The holly blue butterfly

David Chapman / 15 April 2014

Why are there fewer holly blue butterflies on the wing? David Chapman examines a couple of possibilities.

Figures released by Butterfly Conservation reveal that farmland butterflies thrived in 2013 following the first decent summer for seven years. The results, drawn from their annual Wider Countryside Butterfly Survey (WCBS), assesses how butterflies fared at 900 randomly selected locations in the UK.

It showed that many species including Brimstone, Common Blue, Small Copper, Small Skipper, Large Skipper and Small Tortoiseshell bounced back after a low in 2012. Strangely a couple of usually common species, the Holly Blue and Red Admiral, were down on the previous year.  

There could be a very simple explanation for the lower numbers of holly blue butterflies last year. This butterfly has two generations each year and the first is very early, being seen on the wing from late March through May.  Last spring was particularly cold and this might have been 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. I remember seeing a lot of holly blues on the wing during 2011 and 2012 so maybe this was a cause.

The holly blue usually closes its wings when at rest and then it is possible to see that its underwing is silvery-blue with only black spots and this is enough to identify this species. On the upperside the males are violet-blue but the females have dark margins to their wings.

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