Evaluation location: Ohio, United States
A colony of soybean aphid (SA, Aphis glycines Matsumura) was established in a growth chamber at Ohio Agricultural Research and Development Center (OARDC), Wooster, OH, during the summer of 2005 by collecting aphids from nearby soybean fields. The colony was maintained on seedlings of cultivar Williams 82 placed inside the growth chamber at temperatures between 22 and 24C with a photosynthetically active radiation of 330 umol m/2 s/1 for 15 h daily and 60 to 70% relative humidity. The colony was restarted on a fresh batch of seedlings every 3 to 4 wk by transferring aphids from the old seedlings to new ones. Seeds were planted in 15 cm deep and 4 cm in diameter plastic conetainers and thinned to one seedling per conetainer at the unifoliolate-stage. Each conetainer was an experimental unit, and the experiment was arranged in a RCBD with two replicates. At the V1-stage, seedlings were infested with 20 to 30 aphids of all developmental stages except the winged aphids by placing an infested leaf section between the petiole of the youngest expanding leaf and the stem. Aphids migrated to the leaves and stems of the seedlings within several hours after putting the infested leaf sections on the seedlings. The aphids were allowed to multiply and move freely among plants in the experiment. The plants were bottom watered to avoid disturbing the aphids. Fourteen days after infestation (DAI), each experimental unit was assigned an aphid score between 1 and 5, where 1 = <25 aphids per plant, 2 = 25 to 100 aphids per plant, 3 = 101 to 200 aphids per plant, 4 = 201 to 400 aphids per plant, and 5 = >400 aphids per plant. On 28 DAI, the plants were evaluated for susceptibility to SA by using a plant damage score of 1 = <25 aphids and plant appears normal and healthy; 2 = 25 to 100 aphids per plant and plant appears normal and healthy; 3 = 101 to 300 aphids per plant and plant appears slightly stunted with slight yellowing of older leaves; 4 = 301 to 600 aphids per plant and plant appears moderately stunted with yellowing of older leaves and slight curling of young leaves; and 5 = >600 aphids per plant and plant appears severely stunted with severely curled and yellow leaves and most of the stem and leaf surfaces are covered with sooty mold and cast skins. The aphid number per plant was included in the plant damage scores because this was particularly helpful in differentiating plants with scores of 1 and 2. In both cases, the plants were fully healthy and normal looking but a plant with the score of 1 had very few if any aphids (<25) on it while a plant with a score of 2 had up to 100 aphids in some instances. This approach of separating these two types of plants allowed a conservative classification of plants as SA resistant. The plants with relatively higher numbers of aphids were classified as moderately resistant. Also, the aphid numbers at 14 DAI were not always in synchrony with the aphid numbers at 28 DAI. Some moderately resistant plants had the same number of aphids at 14 DAI as the resistant plants, but over the next 2 wk the aphid numbers on these plants increased slowly while the aphid numbers on the resistant plants did not increase at all or declined. The aphid susceptibility index (ASI) was calculated by multiplying the aphid score at 14 DAI by the plant damage score at 28 DAI with a possible ASI between 1 and 25. Resistant lines were tested again with choice and no choice tests.