Holly Blue Celastrina argiolus

This sky-blue butterfly has one of the more unusual seasonal strategies in the butterfly world: the spring generation lays its eggs exclusively on holly, while the summer generation switches entirely to ivy. No other butterfly in Britain routinely changes its larval host plant so completely between generations. The switch may help avoid the specialist parasites and predators that build up on a single plant through the season.

Holly blue populations famously boom and crash in rough ten-year cycles, driven almost entirely by a parasitic wasp that tracks the butterfly's abundance. When blues are plentiful, wasps multiply; wasp pressure then collapses the butterfly population; wasps crash; blues recover. It's a textbook predator-prey oscillation playing out in hedgerows and gardens across Europe.