Purple Emperor Butterfly Report 2018

by Martin Jordan.

Purple Emperor Butterfly.
Purple Emperor Butterfly.

In response to the poster asking for sitings of Purple Emperor we collected three records of adult male Purple Emperor that may otherwise have gone unrecorded. Although the number of sightings are low, it is nonetheless encouraging to know that this rare butterfly is present in the London Borough of Bromley. The survey will be repeated in 2019 with the poster being distributed across Bromley as widely as possible.

Purple Emperor Butterfly Sightings 2018

Distribution

Purple Emperor Distribution.
Distribution - Click to enlarge.
Local records including data from Kent Butterfly Conservation dating back to 2010.
Purple Emperor Hot Spots.
Hot Spots - Click to enlarge.
The Hotspot map of sightings since 2010 shows that although not as densely populated as mid-Kent, Bromley does have higher than average Kent County populations of Purple Emperor.

Larval Food Plants

Larval Food plants are usually Goat Willow, Salix Caprea, or Sallow, Salix cinerea, although other Salix species are occasionally used. The preferred food plant trees have large crowns or grow at ride edges with the eggs being laid in the shady part of the willow canopy. We should look to favourably promote these somewhat unfashionable trees at sites where we have recent sightings.

Salix species require a damp habitat near to rivers or on poor draining soils, and Crofton Wood SSSI would appear be an ideal site for selective management of these trees and is thought anecdotally to be a historic host to Purple Emperor; and we now have a record there for 2018. Caterpillars of a large number of moth species also feed on Salix sp., including Puss Moth, Willow Ermine, Eyed Hawkmoth, Poplar Hawkmoth, several of the Prominent family and Red Underwing.

Extracts from Butterfly Conservation Species Action Plan, Purple Emperor Apatura iris (2000)

The Purple Emperor is univoltine* with adults flying from late June until mid-August. The males occasionally descend to the ground, usually in mid morning, where they probe for salts either from road surfaces or from animal dung. Males set up territories on prominent 'master trees' from mid-day onwards and can occasionally be seen perching on outer, south-facing branches, or soaring gracefully between trees. Some of these trees are used in successive years but others are more temporary.

Contrary to popular belief, such trees are not always oaks and alternatives are nearly always found if a 'master' tree is felled (though loss of such trees is still undesirable). The Purple Emperor requires large blocks of broad-leaved or mixed woodland or clusters of smaller woods and/or dense scrub where willows such as Goat Willow are abundant. The Purple Emperor has always been associated with well-wooded districts, often on clays or in valleys where damp conditions promote abundant growth of suitable willows.

Although the butterfly is often associated with ancient woodland, it can spread to adjacent secondary woodland if suitable willows are abundant. The butterfly has undergone a modest re-expansion during the last two decades and has re-appeared (or has been re-discovered) in several counties after long absences: including east Devon, Kent, Nottinghamshire and Northamptonshire.

Management

Retain numerous willows wherever possible, especially along ride edges, in damp areas and in new woodland planting. In addition to rotational ride management, the provision of willow bushes and trees of different ages away from ride edges, in scallops, glades and at box junctions will help maintain populations. It is also important to ensure continuity because the food plants are comparatively short-lived trees. They can regenerate freely in damp conditions but, if natural regrowth is sparse, they can be readily propagated by pushing cuttings into the ground during the autumn. Unfortunately, low re-growth is often threatened by deer browsing and trees may need to be protected.

— Martin Jordan, Bromley Biodiversity Partnership Species and Habitats Sub-Group, December 2019

Poster

To see or download a poster about Purple Emperors, please click here:

     Downloadable Purple Emperor Poster

* Univoltine: Having one brood or generation in a year