Evidence for a deterrent effect of cardenolides on Nephila spiders

Sequestration of toxic plant compounds by herbivorous insects as a defence against predators has been observed in many tritrophic systems. In this study, we focused on the potential benefit of sequestered cardenolides, potent toxins that block the Na+K+-ATPase against an invertebrate predator. To evaluate the effect of cardenolides we used three different types of cardenolide-containing prey, which were offered to the orb-weaving spider Nephila senegalensis. As a natural system spiders were presented with the cardenolide-sequestering moth Empyreuma pugione. To reduce the complexity of the predator–prey interaction spiders were furthermore fed with ouabain-laced non-toxic flies. Lastly, we developed a new reductionistic assay involving gelatine capsules as insect dummies. Our results show that ouabain elicits significant rejection behaviour and suggest cardenolides to be the basis for rejection of E. pugione by N. senegalensis.