Flower mites and nectar production in six hummingbird-pollinated plants with contrasting flower longevities

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Abstract
Hummingbird flower mites and hummingbirds may compete intensely for the nectar secreted by their host plants. Here, we present the results from field experiments in which flower mites were excluded from flowers of six hummingbird-pollinated plants with contrasting flower longevities. Nectar measurements were taken on flowers from which mites were excluded and those without mite exclusion over their lifespans. The exclusion of mites had a significant positive effect on the amount of nectar available in plants with long-lived flowers. In contrast, nectar availability in short-lived flowers was not significantly reduced after mite exclusion. The significance of the mite-exclusion treatment was independent of floral morph and flower age. Results also suggest that the magnitude of the mite-exclusion treatment depends on the volume of nectar produced by the flower throughout its lifetime. The treatment effect was detected when nectar consumption, presumably by flower mites, exceeded 13% of the nectar produced by the flowers; nectar availability was not significantly reduced when nectar volume was < 7 µL per flower. It appears that flower mites consume proportionately more nectar in long-lived flowers than in short-lived flowers. Parasitic hummingbird flower mites seem to be preferentially taking advantage of plant-pollinator interactions in which flowers last several days and produce large volumes of nectar. The consequences of this finding concerning plant–hummingbird–mite interactions await further investigation. As a working hypothesis, we propose that nectar production has increased over evolutionary time not only by the selective pressures imposed by the pollinators, but also to compensate for the reduction they suffer after exploitation by nectar robbers and thieves such as flower mites.Key words: Ascidae, flower longevity, hummingbird pollination, multiple-species interactions, mutualism exploitation, nectar theft.