Nectar 'theft' by hummingbird flower mites and its consequences for seed set in Moussonia deppeana

1, Mites (Acari: Mesostigmata: Ascidae) that live and feed in the flowers o f about 100 plant species are transported in the nares of hummingbirds (Tro chilidae). Mites may compete with hummingbirds for nectar secreted by the h ost plants, and this could affect the dynamics and reproductive outcomes of the mutualism between plants and their pollinating hummingbirds. 2, Here we combined field observations and experimental manipulations to as sess the role of hummingbird flower mites (Tropicoseius sp. nov.) on nectar secretion and reproductive output of protandrous Moussonia deppeana (Schle cht, & Cham,) Hanst, (Gesneriaceae) during their flowering period in a clou d forest remnant. 3. During the 4 days that the flowers of M. deppeana last, flowers were vis ited exclusively by hummingbirds (Lampornis amethystinus). Bud production p er inflorescence peaked in December. There were few open flowers per inflor escence in November, but numbers increased as the flowering season progress ed (December and January). 4, The availability of each flower phase differed over the flowering season . Staminate-phase flowers were more abundant over the flowering season than pistillate-phase flowers. These differences were statistically significant over time. 5, Nectar availability was reduced by up to 50% in the presence of hummingb ird flower mites. Over the 4 days of observation, significantly more nectar was secreted to flowers from which mites were excluded than to flowers wit h no mite exclusion. The same effect was observed during flowering, but mit es consumed a greater percentage of the total nectar secreted in December. 6, Significantly more nectar was secreted during the staminate phase than i n the pistillate phase, independent of time and treatment. 7, A manual pollination experiment suggested that mites act like secondary pollinators in this self-compatible, non-autogamous plant, at least in flow ers that were not pollinated manually and had no access to pollinating humm ingbirds. 8, Although seed production was not reduced significantly by flower mites, our results suggest that the presence of floral mites can affect pollen tra nsmission, as the amount of nectar available to hummingbirds was reduced dr astically. This can directly affect hummingbird foraging patterns and reduc e the fitness of the host plants.