Episode 335: Large Blue Butterfly vs Ants

We’re kicking off July with a beautiful butterfly that does horrible things to ants!

Further reading:

UK Butterflies – Large Blue

The large blue butterfly (picture taken from page linked above):

Show transcript:

Welcome to Strange Animals Podcast. I’m your host, Kate Shaw.

I recently realized that I have so many weird and interesting invertebrates saved up to feature for invertebrate August that I can’t fit them all into one month, so let’s kick off invertebrate August in July!

This week we’re going to learn about a beautiful butterfly called the large blue, because it is both large and blue. Well, sort of large. The butterfly has a wingspan of up to two inches, or about 5 cm. Its wings are a dusty blue with black spots, although there are a lot of regional differences. Some populations are almost black, some are more tan than blue, and some don’t have spots.

The large blue lives throughout much of Eurasia, although its numbers have decreased in many places in the last 50 years or so. In some places it’s even gone extinct, mainly due to habitat loss. It needs specific host plants for the caterpillars to eat, and it also needs a particular type of ant in order for the caterpillars to survive–because the large blue caterpillar is a brood parasite!

We’ve talked about brood parasites before in birds, where a bird will lay an egg in the nest of a different species of bird. In the case of the large blue butterfly, in summertime the female lays her eggs on wild thyme or marjoram plants near a colony of red ants in the genus Myrmica [meer-mee-kuh]. She usually only lays one egg on any given plant.

When the eggs hatch, the newly emerged caterpillars feed on plants at first, just like any other caterpillar, especially the flowers of the plant. If more than one large blue caterpillar is on a plant and they encounter each other, one of them will grab the other and eat it. Drama among the thyme plants! The caterpillar goes through three growth stages, called instars, as an ordinary caterpillar (except for the cannibalism thing), but once it reaches the fourth instar it starts acting very different.

The caterpillar drops to the ground and releases a chemical that mimics the smell of the Myrmica ant larvae. When an ant finds a caterpillar, the caterpillar will rear up so that it resembles an ant larva. The ant usually takes it back to its nest at this point, but sometimes the caterpillar will just follow an ant trail and enter the nest on its own. Either way, the ants will assume it’s a lost baby and take it to the nesting chamber, where they feed and take care of it.

The caterpillar is bigger than a usual ant larva, but it uses this to its advantage. It mimics the sounds made by a queen ant, which means the ants take extra good care of it. If the ants run out of regular food to feed the caterpillar, they will even start feeding it real ant larvae. But sometimes the caterpillar gets impatient, or maybe just hungry, and will just start eating the other pupating ant larvae.

The system isn’t perfect, because a lot of times the ants figure out that the caterpillar is an intruder and will kill and eat it. If the queen ant encounters the caterpillar, she recognizes that it isn’t an ant larva and will attack it. Sometimes the ants just up and abandon the nest, leaving the caterpillar behind. In that case, the caterpillar will either leave the nest itself and find another one, or it will wait for a new ant colony to find the nest and move in. This can actually happen repeatedly during the nine months or so that the caterpillar requires to finish growing, although during the winter the caterpillar is more or less dormant.

Around the end of spring, the caterpillar spins a cocoon and pupates right there in the ant nest. The ants continue to take care of it, making sure the pupa is clean. When it emerges as a new butterfly after a few weeks, it has to find its way out of the ant nest and to the surface, where it climbs a plant stem and rests while its wings inflate and dry. The adult butterflies only live for a few weeks, eating flower nectar, especially of the thyme plant.

One of the places where the large blue butterfly went extinct was in the British Isles, where it was last seen in 1979. Before that, though, scientists already recognized that the species was in danger in Britain. They knew that the butterflies needed wild thyme and Myrmica ants, and made sure to plant lots of the thyme in areas with lots of Myrmica ant colonies. But the butterflies still declined until none were left in Britain. It turns out that the large blue butterfly requires a particular species of Myrmica ant, Myrmica sabuleti, and if the caterpillars are adopted by other ant species, they aren’t usually successful in surviving to grow up.

Fortunately, a few years later, scientists re-introduced large blue butterflies to Britain from Sweden, and this time it worked. Not only are there still large blue butterflies in Britain again, they’re now more common in Britain than anywhere else throughout its range.

Other butterflies closely related to the large blue also act as brood parasites to Myrmica ants, but to different species. There are probably more butterflies that do this than we know, since it takes a lot of very careful observation of the butterflies, caterpillars, and ants to determine what exactly is going on. Considering that even the ants don’t really know what’s going on, it’s no surprise that scientists have trouble figuring it out too.

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Thanks for listening!