A double defensive mutualism? A case between plants, extrafloral nectaries, and trophobionts

Тhis is the first case reported in the literature of a double defensive mutualism occurring simultaneously on a single plant species.

Guest blog post by Cássio Pereira

The Zeyheria montana shrub is quite common in the Brazilian Cerrado and is known to have extrafloral nectaries on the leaf blade that attract patrolling ants such as the aggressive Ectatomma tuberculatum. The ant, in turn, defends the leaves against the action of herbivores. However, extrafloral nectaries can distract ants on the leaves, segregating them from the reproductive parts and preventing them from driving away pollinators, which can benefit the action of florivores and nectar robbers.

Surprisingly, in southeastern Brazil, we observed a second defensive mutualism occurring on the reproductive tissues of these shrubs between E. tuberculatum and the treehopper Guayaquila xiphias, which provides the ant with honeydew in exchange for protection. This trophobiosis relationship (interaction between ants and phytophagous hemipterans that secrete sugary exudates) seems to be effective not only in the defense of floral buds and flowers, but also of the fruit, which, despite being dry, contains a lot of water in its formation and is attacked by beetles of the Curculionidae family.

The treehoppers G. xiphias at the base of Z. montana fruits.

As far as we know, this is the first case reported in the literature of a double defensive mutualism occurring simultaneously on a single plant species. Given this record, important questions arise regarding these interactions. Is the trophobiosis that occurs in reproductive organs capable of increasing the fitness of these plants? Although these ants are probably also scaring away possible pollinating insects, could the fact that Z. montana is primarily pollinated by hummingbirds offset this loss given that hummingbirds are larger and perhaps immune to ant attacks?

Our record raises more questions than it answers. Long-term Z. montana population studies would help improve our ecological understanding of these interactions.

Accidental tree wound reveals novel symbiotic behavior

Despite significant movement restrictions during the first wave of the pandemic in Panama City, a group of curious high school students roamed their neighborhood drilling holes into Cecropia trees and documenting how Azteca alfari ants responded to damage to their host plant.

During the pandemic, five curious high school students accidentally discovered how Azteca alfari ants respond to damage to their Cecropia host trees. Photo by Donna Conlon

One afternoon, during the early days of the COVID-19 pandemic in Panama, a bored teenager with a slingshot and a clay ball accidentally shot entry and exit holes in a Cecropia tree trunk. These are “ant-plant” trees, which famously cooperate with fierce Azteca ants; the trees provide shelter and food to the ants, and in exchange the ants defend their leaves against herbivores. The next morning, to his surprise, the Azteca alfari ants living within the Cecropia trunk had patched up the wound.

This unexpected occurrence drove five curious high school students, with time on their hands, to participate in the Smithsonian Tropical Research Institute’s (STRI) volunteer program, and they enlisted STRI scientist William T. Wcislo’s help in devising their experiment. Despite significant movement restrictions during the first wave of the pandemic, they roamed their neighborhood drilling holes into Cecropia trees and documenting the ants’ responses to the damage.

They found that as soon as the plants had holes drilled into them, the ants ran to the wound area and began patching it up. Within 2.5 hours, the size of the hole had been significantly reduced and it was often completely repaired within 24 hours.

Although some Azteca ants are known to defend their Cecropia host plants against herbivores, these new results, published in the Journal of Hymenoptera Research, reveal that not only do the ants behave in ways to minimize damage to their hosts, but when damage does occur, they actively work to fix it, particularly when their brood is directly threatened.

“I was totally surprised by the results,” says William Wcislo. “And I was impressed by how they developed a simple way to test the idea that ants repair damage to their home.” 

Azteca ant and Cecropia plant responses to wounds in the stems a a sealed hole after 24 hours, but not yet filled in to the stem surface (arrow) b a fully patched and filled-in hole after 24 hrs, oozing sap from the ant-sealed wound (arrow) c a natural plant scar surrounding a 6.4 mm hole that was fully sealed by the ants, approximately 5 months later d a hole in a plant without ants after 24 hrs, showing the green wall of the opposite side of the stem (arrow).

Sloths and silky anteaters often visit Cecropia trees and their sharp toenails sometimes pierce the wood, so the researchers speculate that these occurrences, which are far more common and ancient threats to the Cecropia than teenagers shooting clay balls at them, could have led Azteca alfari ants to evolve the observed repair behavior when their host plant is damaged. 

Their experiment also left them with new questions, since not all of the ant colonies repaired the damage to their host plants. Understanding what factors influence the ants to take action could be the subject of future research for these budding scientists, although perhaps to be addressed after graduating from high school.

“Sometimes messing around with a slingshot has a good outcome,” said lead author Alex Wcislo. “This project allowed us to experience first-hand all the intricacies behind a scientific study. All in all, it was a great learning experience, especially considering the difficulties associated with fulfilling this due to COVID-19.”

Research article:

Wcislo A, Graham X, Stevens S, Toppe JE, Wcislo L, Wcislo WT (2021) Azteca ants repair damage to their Cecropia host plants. Journal of Hymenoptera Research 88: 61-70. https://doi.org/10.3897/jhr.88.75855

Guardian ants: How far does the protection of a plant-ant species to its specific host go?

Seemingly helpless against their much more lively natural enemies, plants have actually come up with a wide range of defences. In the present research, published in the open-access Journal of Hymenoptera Research, Dr. Adriana Sanchez, Universidad del Rosario, Colombia, and Edwin Bellota, Texas A&M University, USA, focus on the mutualistic relationship developed between a specific Neotropical knotweed and an ant species. During a series of ant-exclusion experiments the scientists observed and subsequently reported an aggressive and highly protective behaviour.

In order to assess the extent of protection these plant-ants provide their exclusive host with, the researchers compared the percentage of herbivory between control plants and experimental ones, which had their resident ants removed. The unambiguous results showed a 15-fold increase in the herbivory in the latter group, which kept on growing even further as the time progressed.

Normally, the studied ants patrol their hosts during both day and night at temperatures sometimes as low as 13C. Every time they found a herbivore, they were seen to attack it aggressively by biting and stinging.

“When an ant encountered a caterpillar, a worker approached and detected it with its antennae, and then recruited more workers. Typically more than 10 workers were recruited around the intruder in less than five minutes,” shared their observations the researchers. “Several workers harassed the herbivore by stinging or biting, until it dropped off the plant. The caterpillars usually hung by a silk thread and attempted to move back onto the plant. However, individuals of Pseudomyrmex continued to chase them until they dropped again. This cycle was repeated several times.”

While patrolling, they were noticed to remove any found debris from the top of the leaves. When they failed to find any signs of mosses, fungi or lichens on the sampled saplings, the scientists suggested that the ants not only protect their host from herbivores, but also from various disease-causing agents.

Plant vitality, growth and reproduction are seriously threatened by herbivores such as, in the case of the hereby studied knotweed, Triplaris americana, caterpillars and grasshoppers. Fighting for their life, plants use structural defenses, toxins, digestibility-reducing compounds, or mutualistic relationship with the enemies of their herbivores.

The herein researched Neotropical plant have found its way of survival through becoming the only host to the ant species Pseudomyrmex dendroicus, characterised with remarkable eyes, light brown body and potent venom, injected through a well-developed sting. In its turn, the knotweed shelters their entire colony in its hollow stems while another symbiont, scale insects, feeds them with the sugary sticky liquid it secrets on digesting plant sap.


Original source:

Sanchez A, Bellota E (2015) Protection against herbivory in the mutualism betweenPseudomyrmex dendroicus (Formicidae) and Triplaris americana(Polygonaceae). Journal of Hymenoptera Research 46: 71-83. doi: 10.3897/JHR.46.5518.