Effectiveness of Tortoise Beetle Larval Shields Against Different Predator Species

Abstract

Larvae of the tortoise beetles Charidotella bicolor and Deloyala guttata carry shields formed from exuviae and feces over their bodies that are thought to provide protection from natural enemies. We investigated the effectiveness of shields as a defense against three groups of invertebrate predators (mandibulate, piercing/sucking, and chelicerate taxa) common in tortoise beetle habitats.Choice experiments, functional response studies, measurements of predator body size and mouthpart length, and a literature review indicated that predators with long piercing! sucking mouthparts (i.e., Heteroptera) were the most effective predators of tortoise beetles. For example, relatively large heteropterans with long, piercing/sucking mouthparts (e.g., Nabis americoferus and Podisus maculiventris) were not deterred by shields whereas small predators with short beaks (e.g., Geocoris punctipes) were. Likewise, coccinellid predators with short mandibles were deterred by the shield, while a carabid predator with long mandibles (Lebiafuscata) was effective at killing tortoise beetle larvae. No clear patterns of the effectiveness of shields against chelicerate predators were evident. Choice experiments also indicated that small tortoise beetle larvae were more susceptible to predator attack than were larger instars. Additionally, the effectiveness of the shields against different predators was quite similar between the two tortoise beetles.Field experiments indicated that predators are an important source of mortality for tortoise beetle larvae in natural settings, and high larval mortality is associated with elevated densities of predators with piercing/sucking mouthparts, such as heteropterans. Thus, shields provide some but not universal protection against the variety of predators present in tortoise beetle communities.

Defensive use of a fecal thatch by a beetle larva ( Hemisphaerota cyanea )

“…Ants must certainly constitute a hazard and might well be thwarted by the thatch, as they are by the fecal shield of other cassidines (1,5). It is noteworthy that, whereas the Hemisphaerota thatch appears to be pentatomid-proof, the shield of other cassidines offers only limited protection against heteropterans (2).…”

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

“…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;.…”

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).…”

“…However, failure-time analyses revealed that specialists enjoyed significantly lower capture rates, even in a brief 5-min bioassay. According to optimal foraging theory, resistance characters that increase the time required to subdue, handle, and assimilate prey may induce opportunistic generalist predators, like bugs, to switch to different, less costly prey that can be more efficiently dispatched (Rabb and Lawson 1957;Pyke 1984;Paradise and Stamp 1990;Olmstead and Denno 1993). Furthermore, some invertebrate predators can learn to avoid unpalatable prey and so may switch to more palatable prey before further direct encounters (Bernays 1989;Paradise and Stamp 1991).…”

“…As external structures, shields were manipulated by procedures that did not affect their maneuverability or harm the larvae (Olmstead and Denno 1993;. Fourthinstar larval sibships of each species-host plant combination were equally divided at random among the following treatment groups: (1) water (H 2 O); (2) methanol (MeOH); and (3) unleached (intact) control.…”