Laboratory Screening Techniques for Evaluation of Soybean Germplasm for Resistance to Twospotted Spider Mite (Acari: Tetranychidae)

Thirty-one soybean, Glycine max (L.) Merrill, accessions from maturity groups II through VIII were evaluated in excised and intact (whole plant) leaf bioassays to determine the ability of these bioassays to detect differences in susceptibility to the twospotted spider mite, Tetranychus urticae Koch. Although there were few significant differences between bioassays for variables measured within maturity groupings, the excised leaf bioassay which was easier to set up and monitor and took three-fourths less growth chamber space also had less variation among replications and repeated tests and detected a greater number of differences among accessions. Although there were significant differences among soybean accessions within a maturity group for specific variables, results suggest that high levels of resistance to the twospotted spider mite are not present in the germplasm screened. Several accessions screened, with known resistance to foliar feeding insects, were significantly less preferred for spider mite oviposition and development. However, it was apparent that the gene(s) controlling insect resistance do not impart the same level of resistance to the twospotted spider mite. Results of this study, based on differences in susceptibility among soybean accessions, demonstrate that the excised leaf bioassay should prove to be an efficient and uniform laboratory bioassay to screen soybean germplasm for resistance to the twospotted spider mite.