Many factors of the test arthropod, host plant, and environment may influence the expression of host plant resistance to arthropods. For example, the plants on which arthropods are reared prior to testing for host plant resistance may directly influence arthropod behavior during the experiment (a phenomenon called preconditioning). Three biotypes of the soybean aphid, Aphis glycines Matsumura, named biotype 1, 2 and 3, and multiple breeding lines of soybean, Glycine max (L.) Merr., resistant to these biotypes have been identified. However, biotypes of soybean aphid were reared on separate pre-test host lines in these prior experiments to avoid mixing biotypes. We tested whether the pre-test hosts of biotype 2 and 3 influenced the expression of host plant resistance to aphids. We reared biotype 2 on three pre-test lines of soybean: PI 567541B (presumably resistant to this biotype) and Williams 82 and LD05-16611 (presumably susceptible). We reared biotype 3 on four pre-test lines of soybean: LD05-16611 and PI 437696 (presumably resistant to this biotype) and Williams 82 and PI 200538 (presumably susceptible). We enclosed aphids from each pre-test line of soybean on five experimental plants from each line of soybean and counted the aphids after 12 days.
Population sizes of biotype 2 were mainly influenced by the test line of soybean, but population sizes of biotype 3 on most test lines of soybean were influenced by the pre-test line on which soybean aphids were conditioned. The test line Williams 82 had large populations of soybean aphids, regardless of the pre-test line of soybean on which aphids were conditioned. The test line PI 437696 was relatively resistant to biotype 3 when aphids were preconditioned on Williams 82 and PI 200538, but intermediate-sized populations developed when aphids were preconditioned on PI 437696 and LD05-16611. The test line PI 200538 was relatively susceptible to biotype 3 when aphids were preconditioned on most pre-test host lines, but only small populations developed on this test line when preconditioned on Williams 82. Finally, the test line LD05-16611 was moderately susceptible when biotype 3 aphids were conditioned on any plants with known resistance genes, but resistant when aphids were conditioned on Williams 82, which has no known resistance genes. The preconditioning effect must be compensated for in experiments so lines of soybean are not erroneously labeled as susceptible or resistant, which could result in accidental deployment of susceptible lines of soybean in the field.