Many small rather than few large sources identified in long-term bee pollen diets in agroecosystems

•Bee colonies compensate in resource-poor landscapes by varying their foraging behaviour and have clear preferences for species of pollen.•Bees aim for a diverse, nutritionally-balanced pollen diet to ensure colony performance.•Diversity is key when considering pollen forage for social bees.•Introduced weeds and even wind-pollinated plants may provide critical nutrients for bees during periods of poor pollen availability.