A common garden visitor, also seen in parks, churchyards, hedgerows and woodland paths.
The Holly Blue (Celastrina argiolus
) is the only blue butterfly regularly seen flying in gardens and city centres. It emerges well before other blue butterflies. and tends to fly high around bushes and trees where it gathers around Holly (in spring) and Ivy (in late summer) to lay its eggs. Once hatched, the caterpillars feed on the buds and flowers then drop to the ground to pupate (turn into chrysalis). Their wings are blue, but females have black wing edges. The numbers of holly blues varies massively every year due to their relationship with a parasitic wasp (Listrodomus nycthemerus).
The wasp injects a single egg into the living Holly Blue caterpillar. The wasp larvae will then feed on the caterpillar, eventually killing it after it has changed into a chrysalis. The wasp only feeds on Holly Blue caterpillars and some years will kill too many, leaving not enough for the next year, so the wasp numbers go down as well. This allows the butterfly numbers to increase again until the wasp numbers catch up and it starts all over again!
For more info see https://butterfly-conservation.org/butterflies/holly-blue