Cardenolides May Affect Herbivory on Milkweeds ( Asclepias spp.) by the Root-Knot Nematode Meloidogyne incognita

Root-knot nematodes (RKNs) of the genus Meloidogyne are important pests in agriculture. RKNs are generalist herbivores with a wide host range including crop and wild plants. The latter are an important source of defensive metabolites that may be helpful for RKN management. Milkweeds ( Asclepias spp.) produce toxic cardenolides that protect them from herbivory. However, it is unclear if cardenolides may defend milkweeds against RKNs. Here, we tested this directly through herbivory assays with the RKN M. incognita on milkweed species that produce negligible and high levels of cardenolides, A. tuberosa and A. curassavica , respectively. We found that M. incognita induces fewer galls on A. curassavica than A. tuberosa and fails to reproduce after reaching maturity on the former but not the latter species. This suggests that the predominantly polar cardenolides in A. curassavica may engender long-term reproductive toxicity. Further toxicity assays with the polar cardenolide ouabain showed that cardenolides can also have more immediate toxic effects on M. incognita at higher concentrations. Ouabain caused a coiled, paralytic phenotype in juvenile RKNs, a sign of neurotoxicity, leading to lethality in a subset of RKNs. Some nematodes recovered upon ouabain removal, confirming that neurotoxic cardenolides have a nematostatic effect that results in death when exposure persists. Taken together, our results provide further evidence that cardenolides may function as anti-herbivore defenses against RKNs. The ‘dead end’ host plant A. curassaviva appears to possess useful properties that may be leveraged for control of RKNs in agriculture.