Chemical Defense of Lupins. Mollusc-Repellent Properties of Quinolizidine Alkaloids

Polyphagous molluscs such as Helix pomatia and Arion rufus generally do not feed on plants containing alkaloids. Of 19 species tested 10 species were totally avoided, the other 9 species were less attacked than Lactuca sativa, which was readily taken. Plants containing quinolizidine alkaloids were studied in detail. Those species with the α-pyridone alkaloids cytisine and N-methylcytisine were avoided to a higher extent than plants with lupanine as the major alkaloid. Since the repellency observed could be due to other natural products present in the plants besides the alkaloids, the feeding response of Helix pomatia was tested on artificial diets containing quinolizidine alkaloids in various concentrations as the only variable. If the snails had the choice they clearly preferred alkaloid-free food or a diet with only low alkaloid concen­trations. Half-maximal repellency of cytisine is less than 2 mᴍ, of sparteine 1-5 mᴍ, and of lupanine 1-8mᴍ. Since the in vivo concentrations of sparteine, cytisine, and lupanine are equal to or higher than the inhibitory concentrations required it is concluded that quinolizidine alkaloids constitute a potential antimolluscan principle of legumes.