Effect of alfalfa saponins and flavonoids on pea aphid

We studied the development and feeding behaviour of the pea aphid, Acyrthosiphon pisum (Harris) (Homoptera: Aphididae), on the Radius and Sapko alfalfa (Medicago sativa L.) (Fabaceae) cultivars. Three saponins and flavones were identified in the alfalfa cultivars after thin layer chromatography separation. Cultivar Radius differed from Sapko in that it had a higher level of saponins, including zanhic acid tridesmoside and 3-GlcA,28-AraRhaXyl medicagenic acid glycoside. The flavones identified, including 7-O-beta-D-glucuronopyranosyl-4'-O-[2'-O-E-feruloyl-O-beta-D-glucuronopyranosyl(1 -> 2)-O-beta-D-glucuronopyranoside] apigenin, 7-O-{2-O-E-feruloyl-[beta-D-glucuronopyranosyl(1 -> 3)]-O-beta-D-glucuronopyranosyl(1 -> 2)-O-beta-D-glucuronopyranoside} apigenin, and 4'-O-[2'-O-E-feruloyl-O-beta-D-glucuronopyranosyl(1 -> 2)-O-beta-D-glucuronopyranoside] apigenin, occurred in tissues of both alfalfa cultivars. However, cv. Radius had very low mean flavonoid concentrations in comparison to cv. Sapko. Pea aphids that fed on cv. Radius plants showed a reduction in reproduction and survival. The aphid pre-reproductive period on cv. Radius was prolonged and the reproductive and post-reproductive periods on cv. Radius were reduced, compared to those on cv. Sapko. Cultivar Radius also negatively influenced aphid probing behaviour. This was especially the case during the initial period of the pathway phase. The results suggested that alfalfa cv. Radius, which had a higher level of saponins and a lower level of flavonoids, was less accepted by the pea aphid.