A Case-Study On Japanese Beetle Resistance Among Aphid-Resistant Soybean Germplasm: Genetics and Beyond.

The soybean aphid, Aphis glycines Matsumura (Hemiptera: Aphididae), is native to Asia, and is a major destructive pest of soybean Glycine max (L.) Merr. Up to 50% yield loss can occur in the United States if fields are left untreated. Several aphid-resistant soybean lines have been developed by many breeding programs, but only very few lines were successful due to breakdown of resistance. Thus far, the aphid-resistant lines developed by the MSU (Michigan State University) soybean breeding program continue to show reliable resistance. However, in a study conducted during 2007-2008, MSU aphid-resistant germplasm (E06901, E06905, and E06906) showed elevated susceptibility to defoliation (>50%, in field choice tests) by Japanese beetle (JB) Popillia japonica Newman, when compared to another aphid-resistant line (LD05-16060) developed by the Uni. of Illinois. Typically, action thresholds for soybean defoliation range from 30%-40% pre-bloom, decreasing to 15% between bloom and pod fill, and 25% thereafter. This issue led to identification of new lines among aphid-resistant germplasm that also show resistance to JB defoliation. A QTL (Quantitative Trait Loci) mapping approach was used to identify sources of JB resistance in a population developed from E06906 x LD05-16060. Pest Severity and Pest Incidence data were collected for two years on the same study site in East Lansing. Preliminary mapping with 94 individuals, showed potential QTL peaks on A1, A2, B2, and M linkage groups, for pest severity data from both years. Seventeen best F4:5 lines that showed good resistance to both insects were selected with the available molecular marker and phenotypic data. Moreover, this new direction also provided an opportunity to stack aphid–resistant genes (Rag1 with rag3 and rag1b), thereby improve the MSU aphid-resistant germplasm to confer resistance to both soybean aphid and JB.