Abstract Plant defence phenotypes commonly integrate physical and chemical traits that may act synergistically against herbivores, but empirical evidence for synergy as a defence strategy remains limited. We experimentally tested the separate and combined effects of two common plant defences, non‐glandular trichomes and leaf metabolites, on the feeding, behaviour and performance of a generalist herbivore. We conducted choice and no‐choice assays that exposed beet armyworm caterpillars ( Spodoptera exigua , Noctuidae) to desert stingbush ( Eucnide urens , Loasaceae) leaves with manipulated trichome cover and raised caterpillars on a diet containing factorial combinations of E. urens trichomes and leaf extracts. We assessed differences in caterpillar behaviour, performance, and survival across treatments. We found a significant interactive effect of trichomes and leaf metabolites that markedly decreased caterpillar growth, providing evidence for a defence synergism. Both defences also reduced caterpillar performance individually. Leaf extracts contained a high proportion of putatively defensive compounds, including iridoid glycosides, which were associated with reduced caterpillar growth and high mortality before pupation. Barbed trichomes impeded caterpillar movement and, when consumed, caused mandible damage and reduced eclosion rates compared to controls. Our results indicate that two commonly co‐expressed plant defences can synergistically reduce herbivore performance. These findings provide a rare example of trait synergy as a defence strategy and represent a step forward in the effort to study plant defence holistically, as suites or syndromes of defences, rather than isolated traits. Read the free Plain Language Summary for this article on the Journal blog.
Glos et al. (Tue,) studied this question.