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

Abstract

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 bug Lygaeus equestris (Heteroptera: Lygaeinae) was recently shown to sequester cardenolides from Adonis vernalis (Ranunculaceae), while its relative Horvathiolus superbus also obtains cardenolides but from Digitalis purpurea (Plantaginaceae). Remarkably, toxin sequestration protects both species against insectivorous birds, but only H. superbus gains 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 lacewing Chrysoperla 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 from D. purpurea but not from A. vernalis. Since the total amount of toxins stored was not different between the plant species in H. superbus and even lower in L. equestris from D. purpurea compared to A. 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.

Dietary cardenolides enhance growth and change the direction of the fecundity‐longevity trade‐off in milkweed bugs (Heteroptera: Lygaeinae)

“…Milkweed bugs are seed feeders primarily found on Apocynaceae species and on unrelated cardenolide‐producing plants (Petschenka et al, 2020 ). Besides feeding on cardenolide‐containing plants, milkweed bugs also sequester cardenolides to ward off predators (Evans et al, 1986 ; Pokharel et al, 2020 ). Alteration of the Na + /K + ‐ATPase in the Lygaeinae is probably correlated with several duplications of the ATPα1 gene resulting in four ATPα1 paralogs (A, B, C, and D) found in Oncopeltus fasciatus and Lygaeus kalmii (Dalla & Dobler, 2016 ; Yang et al, 2019 ; Zhen et al, 2012 ).…”

Antioxidant availability trades off with warning signals and toxin sequestration in the large milkweed bug (Oncopeltus fasciatus)

“…It is possible that the variance in the hue of the black hemelytra patches that correlated with sequestration would be difficult to detect because the Euclidian distance varies by ~2.5, and because of the geometry of Lab color space where the length of the b * axes is reduced for dark colors. Whether the resource‐dependent variation in warning signals and cardenolide concentrations are perceived by natural predators will be important to test in the future to understand how variation and correlations in primary and secondary defenses is maintained in aposematic species (Ottocento et al, 2023 ; Petschenka et al, 2022 ; Pokharel et al, 2020 ; White & Umbers, 2021 ).…”

Stage structured prey-predator model incorporating mortal peril consequential to inefficiency and habitat complexity in juvenile hunting

“…The prey's antipredator behaviour is inferred to be its habitat, without which the prey is innocuous. For example, many insect species such as milkweed bugs sequester phytochemicals as a defensive mechanism against their predators, but the protection gained relies profoundly on the plant species [48] . In the context of spiders, their webs rather than themselves are pernicious to the predators.…”