Holly blue butterflies

Posted: Friday 18 July 2014
by Kate Bradbury

There have been lots of holly blue butterflies flitting around the ivy in my garden lately. I’m hoping they’re laying eggs...

There have been lots of holly blue butterflies flitting around the ivy in my garden lately. I’m hoping they’re laying eggs – despite being called holly blue butterflies, the summer generation of females lays eggs on ivy, rather than holly. (They may also lay on bramble, gorse, snowberry and spindle.)

A smallish butterfly, Celastrina argiolus is the blue we’re most likely to come across in our gardens. Most blues inhabit grasslands, flying low and feeding on plants such as bird’s foot trefoil and vetches. There’s a lovely common blue colony at the tiny local meadow next to the Regent’s Canal in Hackney – I usually tick them off my ‘species to see’ list in one fell swoop along with the large skipper, gatekeeper, meadow brown and six-spotted burnet.

But the holly blue flies at head height. So, if you see a blue butterfly flying among the shrubs in your garden it’s most likely to be a holly blue. And if it’s investigating the ivy in summer, it’s hopefully a female looking to lay eggs.

Holly blue caterpillars are small, flat and pinkish green. They are, apparently, notoriously hard to find, but I might try to look for them in a few weeks, when they have grown a little. The UK Butterflies website suggests looking for signs of damage on developing flower buds. The larvae bore a hole in the side of the flower bud and eat its contents, before moving on. I might take my magnifying glass.

Last year there were very few holly blues, compared to previous years. When all other garden butterflies were thriving, the holly blue was notably absent. Results from the 2013 Big Butterfly Count suggest numbers were down by around 5 per cent from 2012. Populations can fluctuate dramatically due to the parasitic wasp, Listrodomus nycthemerus, which uses only the holly blue as its host. So it would follow that, if last year there were fewer holly blue butterflies, then this year there would be fewer parasitic wasps – perhaps we will see holly blue numbers bounce back this summer.

Whether I’m successful in my hunt for holly blue caterpillars remains to be seen, but I’m looking forward to heading to the local meadow to tick off my ‘species to see’ in one fell swoop, and counting numbers of holly blues flitting around my ivy. ‘Tis the season to count butterflies.

The 2014 Big Butterfly Count launches tomorrow, Saturday 19 June. Download the Count Butterflies app and spend 15 minutes in your garden or local park counting and recording the number of butterflies you see. You can do this as many times as you like and it helps scientists monitor population fluctuations.

Many thanks to Brian Valentine / Butterfly Conservation for kind permission to use their lovely image.

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