WITHIN-PLANT PATTERNS OF THERIOAPHIS-MACULATA ON RESISTANT, TOLERANT, AND SUSCEPTIBLE ALFALFA PLANTS

Plant lines with resistance (antibiosis or antixenosis or both), tolerance, or susceptibility to the spotted alfalfa aphid, Therioaphis maculata (Buckton), were selected from the cultivar OK08 for use in studies to describe the within-plant pattern of the aphid in the foliar canopy of alfalfa. Aphid counts and damage ratings were analyzed for each node (from the crown), and profiles of averages per node for each alfalfa line were prepared to illustrate within-plant patterns. Numbers of spotted aphids per node or per stem on susceptible and tolerant plants were significantly higher than on resistant plants. After 10 d of infestation, damage ratings averaged over nodes were significantly higher for susceptible plants than for those with resistance or tolerance. On all three lines, the spotted alfalfa aphid showed a clear preference for leaf blades over petioles or stems, and the greatest numbers were consistently found on nodes nearest the plant crowns. Little upward movement on stems was evident until damage to leaf blades of lower nodes had resulted in necrosis and leaf drop.