Fruit fly larvae use sugar as bait to attract and devour ants

Researchers describe hitherto unknown strategy used by larvae of small fruit fly to prey on ants infesting plants native to Brazil’s Cerrado biome

By Peter Moon  |  Agência FAPESP – Many trees and bushes in the Cerrado, the Brazilian savanna, have sugar glands to attract ants. Known as extrafloral nectaries, the glands secrete droplets of nectar that provide a nutrient source for ants.

“The ants gather the nectar and, at the same time, patrol the plant’s leaves against attack by other insects, such as caterpillars, for example. All this is very common. What’s surprising is to see an insect taking advantage of ant-plant mutualism to prey on ants, as does a small carnivorous fly,” said Paulo Sergio Oliveira, a professor of ecology at the University of Campinas’s Biology Institute (IB-UNICAMP) in São Paulo State, Brazil.

The fly mentioned by Oliveira belongs to the Drosophilidae family of fruit flies. While studying ant mutualisms between 2008 and 2013, biologist Mayra Cadorin Vidal, whose master’s research was supervised by Oliveira, went with colleagues to a private Cerrado reserve on a farm in the vicinity of Itirapina, a small town in São Paulo State.There, they noticed tiny larvae of insects in some extrafloral nectaries of Qualea grandiflora (Vochysiaceae), a flowering tree native to the Cerrado (local name pau-terra). The larvae fed on ants and belonged to an unknown species of fruit fly.

The researchers placed the new species in the genus Rhinoleucophenga and gave it the epithet myrmecophaga (from Greek myrmex = ant, and phagos = devourer). A description by Vidal et al. was published in 2015 in Annals of the Entomological Society of America.

In a new paper published in July in the journal Environmental Entomology, Vidal details the method used by larvae of R. myrmecophaga to prey on carpenter ants of the genus Camponotus.

The researchers found that adult female flies laid single eggs next to nectaries, where the larvae later hatched. “We began investigating how the presence of these larvae might affect ant-plant mutualism. Initially, we thought the larvae blocked access by the ants to the resource exchanged in mutualism, but we later realized the ants were trapped in shelters built by the larvae,” said Vidal, currently at Syracuse University in New York (USA).

Vidal concluded that what she earlier observed and has now described was a unique predation strategy. “This exploitation of an ant mutualism is unique. In fact, it’s the first known instance of an agent that takes advantage of a resource offered by a mutualistic partner to attract and eat another partner,” she said.

The study was supported by FAPESP via the foundation’s Research Program on Biodiversity Characterization, Conservation, Restoration & Sustainable Use (BIOTA-FAPESP).

Ant trap

During the dry winter months in the Cerrado, the branches of Q. grandiflora are entirely leafless. In the summer, which is the rainy season, the trees are full of green leaves, and their mutualism with ants of the genus Camponotus can be observed.

Sugar droplets produced in extrafloral nectaries are offered to insects only in the summer months. In exchange, the plant makes good use of the aggressiveness of the legions of ants that patrol its branches and attack caterpillars, beetles, aphids and other bugs attracted by its leaves and nectar.

In effect, the carnivorous fly has gatecrashed this mutualistic relationship. During the evolution of R. myrmecophaga, its larvae have adapted to take advantage of the relationship between Q. grandiflora and the carpenter ant to prey on the latter.

“When the larva hatches from the egg, it climbs up and builds a shelter on top of the sugar gland, where it develops until it becomes an adult fly. The shelter is not just the site of larval development but also a trap to capture ants. Inside the shelter, the larva is always lurking for unwary prey,” Vidal said.

The larval shelter, she added, has a hole through which the larva pushes a droplet of nectar from the plant’s gland. This droplet is the bait. The larva sits right in the middle of the shelter, which is extremely sticky.

When a foraging ant enters to collect the nectar, it becomes stuck and eventually dies of exhaustion after struggling to escape. The larva then uses its two mouth hooks to pry open the ant’s exoskeleton and devours the contents.

“The larva eats the ant’s insides. We found several empty exoskeletons that were still stuck to the larval shelter. In a few cases, the larvae had also devoured wasps, beetles and flies,” Vidal said.

Infestation of trees and bushes in the Cerrado is very common, she noted, and 85% of the plants observed in the study were infested with R. myrmecophaga larvae. Each plant had five larvae on average.

According to Oliveira, the predation of ants is a rare phenomenon. “Very few animals are adapted to eat ants, which are aggressive,” he said. “They sting and spray acid. Moreover, they’re social insects, so where there’s one there are always many others, and they all gang up on the enemy until it’s dead. Then, they cut it up and take it back to the nest for food.”

In another study, published in 2016 in the journal Ecology, Vidal analyzed the effects of the presence of R. myrmecophaga on trees of the species Q. grandiflora. She found that trees on which ants spent less time suffered more damage because they were less protected against herbivores.

Vidal said she suspected there might be other hitherto unknown cases of ants being preyed upon while foraging on plants in the Cerrado.

“Both ants and plants with sugar glands are abundant in the Cerrado,” she said. “The ants constantly visit these plants to feed on their secretions. It’s possible that other specialized eaters of ants may yet be discovered.”

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) can be read at https://academic.oup.com/ee/advance-article/doi/10.1093/ee/nvy097/5050820.