Spatiotemporal dynamics and plant-part preference patterns of the plant-visiting ants and the insect herbivores of sponge gourd plants

Field investigations revealed a synchrony between daily and seasonal activity patterns of two Raphidopalpa pests on the extrafloral nectary (EFN)-bearing sponge gourd, Luffa cylindrica (L.) plants. A similar pattern was revealed in six of the most abundant plant-visiting ant species. The spatial activity patterns of the herbivores R. foveicollis and R. intermedia revealed a high frequency on the leaves (except, newly unfolded leaves) and the corolla, but not on the bracts, bracteoles and the calyces of male/female buds and flowers. Young expanding leaves (leaf age class 2) had a significantly higher density of extrafloral nectaries (EFNs), as well as minimum leaf damage compared with the other leaf age classes. The density of EFNs was highest on the bracteoles and lowest on the leaves. In behavioural assays, ants demonstrated significantly greater preference for the 2-day old buds/newly opened leaves than the 2-day old intact unisexual flowers/corolla. Also, young, newly expanding leaves were preferred over young (unopened) and mature leaves. Thus, the higher density of EFNs at vulnerable plant sites, along with the visits of ants to such sites, may influence the spatiotemporal activity patterns of the insect herbivores. These results provide support for the optimal defense hypothesis since they show that younger tissues and reproductive tissues may be getting better protection by the ant species.