Interrelationship between Quinolizidine Alkaloid Producing Legumes and Infesting Insects: Exploitation of the Alkaloid- Containing Phloem Sap of Cytisus scoparius by the Broom Aphid Aphis cytisorum

Abstract Aphids (Aphis cytisorum) which infest broom plants (Cytisus scoparius) accumulate up to 500 μg alkaloid/g fr. wt. The alkaloids, which are similar to those of the plants, consist of 17-oxo-sparteine, sparteine, 12,13-dehydrosparteine, and lupanine. Infested plants contain > 50% less alkaloids than aphid-free plants. In Lupinus aphid resistance which is due to their high alkaloid content is more expressed: Whereas bitter varieties are free from aphids, only the sweet alkaloid-free plants are susceptible to aphid infestation. The accumulation of alkaloids in aphids indicates that the quinolizidine alkaloids are translocated via the phloem in legume plants. This assumption is supported by direct evidence: analysis of phloem sap from Lupinus contains up to 5 mg alkaloid whereas xylem sap is virtually free of alkaloids. The interrelationship between quinolizidine alkaloids and hervivores is discussed.