Rare butterfly larvae found in Millennium Woods

Larvae (caterpillars) of the Black Hairstreak butterfly have recently been discovered in the Millennium Woods. The Black Hairstreak is one of Britain's rarest butterflies and it is found in only about 50 sites that lie in an arc corresponding to the deep clay soil running from Oxford to Peterborough. To our knowledge, this is the first record of the species in Padbury.

Butterflies of the Black Hairstreak are very difficult to spot. They are on the wing for only a few weeks a year from the end of June to the beginning of July. They have a wingspan of less than 4 cm and they spend much of their time high up in the canopy of mature trees feeding on honeydew secreted by tree-living aphids. It is difficult to distinguish (especially at a distance) the Black Hairstreak from the more common White-letter Hairstreak.

In July the butterflies lay eggs singly on Blackthorn stems and the eggs overwinter there until hatching into caterpillars the following Spring when the Blackthorn comes in to leaf and the larvae start to feed. The eggs and the caterpillars are extremely hard to spot. They are small, few in number and extremely well-camouflaged. Luckily, however, it has recently been discovered that the caterpillars can be made to fluoresce bright blue/green if illuminated with ultraviolet (UV) light, and thus they become readily visible at night using UV torchlight. Using this method with torches purchased by Greener Padbury Group, GPG members and Parish councillors have found Black Hairstreak larvae on Blackthorn leaves at sites throughout the Woods, and although few in number the larvae are well-distributed. The larvae of the Black hairstreak are readily identifiable and clearly distinct from those of other closely-related butterflies.

The Black Hairstreak has been in steep decline and it is under threat of disappearing from the UK. It is now a 'Red List' species and all stages of its life-cycle are to be protected.

The presence of the larvae in the Woods raises important questions about management there. Since the eggs, the larvae and the pupae (chrysalises) of the butterfly are present on Blackthorn nearly all-year round, it is vitally important that the plant is very carefully managed. The butterflies require tall and undisturbed stands of mature Blackthorn on which to lay their eggs, and nearby mature broad-leaf trees (particularly Ash) with aphids that in summer supply the secreted honeydew upon which the butterflies feed. We are currently seeking advice on optimal management practises from the UK expert on the butterfly and we will work closely with the Parish Council to implement these. It is important that we do all we can to help the butterfly thrive in the Woods, and if it does so it we can be very proud to have a newly-discovered population of the Black Hairstreak in Padbury.

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