Japanese red elder plants protect their own survival by dropping fruits that contain Heterhelus beetle larvae. Surprisingly, this action also allows the larvae to survive. A study from Kobe University suggests this unusual interaction reshapes how scientists understand the balance between plants and the insects that pollinate them.
In some plant insect relationships, the insect both pollinates the plant and uses the fruit as a place for its offspring to grow. Biologists refer to this type of partnership as “nursery pollination mutualism.” Kobe University botanist Kenji Suetsugu explains, “These interactions are fascinating because they sit on the boundary between cooperation and conflict.”
Classic examples include figs and fig wasps and yuccas and yucca moths. In these systems, plants often control insect populations by dropping fruits that contain too many larvae. Because the larvae typically die when the fruit falls, scientists have long viewed this process as a punishment that keeps the relationship balanced.
Suetsugu began to question whether this explanation applied to Japanese red elder plants. “I once observed Japanese red elder flowers full of Heterhelus beetles mating and feeding, and I also saw fruits infested by the beetles’ larvae dropping in large numbers. With such seemingly great losses to both sides, I wondered whether this was really punishment and how the insects keep their losses contained,” says Suetsugu, voicing suspicion that there is something missing in the current narrative of the sanction-driven balance in nursery pollination mutualisms.
Investigating the Plant Beetle Relationship
To explore this puzzle, Suetsugu and his colleagues focused on two key questions. First, are Heterhelus beetles essential pollinators for the Japanese red elder Sambucus sieboldiana? Second, what mechanism allows this relationship to remain beneficial for both species?
Suzu Kawashima, a master’s student in Suetsugu’s laboratory, describes the complex approach required to answer these questions. “To tackle this issue, one requires an unusual combination of careful field observation of pollination events, exclusion and hand pollination experiments, as well as developmental tracking of the insects even after the fruit drop. Many studies stop at one of these steps, simply because doing all of them takes time, patience and logistical commitment.”
Fruit Drop That Protects Both Plant and Larvae
The research team reports its findings in the journal Plants, People, Planet. Their experiments revealed that the Japanese red elder depends on Heterhelus beetles for pollination. At the same time, the plant aborts nearly all fruits that contain larvae, which helps limit the plant’s resource investment.
However, the larvae do not die after the fruit falls. Instead, they leave the dropped fruit and burrow into the soil, where they continue developing until maturity.
“What our finding shows is a different route to a stable balance, where fruit abortion can function as a compromise that both sides can tolerate. This finding shifts the narrative from dropping fruit as punishment to it being a shared benefit — without denying the underlying conflict that defines nursery pollination mutualisms in the first place,” says Kawashima, who was the first author of the study.
Environmental Factors Shape the Balance
The researchers also calculated the costs and benefits of the relationship between the plant and the beetles. Their analysis showed that this balance varies across locations, suggesting environmental conditions influence how the interaction works.
Kawashima explains: “While all Heterhelus beetle species depend on elder plants for reproduction, the same is not true in reverse, and there is considerable variation in pollinator dependence across elder plant species. In future work, mapping where Heterhelus dominates versus where alternative pollinators are more important should clarify the ecological drivers behind when the ‘fallen-fruit compromise’ is favored and when it is not.”
Rethinking Cooperation in Nature
For Suetsugu, the findings highlight how cooperation in nature can arise from processes that initially appear wasteful or unsuccessful.
“On a personal level, this study makes me feel that we are only beginning to appreciate how much cooperation in nature is maintained by mechanisms that look, at first glance, like failure. A fallen fruit looks like a loss. Realizing that it can instead be the very structure that keeps a mutualism stable is exactly the kind of insight that makes me want to keep following these interactions year after year.”
The research was funded by the Japan Science and Technology Agency (grant JPMJPR21D6) and carried out in collaboration with a researcher from the University of Human Environments.