Leaf-chewing but not sap-feeding herbivores create soil legacies that shape plant resistance through trait-mediated, guild-specific effects in Baccharis salicifolia

1. Herbivory can affect the soil microbiome and create legacies that affect plant resistance, but how these effects vary by feeding guild and the plant traits involved remain underexplored. We tested how soil legacies created by a leaf-chewing caterpillar (Spodoptera exigua) and a sap-feeding aphid (Aphis gossypii) influence resistance in Baccharis salicifolia by evaluating changes in plant traits and soil microbial diversity.2. We conditioned soil with three treatments on B. salicifolia: caterpillar herbivory, aphid herbivory, or control. Then, we grew new plants in sterile soil inoculated with 10% conditioned soil and conducted resistance bioassays, measured plant nutritional and defence traits, and analysed microbial diversity.3. Caterpillar, but not aphid herbivory, created legacies that affected resistance to both herbivores in opposite directions: plants in caterpillar-conditioned soil had 16% smaller aphid colonies (greater aphid resistance) but a non-significant 46% increase in caterpillar performance (lower caterpillar resistance) compared to controls. These plants also showed a 12% higher carbon-to-nitrogen ratio, mediating aphid but not caterpillar resistance. Herbivory did not affect microbial taxonomic diversity, suggesting shifts in microbiome biomass or function underlie the observed effects.4. Our findings highlight that herbivory-induced soil legacies are guild-specific and shape plant resistance via changes in plant traits.