Monarch Performance and Cardenolide Sequestration Decline on Naturally Derived Hybrid Milkweeds (Asclepias syriaca x speciosa)

Plant hybridization often alters herbivore-relevant traits, yet the effects of hybrid feeding on specialist herbivores, especially those that co-opt host-plant defenses, are largely unknown. We examined how naturally derived hybrids between Asclepias syriaca and A. speciosa (A. syriaca x speciosa) affect monarch butterfly (Danaus plexippus) performance and cardenolide sequestration. Across three independent greenhouse experiments, monarch caterpillars reared on genetically heterogeneous A. syriaca x speciosa hybrid milkweeds grew more slowly than caterpillars reared on either parental species. Monarchs reared on hybrids also, on average, sequestered lower total cardenolide concentrations into adult tissues. However, plant measures of cardenolide chemistry did not differ significantly among parental and hybrid host groups, and measured physical defenses (trichome and latex) showed only partial differences among groups, and none in the expected direction. Element analyses of plant leaf tissue, although preliminary, revealed a negative relationship between caterpillar growth and manganese level, with hybrids containing between two- to three-fold higher levels of manganese as either parent species. These results indicate that host-plant hybridization can alter both specialist herbivore performance and acquired chemical defense, even when broad measures of host defenses do not explain those effects. Hybridization may therefore influence plant-herbivore interactions not only by changing host quality, but also by altering how specialist herbivores acquire plant-derived defenses.