Evaluation location: Illinois, United States
Expression of soybean aphid resistance was characterized in choice and nonchoice tests. Choice tests were arranged in a randomized complete block design, 2 replications of 3 plants each, where aphids were allowed to move to preferred host plants, were conducted in an air-conditioned, insecticide-free greenhouse (22-25C, continuous illumination). Leaves from Williams 82 plants infested with the Urbana aphid clone were placed on top of V1-stage seedlings. The aphids moved from the infested leaf used to transfer aphids to the test seedlings within the first day after transfer, and after about 7 to 14 d, they distributed themselves on preferred hosts and developed colonies. An aphid index was calculated by multiplying the estimate of aphid population (0, no live aphids, to 3, high aphid density) by the rating of aphid damage (0, no damage, to 3, severe damage) 21 d after transfer of aphid infested leaves. The type of resistance in resistant germplasm was studied in nonchoice tests. Total aphid populations on each plant were counted