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

Abstract

Studies of multitrophic interactions show that insect faeces may act as a defensive device against predators, as kairomone source for attraction of antagonists and as a significant energy source for micro-organisms. In the present study, we investigated effects of larval faeces from leaf beetles of the subfamiliy Cassidinae towards a generalist predator, the ant Myrmica rubra. Most cassidine larvae collect their faeces together with exuviae as so-called abdominal defensive shields on two movable spines at the posterior tip. The effects of these abdominal shields towards M. rubra were studied in three cassidine species, which feed mono- or oligophagously upon tansy (Chrysanthemum vulgare): Cassida denticollis and C. sanguinosa which possess such faecal shields and, for comparison, C. stigmatica with shields made of exuviae only (=skin shield). Bioassays revealed that larvae with both faecal and skin shields were attacked by the ant M. rubra more often than larvae whose shields had been removed. This attractiveness of shields towards ants contrasts with other studies, which found that abdominal shields of chrysomelid larvae act as defensive mechanisms against generalist predators like ants. To characterize the attractive cues of the shields, we studied possible chemical and physical stimuli. Olfactometer bioassays with M. rubra and chemical analyses revealed that plant-derived volatiles from faecal shields of C. denticollis attracted the ant, whereas odour from skin shields of C. stigmatica did not. Skin shields also emitted volatiles which derived from tansy, but in much lower quantities. Exclusion of contact to surface chemicals of a faecal shield reduced the ants' aggressive behaviour, whereas a change in the moisture content of a faecal shield had no influence. Visual stimuli cannot be ruled out as enhancing the ants' reaction towards faecal shields with their attractive volatiles, and are suggested to play a major role in the ants' response towards skin shields. This novel attractive effect 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 predator M. rubra.

Do fecal shields provide physical protection to larvae of the tortoise beetles Plagiometriona flavescens and Stolas chalybea against natural enemies?

“…Similarly, we found sesquiterpenes both on leaves of C. pinnatifida and in fecal shields of S. chalybea. Sesquiterpenes might cause the deterrent activity in fecal shields (Gomez et al, 1999), although these compounds are also cues to attract predators (Müller & Hilker, 1999).…”

“…Larvae can carry masses of exuviae and feces that accumulate on a fork-like structure located on the last segment of the abdomen, forming a shield-like structure (Eisner & Eisner, 2000). Previous studies have demonstrated that the shield provides protection against various groups of predators (Eisner et al, 1967;Root & Messina, 1983;Olmstead & Denno, 1993;Vencl et al, 1999;Eisner & Eisner, 2000), but there are also cases in which fecal shields can serve as an attractive cue for generalist predators (Müller & Hilker, 1999). Several authors have shown that the protection is provided by chemical compounds found in the fecal shield (Gómez et al, 1999;.…”

Faecal shield chemical defence is not important in larvae of the tortoise beetle Chelymorpha reimoseri (Chrysomelidae: Cassidinae: Stolaini)

“…In the field, we observed some predation events: a Polistes wasp preyed upon C. reimoseri larvae with shields by removing it before taking the prey away, an unidentified predatory bug feeding on larvae with shields, as well as on adults and eggs, and the bird Piaya cayana (Cuculiformes, Coccyzidae) preyed on larvae with shields and on adults. Other authors have shown that some natural enemies may use the shield as a cue to find their prey (Müller and Hilker 1999;Schaffner and Müller 2001). The sequestration of chemicals from the host plant or de novo biosynthesis of defensive compounds and storage in the tissue may, therefore, represent an attempt to overcome the poor protection of larvae offered by shields.…”

Mothers modify eggs into shields to protect offspring from parasitism

“…In terms of allocating resources to reproduction, producing faeces seems less costly to offspring and maternal fitness to produce. However, faeces may be highly effective at concealing eggs from one natural enemy, and a highly effective attractant for other natural enemies that can circumvent the defence [34]. Additionally, in species where the adults are non-feeders and may use most food consumed as larvae for growth and reproduction (as in M. amicus), or the faecal material is not substantial or malleable, faeces may not be the best material for egg concealment.…”