1. Insect pollination is a vital ecosystem service that maintains biodiversity and sustains agricultural crop yields. Social bees are essential insect pollinators so it is concerning that their populations are in global decline. 2. Although pesticide exposure has been implicated as a possible cause for bee declines, we currently have a limited understanding of the risk these chemicals pose. Whilst environmental exposure to pesticides typically has non-lethal effects on individual bees, recent reports suggest that sublethal exposure can affect important behavioural traits such as foraging. However, at present we know comparatively little about how natural foraging behaviour is impaired and the relative impacts of acute and chronic effects. 3. Using Radio Frequency Identification (RFID) tagging technology we examined how the day-to-day foraging patterns of bumblebees (Bombus terrestris) were affected when exposed to either a neonicotinoid (imidacloprid) and/or a pyrethroid (λ-cyhalothrin) independently and in combination over a four week period. This is the first study to provide data on the impacts of combined and individual pesticide exposure on the temporal dynamics of foraging behaviour in the field over a prolonged period of time. 4. Our results show that neonicotinoid exposure has both acute and chronic effects on overall foraging activity. While foragers from control colonies improved their pollen foraging performance as they gained experience, the performance of bees exposed to imidacloprid became worse: chronic behavioural impairment. We also found evidence suggesting that pesticide exposure can change forager preferences for the flower types from which they collect pollen. 5. Our findings highlight the importance of considering prolonged exposure (which happens in the field) when assessing the risk that pesticides pose to bees. The effects of chronic pesticide exposure could have serious detrimental consequences for both colony survival and also the pollination services provided by these essential insect pollinators.