There are 4 hairstreaks that have been seen in the Parish of Westbury, but they are all rare and, maybe, particularly hard to notice as well. Green hairstreaks are over, Purple and White-letter hairstreaks are flying now and Brown Hairstreaks fly from mid-August into September.
Green Hairstreak: The butterfly overwinters as a pupa, unlike the other hairstreaks that over-winter as eggs. These handsome bright green adult butterflies emerge from mid-April through to the beginning of June. It is the underneath of the wings that are green and, settling with wings always closed, you do not see the brown upper sides of the wings. They do well in areas with scrubby vegetation mixed with shrubs. The males are territorial flying out to investigate any passing butterfly for females. The eggs can be laid on a wide variety of food plants such as bird’s foot trefoil and common rockrose, and on the shrubs buckthorn, dogwood and gorse.
By the end of July, the caterpillars are fully developed, and the pupa is formed hidden at the base of the vegetation. Like other members of the Lycaenid – Blue Family, there is some relationship with ants, and it is perhaps as pupae that the ants offer some protection. The pupa is reported to squeak just like an ant!
Photos John Ball Peter Bright
White-letter Hairstreak: This is the rarest of this quartet and has suffered greatly from the loss of elms from Dutch Elm disease. It is entirely dependent on elms and has a preference for Wych elm. The adults, about the size of a common blue, emerge in late June and can fly for the whole of July. They feed on aphid honeydew and can spend their whole life at the tops of elms and so be almost invisible. Some may come down to visit flowers at ground level in the early morning or late afternoon as do Purple Hairstreaks.
The adult butterflies rest with wings closed showing the white W that gives it its name. It has tails and eyespots on the hind wings which is believed to protect it from fatal bird attack with some flying butterflies being seen with this part of the hind wing missing. The eggs are laid on small twigs of any of the various elms and it is in this stage that it overwinters. The newly emerged caterpillars feed high in the canopy during March, April and May, and pupate in late May or early June to emerge at the end of the month. One was seen in Westbury in 2011 and another in 2019 and there are regular reports from Lord’s Wood near Pensford, but there is nowhere you can guarantee to see them.
Photo Peter Bright
Purple Hairstreak: This is a butterfly of oak woods or even single oak trees. During June and early July, the pupae are formed in the vegetation at ground level below oak trees. It may be that at this stage as well as a caterpillar, that there is some relationship with ants to which they can ‘sing’!
The adults emerge from pupae at ground level and having expanded their wings fly up into the oak tree canopy.
There, the silver undersides of the flying butterfly can be seen flashing in the sunlight high in the tree. The upper surfaces of the wings have a purple sheen that shows as they bask in the sunshine. They are a little larger than common blues. The adult butterflies can spend their whole life high in the canopy feeding on aphid honeydew and mating out of sight of human observers. They are reported to come down to ground level to nectar on flowers in the early evening so are hard to find at ‘normal butterfly times’ in the middle of the day.
The eggs are laid at the base of oak buds and it is at this stage they over-winter. The following March, April, and May the caterpillars feed inside the developing buds later transferring to eating the young developing leaves at night after resting camouflaged on the twigs. Fully mature at the end of May the caterpillar descends to ground level once more. This is the most abundant of the hairstreaks but being so hard to see its distribution and numbers are extremely hard to gauge accurately.
Photo Mick Fletcher John Ball Peter Bright Simon Bright
Brown Hairstreak: This handsome butterfly is the largest of the hairstreaks being about the size of a gatekeeper. It is a butterfly that depends on Blackthorn and needs 2 and 3-year-old young shrubby growth on which it lays its eggs in late August and September. Flailing of blackthorn hedges every year is a disaster.
Management for Brown Hairstreaks means cutting blackthorn back on a 3-year cycle. They overwinter as eggs and the easiest way to detect the presence of this elusive butterfly is to look for the bright white eggs laid in the forks of small twigs. On hatching in early May, the caterpillar feeds inside the young leaf buds but transfers to eating the young leaves as they appear in May and June. The pupa is formed in July in the ground and it seems that it may be buried by ant activity in a cell of dry earth. The adults emerge in August and September. The males gather in assembly trees such as tall hedgerow ashes feeding on the aphid honeydew and waiting for the visiting females. After mating the females search out suitable blackthorn so that they are the ones that people come across easily at human eye-level. The males rarely come down from the tall trees where they spend their time and so are hardly ever seen.
Several Brown Hairstreaks have been seen in the Village but on the lower parts facing onto the moor. It is reasonable to speculate that there are colonies in the Parish somewhere on the hedgerow blackthorns that are not cut every year. The Somerset strongholds are in the centre of the county around Taunton.
Photos Peter Bright