Salicylate plant defences reduce aphid nutritional quality and increase predator consumption

Plant-induced defences can shape multitrophic interactions by affecting herbivore performance and altering their suitability as prey. While the direct effects of these defences on herbivores are well documented, their indirect consequences—particularly for predator behaviour and biological control effectiveness—remain less understood. In our study, we examined how activating salicylic acid (SA)-mediated defences in tomato plants affects the performance and nutritional quality of the aphid, Macrosiphum euphorbiae, and how this intraspecific variation in aphid quality as prey impacts the predation by the lady beetle predator, Hippodamia convergens. Aphids on SA-treated plants exhibited reduced body mass, protein and lipid content, indicating lower nutritional value as prey. Correspondingly, we recorded a stronger decline in aphid population on SA-treated plants in the presence of predators. To investigate the effects of reduced aphid quality on predation, we conducted a controlled no-choice assay, which showed that individual lady beetle predators consume more aphids from the SA-treated plants. This increased consumption is consistent with compensatory feeding in response to nutrient-poor prey, although alternative explanations, such as increased prey vulnerability, should also be considered. Our findings demonstrate that SA-mediated defences can shape predator–prey interactions through bottom-up effects on prey quality and underscore the need to consider these cascading effects when evaluating the ecological impact of plant resistance traits in biological control strategies.