Variation in the effectiveness of abdominal shields of cassidine larvae against predators

Abstract

AbstractThe abdominal shields of the larvae of two tansy [Tanacetum (Chrysanthemum) vulgare L., Asteraceae] feeding species of Cassida (Coleoptera: Chrysomelidae) were tested for their ability to deter attacks by predatory insects. Cassida stigmatica Suffr. which carries an abdominal shield made of the exuviae only, and C. denticollis Suffr. which builds a compact faecal shield, were compared. Adult earwigs (Forficula auricularia L., Dermaptera: Forficulidae), adult ladybird beetles (Coccinella septempunctata (L.), Coleoptera: Coccinellidae), and larval lacewings (Chrysoperla carnea Steph., Neuroptera: Chrysopidae) were used as predators and tested in dual choice bioassays. Against earwigs, shields of both cassidine species acted as a protective device. Earwigs preferred to feed on larvae without shields over larvae with intact shields. Adult ladybirds did not differentiate between C. stigmatica larvae with and without exuvial shields. However, C. denticollis larvae with faecal shields were contacted at a higher frequency by ladybird beetles, even though they were less often consumed, i.e., their shield served as protection. In an olfactometer assay, faecal shields of C. denticollis were shown to have no attractive effect on the ladybird beetles. The protection for larvae with shields is likely to be caused by mechanical effects, namely the ability to move the shields. Control insects, where faecal shields of C. denticollis were glued on larvae of Galleria mellonella L. (Lepidoptera: Pyralidae), could not move their shields and were consequently consumed by the predator. Feeding by larvae of lacewings was not influenced by the presence or absence of the faecal shield. Thus, the effectiveness of the abdominal shields of tansy‐feeding cassidine larvae varies with predator species, and might be based more on mechanical, than on chemical modes of action.