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Meddling in the Mutualism: Fly Larva Uses Nectar to Lure Ants for Lunch

Camponotus ant visiting nectary and trapped

Ants of the genus Camponotus visit the extrafloral nectaries of Qualea grandiflora plants in the Brazilian savanna region known as the cerrado (left). In exchange for the nectar, the ants protect the plants from predation by herbivores. However, a fly species, Rhinoleucophenga myrmecophaga, specializes in exploiting this ant-plant mutualism by laying its eggs near the nectaries. The larvae that hatch form sticky shelters atop the nectaries to trap the ants (right) and prey on them. (Images originally published in Vidal et al 2018, Annals of the Entomological Society of America)

In the tropical savannas of Brazil, it’s common to find plants protected by ants, in arrangements that have evolved to be win-win situations for both parties. Through these “ant-plant mutualisms,” the plant provides a reward for the ant, such as nectar to feed on, in exchange for the ants’ defense against herbivores that might otherwise feed on the plant.

But, in at least one case, a third party has entered the mix, with a clever modus operandi: a fruit fly whose larva plays a trick to lure ants to their doom.

In a report published in July in Environmental Entomology, a group of Brazilian researchers detail the behavior of Rhinoleucophenga myrmecophaga fly larvae, which perch atop extrafloral nectaries on the common plant, form a small shelter, and use the nectar to attract the visiting ants. The larvae then sink their mouth hooks into the trapped ants and feed on their insides.

“This exploitation of an ant-plant mutualism is peculiar because it is the first case of an exploiter known to use a resource offered by one partner of the mutualism to attract and eat the other partner,” says Mayra Vidal, Ph.D., a postdoctoral researcher at Syracuse University and lead author on the report; Vidal originally conducted the research with colleagues during her Master’s degree work at the State University of Campinas in Brazil.

While studying ant-plant mutualisms in Brazil’s cerrado region, Vidal and colleagues noticed small insect larvae present on some extrafloral nectaries on Qualea grandiflora plants, and soon found they were of a fruit fly species. The adult female flies laid single eggs right next to the nectaries, where the larvae later hatched. “We started to investigate how the presence of these larvae could affect the mutualism between ants and the plants, because at first we thought that the larvae were blocking the access of ants to the resource exchanged in the mutualism,” says Vidal. “However, after observing them for a while, we noticed that ants were getting trapped to the larval shelters.”

Their curiosity piqued, the researchers conducted further observations, watching each fly egg hatch after about three days, at which point the larva moved to the nectary and formed a sticky shelter (perhaps fabricated from the nectar itself) with a small hole at the top. Then it produced a small droplet at the opening of the shelter. Eventually, an ant visiting the plant—or, in a few cases, another kind of insect—attempted to feed on the droplet but then found itself stuck to the shelter, and that’s when the larva attacked. The ant usually struggled to escape and died of exhaustion before being consumed. Later, the researchers often found the ants’ empty exoskeletons, sucked dry by the fly larvae, still stuck to the shelters.

Vidal and colleagues first reported and described the fly, Rhinoleucophenga myrmecophaga, which turned out to be a new species, in 2015. Informally, they simply call it the “ant-preying larva,” Vidal says, and the species name comes from the Greek myrmex (ant) and phaga (eat). In their new study, they’ve combined detailed observation of the fly larvae’s feeding behavior with chemical analysis of the Q. grandiflora nectar, the proffered droplet, larval specimens, and visiting ants, all of which confirm their initial observations of the larvae’s predation on the mutualistic ants.

Rhinoleucophenga myrmecophaga life stages

The fruit fly Rhinoleucophenga myrmecophaga lays its eggs next to extrafloral nectaries of the Qualea grandiflora plant. (Egg shown at left.) The larva soon hatches and moves atop the nectary, forming a sticky shelter around itself (shown at right). Then it uses the plant’s nectar to attract ants, which normally feed on the nectar and protect the plant from predation by herbivores. But ants attracted to the nectar offered by the R. myrmecophaga larva become trapped atop the sticky shelter and then later become food for the larva. The inset at bottom right shows an adult R. myrmecophaga. (Photo credit: Mayra Vidal, Ph.D.)

“Predators that mainly eat ants are rare, as ants tend to be very aggressive,” says Vidal. One other case is known of a mutualism exploiter preying on ants, though Vidal notes the ant and plant are more dependent on each other in that case (an obligate mutualism) than in the one they’ve found R. myrmecophaga using (a facultative mutualism). The ants “could just stop using the plants with R. myrmecophaga,” Vidal says.

And, in fact, they’ve found that to be the case. In a study looking at the effect of R. myrmecophaga presence on Q. grandiflora plants, published in Ecology in 2016, they observed the ants spending less time on fly-infested plants, which resulted in greater leaf damage to the plants, as they were less protected from herbivores.

“Who would have thought that a drosophilid fly larva could trap and prey on nectar-feeding ants?” Vidal says. “And more so, who would realize that ant predation by fly larvae would be relevant enough to cascade to other trophic levels to a point of affecting leaf damage to the plant?”

She says R. myrmecophaga‘s surprising behavior was only discovered because “paying attention to our surroundings and being curious pay off” and that she suspects the region may host other similar interactions yet to be found.

“Since plants bearing extrafloral nectaries are abundant in ant-rich cerrado, and ants constantly visit such plants for their secretions, it is possible that other specialized ant-eaters are still to be discovered,” says Vidal.

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