This is a photo I took this afternoon of one that settled in my vegetable patch for a minute. It’s been a bit cooler today compared with the hot sun of April! So a brief pause from this usually relentless flier to gather some solar power gave me a chance to snap it with the phone.

We had a holly blue on both of the butterfly transects in April. Comparing to data, which has just been inputted into a spreadsheet by our stalwart volunteer Tony Roberts, it seems we’ve only had two records of holly blue sighting for the month of April, since 2002. And the species has only been recorded in half the years we’ve been surveying.

The butterfly comes out early in spring, to coincide with flowering holly and a few other shrubs where it lays its eggs. These caterpillars will develop into butterflies in late summer (August-ish) which lay their eggs on the developing flowers of ivy, these develop into next spring’s generation of butterflies.

They have a cycle of abundance and rarity which is largely governed by a parasitic wasp. This wasp lays its eggs in the caterpillars, which then fail to turn into butterflies but instead generate more wasps. As the wasp population builds up, the butterfly becomes scarce, until it gets to a point where there are hardly any wasps. Then, the butterfly has a couple of good years where many of the caterpillars turn into butterflies, at which point the wasp numbers start to build up again.

It’s always scarce on the farmland – I see more in my garden, where it’s a bit more sheltered. On our farmland counts, the best years had just four (2011) and three (2005) sightings over the 26 hours of recording (we record 1 hour per week from April to September). We are due another peak year, and early signs suggest this might be one!

The holly blue is not to be confused with the common blue which many people think they see in their garden. The common blue will start to appear in about two week’s time, and is much more associated with meadows and areas of grassland where birdsfoot trefoil grows. I’ve only seen one or two in my garden over the many years, whereas holly blue is seen every year in small numbers.

Dr Vince Lea
Head of Wildlife Monitoring 

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