The chance discovery by the Professor’s son reveals a surprisingly complex interaction between plants and insects.
Two years ago, 8-year-old Hugo Deans, the son of Penn State entomology professor Andrew Deans, found a handful of round wooden beads near an anthill in his backyard. “He thought they were seeds,” and he went to show them to his father, surprised to discover that the ants would collect the seeds and eat them. Instead, Professor Hugo told, they were oak – and in time, this would lead to the discovery of a unique interaction between insects and plants.
Although many plant-insect interactions are well documented at this point, there seems to be more to understand – and our knowledge is not all correct. The present paper describes a type of Mermicuri, the dispersal of seeds by ants, in which wasps, oaks, and ants form a multi-layered and complex system that all work to benefit Wasp larvae.
“In Myrmecochory, the ants get very little nutrition when they eat the elliosomes, and they scatter the seeds of the plants in a place free of enemies,” explains Professor Deans. “This phenomenon was first documented over 100 years ago, and is usually taught to biology students as an example of plant-insect interaction.”
Cynipid wasp breeding involves persuading oak trees to plant tannins (a woody round growth) around their larvae as a way to protect their offspring as they grow. The paper also explains that some plants such as bloodroot, a wildflower native to North America, produce edible supplements on their seeds to attract ants. These growths are known as elliosomes, and their role is to lure the ants to carry the seeds to their nests, helping the plant to disperse to future generations.
Although these behaviors appear to be unrelated on the surface, the paper suggests that they are, in fact, deeply related.
“First, we noted that while these balls typically had a fleshy, pale pink ‘cap’, the tannins near the anthill did not have these caps, indicating that they may have been eaten by ants,” says Professor Deans. “Ultimately, this led us to discover that wasp wasps manipulate acorns to produce mites, and then take another step and manipulate the ants to restore the mites to their nests, where wasp larvae may be protected from predators or receiving other benefits. This multi-layered interaction is astonishing. It’s hard to wrap your mind around it.”
For the study, the team conducted a series of field and laboratory experiments. Their first step was to determine if the oak’s gall caps, which they called kapéllos (Greek word meaning “cap”), were edible and attractive to ants such as elaiosomes. For this, they observed the interaction of ants with oak beams in western New York and central Pennsylvania both directly and through video recordings. Both showed ants transporting mites to their nests, with food coverings removed while the mites themselves remained intact.
Second, they wanted to determine if kapéllos function similarly to elaiosomes. In this step, they studied whether ants preferred acorn acorns to blood-rooted seeds by placing plants containing both nearby. ant colonies. The insects removed the same numbers from each, indicating that they had no preference one way or the other.
But why did the ants collect these capellas? The team assumed they had evolved to be highly nutritious to attract insects, so the team prepared three petri dishes for ants to choose from. One contained whole carcasses, the other contained gallbladder bodies removed from the kapéllos, and the third contained only the kapéllos. When introducing the ants to these Petri dishes, the team found that the insects were equally interested in the control groups and those without kapéllos, but that they showed more interest in ants with intact kapéllos and kapéllos alone than in the controls.
In order to determine why these capelos are so attractive to ants, in particular, the team compared their chemical composition to that of iliosomes. They report that both contain healthy fatty acids in high amounts, which they say mimic the chemical contents of dead insects.
“We showed that sprites with caps were more attractive to ants than those without caps, and that caps in and of themselves were also attractive to ants,” says John Tucker, professor of entomology at Penn State and co-author of the research paper. “This suggests that hats must have evolved as a way to lure ants.”
“Ants are scavengers trying to find and grab anything suitable to bring back to their colony, so it is no coincidence that both gall caps and lysosomes contain fatty acids typical of dead insects.”
The results shed light on a particularly complex system in which wasps interact with oak trees, which in turn interact with ants, to carry the oak balls back to the ant colonies. Since ants only consume kapéllos, wasp larvae in tannins remain undisturbed and have the added protection of being underground and being defended by unaware ants.
It is not yet clear how exactly this system emerged, and what part developed first. But the team believes that it was this interaction between acorns and ants that gave rise to phytoplankton like blood roots. These species make up only a small part of the plant kingdom, while oak beams are abundant – in fact, they were once used as animal feed.
“If these orcs were so abundant and developed this tactic to grow this cover thousands of years ago, it could be a powerful driver of natural selection in ants,” says Professor Warren. “The ants could have long ago been accustomed to picking up tannins with lids, and then when the spring wildflowers started producing seeds that happened to have an edible supplement, the ants were already ready to pick up the stuff with a fatty acid supplement.”
The paper was published with the title “Oak Oak View Convergent Ant Dispersal with Myrmecochorous Seeds” published in the magazine American naturalist.