Attached vs. Excised Trifoliolates for Evaluation of Resistance in Alfalfa to the Spotted Alfalfa Aphid1

Abstract

Spotted alfalfa aphid, Therioaphis maculata (Buckton), adult and nymphal survival was used to compare the level of resistance between caged trifoliolates on the plant and the corresponding plant part which had been detached. The adult and nymphal survival averaged over clones and replications was higher on excised than intact alfalfa trifoliolates. However, the differences in survival varied differentially among the clones tested. Adult and nymphal survival differences between intact and excised plant parts were significant on 24% (adult survival) and 20% (nymphal survival) of the clones. In all cases where the differences were significant, the survival on the excised trifoliolates exceeded that on the intact trifoliolates of the same clone. No significant differences in adult or nymphal survival were detected where the terminal was removed or left intact on the excised plant parts. From a practical standpoint, the results indicated that using the excised-trifoliolate procedure in resistance screening tended to underestimate the resistance level of the plant population tested.