When the sexes of the common blue meet, copulation occurs immediately without any form of courtship ritual. The circular flattened white eggs are usually laid on leaves of plants from the legume family.
Common bird’s-foot-trefoil (Lotus corniculatus) is the main foodplant. The green caterpillars are about 10 mm long and have a shiny black head. Like most Lycaenid species they are often attended by ants, which milk them for sugary secretions. The larvae in exchange are protected by the ants from predatory insects.