Greg Kerr, CCA, Kerr Agronomics; Tom Novak, CCA, Total Crop Management; and Jim Fanta, CCA, NMC^2; for their reports from the LaCrosse; Eau Claire-River Falls; Jefferson County; and Dodge County areas, respectively.

Figure 1. ipmPIPE map of Wisconsin soybean aphid densities through August 1^st.Source: http://sba.ipmpipe.org

• Statewide, soybean aphid densities remain below economic threshold of 250 aphids/plant (Fig. 1). UW Extension and WI DATCP weekly counts range from a few aphids per plant to 20 or 50 aphids per plant.   However, continue to scout fields through R5 soybean growth stage. A lot can happen with soybean aphid during August. In 2008 we saw numbers increase most during August, particularly on later planted fields.

• Economic threshold field averages (250 aphids per plant) are being reported this week. Gregg Kerr confirmed treatable populations near River Falls (Pierce County), and Colfax (Dunn County). Tom Novak has had a few Jefferson County fields reach threshold. And WI DATCP identified 3 fields at economic threshold densities in the Dodge and Columbia county areas. All of these reports stressed the variability from field to field and need to scout.

• Two-spotted spider mites are a factor in drought stressed fields as well as drier areas of farms in regions not as heavily drought stressed. If soybean aphids have reached economic threshold and spider mites are present in the same field, choose an insecticide with the active ingredients chlorpyrifos or dimethoate. Theses active ingredients are labeled for spider mite control, whereas most pyrethroids are not. For a full account of current two-spotted spider mite activity in WI and scouting and management recommendations, please see the article Dry Weather Reminder for Soybeans: Two-Spotted Spider Mites in this issue of the Wisconsin Crop Manager.

• Canopy deposition in soybeans at this time of year is critical for good coverage and efficacy if treating for soybean aphid and/or spider mites. Higher insecticide spray volumes (20 gpa) and higher pressure (e.g., 30-35 psi, depending on nozzle type) help move the insecticide down into the canopy.                                              

• Dave West and Jim Fanta are reporting from the LaCrosse and Dodge County that their acreage scouted is similar to the ipmPIPE sentinel fields, still below threshold. A few scattered plants with high numbers, but field averages far below threshold and not economically justified to treat. Both Jim Fanta and Tom Novak reported parasitized or “diseased” soybean aphids very noticeable in fields below economic threshold. Please see photos in this article for a primer on the different types of aphid mummies and diseased aphids you may encounter in the field. Aphid mummies provide evidence of biological control by parasitoids (tiny wasps) in the field.

   

Figure 2. Soybean aphid mummy. Parasitized by Lysiphlebus testaceipes , a native parasitic wasp. Photo: Dan Mahr, UW   Entomology

Figure 3. Soybean aphid mummy. Parasitized by Aphelinus albipodus , an introduced parasitic wasp. Photo: Dan Mahr, UW Entomology

Figure 5. Paecilomyces lilacinus, one example of a fungal pathogen that infects soybean aphids. Foliar fungicides applied to soybean are detrimental to this group of aphid natural enemies and unnecessary fungicide applications should be avoided to preserve these beneficial fungi. Photo: Bill Stoneman.

• Spraying below economic threshold is not advisable. University research from the North Central Region, including Wisconsin, shows no economic return to treating below threshold. This practice also increases selection pressure for insect resistance to insecticide active ingredients over the long term, and prematurely disrupts natural enemy suppression of soybean aphid (predators, parasitoids, entomopathogenic fungi). Please see the July 23^rd issue of the Wisconsin Crop Manager Considerations for Spraying Insecticides for Soybean Aphid in Soybeans.