The holly and the ivy – and the holly blue butterfly

Aside from the Christmas carol, what brings holly and ivy together? Not just table decorations, but also a thoroughly festive butterfly

Yes, they might both be jostling for position in your homemade Christmas wreath. But did you know that both of these famously festive plants are also required by the holly blue butterfly, Celastrina argiolus

Though ivy doesn’t feature in their common name, these delicate blue butterflies have a close relationship with both wintery evergreens. The prickly leaves of holly (Ilex aquifolium) are a must for this species in spring, whilst ivy (Hedera helix) is sought out in the summer.
By December, the UK population of holly blues are safely ensconced as chrysalises. This is the name given to the pupae of butterflies and moths – the hardened outer layer protects the soft developing insect within.

As the weather starts to warm, the butterflies begin to emerge. They are seen from early spring onwards; fluttering over gardens, hedgerows and woodland pathways in search of holly. Though holly blues prefer to lay eggs on female holly plants, you can often find them on male plants too.

The holly blue on ivy (Hedera helix)
The eggs hatch into inconspicuous, pale green caterpillars that feed on holly flower buds, berries and the newest leaves. The second generation can be spotted during the summer. Eggs of the insects that go on to overwinter as chrysalises are laid on ivy plants in late August.

Holly blues and their caterpillars support birds as part of the food chain, but in a more gruesome relationship, they are also the sole host of a species of small parasitoid wasp (Listrodomus nycthemerus), which lays its eggs inside holly blue caterpillars. The eggs hatch out when the holly blue is in the chrysalis stage – instead of a blue butterfly, a black and yellow parasitoid wasp emerges.

This relationship leads to a boom-and-bust rhythm for the holly blue, in which the butterfly’s populations follow a four-yearly cycle. As numbers of the parasitoid wasp increase, the holly blue population suffers; resulting in a crash in wasp numbers the following year, since they have few hosts left. The holly blues then have a couple of years to recover before wasp populations build up enough to again have a large impact.

The holly blue on its namesake plant (Ilex aquifolium)
Holly blues can complete their life cycle on other plants, including bramble, dogwoods, gorses, snowberry and spindle, but their main host plants are holly and ivy.

  • Best plants for butterflies

    Other blue butterflies generally fly close to the ground, while the holly blue will fly at head height or higher, gathering around host plants

    - Dr Stephanie Bird

Not only does ivy support the holly blue, but it is a fantastic plant for wildlife in general. It provides cover for nesting birds, habitat for invertebrates, and is a valuable late-season pollen source for many pollinators, particularly the ivy bee. Recent RHS research highlighted just how much biodiversity can be supported by growing ivy up a wall – which also helps to keep houses cool in summer and reduces damp, too.

Fancy spotting the holly blue in your garden? Plant a couple of these top 10 AGM-winning hollies and ivies to give not just butterflies, but also birds, bees and other wildlife a helping hand.

About the author – Stephanie Bird

A Senior Scientist in the RHS Plant Health team, Steph loves exploring the natural world and answering members’ enquiries through research. Read more

Save to My scrapbook

You might also like

Get involved

The Royal Horticultural Society is the UK’s leading gardening charity. We aim to enrich everyone’s life through plants, and make the UK a greener and more beautiful place.