RESPONSE OF A LEAF BEETLE TO 2 FOOD PLANTS, ONLY ONE OF WHICH PROVIDES A SEQUESTRABLE DEFENSIVE CHEMICAL

Oreina elongata is a chemically defended leaf beetle. If its food plan t contains pyrrolizidine alkaloids, all life stages of the beetle sequ ester them. However, one of the two known host-plant genera does not c ontain these alkaloids. In this paper we compare the adult feeding pre ference and larval performance of two populations, one feeding on Aden ostyles alliariae (which contains alkaloids) and one on Cirsium spinos issimum (devoid of alkaloids). Adults of the population living on C. s pinosissimum preferred the alkaloid-containing A. alliariae, while adu lts of the population feeding on A. alliariae showed no preference for either plant. On the other hand, larval growth of both populations is better on C. spinosissimum, without alkaloids. This is especially so in the population that never naturally encounters pyrrolizidine alkalo ids; the population living on A. alliariae is apparently better adapte d to its host's secondary compounds. The data are discussed in terms o f cost of defense and trade-offs between growth and defense.