The chemical ecology of multispecies interactions: how secondary metabolites mediate pollination and herbivory

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Flowering plants produce nectar to attract pollinators. It is therefore unusual that floral nectar frequently contains secondary metabolites, noxious compounds commonly used to defend plants from herbivores. Hypotheses to explain this so-called “toxic” nectar include protecting flowers from nectar robbing animals, encouraging specialized pollination and preventing microbial growth, but nectar secondary metabolites may also be an unintended consequence of chemical defenses in leaves. Nectar secondary metabolites can have significant effects on pollinator health and behaviour. Animals can avoid visiting flowers with “toxic” nectar or may reduce the number of visits or length of time spent per flower. Consuming nectar secondary metabolites can reduce nutrient assimilation and mobility in pollinators and, in some cases, cause death. Given their effects on pollinators, nectar secondary metabolites are predicted to have significant consequences for pollination and plant reproduction, however this has rarely been examined. My proposed research will address the role of nectar chemistry in mediating plant-pollinator interactions and plant reproductive success. Research will focus on milkweeds (Asclepias spp), which have become a model system for studying plant chemical ecology. Milkweeds produce secondary metabolites called cardenolides that can be very toxic to herbivores. These compounds are also be found in milkweed nectar, but their effects on pollinators and pollination are currently unknown. The goal of my research program will be to determine whether nectar cardenolides in milkweeds incur an ecological cost to plants by deterring pollinators, inhibiting pollination and reducing seed production. To investigate the effects of milkweed nectar chemistry on pollination, my research group will use a combination of laboratory assays, greenhouse trials and manipulative field experiments. First, we address how nectar cardenolides alter the foraging behaviour of bumble bees, common pollinators of milkweeds. In the laboratory, we will test how nectar cardenolides affect pollinator preference and whether variation in nectar sugar concentration or nectar volume influence this preference. We will then examine the direct effects of herbivory on nectar chemistry and pollination using monarch caterpillars. We will allow caterpillars to feed on milkweed plants in the greenhouse and will subsequently release bumble bee workers to visit milkweed flowers, looking for changes in nectar chemistry, pollinator behaviour and pollination success due to herbivory. Finally, we will plant milkweeds in field plots and look at how herbivory by caterpillars affects wild pollinator behaviour and pollination. We will use molecular techniques to track paternity of seeds within these plots to assess whether the effects of herbivory on nectar chemistry change how genes move within the plant population. Pollination is essential to both wild and agricultural ecosystems and understanding the relationships between plants and pollinators is critical to preserving ecosystem function and biodiversity. Taken together, these experiments will provide new insight into the role of nectar chemistry in structuring complex multispecies interactions in terrestrial ecosystems.