Ecological theory predicts that host-plant traits affect herbivore population growth rates, which in turn modulates predator–prey interactions. However, while vector-borne plant pathogens often alter traits of both host plants and vectors, a few studies have assessed how pathogens may act as interaction modifiers within tri-trophic food webs. By applying a food web motif framework, we assessed how a vector-borne plant pathogen (Pea-enation mosaic virus, PEMV) modified both bottom–up (plant–herbivore) and top–down (predator–prey) interactions. Specifically, we assessed trophic interactions with PEMV-infectious Acyrthosiphon pisum (pea aphid) vectors compared to non-infectious aphids in a factorial experiment that manipulated predator and plant communities. We show that PEMV altered bi-trophic relationships, whereby on certain plant species, PEMV reduced vector performance but also increased their susceptibility to predators. However, on other plant species, PEMV weakened top–down control or increased vector performance. Our results suggest that vector-borne plant pathogens are important interaction modifiers for plant–herbivore–predator dynamics: host-plant response to viruses can decrease herbivore abundance by reducing herbivore performance, but also increase herbivore abundance by weakening top–down control. Broadly speaking, trophic interactions that regulate herbivore outbreaks appear to be modified for herbivores actively transmitting viruses to host plants. Consequently, management and monitoring of outbreaking herbivores should consider the infection status of focal populations.

生态理论预测寄主-植物性状会影响食草动物的种群增长率，进而调节捕食者-猎物的相互作用。然而，虽然媒介传播的植物病原体通常会改变宿主植物和媒介的性状，但一些研究已经评估了病原体如何在三营养食物网中充当相互作用调节剂。通过应用食物网基序框架，我们评估了媒介传播的植物病原体（豌豆花叶病毒，PEMV）如何改变自下而上（植物-食草动物）和自上而下（捕食者-猎物）的相互作用。具体来说，我们评估与PEMV传染性营养相互作用豌豆蚜（豌豆蚜虫）载体与非传染性蚜虫在操纵捕食者和植物群落的析因实验中进行了比较。我们表明 PEMV 改变了双营养关系，因此在某些植物物种上，PEMV 降低了载体性能，但也增加了它们对捕食者的易感性。然而，在其他植物物种上，PEMV 削弱了自上而下的控制或增加了载体性能。我们的研究结果表明，媒介传播的植物病原体是植物-食草动物-捕食者动态的重要相互作用调节剂：宿主-植物对病毒的反应可以通过降低食草动物的表现来降低食草动物的丰度，但也会通过削弱自上而下的控制来增加食草动物的丰度。从广义上讲，调节食草动物爆发的营养相互作用似乎因食草动物将病毒主动传播给宿主植物而改变。