By Kiran Gadhave
Not long ago, the large blue butterfly (Maculinea arion) had fallen to extinction in the UK, largely due to its obligate association with a particular species of ant. It was then saved by a complicated reintroduction program by Natural England and the Centre for Ecology and Hydrology.
The life history of this blue fairy is intriguing. In the late nineteenth century, nature conservationists saw a massive habitat loss as a serious issue for the large blue. Following a severe drop in numbers, the butterfly became extinct in Britain in 1979.
The reason for the disappearance, it was thought, was the loss of wild thyme (Thymus praecox), a major food source of the early larval instars. Even though this was partly true, the central player wasn’t identified until the late 1970s. After an alarming drop in the known number of colonies to just two, an investigation revealed proximate and species-specific associations between the butterfly and a species of ant called Myrmica sabuleti.
Unlike most butterflies, large blue larvae are cannibals and they’re omnivorous. Upon hatching, a caterpillar typically feeds on the flower head of wild thyme, but if two eggs hatch on the same flower, one caterpillar will eat the other. Life often begins with the murder of its own sibling.
The surviving caterpillar typically feeds on the same flower until the third instar. Apparently, this is the only peaceful phase of its life.
In the fourth instar, an abrupt behavioral change occurs and the larva drops itself off the flower. The caterpillar then elicits chemical cues from its “honey gland” to attract M. sabuleti worker ants. Having been sweetly deceived by this secretion, the ants carry the caterpillar to their colony. In order to be carried, the caterpillar must pretend to be dead, and in order to survive, the colony must contain about 200 ant grubs, which the caterpillar will eat.
After feeding, the final instar of the caterpillar either crawls out of the colony or pupates inside it. In the latter case, the ant grubs actually care for it and provide protection by warding off predators.
The caterpillar and the ants live in an inequitable mutualism. The caterpillar secretes sugary food for the ants, but it devours numerous ant grubs and gains protection.
However, despite the range of benefits that the butterfly obtains from the ants, this intimate association has its weaknesses. First, if the ant colony is heavily populated with caterpillars, they will devour all of the ant eggs and grubs, which will wipe out the colony and the caterpillars’ food source. Second, if multiple ant queens are produced in the colony, the workers will kill all of the caterpillars and the symbiosis ends in a strange manner.
The Myrmica ants require favorable environmental conditions to survive. They inhabit a slope with very short grass. A lack of ample sunlight and/or taller grass can severely affect the ant population and, consequently, the large blue population. Such a specialized environment immensely limits the breeding ground for this butterfly, which has been one of the major reasons behind the large blue’s disappearance in the 1970s.
The conservation program that helped revitalize the large blue required the establishment of M. sabuleti ants in addition to the butterflies. This successful effort helped the species go from endangered to near-threatened status.
The unraveling of the large blue’s life-history characteristics opened new avenues for its conservation and led to new approaches for saving other precious butterflies on the verge of extinction. Here are a few healthy habits that you can adopt to conserve our precious flying fairies:
– Grow a range of colorful flowering plants in your garden.
– Volunteer for for local conservation activities.
– Report species that you find interesting to local experts.
– Donate to trusts, societies, organizations, and parks that are actively involved in conservation.
I hope this little essay will help you to understand the complexity of these tiny creatures and to admire their meaningful, complex lives — not just their beauty — the next time you see them basking.
Kiran Gadhave is a PhD student in Prof. Alan Gange’s lab at the University of London. He received his master’s degree in entomology from Tamil Nadu Agricultural University, and in plant breeding and genetics from Cornell University. He has published three peer-reviewed papers and has won four major scholarships, a poster prize, and seven travel awards. He is currently working on a multidisciplinary research project investigating interactions between plant-growth-promoting rhizo-bacteria and plant-insect herbivores.