Purple Emperors on Oak Sap

Emperors and Empresses feed, almost addictively, on small sugar-rich bleeds from distressed veteran oaks, especially during the second half of the flight season and in challenging weather conditions. 

Heslop called favoured trees 'Feeder Trees'. Here's a good one, from Knepp (the left-hand tree) -

The features are usually small whiteish bubbles of discharge, like this -

 Rot hole discharge, like this, are also favoured, but are much rarer -

Big runs of 'black treacle' from wounded trees are exceptionally rare, and dry up quickly. Many formed after the great storms of 1987 and 1990, after trunks twisted and broke in strong winds. Oozes from Noctule bat roosts are also good. 

Hornets, Red Admirals and Commas also visit these sap bleeds, along with a host of flies (hoverflies in particular). Watch for Hornet and Red Ad activity under the oak canopy, and watch them home in on the sap bleed. An Emperor or Empress may well be about...

However, feeding on sap runs is a high risk strategy, as Hornets readily predate Emperors. This is what happened to a large Empress at Knepp, 14/7/23 - 

This is a modern problem. I lived in East Hampshire from the mid-70s into the early 90s, and never saw a Hornet - not even in the long hot summer of 1976: they were common in and around the New Forest only. They colonised Alice Holt Forest during the mid-1990s. Likewise, they were absent from the landscape around Knepp until the mid-90s. 

Also, Heslop (lead author of Notes & Views of the Purple Emperor, published 60 years ago) doesn't mention Hornets at all, despite working the Bentley Wood area (SE Wilts, close to the New Forest) and knowing the butterfly's fondness of oak sap.

Here's a Hornet on a sap bleed -

Here's a photo of happier moments, from 14/7/23 -