Ants defending plants receive sugar and protein

Biologists Laura Carolina Leal and Felipe Pbados conducted a series of experiments to determine the interaction of extra-floral nectary plants with ants in the northeastern region of Brazil, specifically within the state of Bahia, where predominates the semiarid biome of Caatinga.

Extrafloral nectaries are nectar-secreting glands that do not participate in pollination. They provide carbohydrates to insects in exchange for a defense against herbivores. Nectar attracts predatory insects that consume both nectariferous arthropods and plant eaters, acting as bodyguards.

"Contrary to popular belief, we discovered that carbohydrates are just one of the forms of payment offered by plants to the ants that protect them.Another is the protein that ants get by consuming the herbivorous arthropods available on or around plants they visit. "said Leal, a professor at the Institute of Environmental Sciences, Chemicals and Pharmaceuticals of the Federal University of São Paulo (ICAQF-UNIFESP) in Brazil.

"This discovery contradicts the idea that payment only concerns sugar," Leal said. "This shows that what ants are gaining with herbivores also counts.We have found that ants can be more aggressive in environments where arthropods and other protein sources are scarce, defending their food sources and thus protecting plants. "

The study is published in Biological Journal of Linnaean society. The areas of research pursued by Leal and Pbados focus on the different forms of insect-plant mutualism. "Mutualism is a form of interaction between two species in which each one benefits in a certain way from the interaction.If this is not advantageous for both species, but only for the One of them, that is parasitism, "Leal said.

"Several studies have shown that nectarivorous ants expel herbivores and improve the reproductive success of extrafloral nectaries." The more the extrafloral nectar is important for ants, the better for the plants, as it increases the ants' aggressiveness towards the plants. We decided to find out if nectar is the only payment that plants pay for ant protection or if eating herbivores could also be beneficial for ants. "

Leal and Pbados have confirmed the hypothesis that the presence of plants by more aggressive ants and the efficiency of their defense increases when the availability of carbohydrates and / or protein ants is low, thus reinforcing the value relative extra-floral nectaries and protein-rich herbivores. insects.

The study was conducted on the campus of the University of Feira de Santana in Bahia. The region has a semi-arid climate with an average annual temperature of 25.2 ° C and an average rainfall of 848 mm per year. The vegetation of Caatinga is xerophyte (adapted to life in a dry habitat), consisting of a mosaic of thorny shrubs and dry season forests.

The researchers created early 2017 19 study plots of 16 square meters each. They included at least 30 meters from each other and contained mainly Turnera subulata, a plant of the pbadionflower family called white alder. It was the only plant with extrafloral nectaries in plots. Its density ranged from five to 218 individuals per plot.

"T. subulata has a pair of extrafloral nectaries on each petiole [the stalk that attaches the leaf blade to the stem] and base of the inflorescence, "said Leal.The extrafloral nectaries are constantly visited by different species of ants that can defend the plant against herbivores.The relative importance of any resource for animals is influenced not only by its abundance in the habitat, but also by the number of plants, so we started by counting the ant nests that were feeding in our study plots. "

The researchers left five mixtures of carbohydrates and proteins (sardine and honey) as bait in the soil of each plot between 7 am and 11 am, at which time the ants were the most active on the site. A piece of bait was placed in the center and the other four at about three meters from the corners.

"We waited for the ants to find the bait and followed them to their nest, even though these were outside of our study site," Leal said.

They counted ant nests and estimated the abundance of protein and carbohydrate resources for the ants in each plot. As T. subulata is a prostrate herb present in open habitats, it is mainly visited by ant species that feed on the soil.

"We recorded 312 occurrences of 13 species of ants on these plants, most of which were visited simultaneously by two species of ants," Leal said.

The most common species was Camponotus blandus (42% of occurrences), followed by Dorymyrmex piramicus (25.6%). Dead arthropods in the soil are the main source of protein for these ants.

Researchers used soil arthropod biombad as an indirect indicator of protein availability for ants that visited extrafloral nectaries in each plot. To achieve this, they installed five pitfall traps in each plot, one in the center and one in each corner.

"Trap traps remained active for 24 hours, we filtered their contents and dried in an oven at 60 ° C for 24 hours, the lower the average biombad of dry arthropods collected in each plot, the lower the availability. Local protein for the ants was weak, "says Leal.

Less protein, more aggressiveness

The researchers also observed the behavior of ants visiting extra-floral nectaries vis-à-vis a simulated herbivore to determine if the availability of carbohydrates and / or proteins in the habitat affected the environment. ant defense effectiveness.

"We simulated the presence of herbivores on the plants using larvae of Ulomoides dermestoides, a common predator of peanut seeds known as the lizard weevil. peanut or chinese weevil.On the most apical branch of each focal plant, we placed a larva on the leaf We let the larva move freely on the leaf and waited for it to be found by the ants ", said Leal.

Biologists identified the ants present on five plants in each plot and measured their effectiveness in eliminating simulated herbivores from the plants.

"When a larva was located, we observed the behavior of the ants vis-à-vis the larva We observed whether the larva had been removed from the plant, if the ants had brought the larva to the ground had pushed out of the plant or had eaten it where it was, "said Leal.

According to her, the probability of interaction between plants and more aggressive ant species was not influenced by the number of extrafloral active nectaries nor by the biombad of arthropods found in the plots.

"However, simulated herbivores have been eliminated more frequently in plots where arthropod biombad is reduced, suggesting that ants of all species are becoming more aggressive towards other arthropods in the field. Protein-poor habitats.This increase in aggressiveness potentially increases efficiency with extra-floral nectaries being defended against herbivores, "said Leal.

Unlike carbohydrates, protein resources are not renewable and are distributed randomly in the environment. Dead insects, for example, do not have a predictable distribution pattern and can be found almost anywhere. Once eaten, these dead insects are no longer available for other ant species in the community.

"This has led us to suggest that plants with extra-floral nectaries can be defended more efficiently in protein-poor habitats, regardless of the amount of their interactions via nectar secretion," said Leal.

If this is the case, even plants secreting poor quality extrafloral nectar can be effectively defended, as the behavior of ants with regard to herbivores will depend on their protein demand and not on carbohydrates.