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 multi-trophic 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.

在水供应减少的情况下生长的植物可能会对昆虫食草动物产生不同的影响，在某些情况下会使它们受益。但是，调解这些积极影响的力量仍然大多不清楚。为了确定水的可利用性如何影响植物质量和多营养相互作用，我们对两个种群的食草性菜青虫Pieris rapae及其寄主植物Rorippa indica进行了操纵性田间研究。我们发现，菜粉蝶幼虫经历了较高的存活率R.籼稻与水分利用率高的植物相比，水分利用率低的植物生长。卵和幼虫的较高存活率与其他草食动物和天敌数量的减少有关。水的供应量通过改变植物质量而对草食动物群落的其他成员产生不同的影响。可用水量低会使大多数草食动物的印度。树的质量下降，这是由于田间丰度降低和实验室摄食分析中的相对生长速率降低所表明的。相比之下，在不同的水供应条件下生长的同胞R. indica不会影响菜青虫的幼虫性能。这些结果表明，本地菜青虫种群具有生理适应性，以克服宿主质量的波动。我们的发现表明，减少水的供应有益于特殊的草食动物，但不利于大多数其他草食动物。我们的工作突出了节肢动物群落与植物有关的复杂影响，以确定水的可利用性对昆虫食草动物的影响。