The goal of this research was to describe within-plant distribution for Acyrthosiphon kondoi Shinji and Kondo on alfalfa (Medicago sativa L.). Alfalfa entries used in this study were from breeding lines selected for moderate and high levels of resistance to A. kondoi and from the susceptible cultivar ‘OK08’. From each of the three entries, three stems on each of 12 established alfalfa plants grown in pots were infested with aphids when stems had four to five nodes. At 2-d intervals through 10 d after infestation, stem heights, numbers of nodes per stem, and numbers of trifoliolates per node (as axillary branching occurred) were recorded. Also recorded for each node were numbers of A. kondoi on leaf blades, petioles, and the internodal stem section below the node. Aphid counts were analyzed to determine significant differences among alfalfa entries and among plant parts (leaf blades, petioles, and stem sections) within entries. Plant profiles were prepared to show within-plant distributions. Regression of mean aphid numbers versus days after infestation gave a highly significant (P < 0.01) slope for the susceptible alfalfa entry. Slopes for regressions calculated for moderately resistant and highly resistant entries were not significant (P = 0.07 and P = 0.89, respectively), indicating that aphid numbers did not increase significantly on these entries. Leaf blades were used most frequently as feeding sites on the resistant entries over the 10-d infestation period. However, the highest proportions of aphids were found on stems of susceptible plants. Plant profiles show that A. kondoi preferred stems or leaves along the mid-portions of the foliar canopy and trifoliolates near the plant terminals on all three alfalfa entries. Relatively few aphids occurred on nodes near the soil surface or on nodes near the tops of stems aside from the plant terminal.
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