Family matters: variation in defensive chemistry in the pipevine swallowtail (Battus philenor)

DIMARCO ROMINA D.; NICE CHRIS C.; FORDYCE JAMES A.

Conferencia; Evolution 2009 Conference; 2009

Insect herbivores contend with various plant traits that are presumed to function as feeding deterrents. As a consequence, some insects avoid certain parts of the plant in order to avoid plant defenses. Some insect herbivores might benefit from defensive traits, such as specialist herbivores that sequester plant chemical defenses that they use as their own defense against natural enemies. Caterpillars of the butterfly 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. In this study, we examine the importance of two traits that might serve as a defense function in Aristolochia, leaf toughness and leaf secondary chemistry that Aristolochia possesses to deter caterpillar of B. philenor. It has been proposed that genetic variation for sequestration ability is "invisible to natural selection" because intra- and inter-individual variation in host plant chemistry will largely eliminate a role for genetic variation in determining an herbivore’s chemical phenotype. We also examine the importance of intraplant chemical variation for determining the chemical phenotype of pipevine swallowtail caterpillars. Previous studies have shown that sequestration ability of B. philenor is heritable, and that sequestration enhances survivorship under natural field conditions. We found substantial intra- and inter-plant variation in leaf toughness and in the aristolochic acid chemistry of three Aristolochia species. Based on field observations and laboratory experiments, we show that caterpillars preferentially feed on young, less tough leaves and avoid more tough, older leaves, and found no evidence that aristolochic acid content influence caterpillar foraging. We also find that a majority of variation in sequestered aristolochic acids is explained by caterpillar family, not by host plant aristolochic acid content. Physical barriers, such as leaf toughness, have a greater influence on caterpillar foraging and development compared to the quantity of aristolochic acids.