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

Abstract

The 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.

Defecation Behavior and Ecology of Insects

“…A number of manipulative studies have demonstrated that larvae with intact fecal coverings are better protected against predators than are larvae from which the coverings have been removed (45,56,95,104,142), though fecal coverings are not equally effective against all predators (95,104). In some instances the defense appears to be mechanical in nature, as the predator is unable to reach the larva through the fecal covering or is beaten back by the larva's active wielding of its fecal shield (29,45,104).…”

Dietary specialization influences the efficacy of larval tortoise beetle shield defenses

“…Some predators can inflict high mortality on tortoise beetle larvae, especially under experimental conditions (Olmstead 1996). For example, tortoise beetle shields have been shown to be ineffective at thwarting bug attacks because heteropterans can insert their long rostra through or under shields (Olmstead and Denno 1993;Mu¨ller 2002). Although generalist predators are considered important determiners of diet breadth (Price et al 1980;Bernays and Graham 1988), our results might have been different if we had used predators that specialize on tortoise beetle larvae (e.g., Gross et al 2004).…”

“…Tortoise beetles that vigorously wave their shields can beat back attacking predators (Eisner et al 1967;Go´mez 1997;Chaboo 2002;Mu¨ller 2002;Nogueira-de-Sa´and Trigo 2002); however, shields appear to be more than simple barriers. Chemical factors contribute to effective shield function in all our study species.…”

Biology and Morphology of Neochlamisus gibbosus (Fabricius, 1777) (Coleoptera: Chrysomelidae: Cryptocephalinae: Fulcidacini)

“…This fecal case is retained from instar to instar, with expansion continuing; the final instar case is converted to a pupation chamber by adding fecal material to seal the case and attaching it to the host or non-host plants. This fecal case is hypothesized to serve as a physical barrier to protect the juvenile stages from the environment, as a deterrent to predators and parasitoids, or as a disguise (e.g., the fecal case of N. bebbianae resembles a plant bud) (Wallace, 1970;LeSage, 1984;Erber, 1988;Olmstead, 1994;Mü ller, 2002;Brown and Funk, 2005;Chaboo et al, 2008). Brown and Funk (2010) demonstrated that these cases deter predator attacks in N. bimaculatus Karren and N. platani (Brown).…”