Selection and Characterization of Tolerance to the Spotted Alfalfa Aphid (Homoptera: Aphididae) in Alfalfa

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Abstract
Selection for tolerance to the spotted alfalfa aphid (SAA), Therioaphis maculata (Buckton), was investigated in an alfalfa, Medicago sativa L., cultivar ‘OK 08.’ Seedlings were infested with SAA at three stages of development (cotyledon, unifoliolate, and trifoliolate) for each of five durations (2, 4, 6, 8, and 10 d). Of 1,200 seedlings tested, 54 that survived the 6-, 8-, and 10-d infestation periods were retested to confirm and characterize tolerance. Tolerant plants supported SAA infestation levels equivalent to those of susceptible plants but sustained significantly less plant damage. Plant damage was indexed by measuring stem height, number of trifoliolates per plant, and percentage of leaves exhibiting chlorosis. Thirty-two of the 54 originally selected plants supported SAA infestations equivalent to the susceptible standards. Of these, five were judged to have a high degree of tolerance. Aphid feeding behavior on highly tolerant plants was similar to feeding behavior on susceptible standards when tested with electronic feeding monitors, except for reduced phloem ingestion time.