Plants growing under reduced water availability can affect insect herbivores differently, in some instances benefitting them. However, the forces mediating these positive impacts remain mostly unclear. To identify how water availability impacts plant quality and multitrophic interactions, we conducted manipulative field studies with two populations of the specialist herbivore Pieris rapae, and its host plant, Rorippa indica. We found that P. rapae larvae experienced higher survival on R. indica growing under low water availability compared with plants grown under high water availability. Higher survival of eggs and larvae was related to the reduced abundance of other herbivores and natural enemies. Water availability had differential impacts on other members of the herbivore community by altering plant quality. Low water availability decreased the quality of R. indica to most herbivores, as indicated by reduced abundance in the field and decreased relative growth rate in laboratory feeding assays. In contrast, P. rapae larval performance was not affected by sympatric R. indica grown under different water availability. These results indicate that local P. rapae populations possess physiological adaptations to overcome fluctuations in host quality. Our findings illustrate that reduced water availability is beneficial to a specialist herbivore but detrimental to most other herbivores. Our work highlights the complex effects of the arthropod communities associated with plants in determining the impacts of water availability on insect herbivores.

AbstractPlants growing under reduced water availability can affect insect herbivores differently, in some instances benefitting them. However, the forces mediating these positive impacts remain mostly unclear. To identify how water availability impacts plant quality and multitrophic interactions, we conducted manipulative field studies with two populations of the specialist herbivore Pieris rapae, and its host plant, Rorippa indica. We found that P. rapae larvae experienced higher survival on R. indica growing under low water availability compared with plants grown under high water availability. Higher survival of eggs and larvae was related to the reduced abundance of other herbivores and natural enemies. Water availability had differential impacts on other members of the herbivore community by altering plant quality. Low water availability decreased the quality of R. indica to most herbivores, as indicated by reduced abundance in the field and decreased relative growth rate in laboratory feeding assays. In contrast, P. rapae larval performance was not affected by sympatric R. indica grown under different water availability. These results indicate that local P. rapae populations possess physiological adaptations to overcome fluctuations in host quality. Our findings illustrate that reduced water availability is beneficial to a specialist herbivore but detrimental to most other herbivores. Our work highlights the complex effects of the arthropod communities associated with plants in determining the impacts of water availability on insect herbivores.

Additional file 2: Table S1. Statistics of raw reads and clean reads per RNA-Seq sample mapped to the rice reference genome.

Additional file 2: Table S1. Statistics of raw reads and clean reads per RNA-Seq sample mapped to the rice reference genome.

Additional file 3: Table S2. List of differentially expressed genes. Up- and downregulated genes are shown for a leaffolder vs control_leaf comparison, b leaffolder vs control_sheath comparison, c BPH vs control_leaf comparison, d BPH vs control_sheath comparison, common DEGs in e leaves and f sheaths under separate leaffolder and BPH infestations.

Additional file 3: Table S2. List of differentially expressed genes. Up- and downregulated genes are shown for a leaffolder vs control_leaf comparison, b leaffolder vs control_sheath comparison, c BPH vs control_leaf comparison, d BPH vs control_sheath comparison, common DEGs in e leaves and f sheaths under separate leaffolder and BPH infestations.

Additional file 4: Table S3. The expression changes of defensive phytohormone-responsive genes in Qingliu under leaffolder and BPH infestations.

Additional file 4: Table S3. The expression changes of defensive phytohormone-responsive genes in Qingliu under leaffolder and BPH infestations.

Additional file 5: Table S4. Details of GO enrichment results for the DEGs in Qingliu at 24 h of leaffolder herbivory. a Upregulated genes in the leaffolder vs control_leaf comparison, b downregulated genes in the leaffolder vs control_leaf comparison, c upregulated genes in the leaffolder vs control_sheath comparison and d downregulated genes in the leaffolder vs control_sheath comparison.

Additional file 5: Table S4. Details of GO enrichment results for the DEGs in Qingliu at 24 h of leaffolder herbivory. a Upregulated genes in the leaffolder vs control_leaf comparison, b downregulated genes in the leaffolder vs control_leaf comparison, c upregulated genes in the leaffolder vs control_sheath comparison and d downregulated genes in the leaffolder vs control_sheath comparison.