The curious case of leaf-rolling nematodes

Several arthropod species, including caterpillars and spiders, commonly construct leaf-based shelters in the form of rolls, tents, and tiers for protection from predators and extreme physical conditions, affording them safety during development and reproduction. By building such shelters, these organisms qualify as ecosystem engineers (Neotrop Entomol 2016; doi.org/10.1007/s13744-015-0348-8), indirectly facilitating arthropod diversity on host plants (Arthropod-Plant Interact 2019; doi.org/10.1007/s11829-018-9661-6).

In the Cerrado Rupestre vegetation of southeastern Brazil (Nat Conserv -Bulgaria 2022; doi.org/10.3897/natureconservation.49.89237), we observed a gall-forming species of nematode that also induces a plant's gall-infested leaves to roll – the first recorded case, to the best of our knowledge (Ecol Entomol 2021; doi.org/10.1111/een.12993). The microscopic (600 μm) nematode Ditylenchus gallaeformans induces galls on the shrub Miconia ligustroides. As the galls develop over time, they cause the undersides of the leaves to curl, forming rolls roughly 20 mm in diameter (top). The interiors of the rolled leaves with attached galls are frequently colonized by many arthropod species, especially spiders, which deposit thick layers of silk to envelop and protect their egg sacs (bottom). As compared to host plants with intact (unmodified) leaves, host plants with gall-induced rolled leaves, which remain on the plants for approximately eight months, are associated with higher arthropod abundance and diversity (Ecol Entomol 2021; doi.org/10.1111/een.12993).

By diverting nutrients to feed the nematode larvae within them, the galls directly damage the host plants. At the same time, however, the galls may indirectly protect host plants from herbivory, given that the spiders that take refuge in these rolled structures repel sap-sucking and chewing insects (Ecol Entomol 2021; doi.org/10.1111/een.12993). Does gall presence have a net positive or negative effect on host plants? In addition, could galls accelerate the decomposition rates of the fallen infected leaves?