The holly blue, the only blue butterfly regularly seen flying in gardens and city centres, increased by 151% on 2014 in Butterfly Conservation’s Big Butterfly Count.
It was a welcome return for the azure insect, whose populations boom every six years or so due to its relationship with a parasitic wasp, Listrodomus nycthemerus. The wasp injects a single egg into the living Holly Blue caterpillar via a syringe-like sting. The wasp larva develops within the caterpillar, eventually killing it after it has changed into a chrysalis.
The parasite is completely dependant on the holly blue and can be almost too successful – killing all 100 caterpillars monitored in some scientific studies. Once holly blue numbers have been drastically reduced, the wasp’s population crashes as well.
This allows the butterfly to recover again – holly blues previously boomed in 1976, 1984, 1990, 1996 and more recently in 2010.
This year, the butterflies were seen in St James’ Park and Battersea Park in central London and as far north as Edinburgh.
Despite an often miserable July and August, a record 52,204 people took part in the count this year, spotting almost 600,000 butterflies.
The gatekeeper was the most commonly seen species, with numbers up 17% compared to last summer, with the large white in second place with a 46% rise.
Despite large numbers of peacocks and small tortoiseshells surviving a mild winter to emerge in good numbers this spring, a bumper summer generation of these colourful garden butterflies failed to materialise.
“My biggest surprise is where the heck have all the peacocks and small tortoiseshells gone?” said Richard Fox, Butterfly Conservation’s head of recording. “The peacock won last year’s Big Butterfly Count, it’s had a couple of amazing years, large numbers came through hibernation and it was easy to find its caterpillars on nettles in late spring. I thought there was going to be a massive emergence but they just disappeared.”
One possibility is that the weather was so poor that peacocks immediately went into hibernation after hatching from their chrysalises, but the butterfly’s population may also have been hit by other parasitic wasps and flies.
The peacock was 61% down on last year’s count, but was barely seen in Northern Ireland and Scotland, where numbers recorded slumped by 98% and 97%.
According to Fox, the holly blue may do well again next year – as long as gardeners don’t cut ivy back this autumn while it is flowering. The caterpillars feed on the ivy flowers and then drop to the ground to pupate, which means that ivy can be safely cut back in midwinter. In spring, holly blue caterpillars are more likely to feed on holly but also use dogwood, spindle, gorse, bramble and even the buds of cotoneaster.