Nectar chemistry modulates the impact of an invasive plant on native pollinators

Abstract

1. Invasive species are considered a main driver of pollinator declines, yet the direct effects of invasive alien plants on pollinators are poorly understood. 2. Abundant, invasive plant species can provide a copious nectar resource for native pollinators. However, t he nectar of s ome plants contain s secondary compounds, usually associated with defence against herbivores . T he impact s of these compounds on pollinators are often unknown . 3. We compared how consumption of grayanotoxin I and III , natural secondary compound s in the nectar of invasive Rh ododendron ponticum L. , affected three native bee species: a honeybee, ( Apis mellifera L. ), a solitary mining bee ( Andrena carantonica , Pérez ) and a bumblebee, ( Bombus terrestris , L. ). 4. Survival of the solitary bee and the bumblebee species was not affected by either grayanotoxin , but honeybees were ~20x more likely to die when fed solutions containing grayanotoxin I . Furthermore, s olitary bees were deterred from feeding and exhibited malaise behaviours indicative of sublethal toxicity in response t o consumption of grayanotoxin I. In contrast, g rayanotoxin s did not affect bumblebee survival or behaviour, even when bees were subjected to multiple stressors ( parasite infection or food stress ) . 5. Our experiments suggest that w hile R. ponticum provides abundant floral nectar, it is only available as a food resource to pollinators that tolerate grayanotoxin s . Pollinators whose health is negatively affected by grayanotoxin s may experience negative impacts from R. ponticum invasion directly (if th ey consume R. ponticum nectar) or indirectly (if native floral resources are replaced by R. ponticum ). 6. Our study makes a novel comparison of the effects of a natural nectar secondary compound on three pollinator s pecies and clearly demonstrate s drastic variation in the responses of different key pollinator taxa to a nectar toxin . Our findings are thus in congruence with literature demonstrating the varying effects of invasive plant chemistry on native foliar 3 herbivores and our work demonstrates that nectar chemistry should be taken into account when determining the impacts of plant invasion for native pollinators.