The holly blue butterfly (Celastrina argiolus) is a small blue British butterfly that emerges early in spring, with a second generation on the wing in summer.
Holly blues are known for having wildly fluctuating numbers. Figures released by Butterfly Conservation reveal that farmland British butterflies thrive in some years and face serious decline in others, such as in 2013 which was the first decent summer for seven years and saw an improvement in many species, including small skipper, large skipper and small tortoiseshell. The results, drawn from their annual Wider Countryside Butterfly Survey (WCBS), assesses how butterflies fare at hundreds of randomly selected locations in the UK. Sometimes, however, the holly blue numbers do not bounce back as expected.
There could be a very simple explanation for the lower numbers of holly blue butterflies in certain years. This butterfly species has two generations each year and the first is very early, being seen on the wing from late March through May, and a particularly cold spring might have be enough to depress numbers. But there is another slightly less obvious possibility.
The holly blue butterfly has a parasite, a type of ichneumon wasp, Listrodromus nycthemerus. This wasp lays its egg in the caterpillar of the holly blue and the resulting grub develops inside the caterpillar feeding on its body tissue. Outwardly, the caterpillar develops normally and pupates as usual but from the pupa emerges an adult wasp. This wasp then seeks out the next generation of holly blue larvae in which to lay its eggs.
The wasp population gradually builds up in response to higher than usual numbers of holly blues until it is so numerous that it causes a crash in the population of the butterfly. As the butterfly diminishes in number so does the parasitic wasp, therefore the butterfly population recovers and the cycle begins again.
Attracting holly blue butterflies into the garden
Holly blue butterflies are frequently seen in gardens, parks and woodlands, particularly in the south of England. Holly and ivy are favoured food sources of the caterpillar, which explains why holly blues are often seen in churchyards. Adults will feed on aphid honeydew and the nectar of holly, bramble and forget-me-not.
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Holly blue butterfly life cycle
Egg laying time
Eggs are laid singly on the buds of holly (in spring) and ivy (in summer). The eggs are white with a bumpy surface and hatch within two weeks.
Holly blue caterpillars
The larva of holly blues are creamy white, turning green and then dusky pink as they mature and shed their skin. The caterpillars are fully grown after about 25 days.
Holly blue chrysalis
Holly blues pupate on the ground. The pupa is mottled brown with a smooth surface and rounded edges. The first generation of holly blues pupate for two to three weeks, but the generation laid in the summer months will overwinter in this state.
Transformation into butterflies
After two to three weeks (in spring) or after overwintering the holly blue butterfly will emerge from the chrysalis. They are a small species, with a wingspan of about 3.5cm. They can be easily mistaken for their relative the common blue butterfly, but you can tell the apart from the underwing. The holly blue has a pale blue underside with black speckles, while the common blue has a brown underside with black marks circled with white.
Male and female holly blue butterflies
On the upperside the male holly blues are violet-blue but the females have dark margins to their wings.
Holly blue butterfly and hibernation
Holly blues overwinter in their chrysalis on the ground near their food source plant, ivy. It is the generation born in late summer which overwinters.
Find out more about wildlife gardening, including how to choose the best plants for attracting butterflies and great tips for photographing butterflies