Lemon Ants (Myrmelachista schumanni) Photo from https://the-most-exteme.fandom.com/wiki/Lemon_Ant

Dear Readers, I received a comment a few days ago from someone who had accidentally nearly eaten an insect of some kind. They described how it tasted of cinnamon, and I was instantly intrigued. On a holiday to Costa Rica many years ago, I tasted a lemon ant (sorry!) and was surprised at how ‘lemony’ it tasted – formic acid is presumably chemically similar to the citric acid in citrus fruit, so maybe it isn’t that unlikely. But when I started to look into the natural history of the Lemon Ant, I found that they used their formic acid for something much more interesting than dissuading predators (except humans, clearly) from eating them.

Lemon Ants create ‘devil’s gardens’ – these are areas where all the natural biodiversity of the rainforest has disappeared, leaving nothing but the ants and three species of tree. One of them is Duroia hirsuta, and it plays a very special role in the life of the ants.

Duroia hirsuta fruits (Photo by Karsten Thomsen at https://www.flickr.com/photos/94052068@N06/9124372155

Firstly, the plant itself produces chemicals which deter other plants from growing close to it, but this is greatly aided by the ants, who  inject other plants with their formic acid and make sure that nothing else can grow around the Duroia trees. Lemon Ants are the only insects known to use formic acid as a herbicide. The Lemon Ants also kill any other ant  species which would otherwise harvest the leaves of the tree, and will usually nest in a Duroia tree, as well as eating its leaves.

The devil’s gardens that the ants create can be enormous – the largest garden ever found contained over 300 trees over an area of 1,300 square metres, and was estimated to be 800 years old.

Although the Duroia trees have to put up with having their leaves eaten by the ants, there is a clear overall benefit to the plants: in the absence of ants, 94% of Duroia trees die. The trees benefit from having the competitive plants around it killed by the Lemon Ants, and also by the elimination of other invertebrate species who might munch on the leaves. The waste from ants’ nests can also be an important source of nutrients for the tree. In return, the Duroia trees offer food, and a place to nest. This is a relationship that has probably been ongoing since the Cretaceous period, 108 million years ago. There is still so much that we can learn about the complex interrelationships of different species.

And next time I’m offered a lemon ant, I shall definitely pass.