A simple story, isn’t it…

We’ve probably all heard about the butterfly lifecycle at some point, with the great transformation of the humble caterpillar into the majestic butterfly (1). The most common image in our head is that of a little caterpillar munching away on green leaves growing bigger and bigger until it spins its chrysalis, hiding away from the world until the time for it to re-emerge. While this is the case for many caterpillars, as with most things, there is a much more complex story behind this simple idea.

Many butterflies and caterpillars are associated with specific species for feeding and not all of them stick to green leaves and colourful flowers for their main course (1). This is the case for the caterpillar of the large blue butterfly which has a slightly different tale to tell. Less childhood story, more horror story is how you would describe these hungry caterpillars, with a myriad of tricks up their sleeves and devious tactics which they’ve evolved to use (2).

Their tale starts out as many others, with the adult female butterflies laying small white eggs on the purple flowers of wild thyme or marjoram in the mid-summer sunshine (3). From these eggs emerge small light brown caterpillars which happily eat the flower head and seeds of the wild thyme for the first few weeks of their lives. While the caterpillar is happy up within the grasses and wildflowers, down below the workers of the red ant Myrmica Sabuleti are busy bringing back food for their young which are meticulously cared for, safe for now in their underground nests (4).

A caterpillar or an ant larvae?

A true disguise artist, the cunning large blue caterpillar, falls to the ground and lies in wait for these red ants to stumble across them (5).  When this happens, the caterpillar tricks the red ant into believing they are their own young escaped from the nest, by distorting their bodies and releasing sweet smelling pheromones mimicking the ant larvae (6). After a thorough inspection of the caterpillar by the ant to insure it isn’t an impostor (but it is!), it is quickly gathered up and taken into the brooding chamber of the ant nest (7). Here it is cared for by the worker ants happily surrounded by its new source of food. This is known as social parasitism and is often seen in many wasp, bee and ant species, where one species is reliant on another to survive (8).

Hidden in plain sight and cared for by its adoptive family for several months, the large blue caterpillar feeds on its ant siblings, starting with the biggest ones. This crafty caterpillar cleverly leaves the smaller larvae and eggs alone giving them a chance to grow, ensuring it has a constant supply of food until it forms its chrysalis several months later(4). Although not such a happy story for the ants, this process produces a beautiful blue butterfly that emerges from underground and crawls up into the golden grassland to warm in the summer sun.

A more civilized adult

In comparison to their devious younger selves the adult large blues have a much more harmless existence. Having spent roughly 11- 22 months, over 90% of their lives, underground the large blue’s return to the wild thyme and marjoram where they feed, mate and the females lay their eggs. All of this is done over 5 to 7 short days and then the cycle begins all over again with hungry caterpillars awaiting their unsuspecting red ant hosts (9).

Too dependent to survive?

This shows the great lengths some species go to survive and to think, that without two specific plants and one type of ant these amazing and rare butterflies would not be able to exist. It may be a strange way to have evolved, to be so dependent on a particular species but this is seen quite often in the natural world.

However, as with many species, this does make the large blue’s more susceptible to slight changes in their habitat whether from change in management or climate. If their food plant does not flower when the females are ready to lay their eggs, if the eggs are not laid near a red ant nest or if the red ants’ nest is not big enough to sustain the hungry caterpillar, then the large blue butterfly is unlikely to survive in that location for long (10). That is why it is very important that there is suitable habitat available to them, where all these requirements are met, and that it is managed correctly to ensure all the large blues have all they need to thrive.

We will explore this further in next week’s blog, where we discuss how we manage the habitat on our reserves, providing good habitat for this rare species and others to live in!

References:

1. Vane-Wight D. (2015). Butterflies A Complete Guide to Their Biology and Behaviour 2^nd Edition. Natural History Museum Life Series, London Natural History Museum.
2. Thomas JA, Schonrogge K, Bonelli S, Barbero F, Balletto E. (2010) ‘Corruption of ant acoustical signals by mimetic social parasites Maculinea butterflies achieve elevated status in host societies by mimicking the acoustics of queen ants’, Commun Integr Biol, 3, p169–171
3. Hayes, M.P. (2015) ‘The biology and ecology of the large blue butterfly Phengaris (Maculinea) arion: a review.’ J Insect Conserv, 19, 1p037–1051
4. 4Elmes, G.W, et al., (2001) ‘Larvae of lycaenid butterflies that parasitize ant colonies provide exceptions to normal insect growth rules’, Biological Journal of the Linnean Society, 73(3), p259–278
5. Thomas, J.A. & Settele, J. (2004) ‘Butterfly mimics of ants’. Nature, 432, p283-284
6. Schonrogge K, et al., (2004) ‘Changes in chemical signature and host specificity from larval retrieval to full social integration in the myrmecophilous butterfly Maculinea rebeli’. J Chem Ecol, 30, p91–107
7. Thomas, J.A. (2002) ‘Larval niche selection and evening exposure enhance adoption of a predacious social parasite, Maculinea Arion (large blue butterfly), by Myrmica ants’, Oecologia, 132, p531-537
8. Buschinger, A. (2009) ‘Social parasitism among ants:  a review (Hymenoptera: Formicidae)’. Myrmecological News, 12, p219-235
9. Tomlinson, D. et al., (2020). Britain’s Butterflies. Princeton University Press
10. Diamond, S.E. et al., (2011) ‘Species' traits predict phenological responses to climate change in butterflies’. Ecology, 92(5), p1005-1012