1.Plant-insect food webs tend to be dominated by interactions resulting from diffuse coevolution between plants and multiple lineages of herbivores rather than by reciprocal coevolution and co-cladogenesis. Plants therefore require defence strategies effective against a broad range of herbivore species. In one extreme, plants could develop a single universal defence effective against all herbivorous insects, or tailor-made strategies for each herbivore species. The evolution and ecology of plant defence has to be studied with entire insect assemblages, rather than small subsets of pairwise interactions.2.The present study examines whether specialists and generalists in three coexisting insect lineages, forming the leaf-chewing guild, respond uniformly to plant phylogeny, secondary metabolites, nutrient content and mechanical anti-herbivore defences of their hosts, thus permitting universal plant defence strategies against specialised and generalist folivorous insects from various taxa.3.The extensive data on folivorous assemblages comprising three insect orders and 193 species are linked with plant phylogeny, secondary chemistry (salicylates, flavonoids and tannins), leaf morphological traits (SLA and trichome coverage), nutrient (C:N) content and growth-form of eight willow (Salix) and one aspen (Populus) species growing in sympatry.4.Generalists responded to overall host-plant chemistry and trichomes, whilst specialists responded to host-plant phylogeny and secondary metabolites that are unique to willows and that are capable of being utilised as an anti-predator protection. We did not find any significant impact of other plant traits, i.e. specific leaf area, C:N ratio, flavonoids, tannins and growth-form, on the composition of leaf-chewing communities.5.Our results show that the response to plant traits is differential among specialists and generalists. This finding constrains the ability of plants to develop defensive traits universally effective against herbivores and may lead to diversification of plant defensive mechanisms into several complementary syndromes, required for effective protection against generalists and specialists from multiple insect taxa comprising most leaf-chewing assemblages. These results point to the necessity of broad studies of plant-herbivore interactions, across multiple insect taxa and guilds.This article is protected by copyright. All rights reserved.