The goal of this proposal is to provide growers with the facts needed to successfully manage soybean aphids, Aphis glycines, and soybean cyst nematode (SCN), Heterodera glycines, in their fields in ND. By understanding IPM strategies for control of these pests, soybean yields can be maximized. The first objective will determine efficacy of different insecticide management strategies (seed treatments versus foliar applied insecticides including biorational insecticides) and compare insecticides to the utility of Ragl aphid resistant soybean. For biorational insecticides, sugar-based products will be tested for soybean aphid control. The benefits of using the established economic threshold of 250 aphids per plant will be demonstrated. A second objective is to determine the interaction between soybean aphids and soybean cyst nematode, and corresponding yield loss for SCN resistant varieties. Soybean aphids affect SCN management by increasing SCN reproduction. Future management recommendations need to consider both soybean aphids and SCN. In summary, this research will identify the best IPM strategy for soybean aphids and SCN in ND. By understanding the producer's economic risk with different insecticide strategies and the presence of SCN, management of soybean aphid can optimized for ND's soybean producers.

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Management of Soybean Aphids and Interaction with Soybean Cyst Nematode

Executive Summary, 30 June 2016

Investigators:
Dr. Janet Knodel, Extension Entomologist, Department of Plant Pathology, NDSU
Dr. Sam Markell, Extension Plant Pathologist, Department of Plant Pathology, NDSU
Cooperators:
Dr. Ted Helms, Soybean Breeder, Department of Plant Sciences, NDSU
Mr. Patrick Beauzay, Research Specialist, Department of Plant Pathology, NDSU

Soybean aphid is well-established in the upper Midwest and continues to be a serious pest of soybean production in North Dakota. Soybean producers need to be vigilant and scout their fields regularly, especially from July through mid-August when soybean aphid populations can increase rapidly. The goal of this portion of our research was to compare alternate management strategies, including the use of an insecticide seed treatment, early (R1) foliar insecticide applications and a Rag1 aphid-resistant variety, to a foliar insecticide application made at the economic threshold of 250 aphids per plant. We also examined the use of a 10% sugar solution, and a sugar-based biorational insecticide called sucrose octanoate, because there have been recent questions about whether these products can provide adequate soybean aphid control.

Under heavy soybean aphid pressure, our results demonstrated that the use of an insecticide seed treatment (thiamethoxam) and R1 foliar insecticide applications (lambda-cyhalothrin) did not result in a yield advantage compared to foliar insecticide applications made at the economic threshold of 250 aphids per plant, even when the seed treatment and/or R1 applications appeared to keep soybean aphid numbers below the economic threshold. The use of an insecticide seed treatment and early foliar insecticide applications are no guarantee to keep aphid populations in check through the R6 growth stage when soybeans are no longer susceptible to aphid feeding. These treatments will be ineffective for late July and early August aphid influxes, and producers may well need to make a second foliar application, with no added yield benefit to offset the additional cost. Furthermore, early foliar insecticide applications can reduce natural enemies of soybean aphids, may flare spider mites, and may promote the development of insecticide resistant aphids. Sugar and sucrose octanoate did not provide acceptable aphid control and we do not recommend using these products.

Soybean cyst nematode (SCN) is a major threat to soybean production in the upper Midwest and has expanded significantly in North Dakota, particularly in the eastern part of the state. Recent research suggests that there is a relationship between soybean aphid density and SCN reproduction, and soybean aphid feeding may result in increased SCN, even in SCN resistant varieties. The goal of this portion of our research was to examine SCN population growth in SCN resistant and susceptible varieties under differing soybean aphid pressure (aphid free, economic threshold, and untreated). We found that in the SCN resistant variety, SCN increased when soybean aphid numbers increased, but we did not observe the same pattern in the SCN susceptible variety. For SCN levels, the SCN resistant variety had significantly less SCN than the susceptible variety at the end of the growing season. At this time, there is no compelling evidence to change Extension-recommended management practices for either soybean aphid or SCN.