Biologists Laura Carolina Leal and Felipe Passos have performed a series of experiments to determine how plants with extrafloral nectaries interact with ants in northeastern Brazil, particularly in the interior of the state of Bahia, where the semiarid caatinga biome prevails.
Extrafloral nectars are non-pollinating nectar-secreting glands that provide insects with carbohydrates in exchange for herbivores' defense. The nectar attracts predatory insects that receive both the nectar and herbivorous arthropods and act as a bodyguard.
"Contrary to the previous assumption, we found that carbohydrates are just one of the payment methods that plants offer to ants. Another reason is the protein, the ants, by eating herbivorous arthropods at or near those of them visited plants, "said Leal, a professor at the Institute of Environmental, Chemical and Pharmaceutical Sciences of the Federal University of São Paulo (ICAQF-UNIFESP). in Brasil.
"This finding contradicts the notion that payment is only in sugar," said Leal. "It shows that it depends on what ants from herbivores do, and we've found that in environments where arthropods and other protein sources are scarce, ants are more aggressive, defend their food sources and protect crops."
Biological Journal of the Linnean Society . The research interests pursued by Leal and Passos focus on the different forms of insect-plant reciprocity. "Mutualism is a form of interaction between two species, each of which benefits in some way from the interaction, and if it's not beneficial to both, but only to one, it's parasitism," Leal said.
"Several studies have shown that nectar-eating ants drive off herbivores and increase the reproductive success of plants with extrafloral nectars. The higher the importance of extrafloral nectar for the ants, the better for the plants, as this increases the aggressiveness of the ants towards herbivores "We decided to find out if nectar is the only payment of the plants for the protection of the ants or whether the feeding of herbivores could also be beneficial for the ants."
Leal and Passos affirmed the hypothesis that the plant visit by more aggressive ants and the efficiency of their defense increases when the availability of carbohydrates and / or proteins to the ants is low, which increases the relative value of both the extrafloral nectaries and the high protein herbivores for these 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 in the caatinga is xerophytic (adapted to living in a dry habitat) and consists of a mosaic of thorny shrubs and seasonally dry forests.
In early 2017, 19 study plots, each 16 square meters in size, were at least 30 meters apart and contained mainly Turnera subulata, a cluster plant known as the Weißerler, from the passion flower family. This was the only plant with extrafloral nectar in the plots. Its density varied 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 inflorescence base," said Leal. The extrafloral nectaries are constantly visited by various ant species that can defend the plant against herbivores. The relative importance of an animal resource is influenced not only by its abundance in the habitat, but also by the number of people sharing that resource. So our first step was to count the nests of the ants that visited our study areas.
The researchers left five mixtures of carbohydrates and proteins (sardine and honey) as baits in the soil of each plot between 7 and 11 o'clock in the morning.When the ants were most active on the site, a piece of bait in the middle and Place the other four in the corners about three yards away.
"We waited for the ants to find the bait and followed them back to their nests. Even though they were outside of our study area, "said Leal.
They counted the ant nests and estimated the ants' protein and carbohydrate occurrences in each plot, because T. subulata is a herbaceous prostate that occurs in open habitats visited mainly by soil-eating ant species.
"We have registered 312 occurrences of 13 ant species on these plants. Most were visited by two or more ants simultaneously, "said Leal.
The most common species was Camponotus blandus (42 percent of the occurrences), followed by Dorymyrmex piramicus (25.6 percent) .Total arthropods in the soil are the major source of protein for these ants.
The researchers used soil arthropod biomass to replace the protein availability of the ants, who visited extrafloral nectaries in each plot and one at each corner.
"The pitfalls remained active for 24 hours. We filtered their contents and dried them in an oven at 60 ° C for 24 hours. The lower the average dry arthropod biomass collected from each plot, the lower the local availability of protein for ants, "said Leal.  Less Protein, More Aggressiveness
The researchers also observed the behavior of ants visiting extrafloral nectaries with respect to a simulated herbivore to determine if the availability of carbohydrates and / or proteins in the habitat is Antiviral efficiency influenced.
"We simulated the presence of herbivores on the plants using the larvae of Ulomoides dermestoides, a common predator of peanut seeds known as peanut beetles or Chinese weevils. On the apical branch of each plant, we placed a larva on the leaf that provided the best horizontal or near-horizontal platform for the insect. We let the larvae move freely on the leaf and waited for it to be found by the ants, "said Leal.
The biologists identified the ants present on five plants in each plot and measured their effectiveness in removing the simulated ones Herbivores from the plants.
"When there was a larva, we observed the behavior of the ants towards the larva. We observed whether the larva was removed from the plant, whether the ants brought the larva to the ground, pushed it from the plant, or consumed it where it was, "said Leal. The plants and more aggressive ant species were not affected by the number of active However, the simulated herbivores were more frequently removed in plots with less arthropod biomass, indicating that ants, regardless of their species, become more aggressive towards other arthropods in low-protein habitats The aggressiveness potentially increases the efficiency with which plants with extrafloral nectars are defended against herbivores, "said Leal.
Unlike carbohydrates, protein resources are nonrenewable and are randomly distributed in the environment. For example, dead insects These dead insects are no longer available to other ant species in the community after being consumed.
"This has led us to suggest that plants with extrafloral nectar can be more efficiently defended in protein – poor habitats, regardless of how much they invest in the interaction via nectar secretion," said Leal.
In this case, even plants that secrete inferior extrafloral nectar can be defended effectively, since the behavior of the ants towards herbivores is determined by them. Demand for protein and not for carbohydrates.
Wasps, ants and Ani DiFranco
Felipe Cs Passos et al., Protein Issues: Ants More Commonly Remove Herbivores from Extfloral Nectarine Plants in Low-Protein Habitats, Biological Journal of the Linnean Society (2019). DOI: 10.1093 / Biolinnean / blz033
Ants defending plants get sugar and protein (2019, July 18)
retrieved on 18 July 2019
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