Do native and invasive herbivores have an effect on Brassica rapa pollination?

Abstract

Mutualistic (e.g. pollination) and antagonistic (e.g. herbivory) plant-insect interactions shape levels of plant fitness and can have interactive effects. By using experimental plots of Brassica rapa plants infested with generalist (Mamestra brassicae) and specialised (Pieris brassicae) native herbivores and with a generalist invasive (Spodoptera littoralis) herbivore, we estimated both pollen movement among treatments and the visiting behaviour of honeybees versus other wild pollinators. Overall, we found that herbivory has weak effects on plant pollen export, either in terms of inter-treatment movements or of dispersion distance. Plants infested with the native specialised herbivore tend to export less pollen to other plants with the same treatment. Other wild pollinators preferentially visit non-infested plants that differ from those of honeybees, which showed no preferences. Honeybees and other wild pollinators also showed different behaviours on plants infested with different herbivores, with the former tending to avoid revisiting the same treatment and the latter showing no avoidance behaviour. When taking into account the whole pollinator community, i.e. the interactive effects of honeybees and other wild pollinators, we found an increased avoidance of plants infested by the native specialised herbivore and a decreased avoidance of plants infested by the invasive herbivore. Taken together, our results suggest that herbivory may have an effect on B. rapa pollination, but this effect depends on the relative abundance of honeybees and other wild pollinators.