How do leafcutter ants decide on their menu?

These admirable insects know which plants to cut and which not to cut. How do they perceive it and assimilate it? Although the answer remains a mystery, a study by FAUBA revealed that they could distinguish between “good” and “bad” fungus-infected plants.

Cutter ants, those black ones that decimate the plants in our gardens, have been cultivating the fungus on which they feed in their anthills for 65 million years, like real farmers. To do this, they chop plant material, transport it to the mushroom and offer it to grow. However, not all plants are beneficial for this fungus. How do ants determine and learn which plants to attack and which not to attack? A study by the UBA’s Faculty of Agronomy (FAUBA) looked at the role played by certain plant chemical compounds that could act as “alerts” in the link between ants and their environment.

I was interested in studying how the habit of leafcutter ants of chopping leaves and taking them to their anthill – known as foraging – was modified by the presence of a fungus that lives inside the leaves of Lolium multiflorum grass, or annual ryegrass. . This grass is widely used as cattle feed in the Pampas grasslands, and the leafcutter ants I have studied, of the species Acromyrmex ambiguus, often damage its leaves.said Juan Fiorenza, professor in the Department of Quantitative Methods and Information Systems at FAUBA.

Fiorenza, a brand new doctor of agronomic sciences (EPG-FAUBA) led by Marina Omacini, professor at this faculty and researcher at IFEVA-CONICET, explained that it has long been known that the ryegrass fungus – called Epichloe occultans and known as an endophyte because it lives inside the plant, it produces substances called alkaloids, which are toxic to insects. Since there are ryegrass plants not infected with this fungus, one of the researcher’s ideas was that leafcutter ants would prefer ryegrass that was not infected with endophytes – and therefore without toxic alkaloids – and would reject the one who is infected and toxic.

In the first experiment, Fiorenza found that after a few days, ants were able to differentiate infected ryegrass from uninfected. “For 5 consecutive days, we offered them L. multiflorum plants infected and uninfected with the endophyte. On the first day, the ants cut all the plants equally. But by day 3, the proportion of damaged leaves on plants with endophytes had decreased. By day 5, less than 50% of infected plants were attacked, while those without endophytes were still preferred. Somehow they learned to distinguish which plants were not suitable for them to forage”.

learning takes time

The fungus extracted from the anthill is placed in a pot to be transferred to the laboratory, where it will be grown under controlled conditions.

Before explaining the result of his test, Juan Fiorenza spoke about the eating habits of A. ambiguous. “Contrary to popular belief, leafcutter ants do not eat the pieces of plants they cut. With these pieces of vegetables, they actually feed a fungus that they cultivate in the anthill. They feed on this fungus, scientifically called Leucoagaricus gongylophorus. This is key to understanding how they learned to forage ryegrass plants that were not poisonous to them.”.

The selectivity of ants for ryegrass plants with and without endophyte depended on how the two plant materials affected L. gongylophorus. If a plant is good for this fungus, the ants keep cutting it. If it’s bad, they leave it. Therefore, “learning” involves the passage of time. In our case, five days were enough”.

The researcher said this behavior of ants is called deferred rejection and that their results show this for the first time for science in leafcutter ants facing endophyte-infected plants.

After establishing that ryegrass infected with E. occultans is bad for the fungus that ants cultivate, we wanted to venture to answer a new question: What exactly is the compound produced by these plants that affects the growth of the fungus? ants ? The pieces of ryegrass with endophyte had to carry a message inside”.

Letter from one mushroom to another

ants, cutters, mushrooms, foodThe photos show how the growth of the L. gongylophorus fungus took place after 6 weeks of culture in Petri dishes containing (or not) the volatile compound Z3-HAC. In the left panel, the paper did not contain this substance and the fungus grew equally in all directions. In the right panel, it can be seen that the fungus has grown further to the opposite side of the paper with the Z3-HAC.

Based on previous studies carried out by “Simbiosfera”, the research group of which he is part of FAUBA, Fiorenza knew that in addition to alkaloids, ryegrass infected with endophytes produces large amounts of a chemical compound volatile called Z3-HAC, responsible for “the typical smell of freshly cut grass”. So he decided to test whether this particular substance affected the growth of the anthill fungus.

Our second experience was in the lab. We grew the fungus that ants grow in petri dishes. In some plates we put a piece of paper impregnated with the volatile compound, while in others we put a piece of paper without volatile compound, and we measure the growth of the fungus for 6 weeks.”.

In this case, what we are verifying is that some sort of repulsion of the fungus towards Z3-HAC has occurred. In plates that contained this volatile compound, L. gongylophorus grew more towards the opposite side of the paper.”.

The results of both experiments were presented at the 6th Congress of the Latin American Association of Chemical Ecology, where Juan won the award for best poster presentation.

waiting for more results

According to Fiorenza, it is complex to say what this result implies in ecological terms, because in the laboratory the fungus of the anthill does not develop naturally or does so very slowly. What is clear, however, is that the fungus detected and responded to this particular volatile compound, which may be involved in communication between the infected plant, the ant fungus and the ants.

By way of conclusion, Juan pointed out that his results consist of a first approximation in the study of the role played by volatile compounds as mediating signals between ryegrass plants that harbor the endophyte inside their leaves. E. occultansleaf cutter ants A. ambiguous and the fungus they feed on, L. gongylophorus. “We are happy to have detected the effect of delayed rejection in our study, which is new. But to dig deeper into the effects of Z3-HAC will require much more investigation.”.

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