Holly Blue

Celastrina argiolus

General Distribution and Status

The Holly Blue is widespread and common in England and Wales. It is gradually extending northwards into Scotland. It fluctuates widely in numbers giving rise to a regular cycle of a few years, believed to be caused by a parasitic wasp. There appears not to be any significant decline in numbers in the 20th century like most species although many colonies became extinct at a local level but new sites would be colonised quickly (Heath et al.). Since the 1970s there has been an increase in numbers and expansion in range but since 2007 a slight reversal has occurred (Brereton et al.). In Hertfordshire and Middlesex the long-term trend in distribution also appears to be positive although abundance is more variable (Wood, 2016).

UK distribution map
UKBMS Species summary (2020)

Habitat Requirements

The species is often seen in parks and gardens as well as churchyards, woodland rides and hedgerows. This is the only British butterfly to select different foodplants depending upon the season.

Larval Foodplants

Holly Ilex aquifolium in the spring and Ivy Hedera helix in late summer. Alternative plants occasionally used include Gorse Ulex europaeus, Dogwood Cornus sanguinea, Bramble Rubus fruticosus and Raspberry R. idaeus (Sawford). Dogwood C. sanguinea is perhaps the preferred choice at Shackledell Grassland.

Adult Food Sources

Buddleia Buddleja davidii (93), Bramble R. fruticosus agg. (49), Forget-me-not Myosotis sp. (39), Veronica Hebe x francisana (39).

Historical Records

Gibbs' 1902 survey includes Matthews' record of the species for Norton Green Woods. Foster's 1934 report describes it as 'usually abundant'. A few reports in the 1940s suggest that the butterfly was more numerous than usual. Even in the 1980s from Waterton's survey for 1970-1981 the author writes of the species' fluctuating numbers on about a six-year cycle.

Local Distribution and Abundance

As shown on the map, the butterfly is well distributed in the Stevenage district. A report came from an observer for a sighting north of Graveley (TL22J tetrad) on 31 July 2021, the only previous record in this tetrad during the survey was on 18 August 1996 at the Graveley churchyard. Numbers have declined since 2018 although I found 12 specimens in Fairlands Valley Park on 20 April 2020. Numbers fell in 2021 but recovered in 2022 including 12 individuals reported from a Great Ashby garden on 18 May. 2023 proved to be an even more productive year with 227 reports, a record for the TL22 square.

Transect weekly counts

Stevenage (South Fairlands Valley Park) transect 1993-2023

The transect diagram shows fluctuating numbers and a slight negative trend although the exceptional year of 1996 distorts the picture slightly. A cycle of peaks is also apparent (1996, 2004, 2011 and 2015). That great summer of 1996 includes a count of 38 on 3 August with about half of them flying along the hedgerows bordering Shackledell Grassland and Canyon Field as it was then known. 43 sightings were noted in 2023, the second best year ever here with a particularly strong spring brood.

Knebworth Park transect 1996-2010 and 2017-2023

A small negative trend is evident on this transect although as usual with this species numbers vary widely year by year. Only five records since the resumption in 2017 - one in 2018, and two in 2019 and 2023.

Knebworth Woods transect 2017-2023

Only eight reports of this species including three in 2023.

Pryor's Wood transect 2000-2022

Only five records were reported: one each on 30 July 2001, 17 June 2012, 14 May 2017, 7 May 2018 and two on 25 July 2018.

Life History

Earliest date: 20 March 2017 near Shephalbury Park
Latest date: 18 October 1997 at Watery Grove

The Holly Blue produces two generations a year but a partial third brood is sometimes produced as in 1997, 2004, 2014 and 2019 when reports came in for October sightings. The chart shows that the second brood may be larger but the last few years have indicated that both main broods are broadly similar in abundance. Eggs are laid singly on unopened flower buds of any number of foodplants in the spring, and ivy in late summer. Larvae feed on the buds leaving holes which is a clue to their presence on neighbouring buds. Larvae leave the foodplant when fully grown to pupate on the ground. Those which pupate from the late summer brood overwinter in this stage.
More details on the UK Butterflies website

Behaviour/Observation notes

Unlike the other 'blues' in Britain, in some ways the Holly Blue behaves more like a 'hairstreak' as it frequents holly and ivy trees. Owing to its strong flight it can be frustrating to get a close-up view but if females are egg-laying then opportunities for taking a photograph are greater. It often basks with its wings half open in the early morning but later in the day they are usually closed. Males sometimes visit the ground on mud or puddles, and animal droppings.

Variations/Aberrations

There are aberrations regarding the amount of blue on the wings and the spot pattern on the undersides but I am unaware of any found in Hertfordshire.
Find out more on the UK Butterflies website