The holly blue

In previous blogs I have written about butterflies that you can see early in the year: the brimstone (Gonepteryx rhamni), the peacock butterfly (Aglais io), the comma butterfly (Polygonia c-album) and the small tortoiseshell (Aglais urticae). All four butterflies that hibernate as imago and emerge with the first spring sunshine. And around mid-March other species start flying, including a small blue butterfly: the holly blue (Celastrina argiolus).

Blues

As the name suggests, the holly blue belongs to the subfamily of the blues (Polyommatinae), just like for example the common blue (Polyommatus icarus) and the scarce silver-studded blue (Plebejus argus). Incidentally, the species name of the holly blue is derived from the silver-studded blue. Argiolus is the diminutive of argus, with which Linnaeus indicated that the holly blue was smaller than the silver-studded blue. The upper sides of the males’ wings of the holly blue are a somewhat watery light blue with a narrow dark edge, while those of the females are light blue with a wide dark edge.

Shrubs

The holly blue can be found in thickets in forests, parks and gardens in almost all of the Netherlands. You can also find them in agricultural areas where shrubs and hedges are used to indicate field boundaries. Unfortunately, many of these kinds of small landscape elements have disappeared during the process of agricultural intensification. In recent years however, there are initiatives to replant these structures. The holly blue is certainly completely satisfied if there is alder buckthorn (Frangula alnus) and common buckthorn (Rhamnus cathartica) among them, because these are among its host plants. Incidentally, the butterfly is not that picky about host plants, because ivy (Hedera helix), common heather (Calluna vulgaris), loosestrife (Lythrum spp.) and holly (Ilex aquifolium) are also among the favourites. The holly blue owes its English name to the last plant.This butterfly is not the only one satisfied by the planting of hedgerows and thickets. They offer nesting and shelter opportunities for birds and attract many insects. On top of that those shrubs full of life also look very nice.

Three generations

The holly blue overwinters as a pupa in the litter layer or in cavities of the host plant. The first imagines fly out around mid-March. This first generation flies until early June. After mating, the eggs are laid on the host plant and after three to five days the little caterpillars crawl out of the eggs. They immediately start eating the buds of the host plant, making a small round hole in the bud and eating the inside. At the end of the caterpillar stage, just before the caterpillar pupates, the caterpillar also eat the leaves of the host plant. Depending on the food supply, the caterpillar pupates after two to four weeks. This pupal stage lasts eleven to twenty days and then a new butterfly emerges again. This second generation flies from the second half of June until early September. Sometimes there is also a third generation, depending on the weather. Incidentally, the imagines do not have a long life ahead of them; they live for about nine to eighteen days.

Horror story

Some blue butterflies have a special relationship with ants. The holly blue is one of them. Its caterpillar secretes a sweet substance that the ants love. The ants in turn protect the caterpillar from attackers. This protection does not always work, however, because there is a parasitic wasp, the holly blue Darwin wasp (Listrodomus nycthemerus), that targets the caterpillars nonetheless. This parasitoid wasp lays eggs in the caterpillar and, as if that were not bad enough, it also injects a virus. The virus ensures that the immune system of the caterpillar is taken over and does not resist these intruders. In this way, the wasp larvae have an ideal home to grow. Because the caterpillar offers them food and protection. The horror story does not end there, because even after the larvae have left the body of the caterpillar to pupate, they need the caterpillar. It spins a mat over the wasp cocoons and guards them until the imagoes fly out. The caterpillar of the holly blue then dies. This holly blue Darwin wasp is completely dependent on the holly blues to survive. If there are fewer holly blues, there are also wasps of this species and vice versa.

Nationale vlinder

The holly blue is a very common species in the Netherlands. Compared to 1990, the population has more than tripled. Especially between 1990 and 2010 holly blue numbers grew considerably. Since then the numbers have been more or less stable. With good and bad years, as is often the case. Incidentally, you can find this butterfly in large parts of Europe up to the Arctic Circle in Sweden and Finland. In recent decades, the butterfly seems to have expanded its habitat further and further north. This is most likely related to climate change. The butterfly is almost completely absent in Norway. This can be clearly seen on the map on the left with the distribution in Europe. Finally, a fun fact: the holly blue was declared the national butterfly in Finland in 2017.

Butterfly-friendly garden

The holly blue is one of the many butterfly species that you can see in your garden. Provided that your garden is somewhat butterfly-friendly of course. You do have to do something for it, because it doesn’t happen automatically. For example, by planting host plants. Not only for the butterflies, but also for moths. Shrubs such as common and alder blackthorn and spindle (Euonymus europaeus) for example. And let’s not forget ivy and common nettle (Urtica dioica), which serve as host plants for many butterflies. Do you want to know what the host plants are for different butterfly species? On this page of UK Butterflies you can search for all kinds of host plants. And what’s more, the British researcher Harry Clarke wrote an article about the European butterfly larval foodplants. And with this article he also published a list of all European species of butterflies and their host plants.

So to continue about making your garden butterfly-friendly, plant some nectar plants too, so that the imagines also have something to enjoy. For example, purpletop vervain (Verbena bonariensis), hemp-agrimony (Eupatorium cannabinum), Guelder-rose (Viburnum opulus) and honesty (Lunaria annua). With a little effort and planting, you can also transform your garden into a butterfly-friendly paradise.