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Crop wild relatives (CWRs) have high levels of genetic diversity compared to their domesticated descendants. Soybean (Glycine max) has over 20 species of CWRs, most of which are in secondary and tertiary gene pools. Glycine soja, hereafter ‘soja,’ is the only wild relative in the primary gene pool, i.e., species that readily cross with soybean. Soja has many advantageous traits that may be transferrable to soybean, including resistance to insect pests, with particularly strong sources of resistance to the soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae). Soybean aphid has been a major soybean pest in the United States and Canada since 2000 and a longstanding pest in East Asia. This paper reviews the challenges of developing soybean with durable resistance to soybean aphid in light of multiple, virulent biotypes in North America and China. It also examines particular challenges in evaluating soja germplasm for soybean aphid resistance and resultant solutions to those challenges. Soja germplasm is widely available, but from our experience, the logistics associated with reliably procuring high-quality soja seed has posed the main challenge in working with this CWR. This review highlights soja accessions identified with strong resistance to soybean aphid and their genetic bases, and it discusses possible strategies for exploiting aphid-resistant soja accessions to improve soybean pest management.

Soybean aphid (Aphis glycines Matsumura) is a pest of soybean [Glycine max (L.) Merr] in the Midwestern U.S. and southern Canada. Host plant resistance provides an alternative to insecticides for soybean aphid management. Soybean with Rag1 + Rag2 aphid resistance is available commercially but does not protect against soybean aphid biotype 4 colonization. To find sources of resistance to soybean aphid biotype 4, we turned to soybean’s progenitor Glycine soja (Sieb. and Zucc.). Twenty soja plant introductions (PIs) with previously identified resistance to avirulent soybean aphid biotype 1 were assessed in free-choice tests against biotype 4, followed by a no-choice test for promising PIs. Experiments were repeated using three soybean aphid biotype 4 colonies, collected from separate site-years (Lomira, WI 2013; Volga, SD 2015, 2016). Free-choice tests identified six, three, and eight soja PIs with putative resistance to Lomira13, Volga15, and Volga16 aphid colonies, respectively. No-choice tests identified six, two, and six soja PIs that continued to suppress Lomira13, Volga15, and Volga16 aphids, respectively. PI 65549 and PI 101404A were resistant to each of the three colonies of soybean aphid biotype 4 and may contribute to durable resistance against soybean aphid.

Soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), infestations of soybean, Glycine max (L.) Merr. (Fabales: Fabaceae), and the associated yield loss have led to a large dependence on insecticidal management in soybean throughout the Midwestern United States. However, several populations of pyrethroid-resistant soybean aphids have recently been found in Iowa, Minnesota, North Dakota and South Dakota, which highlights the importance of alternative management approaches. One such alternative method is host-plant resistance, which uses naturally occurring plant defenses in crop cultivars to reduce the potential for yield loss from a pest population. Current soybean aphid-resistant cultivars do not protect against all soybean aphids due to the presence of virulent biotypes. In particular, soybean aphid biotype 4 is virulent to Rag1 and Rag2 resistance genes both individually and in combination. However, we hypothesized that resistance to biotype 4 may exist in previously identified, but uncharacterized resistant soybean plant introductions (PIs). To test this, we evaluated 51 previously identified but uncharacterized soybean aphid-resistant PIs for their resistance to colonies of soybean aphid biotype 4 collected in separate site-years (Lomira, WI 2013; Volga, SD 2015, 2016). Free-choice tests identified 14 PIs with putative resistance to ‘Lomira13’, two to ‘Volga15’, and eight to ‘Volga16’ soybean aphid colonies. Follow-up, no-choice tests corroborated two to three resistant PIs per colony, and PI 437696, which was resistant to each of the three colonies and could aid in breeding efforts and an integrated approach to soybean aphid management.