Impacts of brood parasites, floral abundance, and bee age on maternal investment in a solitary bee, <i> Osmia iridis </i>

Abstract

Parents can adjust both the size and number of offspring they produce in order to maximize their own fitness payoff (Smith & Fretwell 1974). Optimal parental investment is dependent in part on likelihood of offspring survival, which is impacted by predation and parasitism. When risk of predation or parasitism is high, it may be adaptive for parents to reduce either the number of offspring they produce or their investment in individual offspring. Brood parasitism by sapygid wasps represents a significant fitness cost for Osmia bees. Sapygid wasps enter brood cells during the provisioning or brood cell construction stages, when mother bees are absent collecting pollen and nectar provisions for their larvae or materials for constructing brood cell walls. In order to avoid losing their investment in offspring, Osmia bees should reduce provisioning time when parasitism risk is high, providing pollen provisions smaller than what would be optimal for offspring development. Alternatively, instead of investing less per offspring, bees may produce fewer offspring overall when they detect parasites. In this study, we tested these predictions in a naturally occurring population of Osmia iridis bees at the Rocky Mountain Biological Laboratory in Gothic, Colorado, measuring the response of O. iridis to both spatial and temporal variation in sapygid wasp abundance. In addition, we measured other factors which might impact provisioning efficiency, including floral abundance and bee age, and uniquely marked individuals to control for bee identity. Floral abundance was found to be most predictive of pollen provision size, while Sapyga abundance was most predictive of the number of brood cells per nest a bee constructed. These results suggest that bees respond to parasite presence by reducing the number of offspring they produce per nest rather than investing less per offspring. Further research is needed to confirm these findings and to investigate the impacts this investment strategy may have on both lifetime reproductive success and nesting site selection.

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