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Title: Enemy targeting, trade-offs, and the evolutionary assembly of a tortoise beetle defense arsenal

item VENCL, FREDERIC - Smithsonian Research Institute
item Srygley, Robert

Submitted to: Evolutionary Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/24/2012
Publication Date: 3/1/2013
Citation: Vencl, F.V., Srygley, R.B. 2013. Enemy targeting, trade-offs, and the evolutionary assembly of a tortoise beetle defense arsenal. Evolutionary Ecology. 27(2): 237-252. Available: DOI 10.1007/s10682-012-9603-1.

Interpretive Summary: Insects that eat plants have a variety of defenses against their natural enemies. The tortoise beetle Acromis sparsa is noted for having maternal guarding of offspring, and the manufacture of a larval shield that deters predators. It is also gregarious. Here we investigate how this herbivorous beetle uses these defenses from embryo to adult, and the ability of each defense to deter the attacks of its key predators: ants, parasitoid and vespid wasps, and bugs. In order to determine the effect of each defense alone and in combination with the others, defenses were manipulated without harming the insects: mothers were removed, shields were removed, and broods were reduced to half their normal group size. Maternal guarding was essential for survival of eggs, but exclusion experiments demonstrated that guarding was effective against ants but not egg parasitoids. Larval defenses also showed narrow enemy targeting with large groups being effective against bugs, larval shields against ants, and maternal guarding against vespid wasps. Because of ant recruitment, however, large groups were a detriment against ants. In combination, some traits compromised the effectiveness of others, whereas others worked together. Predation has selected for a defensive arsenal of increasing complexity, and the historical sequence of evolutionary origins of shields, grouping, and then guarding in tortoise beetles is matched with evolutionary appearance in the fossil record of predation by ants, bugs and then vespid wasps.

Technical Abstract: In response to intense enemy selection, immature folivorous insects have evolved elaborate, multi-trait defense arsenals. How enemies foster trait diversification and arsenal assembly depends on which selective mode they impose: whether different enemies select for the same defense or exert conflicting selection on a prey species. Theory has long supposed that the selective advantage of a defense depends on its efficacy against a broad spectrum of enemies, which implies that predator selection is more diffuse than pairwise. Here, I use the multi-trait defense arsenal of the tortoise beetle, Acromis sparsa, which consists of shields, gregariousness and maternal guarding, to test whether: (1)diverse enemies can select for narrowly targeted defenses; (2)newer traits out-performed older ones or vice versa, and; (3)if selection by different enemies results in positive (escalation) or negative directional trends in defense effectiveness. Because their defenses could be modified or ablated without otherwise causing harm, individuals were rendered more, or less, protected and their survival was quantified in a long-term field study. Exclusion experiments were used to evaluate the defense efficacy against particular enemy guilds. Logit regression revealed that: (1)no single trait increased survival against the entire enemy suite; (2)trait efficacy was strongly correlated with a particular enemy, consistent with narrow targeting; (3)traits lacked strong cross-resistance among enemies; (4)traits performed synergistically, consistent with the idea of escalation, and; (5)traits interacted negatively to decrease survival, indicative of perfomance trade-offs. Collation of the phylogenetic histories of arsenal and enemy community assembly indicated that older traits performed better against older enemies and that both the patterns of trait and enemy accumulation were consistent with defense escalation. Trade-offs and the lack of cross-resistance among defenses imply that enemy selection has been conflicting, at the guild level, and these functional conflicts have fostered the evolution of a defense arsenal of increasing complexity.