The role of host plant usage and the accumulation of toxic secondary chemical compounds across the life cycle of a passion vine specialist butterfly

Abstract

Abstract Host plant specialisation by herbivorous insects is ubiquitous, especially among the Lepidoptera. Many taxa have the ability to accumulate toxic compounds from their host plants that serve as chemical defences against natural enemies. Despite common knowledge of this pattern among insect ecologists, we still have much to learn about how dietary variation affects an insect's ability to acquire toxic plant chemicals. Longwing butterfly larvae (Heliconius) accumulate toxic cyanogenic glucosides from their host plants, passion vines (Passiflora), that make them toxic to most predators. Here, we present on zebra longwing (H. charithonia) caterpillar cohorts that were reared on native P. affinis, P. biflora, P. lutea, or P. suberosa to determine how host usage affected cyanide accumulation over larval developmental stages, and ultimately the toxicity of adult butterflies. Samples were collected at third, fourth and fifth instars, and teneral adults. Cyanogenic glucosides were extracted, the cyanide molecules were captured as sodium cyanide (CN), and CN content was quantified colorimetrically. The quantity of CN acquired increased linearly over larval ontogeny and was variable among host plants. CN concentration increased over larval development for two hosts but declined on the third. There was no clear relationship between host plant and adult CN concentrations, but adults that consumed certain hosts as larvae ended up accumulating significantly more CN, and thus expressed higher CN concentrations, than others. We demonstrated that some host plants provide specialist insects with more toxins than others and that larval secondary chemical accumulation may predict which life stages are more vulnerable to natural enemies. These conclusions support the hypothesis that host choice affects the survival of specialist insect herbivores.