Supplementary material from "Insect-driven stabilisation of a temperate Glochidion–Epicephala nursery pollination mutualism"

Obligate nursery pollination mutualisms are conflict-prone because pollinators reproduce by consuming host seeds. In classic systems such as figs/fig wasps and yucca/yucca moth, stability is attributed to host-mediated control (selective abortion, sanctions), but how stability is maintained without such traits is unclear. Here, we address this question in a temperate association between Glochidion pseudo-obscurum and Epicephala obovatella, combining multi-year field data with fitness trade-off modelling. Epicephala obovatella is the sole pollinator and obligate seed predator of G. pseudo-obscurum. Pollen deposition increases linearly with oviposition events, indicating tight behavioural coupling. Despite multiple oviposition events, 58% of mature fruits contain a single larva, indicating strong density-dependent larval mortality. We show that density-dependent larval competition limits surviving larvae to ~one per fruit, and that single-larva outcome minimises the fitness cost to the plant while still ensuring moth reproduction. Model comparison found no support for host sanctions: fruit success did not decline with oviposition intensity, and larval survival followed a saturating function consistent with strong density dependence. Comparative analysis across Glochidion-Epicephala interactions suggests that density-dependent regulation may operate more broadly. Our results suggest an insect-mediated stabilisation mechanism via conservative oviposition and density-dependent larval competition, showing that obligate mutualisms can persist without host control.