An Assessment of Japanese Beetle Defoliation on Aphid‐Resistant and Aphid‐Susceptible Soybean Lines

As public institutions and seed companies incorporate soybean aphid ( Aphis glycines Matsumura) resistance genes into soybean [ Glycine max (L.) Merr.] cultivars, it is important to retain resistance to defoliators. This study compared Japanese beetle ( Popillia japonica Newman) defoliation among rag1b and rag3 aphid‐resistant lines (E06901, E06905, and E06906) with a Rag1 aphid‐resistant line (LD05‐16060) and aphid‐susceptible lines (DKB27‐53, SD01‐76R, and Titan RR). Under natural insect pressure, the percentage of leaflets consumed by Japanese beetle was greater on rag1b and rag3 lines (50–86%) than LD05‐16060 (11%) and SD01‐76R (5%). Defoliation on the three most damaged trifoliates was higher on rag1b and rag3 lines (49–54%) and its aphid‐susceptible parent Titan RR (35%) than on LD05‐16060 (5%) and its aphid‐susceptible parent, SD01‐76R (1%). Similarly in laboratory choice and no‐choice tests, greater leaf area was removed from rag1b and rag3 lines. There was more feeding on LD06‐16060 under no‐choice conditions than under choice conditions, suggesting LD05‐16060 was more attractive to Japanese beetle in the absence of a preferred line. At present, the differential susceptibility among these lines cannot be attributed to a specific compound or compounds until sufficient genetic and biochemical studies are conducted. This study shows the importance of monitoring Japanese beetle defoliation in breeding programs to determine the severity of threat posed by this insect on new lines.