The shared benefits of fallen fruits: A novel mechanism stabilizing a nursery pollination mutualism between Sambucus and kateretid beetles

Societal Impact Statement Understanding how cooperative interactions remain stable matters for biodiversity because many plants rely on specialist insects that can also impose reproductive costs. We studied the interaction between Sambucus sieboldiana and seed‐consuming Heterhelus beetles through detailed field observations and pollination experiments. We found that the plant depends largely on Heterhelus for pollination and aborts most infested fruits before they ripen, which likely redirects plant resources to healthier fruits, while the fallen fruits still shelter larvae until they complete development. This finely balanced compromise helps explain how nursery pollination mutualisms persist and can inform practice, including guidance on the timing of fruit and litter removal in gardens, parks, and restoration sites to avoid disrupting the interaction. Summary Pollinating seed‐consuming mutualisms (nursery pollination mutualisms) can shift toward antagonism because seed consumption by pollinators reduces plant fitness. Accordingly, mutualisms that persist under such conditions provide insight into the mechanisms that promote and maintain cooperation. We examined the interaction between Sambucus sieboldiana and its seed‐consuming Heterhelus beetles through field observations, exclusion experiments, and controlled pollination. We also compared larval entry into ovaries containing unfertilized vs. fertilized ovules, reared larvae from developing fruits, and tested whether infestation increased fruit abortion and affected larval development and seed germinability. Sambucus sieboldiana is self‐incompatible, and seed production relies largely on Heterhelus beetles for pollination. Larvae preferentially enter ovaries containing fertilized ovules and consume developing seeds. Nearly all infested fruits abort, yet larvae complete development in the aborted fruits. Seed weight was already lower in infested than in uninfested fruits at the time of abortion, even when larvae did not consume the seeds. Despite the substantially reduced germinability of seeds in infested fruits, fruit abortion likely functions as a cost‐mitigation strategy that limits maternal investment, because S. sieboldiana experiences both pollinator limitation and resource limitation. These findings suggest that reciprocal benefits are maintained through multiple mechanisms, including larval avoidance of ovaries with unfertilized ovules, fruit abortion that reduces maternal resource investment, and larval development within aborted fruits. Fruit abortion in this system differs from that in other pollinating seed‐consuming mutualisms, where abortion causes larval mortality. Nonetheless, the shared benefits of fruit abortion likely contribute to stabilizing the mutualism.