HollyBlue

HollyBlue photo DHardiman
© DHardiman

Habitat:  Locally found where remnants of woodland contain Holly. It is sometimes found in urban gardens and parks with suitable habitats.
It is double brooded from Dublin southwards and single brooded in the north.
Larval Food Plant:    Holly Ilex aquifolium
                                              Ivy Hedera helix
Flight Time:   April and May
                           Mid-July to September (second generation)

Hibernation:   Overwinters as a pupa.

In both sexes the ground colour of the upper wings of the adult is silvery-blue and tinged with lilac.
In the male upperwings there is a narrow black band on the margins of the forewings, wider at the apex and becoming thinner at the tornus. Chequered margins at termination of veins on forewings. The black marginal band is absent on the hindwings.

The female upperwings are darker than in the males and have a more pronounced and wider marginal band on the forewing. Each hindwing has a series of 6 submarginal black spots. Chequered margins at termination of veins on the forewings.

The underside in both sexes is similar the ground colour being a bluish-white with black spots.

Life Cycle of the Holly Blue

Ovum:
The white disc shaped egg is laid singly at the base of unopened flower buds on Holly in the spring brood and on Ivy in the summer brood.
Eggs hatch after 10-16 days, depending on the season.

Larva:
This fairly stout larva measures up to 16 mm in length, tapering towards the extremities.
The larva has a few recognised colour forms but usually when fully grown it is a translucent pale green sometimes with purplish-pink dorsal and lateral stripes. It has a shiny black retractile head.
On the dorsal surface of the 10th segment there is a honey gland (Newcomer's gland) whose secretions are attractive to ants. The body is covered with short whitish setae.

The larva emerges in May and feeds until early July inside the developing drupes on the female Holly tree and on the young terminal leaves of the male Holly tree.
The second brood larva feeds on the developing buds or flowers of Ivy during late August and September.
Prior to pupation the larva becomes a dull purplish colour and wanders from its food plant to pupate.
This larval stage lasts c.26 days.

See Holly Blue parasite - Listrodromus nycthemerus below.

Pupa:
Pupation probably takes place secreted by twigs and bark among the tangled roots and dead leaves within the growth of Ivy or on the undersurface of a Holly leaf. The pupa is attached by cremasteral hooks to a silk pad and supported by a silken girdle.
Pupa from the spring generation hatch within 10-18 days. Those from the summer generation overwinter , spending about 6 months in the pupal state.

Adult:
The adult emerges and is on the wing from mid-April to June and again in mid-July to September where it may be seen flying around holly bushes above head height.


♀ Holly Blue ♂ Holly Blue © DHardiman

Holy Blue Parasite:

The host specific parasitic Ichneumon wasp, Listrodromus nycthemerus, targets the Holly Blue butterfly by laying its egg in the larvae.
Here the Listrodromus grub lives and feeds on the body tissue of the developing butterfly larva.

Eventually the life cycle of this parasitoid, which is approx. 11mm in length, is completed inside the host and results in the emergence of a single adult Listrodromus wasp from what appears to be a normally formed Holly Blue pupa. The pupa dies soon after the emergence of the wasp.

The wasp will then seek new generation Holly Blue larvae in which to inject its egg.

There is evidence that the wasp population gradually builds up over a number of years and eventually, when it gets plentiful, kills a large proportion of pupae resulting in a 'crash' in Holly Blue numbers.
This produces a shortage of food for the parasitoid and its numbers also crash allowing the Holly Blue butterfly population return to normal.
The length of the Holly Blue-Ichneumon wasp cycles observed in Britain - from boom to bust - is about seven years.

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