Flower-dwelling spiders are often regarded as antagonists in pollination systems because they prey on pollinators. However, their ecological roles may vary across developmental stages, particularly if juveniles preferentially target floral exploiters rather than effective pollinators. We examined ontogenetic variation in flower use and prey composition of flower-dwelling spiders. Field surveys conducted in 2023 and 2024 recorded 792 individuals, with Diaea subdola and Xysticus spp. being most abundant. Clear ontogenetic shifts were detected: spiderlings (early-stage juveniles) predominantly occupied flowers of Lespedeza bicolor and Hosta sieboldiana, whereas mature individuals more frequently used Patrinia scabiosifolia and Libanotis coreana. Thrips (Thysanoptera), generally considered floral exploiters, were overwhelmingly abundant on L. bicolor and H. sieboldiana, and prey records showed that spiderlings primarily consumed thrips, later-stage juveniles fed mainly on flies, and adults preyed predominantly on bees and wasps. These results suggest that spiderlings may function as selective predators of floral exploiters, potentially reducing nectar and pollen loss and indirectly benefiting plants, whereas adults may exert antagonistic effects through predation on pollinators. Our findings demonstrate that the ecological impacts of flower-dwelling spiders are strongly stage dependent, highlighting the importance of incorporating predator ontogeny when evaluating their roles in pollination networks.

Host–parasite interactions are strongly influenced by the dispersal behaviour of both partners, yet direct evidence of parasite-driven active suppression of host dispersal has been lacking. Here, we provide evidence that the parasitic nematode Sphaerularia bombi manipulates its hosts, bumble bee queens (Bombus spp.), to limit their dispersal. We conducted genetic analyses of S. bombi and its hosts in Japan and the Netherlands, and found pronounced genetic differentiation among local S. bombi populations, despite minimal structure among host populations, indicating that infected queens exhibit severely restricted movement. Mark–recapture experiments further confirmed that infected queens tend to remain near their hibernation sites. We also identified cryptic species within S. bombi, based on concordance between mitochondrial haplotypes and nuclear clades, suggesting weak host specificity. Our findings provide the first empirical demonstration that a parasite can actively constrain host dispersal—a strategy likely enhancing mating opportunities and local transmission at the cost of broader range expansion. Parasite populations are probably established through rare, accidental transport of infected hosts, leading to strong founder effects and rapid cryptic speciation. This work highlights a novel mechanism by which parasites can shape host movement ecology and drive their own evolutionary trajectories.