Holly blue
Male
Female both Cumnor Hill, Oxford, England
Conservation status
Least Concern (IUCN 3.1)[1]
Scientific classification
Kingdom:	Animalia
Phylum:	Arthropoda
Clade:	Pancrustacea
Class:	Insecta
Order:	Lepidoptera
Family:	Lycaenidae
Genus:	Celastrina
Species:	C. argiolus
Binomial name
Celastrina argiolus (Linnaeus, 1758)
Synonyms
Papilio cleobis Sulzer, 1776 Papilio thersanon Bergstrasser, 1779 Papilio argyphontes Bergstrasser, 1779 Papilio argalus Bergstrasse, 1779 Papilio (Argus) marginatus Retzius, 1783 Lycaenopsis argiolus calidogenita Verity, 1919 Lycaenopsis argiolus britanna Verity, 1919

The Holly blue (Celastrina argiolus)^[2] is a butterfly that belongs to the lycaenids or blues family and is native to the Palearctic.

The common name Holly blue is derived from the association of the larval stage with the Holly tree.^[3]

In India, C. argiolus is known as the hill hedge blue.^[4]

The holly blue has violet blue wings, and pale silver-blue under-wings spotted with black dots.^[5] Seitz describes it:

Male above shining violet blue, only the apical portion of the costal margin being minutely edged with white. The female has both wings broadly bordered with dark, the margin of the hindwing bearing vestiges of ocelli. Underside silver-white, in the disc a row of black dots, some of which are elongate, and before the margin blackish shadowy dots. Egg very flat, whitish. Larva green or brown, marked with yellowish white, bearing catenulate (chain-like) stripes on the back, on segment 7 a gland to attract ants; head brown. On Ivy, Ilex, Euonymus, Rhamnus, Robinia, Genista, Spartium, Astragalus, Rubus, Erica, Pyrus and many other plants; in Europe visited usually by ants of the genus Lasius; in June and the autumn. Pupa mostly fastened to the underside of a leaf, ochreous with brown spots and markings. The butterflies in the spring and again in July, occasionally a third time at the end of August and in September, everywhere common, particularly at the flowers of ivy and brambles^[6]

In Europe, the larvae of the first generation (the spring generation) feed mainly on the holly species Ilex aquifolium (the flower buds, berries, and leaves), but the second generation (the summer generation) uses a range of food plants including Spindle, Dogwoods, Ivy, and Bramble.^[7][3]

The wingspan is around 2.6 to 3.4 cm.^[3][8][9]

The Holly blue usually perches with its wings almost fully closed or around 90 degrees.^[10]

The holly blue is the national butterfly of Finland.^[11]

This species was originally described as Papilio argiolus by Carl Linnaeus in 1758, and refers to the examples flying in Europe. In their monograph on the Lycaenopsis group of polyommatine genera, Eliot & Kawazoe, 1983, list 14 taxa as valid subspecies names, plus many further synonyms to which they accord lesser status. According to Eliot & Kawazoe, 1983, these 14 subspecies are divided into four groups as follows:

Found throughout Eurasia.^[12] It is particularly common in the south of Great Britain.^[13] In South Asia, it occurs from Chitral in Pakistan to Kumaon in India.^[4]

The holly blue is subject to marked population cycles which appear to be caused by interactions with the parasitoid ichneumon wasp Listrodromus nycthemerus. The wasp lays an egg on a holly blue caterpillar, inserting the egg into the caterpillar's body and the adult wasp emerges from the chrysalis, killing the chrysalis before its emergence. The population of the butterfly is tracked by that of the wasp, when butterfly populations are low the wasp population falls and this allows the butterfly population to increase and the population of L. nycthemerus can increase as there are more hosts available to be parasitised. As more caterpillars are parasitised the butterfly population crashes.^[14][15][16]

• List of butterflies of Great Britain

• 
  female laying egg, Bulgaria
• 
  Egg
• 
  Imago
• 
  Female
• 
  Bottom sides of the wings
• 
  Male

• Beccaloni, George; Scoble, Malcolm; Kitching, Ian; Simonsen, Thomas; Robinson, Gaden; Pitkin, Brian; Hine, Adrian; Lyal, Chris. "The Global Lepidoptera Names Index (LepIndex)". Natural History Museum, London. Retrieved 2016-10-15.
• Eliot, J. N. and Kawazoe, A., 1983. Blue butterflies of the Lycaenopsis group: 1–309, 6 pls. London.
• Evans, W.H. (1932). The Identification of Indian Butterflies (2nd ed.). Mumbai, India: Bombay Natural History Society.
• Willmott, Ken and Freed, Tim (1999). The Holly Blue Butterfly. Butterfly Conservation, Colchester, UK, 20p. ISBN 0-9522602-7-1.
• "Celastrina argiolus". Integrated Taxonomic Information System. Retrieved 6 February 2006.

• Savela, Markku. "Celastrina argiolus (Linnaeus, 1758)". Lepidoptera and Some Other Life Forms. Archived from the original on April 3, 2018. Retrieved April 3, 2018.