Epicuticular Lipids of Alfalfa Leaves Relative to Position on the Stem and Their Correlation with Aphid (Homoptera: Aphididae) Distributions

The epicuticular lipids of alfalfa ( Medicago sativa L.) were analyzed by gas chromatography to determine if the distribution of alfalfa leaf epicuticular lipid and its components could be correlated with the previously reported preference of spotted alfalfa aphids, Therioaphis maculata (Buckton), for leaves on lower portions of alfalfa sterns. Older leaves near the crowns of alfalfa sterns, on which spotted alfalfa aphids prefer to feed, were covered with less epicuticular lipid per unit of leaf area than those farther up the stern. These differences were primarily due to differences in amounts of the primary alcohol, triacontanol, found in epicuticular lipids. In addition, leaves near the crown had greater aldehyde/alcohol ratios than did those at the top of sterns. Scanning electron micrographs of alfalfa leaf surfaces confirmed that lower leaves on stems contained large patches of leaf surface which were not covered with the crystalline lipid that formed a nearly uniform layer on leaves arising from the upper nodes of stems.