Lack of Evidence for the Toxic Nectar Hypothesis: A Plant Alkaloid Did Not Deter Nectar Feeding by Lepidoptera

Abstract

Floral nectars of many plant species contain alkaloids and other allelochemics that might deter butterfly visitors and promote flower constancy by specialized pollinators. The pyrrolizidine alkaloid, monocrotaline, has been implicated as such a feeding inhibitor in nectar, in support of this toxic nectar hypothesis. We tested this hypothesis by evaluating monocrotaline for deterrence of nectar feeding in the tobacco budworm moth, Heliothis virescens (Fab.), the cabbage looper moth, Trichoplusia ni (Hubner), and the gulf fritillary butterfly, Agraulis vanillae (L.). Concentrations of monocrotaline added to aqueous sucrose solutions did not reduce consumption in these three species, even with near saturation concentrations of monocrotaline. Also, gulf fritillary butterflies did not alter their preference for visiting orange artificial flowers when given the choice between yellow flowers with a sugar solution and orange flowers with a monocrotaline and sugar solution. Patterns of flower visitation by Lepidoptera are likely due to a combination of factors, such as attractants, feeding stimulants and deterrents, visual stimuli, flower morphology, and ecological factors that control nectar availability.