A growing appreciation of contemporary evolution has encouraged researchers to pay attention to eco-evolutionary dynamics. However, few studies have addressed eco-evolutionary dynamics by explicitly tracking the dynamics of evolution in a community context in the field. In this study, we conducted a field experiment to examine following two questions: (i) How does genetic variation in adaptive traits of an herbivorous insect, Plagiodera versicolora, affect tree-associated arthropod communities? (ii) How does community dynamics feed back to evolutionary dynamics? Plagiodera beetle has between-population genetic variation in feeding preference for leaf-age types. We set up tree-canopy enclosures of mature willows and released genetically distinct Plagiodera populations with single-source populations and mixed-source populations, respectively. Our results showed that the genetic variation in the beetle affected arthropod community dynamics through plant induced defense. Furthermore, we found feeding evolution in a very short time scale. This study provides the first evidence for eco-evolutionary dynamics in a complex community in the field.