Feeding niches drive specialist herbivore responses of conspecific insects to plant defenses in biocontrol

1. Biological invasions threaten global ecosystems and economies, prompting the need for sustainable management approaches such as releasing natural enemies from the native range (i.e., classical biological control; CBC). While the release of multiple natural enemies can enhance the effectiveness of CBC, interactions between these agents and their host plants defenses are poorly understood. 2. This study explored the interactions between two congeneric specialist herbivores introduced for the control of Dioscorea bulbifera (air potato) in Florida, USA: Lilioceris cheni (Coleoptera: Chrysomelidae), which feeds primarily on leaves and L. egena, which feeds primarily on the aerial reproductive structures (i.e., bulbils). Specifically, we investigated whether foliar herbivory by L. cheni or induced defenses by jasmonic acid (JA) or salicylic acid (SA) influenced L. egena's feeding behavior and performance on bulbils. 3. We conducted feeding assays to measure bulbil consumption and its relationship to pupal mass and adult eclosion as indicators of beetle performance. Additionally, we performed dose-dependent feeding assays to test how varying concentrations of saponins, a key defense compound in air potato, influenced feeding and survival of L. cheni and L. egena. 4. Our results indicated no significant differences in bulbil feeding by L. egena across treatments; however, SA-treated plants produced significantly heavier pupae and adults compared to controls. Dose-response assays revealed non-linear, hump-shaped, feeding and survival patterns in L. cheni, while L. egena experienced reduced feeding and survival with increasing saponin concentrations. 5. These findings suggest that the two beetles respond differently to plant defenses while occupying distinct feeding niches, likely allowing for additive impacts on D. bulbifera. This study provides insights into the role of plant defenses in CBC and informs strategies to optimize biocontrol programs for invasive species management.