Unexpected reactions of a generalist predator towards defensive devices ofcassidine larvae (Coleoptera, Chrysomelidae)

Studies of multitrophic interactions show that insect faeces may act as a d efensive device against predators, as kairomone source for attraction of an tagonists and as a significant energy source for microorganisms. In the pre sent study, we investigated effects of larval faeces from leaf beetles of t he subfamily Cassidinae towards a generalist predator, the ant Myrmica rubr a. Most cassidine larvae collect their faeces together with exuviae as so-c alled abdominal defensive shields on two movable spines at the posterior ti p. The effects of these abdominal shields towards M. rubra were studied in three cassidine species, which feed mono- or oligophagously upon tansy (Chr ysanthemum vulgare): Cassida denticollis and C. sanguinosa which possess su ch Faecal shields and, for comparison, C. stigmatica with shields made of e xuviae only ( = skin shield). Bioassays revealed that larvae with both faec al and skin shields were attacked by the ant M. rubra more often than larva e whose shields had been removed. This attractiveness of shields towards an ts contrasts with other studies. which found that abdominal shields of chry somelid larvae act as defensive mechanisms against generalist predators lik e ants. To characterize th attractive cues of the shields, we studied possi ble chemical and physical stimuli. Olfactometer bioassays with M. rubra and chemical analyses revealed that plant-derived volatiles from faecal shield s of C. denticollis attracted the ant, whereas odour from skin shields of C . stigmatica a did not. Skin shields also emitted volatiles which derived f rom tansy, but in much lower quantities. Exclusion of contact to surface ch emicals of a faecal shield reduced the ants' aggressive behaviour, whereas a change in the moisture content of a faecal shield had no influence. Visua l stimuli cannot be ruled out as enhancing the ants' reaction towards faeca l shields with their attractive volatiles, and are suggested to play a majo r role in the ants' response towards skin shields. This novel attractive ef fect of the abdominal shields of cassidine larvae is discussed, especially with respect to host plant chemistry and possible functions of the shields that might outweigh the negative consequences of the attraction of the pred ator M. rubra.