Response of a leaf beetle to two food plants, only one of which provides a sequestrable defensive chemical

Oreina elongata is a chemically defended leaf beetle. If its food plant contains pyrrolizidine alkaloids, all life stages of the beetle sequester them. However, one of the two known host-plant genera does not contain these alkaloids. In this paper we compare the adult feeding preference and larval performance of two populations, one feeding on <i>Adenostyles alliariae</i> (which contains alkaloids) and one on <i>Cirsium spinosissimum</i> (devoid of alkaloids). Adults of the population living on <i>C. spinosissimum</i> preferred the alkaloid-containing <i>A. alliariae</i>, while adults of the population feeding on <i>A. alliariae</i> showed no preference for either plant. On the other hand, larval growth of both populations is better on <i>C. spinosissimum</i>, without alkaloids. This is especially so in the population that never naturally encounters pyrrolizidine alkaloids; the population living on <i>A. alliariae</i> is apparently better adapted to its host's secondary compounds. The data are discussed in terms of cost of defense and trade-offs between growth and defense.