Leaf herbivory causes abortion of selfed seeds in <i>Rhododendron kaempferi</i> var. <i>kaempferi</i>

Abstract Plant reproduction is often inhibited by herbivorous leaf damage, which affects the resource pool allocated for reproduction. Rhododendron kaempferi var. kaempferi is a semi‐evergreen shrub widely distributed in Japan, and its leaves are occasionally damaged by sawfly larvae ( Arge similis ). This species is partially self‐compatible, but its selfed seedlings suffer from inbreeding depression. Our study aimed to clarify (1) the magnitude of herbivorous damage under natural conditions, (2) the effects of foliage damage on seed number and mass, and (3) the possibility of selective abortion of selfed seeds to maintain outcrossed seed production under resource limitation. In the 2022 survey, 2 of the 20 ramets observed were collectively parasitized by sawfly larvae, and the most damaged ramet had a 59.6% foliage loss. In contrast, the herbivorous damage in 2023 was consistently low throughout the growing season, averaging a 3.2% loss. Leaf excision reduced the seed number per fruit (i.e., low seed‐set rate) but did not affect the individual seed mass. The outcrossing rate of the excision treatment (81.7%) was significantly higher than that of the control (72.0%). The values of seed‐set rate × outcrossing rate were 0.172 for the excision treatment and 0.171 for the control, nearly equal. Our results indicated that (1) sawfly larvae damage was concentrated on a few specific plants, (2) intense foliage damage reduced seed number but not seed mass, and (3) R. kaempferi mitigated the decline in outcrossed seed production by the abortion of selfed seeds when leaves were intensively damaged.