Defense of Milkweed Bugs (Heteroptera: Lygaeinae) against Predatory Lacewing Larvae Depends on Structural Differences of Sequestered Cardenolides

Predators and parasitoids regulate insect populations and select defense mechanisms such as the sequestration of plant toxins. Sequestration is common among herbivorous insects, yet how the structural variation of plant toxins affects defenses against predators remains largely unknown. The palearctic milkweed bugLygaeus equestris(Heteroptera: Lygaeinae) was recently shown to sequester cardenolides fromAdonis vernalis(Ranunculaceae), while its relativeHorvathiolus superbusalso obtains cardenolides but fromDigitalis purpurea(Plantaginaceae). Remarkably, toxin sequestration protects both species against insectivorous birds, but onlyH. superbusgains protection against predatory lacewing larvae. Here, we used a full factorial design to test whether this difference was mediated by the differences in plant chemistry or by the insect species. We raised both species of milkweed bugs on seeds from both species of host plants and carried out predation assays using the larvae of the lacewingChrysoperla carnea. In addition, we analyzed the toxins sequestered by the bugs via liquid chromatography (HPLC). We found that both insect species gained protection by sequestering cardenolides fromD. purpureabut not fromA. vernalis. Since the total amount of toxins stored was not different between the plant species inH. superbusand even lower inL. equestrisfromD. purpureacompared toA. vernalis, the effect is most likely mediated by structural differences of the sequestered toxins. Our findings indicate that predator-prey interactions are highly context-specific and that the host plant choice can affect the levels of protection to various predator types based on structural differences within the same class of chemical compounds.