Researchers describe previously unknown strategy; work was published in the journal “Environmental Entomology”
In the Cerrado, there are many species of trees and shrubs with sugar glands to attract ants. These glands, called extrafloral nectaries, produce sugar droplets that the ants collect.
“By doing so, the ants end up patrolling the leaves of the plant against attacks from other insects, such as caterpillars, for example. This is all very common. It is surprising to see an insect taking advantage of the mutualism that exists between plants and ants to prey on ants. That’s what a small carnivorous fly does,” he said. Paulo Sergio Oliveira, professor of Ecology at the Department of Animal Biology at the State University of Campinas (Unicamp).
The fly that Oliveira refers to belongs to the drosophila family. While studying ant mutualisms between 2008 and 2013, the biologist Mayra Cadorin Vidal, who was supervised by Oliveira during her master's degree, visited with colleagues a private Cerrado reserve within a farm in the Itirapina region (SP).
At the site, they noticed small insect larvae present in some extrafloral nectaries of a very common tree in the Cerrado called pau-terra (Qualea grandiflora). The larvae – which fed on ants – were from an unknown species of fruit fly.
The “ant-eating larva”, as called by researchers, ended up being described by Vidal in 2015, in the Annals of the Entomological Society of America. Received the name Rhinoleucophenga myrmecophaga, from the Greek myrmex (ant) and phaga (to eat), that is, ant eater.
In a new work published in July in the magazine Environmental Entomology, Vidal describes the method used by larvae of R. myrmecophaga to prey on ants of the genus Camponotus, popularly known as termite ant or sarassará.
The researchers observed that adult female flies laid isolated eggs next to the nectaries, where the larvae later hatched.
“We began to investigate how the presence of these larvae could affect the mutualism between ants and plants. At first, we thought that the larvae were blocking the ants' access to the resource exchanged in the mutualism. However, we later realized that the ants were trapped in the larval shelters,” said Vidal, currently at Syracuse University in New York.
What the biologist witnessed, and what she now describes, is an unprecedented predation strategy.
“This exploitation of an ant mutualism is peculiar, as it is the first known case of an agent taking advantage of a resource offered by a mutualism partner to attract and eat the other partner,” he said.
The work included support from FAPESP through the Biota-FAPESP Program.
Ant trap
In the Cerrado, in the dry winter months, the branches of the pau-terra tree are entirely leafless. In summer, when it rains, the earthwood is full of green leaves. That's when ants of the genus Camponotus exhibit their mutualistic relationship with pau-terra trees and shrubs.
It is only during the summer that the plant offers insects sugar drops produced in extrafloral nectaries. In return, the pau-terra takes advantage of the aggressiveness of the legions of ants that patrol its branches and branches and attack any and all caterpillars, beetles, aphids or stink bugs that dare to eat the plant's leaves.
What the carnivorous fly did was interfere in this relationship. Throughout their evolution, larvae of R. myrmecophaga they ended up adapting to make use of the mutualism that exists between the pau-terra and the ants, and prey on them.
"When the larva hatches from the egg, it climbs onto the sugar gland and builds a shelter there where it will develop into an adult, when it will then have transformed into a fly. It turns out that the shelter is not just the site of larval development , it is also a trap to catch ants. Inside, the larva is always lurking", said Vidal.
The researcher says that the larva's shelter has a hole through which it places a droplet of nectar from the plant's gland. It's the bait. The larva is positioned right in the middle of the shelter, which is extremely sticky.
When ants come to collect nectar, they stick together. When struggling to escape, the ants end up dying of exhaustion. Then, the larva uses two hooks in its mouth to open the ant's exoskeleton and devour it.
“The larva eats the ant from the inside. We found several examples of empty exoskeletons that remained attached to the larval shelters. In a few cases, the larvae had also devoured wasps, beetles and flies," said Vidal.
The biologist says that this is a very common infestation in the Cerrado and that 85% of the plants observed in the study were infected with larvae of R. myrmecophaga. Each plant had an average of five larvae.
Oliveira highlights that this is a rare case. “Very few animals are adapted to eat ants, which are aggressive. They sting, squirt acid and are social insects. Where there is one, there are always many others. And they will all come together to fight a common enemy, until they kill it and cut it up to serve as food inside the anthill,” he said.
In another study, published in 2016 in Ecology, Vidal analyzed the effect of the presence of R. myrmecophaga in the pau-terra trees. She observed that the trees on which the ants spent the least time were those that suffered the most damage, as they were the least protected against herbivores.
Vidal suspects that there may be other unknown examples of this form of ant predation on Cerrado plants.
“As the Cerrado is rich in ants and plants containing sugar glands are abundant, ants constantly visit these plants to feed on their secretions. It is possible that other specialized ant eaters have yet to be discovered,” she said.
The article
The article Natural History of a Sit-and-Wait Dipteran Predator That Uses Extrafloral Nectar as Prey Attractant, by Mayra C. Vidal, Sebastián F. Sendoya, Lydia F. Yamaguchi, Massuo J. Kato, Rafael S. Oliveira and Paulo S. Oliveira (doi: https://doi.org/10.1093/ee/nvy097), is published in: https://academic.oup.com/ee/advance-article/doi/10.1093/ee/nvy097/5050820.