Conservationists first noticed the decline in large blue numbers in the early 1900s and became alarmed in the 1950s when numbers dropped significantly. They spent 50 years trying to halt the decline in numbers of the large blue and declared it extinct in 1979. It was only in 1984 when the large blue was reintroduced from Sweden that conservationists discovered more about its life cycle and that it could only survive in the nest of one species of red ant.
The large blue has a very unusual life cycle. It feeds for three weeks on the flower buds of wild thyme or marjoram and the caterpillar then produces scents and songs to fool a certain species of red ant (Myrmica sabuleti), into believing that it is one of their own grubs and is carried underground into the ants' nest and incorporated into their brood. It then feeds on ant grubs for 10 months before pupating in the nest and emerging to crawl above ground as a butterfly.
This year about 10,000 adult large blue butterflies were recorded and estimated to be active in Gloucestershire and Somerset based on the observations of huge numbers of eggs laid on the thyme and marjoram flowers which are abundant in the reserves in those areas. If numbers remain at their present levels then the large blue - Britain's most endangered butterfly - could be removed from the International Union for the Conservation of Nature's red list of threatened species. It is not only the large blue that are benefitting and flourishing due to the knowledge and applications of scientists and conservators, but also other species of plants, flies and frogs who were also thought to be in terminal decline.