The cowpea aphid, Aphis craccivora Koch (Aphididae), is considered to be one of the major pests in the semiarid Pampas of Argentina and in other alfalfa (Medicago sativa L.) growing areas of the world. In the present study the antibiotic, antixenotic, and tolerance resistance of eight alfalfa cultivars to A. craccivora, were investigated under laboratory conditions at 24 ± 1 °C, 65 ± 10% relative humidity, and 14:10 h photoperiod. Antibiosis experiments showed significant differences in the

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... lopmental time and adult longevity of the cowpea aphid among the alfalfa cultivars. Intrinsic rate of natural increase (rm) for apterous aphids varied significantly with alfalfa cultivars on which aphids were reared. This value ranged from 0.04 to 0.16 females female -1 d -1 , which was the lowest on 'Medina'. Additionally, the estimated net reproductive rate (R0) and finite rate of increase (λ) for apterous aphids were the lowest on 'Medina'. For the antixenosis experiment significant difference was found in aphid's preference to the alfalfa cultivars. 'Carmina' and 'Victoria' were the least preferred by the apterous aphids. For the tolerance experiment 'Carmina', 'Monarca', 'SPS6550' and 'Victoria' were more tolerant than the other cultivars to A. craccivora. Therefore, our results demonstrated that among the investigated cultivars 'Medina' displayed antibiosis to A. craccivora, 'Carmina' and 'Victoria' expressed antixenosis and tolerance, and 'Monarca' and 'SPS6550' were tolerant to this aphid pest.