Host plant specialization across herbivorous insects varies dramatically, but while the molecular mechanisms of host-plant adaptations are increasingly known, we often lack a comprehensive understanding of the selective forces that favor specialization. The milkweed bugs (Heteroptera: Lygaeinae) are engaged in ancestrally specialized associations with plants of the Apocynaceae from which they commonly sequester cardiac glycosides for defense, facilitated by resistant Na⁺/K⁺-ATPases and adaptations for transport, storage and discharge of toxins. Here, we show that three Lygaeinae species independently colonized four novel non-apocynaceous hosts that convergently produce cardiac glycosides. A fourth species shifted to a new source of toxins by tolerating and sequestering alkaloids from meadow saffron (Colchicum autumnale, Colchicaceae). Across three milkweed bug species tested, feeding on seeds containing toxins did not improve growth or speed of development, and even impaired growth and development in two species, but sequestration mediated protection of milkweed bugs against two natural predators: lacewing larvae and passerine birds. We conclude that physiological preadaptations and convergent phytochemistry facilitated novel specialized host associations. Since toxic seeds did not improve but either impaired growth or at most had neutral effects, selection by predators on sequestration of defenses, rather than the exploitation of additional profitable dietary resources, can lead to obligatory specialized host associations in otherwise generalist insects.

Details on data collection are described in detail in the manuscript. All data are unprocessed. Data transformation is described in the manuscript.

All archived files are listed in a text-file (_README_general) comprising the metadata for each dataset alongside with an explanation of all variables. Datasets are presented in individual csv-files and are named with consecutive numbers (e.g., 1_growth_3_weeks, 2_growth_continuous, 3_development, etc.). Text-files with corresponding R-codes are named accordingly (e.g., 2_growth_continuous, 2_growth_continuous_R-code). In addition all outputs from the statistical analyses computed with JMP are compiled in a pdf named 14_JMP_outputs. Furthermore, a text-file named 14_README_JMP_outputs is provided containing all scripts exported from JMP (JMP v. 13, SAS Institute, Cary, NC, USA) as well as some additional information on excluded outliers.