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Dietary specialization influences the efficacy of larval tortoise beetle shield defenses

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Abstract

Plant chemical defenses and escape from natural enemies have been postulated to select for dietary specialization in herbivorous insects. In field and laboratory bioassays, we evaluated the effectiveness of intact and chemically modified larval shield defenses of the generalist Chelymorpha alternans and the specialists Acromis sparsa and Stolas plagiata (Chrysomelidae: Cassidinae) against three natural predators, using larvae reared on two morning glory (Convolvulaceae) species. We assessed whether: (1) specialists were better defended than generalists when both were fed and assayed on the same plant; (2) larval shield defenses were chemical, physical, or both; and (3) specialists exploit chemistry better than generalists. Live specialist larvae survived at higher rates than did generalists in predator bioassays with the bug Montina nigripes (Reduviidae), but there were no differences among groups against two species of Azteca ants (Hymenoptera: Dolichoderinae). Solvent leaching by H2O or MeOH significantly reduced shield efficacy for all species compared to larvae with intact shields. In contrast, freshly killed specialist larvae exhibited significantly lower capture rates and frequencies than the generalists. Although solvent leaching significantly reduced overall shield efficacy for freshly killed larvae of all species, the pattern of leaching effects differed between specialists and generalists, with H2O-leaching having a greater impact on the specialists. The overall vulnerability of the generalists appears due to lower chemical protection, which is ameliorated by increased escape behaviors, suggesting a selective trade-off between these defensive components. These experiments indicate that shield defenses are essential for larval survival and that specialists are superior at exploiting plant compounds residing in the aqueous fraction. Our results support the hypothesis that diet-specialized herbivorous insects have more effective defenses than generalists when both feed on the same plant due to the differential ability to exploit defensive precursors obtained from the host. The evolution of dietary specialization may therefore confer the advantage of enhanced enemy-free space.

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Acknowledgements

We gratefully thank Adam Ehmer, Nélida Gómez, Kamal Kamalrage, Jeff Sosa, and Anayansi Valderrama for their help in the study’s lab and field aspects. The manuscript benefited from the comments by André Levy, an editor, and two anonymous reviewers. Fumio Aoki assisted with software use. We thank the Gamboa Rainforest Resort staff for their assistance with plant husbandry. Experiments, sample exportation, and specimen collection were done under permits issued by the Authoridad Nacional del Ambiente de Panamá (ANAM). This is contribution#1123 from the Graduate Program in Ecology and Evolution at Stony Brook University. This research was supported by the U. S. National Science Foundation, Grant IBN-108213 to FVV and DJF.

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Correspondence to Fredric V. Vencl.

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Communicated by Jim Cronin

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Vencl, F.V., Nogueira-de-Sá, F., Allen, B.J. et al. Dietary specialization influences the efficacy of larval tortoise beetle shield defenses. Oecologia 145, 404–414 (2005). https://doi.org/10.1007/s00442-005-0138-9

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Keywords

  • Chemical defense
  • Chrysomelidae
  • Enemy-free space
  • Failure-time analysis
  • Plant/herbivore