Where are they found ? (in Britain and Ireland)
Southern and central Britain and Ireland
What is their preferred habitat outside gardens?
Although very common in gardens – any small blue butterfly you see is most likely to be a holly blue – they are also found in parks, churchyards, hedgerows and woodland rides provided their foodplants and nectar sources are nearby.
Resident or migrant?
Resident
When to see them?
There are normally two broods each year. The adults of the first brood emerge in April and fly until early June with the second brood on the wing from mid-July until early September. A third brood can occasionally appear in the south in October; conversely some northern sites may only produce a single brood. The butterflies tend to fly high around bushes and trees, unlike other blues which usually stay near ground level.
What happens in winter?
Holly blues overwinter as pupae from the second brood on the ground near their foodplants (usually ivy).
What do the early stages look like?
The eggs are very small and white, laid one-by-one under unopened buds of the foodplant (holly in spring and ivy in summer). The first
instar caterpillars are very pale, almost white, and extremely small – 1mm or even less. Later stages can grow up to 15 mm and are usually pale green. There are four instars in total and the fourth ones turn dusky pink before pupation. Pupae are found on or close to the ground, about 8-9mm long. See
UKbutterflies for images of the early stages.
What do the caterpillars eat?
As their name suggests, the main foodplant of the first brood is holly
Ilex aquifolium. The female adults that emerge from pupae in spring will mate and then lay their eggs on the unopened flower buds. The larvae eat flower buds and young holly leaves throughout their instars and then pupate for two to three weeks until the second brood of adults emerges. These females usually lay their eggs on ivy
Hedera helix, which is the second brood caterpillars’ main foodplant until their long overwinter pupation. Other plants are occasionally used including
spindle, dogwoods, gorses and
brambles1. .
Flowers they take nectar from
Holly blues visit a variety of nectar sources such as brambles, holly, ivy, forget-me-nots, bugle, buttercups and thistles. However, they seem to prefer honeydew from aphids rather than nectar. The males will also come down to the ground to take salts and minerals from damp mud and animal waste2..
How are they doing?
The holly blue appears to be doing well at present. The 2022 State of UK butterflies report 3. shows a 109% increase in numbers from 1976 and the distribution has also increased as it expands its range northwards. Within this general increase the population shows large fluctuations from year to year which are thought due to parasitism. (see below).
Other interesting facts
The holly blue butterfly has a
parasitoid, a type of ichneumon wasp,
Listrodromus nycthemerus, that lays its egg in the caterpillar of the holly blue
4.. The wasp larva then feeds on the caterpillar from inside and after pupation a single adult wasp emerges instead of a holly blue butterfly.
As the holly blue is the sole host of this wasp, its population can be badly affected. The wasp population tracks that of the holly blue: as the holly blues increase so do the wasps until they are so numerous that this causes the population of the butterfly to crash. The wasp numbers then decrease, the butterfly population recovers, and the cycle begins again 5..
References
3. State of UK Butterflies 2022
report
Page written by Judy Skinner, reviewed and compiled by Steve Head