Butterfly of month - The Holly Blue

I always think that spring has arrived when I see the first Holly Blue butterfly. Unlike other ‘blues’ which emerge later in the year, the Holly Blue tends to fly fairly high and can often be seen fluttering around Holly in the spring and Ivy later in the year. The reason for this is that eggs are laid singly below the buds of Holly in the spring, with the second generation laying their eggs on Common Ivy buds during the summer. No other British butterfly uses two distinct foodplants at different times of the year in this way.

This butterfly is called a ‘blue’ because of the colour of its upper side, which is a very pale blue. The female can be distinguished from the male by the black borders to its forewings. When the butterfly is at rest, none of the blue is normally visible, as it usually sits with its silvery underside wings closed, which have a few black spots dotted around both the forewings and the hindwings.

The ‘wood-louse’ shaped green caterpillar has a retractable head, a groove along its back and a ridge all round the body, which hides its legs. In May, after emerging from its egg and then eating the egg’s ‘crown’, it munches the Holly flowerbuds, unripe berries or even young leaves. After the first moult, it eats its old skin and has a further three moults. When fully grown, it has turned a purplish-pink in colour and stops eating for a day or two before pupating along the underside of a leaf or by hanging from another object.

After about three or four weeks, the second generation butterfly emerges and these are the ones you will normally see from late-July onwards. It is strange that the females know that Common Ivy is the plant to lay their eggs on at that time of year. Perhaps Holly would be less palatable then and this is the reason for laying their eggs on Ivy buds. In some years, there may be a third generation of butterflies.

Although the Holly Blue is the highest flying of the ‘blues’, I have seen a group of about a dozen or so on mud at the edge of a stream in the Pamber Forest. They would have been consuming minerals contained in the mud, which can give butterflies extra vitality.

Keep an eye open for these delicate little butterflies when out walking in the Blackwater Valley.

Peter Martin

Peter Martin is the author of  Blackwater Valley Butterflies a full-colour guide about the 32 different butterfly species butterflies found in the Valley.      More details >>>

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