Predator Responses to Sequestered Plant Toxins in Buckeye Caterpillars: Are Tritrophic Interactions Locally Variable?

Abstract

Recent studies demonstrate that generalist insect herbivores may be more subject to predation than specialists and suggest that this pattern is related to the availability and utilization of plant-derived natural products in diets of the latter. This intuitively attractive hypothesis has seldom been tested under natural conditions, however. A well-described plant-insect system, the buckeye butterfly, Junonia coenia, and its iridoid glycoside-containing host plants, was used to test the hypotheses: (1) that larval sequestration of iridoids reduces predation in the field, and (2) that this protection varies among populations. Fifth-instars reared in the laboratory on leaves of two host-plant species, Kickxia elatine (Scrophulariaceae) or Plantago lanceolata (Plantaginaceae), or on artificial diets were attached to monofilament tethers and set out into four natural populations. Only larvae fed on the locally available host plant and artificial diet-fed caterpillars were used at each site. Despite high overall predation, significantly more leaf-fed caterpillars survived than did artificial diet-fed caterpillars in Plantago sites, but the trend was not significant in Kickxia sites. The results reflect the differences in iridoid glycoside availability in these two host-plant species. They indicate that predators exert a strong selective force on this insect, that sequestered iridoid glycosides can be effective deterrents to predation, and that host-plant choices by ovipositing females and feeding larvae have consequences for mortality from natural enemies. This protection, however, is not absolute, and depends on local predator abundance and/or selectivity. The ecological interactions between buckeye caterpillars, their host plants, and their natural enemies are locally variable in nature.