Northwest and west-central Minnesota soybean IPM survey results

This article was written by Angie Peltier and Jared Goplen, UMN Extension crops educators and Anthony Hanson, UMN Extension IPM educator.

Soybean aphids. 

The Minnesota Soybean Research & Promotion Council has sponsored a UMN Extension-run soybean IPM survey. Scouts Emily Schmiedeberg, Kyle Reese and Tyler Goplen have been busy scouting soybean fields throughout west-central and northwest Minnesota and have encountered what is turning out (so far) to be an uneventful growing season from a soybean aphid perspective (Figure 1).

Figure 1. Soybean fields scouted for pests in the two-week period of June 28 through July 9, 2021. Map credit: NDSU IPM Program.

Very few fields with soybean aphid infestations and very low population densities have been observed in infested fields.  In northern Minnesota, cold winter temperatures may have reduced survival of overwintering eggs. Statewide, hot, windy weather when aphids were moving from buckthorn to soybeans likely affected colonization success. Additionally, high rates of wasp parasitism have been observed on some founding colonies in soybean. Soybean aphid growth and development is influenced by temperature (McCornak et al., 2004). Soybean aphid reproductive potential and rate are maximized at 82 °F, with cooler temperatures favoring more nymphs and females produced and longer generations times and warmer temperatures favoring fewer nymphs and females produced and longer generation times. When very high temperatures prevail (above 95 °F) for some time, both growth and reproduction stop. However, as soybean growth and development progresses, fuller canopy soybeans provide a cooler, shaded micro-climate under the canopy.  

Between vegetative (V) through the beginning seed growth stage (R5) consider treating fields with an insecticide when more than 80% of plants are infested, plants have an average of 250 aphids per plant and population densities are growing. At 250 aphids per plant, significant damage to the plant is not yet occurring. Instead, this threshold gives growers enough time to arrange an insecticide application before populations reach the economic injury level of around 670 aphids per plant at which point significant damage can occur (Ragsdale et al. 2007). Throughout western Minnesota, soybeans are just beginning to reach the beginning pod (R3) growth stage (Figure 2) and so there is time in the 2021 growing season for soybean aphid population densities to build.  Moderate temperatures, slight moisture stress, and R3-4 stage soybeans will favor aphids.

Figure 2. Soybean growth stages for the two weeks between June 28 and July 9, 2021. Map credit: NDSU IPM Program.

Grasshoppers. 

Scouts have noted that grasshopper population densities on field edges have been increasing. As small grains begin to mature, or CRP and other areas are defoliated, these insects may be looking for a living crop to feed on. Should you encounter grasshoppers in your fields, UMN Extension has a webpage devoted to how best to scout for and determine whether grasshoppers population densities or feeding injury severity have reached treatment thresholds. Pay close attention to fields adjacent to CRP and alfalfa.

Spider mites. 

Scouts have also been looking for two-spotted spider mites (Figure 3) in soybean, finding them on field edges (where they typically become established; Figure 4) but not in the interior of fields (where they spread as infestations grow). With drought continuing and intensifying throughout parts of Minnesota over the coming week, keep your eyes peeled for the distinctive stippling injury and webbing caused by spider mites and consult information compiled by UMN Extension regarding spider mite scouting,  treatment thresholds and miticide options (Potter et al., 2020). Briefly, research has shown that treating with pyrethroid insecticides, which may no longer be effective against soybean aphids, can cause two-spotted spider mite populations to flare. Additionally, there is the potential for neonicitinoid insecticides to favor mites, spider mite populations in Minnesota to be resistant to chlorpyrifos (a.i. in Lorsban, for example) and cross-resistant to dimethoate. Lastly, unneeded fungicide applications can reduce entomopathogenic fungi that help to keep mite populations in check. Careful scouting and decision-making regarding which active ingredients to use when spider mite populations have reached treatment thresholds is worthwhile.  

Figure 3. Two-spotted spider mites (red arrows) and eggs (blue arrows) on the underside of a soybean leaf. Photo credit: Angie Peltier.

Figure 4. Locations that were positive for spider mite infestations on the edge of fields between June 28 and July 9. Map credit: NDSU IPM Program.

Please note that while this scouting program can identify regions in which soybean aphid population densities are growing, it is no substitute for diligently scouting your own fields. Fields scouted in this program were selected randomly without prior knowledge regarding seed- or post-applied insecticides, which can affect early-season aphid colonization.

Literature cited

McCornak, B.P., Ragsdael, D.W. and Venette, R.C. 2004. Demography of soybean aphid (Homoptera: Aphididae) at summer temperatures. J. Econ. Entomol. 97(3): 854-861.

Potter, B., Koch, R. and Ostlie, K. 2020. Will two-spotted spider mites throw a curve ball at the 2020 Minnesota crop? Minnesota Crop News article. Online.

Ragsdale, D.W., McCornack, B.P., Venette, R.C., Potter, B.D., MacRae, I.V., Hodgson, E.W., O’Neal, M.E., Johnson, K.D., O’Neil, R.J., DiFonzo, C.D., Hunt, T.E., Glogoza, P.A., and Cullen, E.M. 2007. Economic thresthod for soybean aphid (Hemiptera: Aphididae) J. Econ. Entomol. 100(4):1258-1267.

Special thanks to UMN Extension IPM specialist Bruce Potter for reviewing this article.