The Lime tree (Tilia Cordata, or the Linden tree) is found all around Oxford. As I cycle under the avenue of them on South Parks Road I can feel the slight prickle of honeydew falling.
Honeydew is a sugar-water liquid secreted by the aphids who live on the Lime trees. They suck the sap and excrete the liquid which falls in a light spray from the trees. The leaves quickly get a shine where large amounts of this liquid has dried on them. I've seen this in other places around Oxford.
Honeydew can be forage for bees. They collect it when there are fewer nectar sources. The flavour of the honey is said to be distinctive -- "very dark brown in colour, with a rich fragrance of stewed fruit or fig jam" (source: Wikipedia). Hopefully there will be some to collect from my city hives this season