Ants visiting extrafloral nectaries and pyrrolizidine alkaloids may shape how a specialist herbivore feeds on its host plants

The presence of extrafloral nectaries (EFNs) attracts predators and parasitoids, and protects the plant against herbivorous insects. By improving plant defences, EFNs reduce the fitness of herbivores. The use of similar host plants with no EFNs or adaptations in response to predators and parasitoids may enhance herbivore fitness. In this context, we studied the feeding habit (on leaves or on unripe seeds inside the pods) of larvae of the specialist moth Utetheisa ornatrix in two Crotalaria host plant species in which EFNs are present (C. micans) or absent (C. paulina). We hypothesized that the moths’ feeding habit was influenced by its natural enemies via their presence on EFNs. In C. micans, we found more larvae feeding inside the pods rather than on the leaves, while in C. paulina, larvae were found in both parts of the plant. There was greater activity of natural enemies in C. micans than in C. paulina. The moth sequesters enough pyrrolizidine alkaloid (PAs) to defend against predators in the leaves and seeds of C. paulina, but only in seeds of C. micans. Therefore, a change in the feeding habit in U. ornatrix larvae is a plastic response that depends on whether EFNs are present or not, or whether PA concentrations are low or high. This change does not affect overall moth performance. However, other factors, such as pod hardness, predation by organisms other than those visiting EFNs or even parasitoids cannot be ruled out as being responsible for the change in feeding habit. To date, both the EFNs and PAs in Crotalaria species are a parsimonious explanation of how larvae of U. ornatrix use different species of Crotalaria for feeding.