Holly Blue Butterfly (Focus - May)

The UK Butterfly Monitoring Scheme has recently released its report for the 2023 season and it was a mixed bag. The results provide a picture based on recording at more than 3000 sites nationwide. Some species had very good years while others had a particularly bad year. In some cases, it was easy to put the poor showing down to the impact of the summer drought of ’22. Those butterflies associated with damp conditions, such as the Green Veined White and the Ringlet, did particularly badly last summer.  Their larvae probably found it difficult to find sufficient plant material on which to feed in the parched landscape.  Particularly disappointing is the continued decline of the Small Tortoiseshell, a lovely looking insect, familiar to most but becoming increasingly uncommon. As it is faring better in the moister north and west of the country, this decline could be a result of climate change. That impact may be expressed on the insect itself or it may be that the parasites that attack it are favoured by the changing climate.

One species which did well in ’23 was the Holly Blue. On my transect outside Tisbury, I recorded more than twice as many in ’23 than in any of the previous 8 years. I only saw one there in ’22. It is another butterfly that is regularly affected by parasites, and this leads to a boom-and-bust cycle every 4-6 years.  The caterpillars may be attacked by two species of parasitic wasp, which lay their eggs into the larva’s body where they hatch and feed.  When the wasps are rare, the population of Holly Blues is able to increase but soon, with increased numbers of caterpillars to prey on, the population of the parasitic wasps grows. Eventually, the parasitic wasps become so abundant that they cause the population of the host butterfly to plummet. That then causes the wasp populations to crash for lack of larvae and the cycle begins again.

Now is the time to look out for Holly Blues in sheltered sunny gardens, hedgerows and woodland glades. They will often be seen flying amongst the tops of bushes and hedgerows, often where there is holly or ivy present. Although by no means a certain guide, if you see a small blue butterfly skipping around at height, amongst bushes and trees, it will be a Holly Blue; other blues are more likely to be seen close to the ground. In spring, eggs are often laid on holly flower buds. These give rise to a second brood flying later in the summer and, by contrast, the females of this brood generally lay their eggs on the flower buds of ivy.

A Holly Blue flitting around the garden is a sure sign of spring and it will be interesting to see if the numbers of last year are exceeded or whether this season brings a crash.

​Andrew Graham