Floral parasites: mutualism or exploitation in pollination?

Floral rewards are essential in attracting pollinators for recurrent visits to flowers, promoting the likelihood of reproductive success in angiosperms. Pollen and nectar serve as the primary incentives in these interactions, but in rare instances, flowering plants provide brood sites for the offspring of their pollinators, thereby fostering interdependent relationships in which both partners rely on one another for reproduction. Historically, the fig (Ficus spp)–fig wasp (family Agaonidae) and yucca (Yucca spp)–yucca moth (Tegeticula and Parategeticula spp) interactions were identified as examples of such mutualisms. In these scenarios, female insect pollinators both pollinate the flowers and deposit eggs within them, resulting in a subset of the plant's seeds being sacrificed to nourish the pollinator larvae. This trade-off, however, can impose substantial costs to the plants, especially when the larvae act as seed parasites and thus undermine the mutualistic effect of pollination.

The relationship between the achlorophyllous plant Balanophora tobiracola and the snout moth Assara balanophorae observed on Yakushima Island, Kagoshima Prefecture, Japan, might represent another type of nursery pollination mutualism. Here, female moths lay eggs within the Balanophora inflorescences, and pollination is likely achieved passively through nectar-feeding and oviposition. As revealed by dissected mature inflorescences, moth larvae hatched after approximately two weeks and were documented feeding on the swollen, fleshy axes of inflorescences without consuming the many small seeds (indicated by white arrowheads) that constitute a thin surface layer on the inflorescence. Therefore, relative to their respective plant partners, snout moths may be less costly pollinators than fig wasps or yucca moths. Nevertheless, intense herbivory by snout moth larvae could restrict the nutrient transfer required for seed maturation, potentially resulting in an antagonistic relationship. Thus, while this case seemingly exhibits a cooperative relationship, further research is necessary to thoroughly comprehend its costs and benefits.