Nature's Farmers | Sarah Worsley
The transition from human hunter-gathering to agriculture occurred approximately 10,000 years ago and is often heralded as a major transition in human evolution. However, millions of years earlier, several other unsuspecting species had already taken the step towards intensive agriculture and were busy cultivating their own crop varieties.
Termites, ants, bees and wasps comprise a large group of insects called the Hymenoptera, many of which form vast societies, each made up of many “worker” individuals and headed by one or a few monarchs, often female Queens, that are responsible for the reproductive output of colonies. As with human evolution and agriculture, many of the hymenopteran species that form the largest societies are also known to engage in agricultural activities which provide the fuel to support the Queen as well as the relentless work-force of the colony.
In South America, colonies of Attine leafcutter ants can number up to 4 million individuals with each ant organized into a particular working group that, collectively, form part of an intensive agricultural chain. Older individuals are typically given the risky responsibility of foraging for leaf material which, in the case of Atta ants, they carry back to their nests in droves, leaving a trail of dropped berries and debris in their wake. Once back at the nest, the plant fragments are taken below ground and processed by smaller ants. Leaf material is chewed into pulpy pieces and then added, along with a fertilizing fecal drop, to a massive growth of fungus- the ants’ agricultural haven. The precious fungus, Leucoagaricus gonglyophorus, grows on the ants’ leaf material compost and provides the Queen and developing worker ants with sustenance. Needless to say the ants were not too keen to share their bounty when I took a trip to Panama to collect leafcutter colonies during the first year of my PhD. The task involved a dessert spoon and a sieve and many angry ants!
The Attine ants have been gardening fungus in this way for almost 50 million years, along with several other ant and termite species found in various regions of the world. For example, species of Tetraponera ants farm their fungus in the swollen glands of African Acacia trees, in turn providing their host tree with protection against herbivores. Anything that tries to eat the tree has to confront one very angry ant colony! The Southern Pine Beetle is also particularly partial to a diet of mushrooms. These beetles chisel out narrow galleries in the bark of pine trees which are then used as a living larder. Their fungal cultivar, Entomocorticium, grows prolifically in the galleries, feeding the beetle larvae.
In addition to fungus, insects have also developed a taste for more elaborate food crops. Three million years ago a species of Fijian ant, Philidris nagasau, evolved the ability to farm the fruit of Squamellaria trees. The ants take the seeds, plant them in the bark of other trees and then harvest the new fruits as they develop.
However, farming food is not without its hazards. Like many of our crop systems today, insect species tend to farm just a single food variety. Fragments of this same food crop may additionally be passed down between Queens and their dispersing daughters as they set up new colonies. As humans are well aware, such monocultures are prone to rapidly spreading outbreaks of pests and pathogens as they lack the genetic diversity to resist any attack. In response, humans have synthesized chemical pesticides, however insects have come up with an ingenious alternative to the problem of crop disease. Over millions of years, many of them have developed close partnerships with groups of antibiotic-producing bacteria. In the case of leafcutter ants, these bacteria are recruited to live on their cuticle (often visible as a white bib on their chest plates) and the antibiotics that they produce are used as weed killers within the fungal garden. As more and more of the pathogens that infect our own crops are becoming resistant to chemical pesticides, scientists like me are hoping that we can take inspiration from the systems used in insect societies. Applying specially formulated groups of antibiotic-producing bacteria to the roots of crop plants, or engineering plants that are better able to recruit these friendly bugs, which are common in the soil, may help us to compete in the constant arms-race with crop pathogens and improve food security into the future.