Each winter, more or less at this time of year, we treat our honeybees with Oxalic Acid. It is a treatment for Varroa - Varroa mites feed off the bodily fluids of adult, pupal, and larval honey bees, and may carry viruses that are particularly damaging to the bees (e.g., deformed wings, and IAPV), and accordingly they have been implicated in colony collapse disorder. 

Below is a video I made of me giving my only surviving hive the Varroa treatment and my delight at the amount of bees being out and about on a mild, sunny winters morning is obvious!