Fresh fungus garden in which termites tend white nodules of Termitomyces. Photo: Dr. Angshuman Roy Choudhury
With hard labour and microbes, termites keep their agriculture running. Anyone trying to take over their fungal fields gets a layer of soil dumped on top of them.
Termites protect their fields against weeds by covering sections of farmland with soil teeming with microbes. In doing so, the species Odontotermes obesus suppresses the invasive fungus Pseudoxylaria, whilst their edible fungal partner Termitomyces continues to grow. Only the infected sections receive a layer of soil; the rest of the garden is left untouched. This is how they have kept their fungal fields healthy for millions of years, as new research published in Science on 26 September reveals.
Monoculture
This form of agriculture is a textbook example of symbiosis. The termites collect leaves, grass and wood, chew it into fine pieces and stack it in underground chambers. There, the fungus Termitomyces grows into dark, spongy structures. For the termites, this provides a stable food source, whilst the fungus receives a perfect microclimate. The system resembles human monocultures in certain respects: productive, but vulnerable to disease.
The greatest threat is Pseudoxylaria, a fungus that can overrun a garden within two to three days, much like a weed. But as long as termites are present to manage the garden, the fungus has no chance. Exactly how colonies manage to prevent this had remained unclear until now.
Sealing off
Indian biologists therefore presented their termites with a series of infection scenarios in the laboratory. In the case of small outbreaks, the termites removed the threads of Pseudoxylaria, scraped the contact site clean and packed the removed hyphae in soil. In 94 per cent of trials, this halted growth. In larger outbreaks, they covered entire sections of farmland with soil or separated healthy parts from overrun ones. In more than two thirds of cases, this kept the infestation contained. When researchers artificially glued a healthy section of ground to an overrun one, the termites broke the connection to save the crop.
Crucial was the role of the soil used. These self-made soil pellets turned out to be packed with hundreds of species of micro-organisms, including bacteria such as Burkholderia and Serratia that strongly inhibit the growth of Pseudoxylaria. Sterile soil did not work. The insects are in effect applying a form of integrated pest management: mechanical removal, isolation and microbial assistance interlock seamlessly.
‘It is a very elegant, systematic study. Early observations of this behaviour did exist – including those made in 2010 by my PhD student Anna Visser – but they had never been substantiated so convincingly. The researchers have uncovered it step by step, using relatively simple but thorough experiments’, says Duur Aanen, evolutionary biologist at Wageningen, who has himself conducted extensive research on fungus-growing termites and is cited several times in the paper.
Hard work
Aanen emphasises that lessons can also be drawn for human agriculture. ‘Our monocultures are equally vulnerable. Termites solve that with labour-intensive care, compartmentalisation and biological control through collaboration with microbes. For us, it means that sustainable agriculture is likewise labour-intensive – it is simply hard work and does not happen by itself. Strip cropping and other forms of risk spreading are just as logical a strategy as the compartments of a termite garden.’