Picky predators and the function of the faecal shield of a cassidine larva

1. The faecal shield of cassidine larvae is generally thought to have evolved as a defence against predators, but field evidence for this is only anecdotal. We investigated the effectiveness of the shield as defence against natural enemies and as protection from ultraviolet (UV-B) radiation in the shield beetle Cassida rubiginosa Muller (Coleoptera, Chrysomelidae). We also investigated if the construction and bearing of the shield is associated with fitness costs for the larvae in the absence of natural enemies. 2. Using continuous video surveillance of individual prey, we determined the predator complex of the larvae in the field. Against the main predator, the paper wasp Polistes dominulus Christ (Hymenoptera, Vespidae), the faecal shield was not effective at all. Only few other generalist predators attacked the larvae and thus represented no major mortality factor. By contrast, the faecal shield was highly effective against parasitoids, an unexpected trait described here for the first time. 3. In dual-choice bioassays we found no attractive effect of the faecal shield on the endoparasitoid Foersterella reptans (Hymenoptera, Tetracampidae), but a removed faecal shield facilitated oviposition. 4. There was no protective effect of the faecal shield against UV-B radiation in a laboratory experiment. However, irradiated larvae suffered from a higher mortality rate and reached a lower pupal weight, independent of whether or not they carried a shield. 5. We found no measurable fitness costs for the larvae associated with bearing the faecal shield. Advantages (protection from desiccation, wind) seem to outweigh the costs, even in the absence of enemies. 6. Although many studies on the defensive function of Cassidine shields have been published, this is to our knowledge the first to consider the actual predation pressure in natural situations. This step is rarely undertaken in studies on proposed defensive traits. However, verification that a proposed selection pressure is in fact present in the field is crucial for demonstrating the value and function of any trait.