A mismatch of resource availability in certain periods can lead to spillover of insects between habitats, resulting in temporal differences in insect diversity. Urban gardens are important anthropogenic habitats but it is unknown whether, when and why spillover of beneficial insects occurs between gardens and agricultural habitats. We used trap nests for Hymenoptera to monthly monitor bee and wasp abundance and species richness in 12 gardens and 12 rapeseed fields. Half of the gardens and rapeseed fields were located in the urban-rural interface and bordered each other (a garden paired with a rapeseed field) and the other half were isolated in the rural landscape (isolated rapeseed fields) and in the urban city centre (isolated gardens). In general, gardens in the urban-rural interface comprised the highest richness of bees and wasps. The abundance of bees but not of wasps was highest in paired habitats and peaked at full rapeseed blooming, indicating that mass-flowering rapeseed offers foraging resources for bees nesting in adjacent gardens. Thus, bees nest and increase their populations in both areas, benefiting from the mass-flowering resource in the agricultural habitat as well as the nesting resources from gardens, suggesting spillover of bees but not of wasps between paired gardens and rapeseed fields. Our study highlights the value of gardens in the urban-rural interface for the biodiversity of functionally important insects. Implementing urban gardening and small-scale agriculture in cities and suburban habitats can promote local pollinator populations and benefit adjacent croplands.