Importance of Oak Sap

Heslop invented the term 'feeder tree', for ageing, diseased or otherwise stressed oaks which produce small bleeds of sap. Such oaks are a major source of adult sustenance in landscapes where veteran oaks are common. 

These feeder trees are visited intermittently, as the supply of sap bleeds appears to be intermittent, and only when conditions are relatively still. Feeder trees may be unused on windy days.

Oak sap seems to become increasingly important as the season ages, for older adults. Both males and females visit, often for lengthy periods, particularly during the afternoon and, in hot weather, the evenings. During the recent heatwave iris has been visiting small bleeds of sap on veteran oaks at Knepp as late as 8pm.  

Large sap runs are rare (I've seen none this year) but can attract Emperors in numbers (the most I've seen together is four, females). These photos are all from Knepp this season (one male, three females) -

Both sexes adopt a distinctive zig-zag flight when homing in on an oak to feed on sap bleeds. 

The males seem to get plastered on fermented sap. This may explain why the Wealden males are far more aggressive than their cousins elsewhere - they're primarily oak sap feeders, inhabiting a landscape where veteran oaks are frequent.