Invasive plants are often attacked by both introduced specialist and native generalist natural enemies in new ranges. Soil fertility can potentially alter the interactions of these natural enemies with native versus invasive plants in ways that have largely unexplored implications for biological invasions and biological control. A common garden experiment was conducted to compare the performance of an introduced specialist flea beetle, Agasicles hygrophila, and/or a native generalist nematode, Meloidogyne incognita, on invasive alligator weed, Alternanthera philoxeroides, and its native congener sessile joyweed, Alternanthera sessilis, under different levels of soil nitrogen (N) and phosphorus (P). At a relatively low or moderate N and P levels, the flea beetle and the nematodes were not significantly affected by each other. Under these conditions, alligator weed responded plastically by producing more branches and biomass, as well as longer stolons, in response to attack by the flea beetle and/or the nematode, compared to sessile joyweed responses to similar levels of damage. However, under a relatively high N and P levels, nematode infestations significantly reduced flea beetle damage on alligator weed, resulting in significantly greater above‐ and below‐ground biomass and longer stolons than plants without herbivory. In contrast, beetle herbivory significantly increased the level of nematode infestations on sessile joyweed, resulting in significantly fewer fine and coarse roots, and lower above‐ and below‐ground biomass compared to plants without herbivory. Synthesis and applications. Our findings illustrate the importance of soil fertility in mediating interactions between specialist biocontrol agents and native generalists on native versus invasive plants. High phenotypic plasticity seems to be an important attribute that contributes to the success of invasive plants like alligator weed in relatively nutrient‐poor environments. However, nutrient‐rich environments could potentially confer greater growth benefits on invasive plants than on native ones by changing herbivore–herbivore interactions on plants differently. There is a great need to fully investigate the direct and indirect interactions between biocontrol agents and generalists across food webs following classical biocontrol releases. Nutrient measurements of both soil and water bodies should also be incorporated into all stages of biocontrol programmes. 摘要 在新生境中，入侵植物常常会遭受引入的专食性生防天敌与本地广食性天敌的共同胁迫。土壤肥力可能潜在介导这些天敌在入侵植物与本地植物上的表现与相互作用，但这种介导作用对生物入侵和生物防治的影响机制仍缺乏深入研究。 本文通过同质园试验比较了在不同土壤氮、磷有效性水平下，专食性莲草直胸跳甲与本地广布种南方根结线虫对入侵杂草空心莲子草及其本土同属种莲子草的危害差异。 在相对较低或中等的氮磷水平下，莲草直胸跳甲与根结线虫的相互影响作用并不显著。在莲草直胸跳甲和/或根结线虫的危害下，与莲子草相比，空心莲子草的分枝数、生物量和匍匐茎长度均显著增高。而在相对较高的氮磷水平下，与无取食危害相比，根结线虫显著降低了莲草直胸跳甲对空心莲子草的危害，使得空心莲子草的地上地下生物量和匍匐茎长度均显著增高。与之相反，与无植食性生物危害相比，莲草直胸跳甲取食促进了根结线虫对莲子草的危害，莲子草的细根数、粗根数和地上地下生物量均显著降低。 整合与应用:我们的研究结果表明土壤肥力在调节入侵植物和本土植物介导的专食性生防天敌与本地广食性天敌间互作中的重要性。高表型可塑性可能是入侵植物如空心莲子草在相对营养贫乏的环境中成功入侵的重要因素，然而营养丰富的环境可能通过改变植食性动物间的互作使入侵植物潜在获益。因此，在生防天敌释放以后，有必要深入探究生防天敌与本地广食性天敌在整个食物网中的直接和间接相互作用。其次，对土壤和水体的养分测量应贯穿生物防治项目的各个阶段。 Our findings illustrate the importance of soil fertility in mediating interactions between specialist biocontrol agents and native generalists on native versus invasive plants. High phenotypic plasticity seems to be an important attribute that contributes to the success of invasive plants like alligator weed in relatively nutrient‐poor environments. However, nutrient‐rich environments could potentially confer greater growth benefits on invasive plants than on native ones by changing herbivore–herbivore interactions on plants differently. There is a great need to fully investigate the direct and indirect interactions between biocontrol agents and generalists across food webs following classical biocontrol releases. Nutrient measurements of both soil and water bodies should also be incorporated into all stages of biocontrol programmes.