Family matters: eVect of host plant variation in chemical and mechanical defenses on a sequestering specialist herbivore

Insect herbivores contend with various plant traits that are presumed to function as feeding deterrents. Paradoxically, some specialist insect herbivores might beneWt from some of these plant traits, for example by sequestering plant chemical defenses that herbivores then use as their own defense against natural enemies. Larvae of the butterXy species Battus philenor (L.) (Papilionidae) sequester toxic alkaloids (aristolochic acids) from their Aristolochia host plants, rendering larvae and adults unpalatable to a broad range of predators. We studied the importance of two putative defensive traits in Aristolochia erecta: leaf toughness and aristolochic acid content, and we examined the eVect of intraand interplant chemical variation on the chemical phenotype of B. philenor larvae. It has been proposed that genetic variation for sequestration ability is “invisible to natural selection” because intraand interindividual variation in host-plant chemistry will largely eliminate a role for herbivore genetic variation in determining an herbivore’s chemical phenotype. We found substantial intraand interplant variation in leaf toughness and in the aristolochic acid chemistry in A. erecta. Based on Weld observations and laboratory experiments, we showed that Wrst-instar larvae preferentially fed on less tough, younger leaves and avoided tougher, older leaves, and we found no evidence that aristolochic acid content inXuenced Wrstinstar larval foraging. We found that the majority of variation in the amount of aristolochic acid sequestered by larvae was explained by larval family, not by host-plant aristolochic acid content. Heritable variation for sequestration is the predominant determinant of larval, and likely adult, chemical phenotype. This study shows that for these highly specialized herbivores that sequester chemical defenses, traits that oVer mechanical resistance, such as leaf toughness, might be more important determinants of earlyinstar larval foraging behavior and development compared to plant chemical defenses.