Abstract
Larvae of most bee species consume individual provision masses composed of pollen mixed with nectar. For simple metabolic reasons, mature larvae should weigh less than their consumed provision. However, past research reported a remarkable result: mature larvae of three ground-nesting halictid bees weighed 60% more than their original provision masses. This surprising paradox could result from the expected hygroscopic nature of nectar. Sugar solutions absorb water vapor at rates defined by their osmolarity and ambient humidity. Our experiments tested this hypothesis, showing that larval provisions of a ground-nesting bee, Nomia melanderi, are strongly hygroscopic. They consequently absorbed substantial water vapor from this bee’s preferred nesting soil. Mature larvae weighed 65% more than their original provision because hygroscopy had greatly augmented available dietary water. Liquid accumulating around isolated provisions was a sweet nutritious broth that included amino acids leached from the pollen. Hygroscopy was most intense during the egg and early larval stages. However, provision liquefaction (and possible drowning) was partly offset by rapid hydration of cached pollen, whose weight could double after absorbing free water. Larval provisions of two cavity-nesting Osmia species also readily absorbed water vapor from a soil atmosphere. However, at humidities measured within tunnels of their natural deadwood nesting substrates, they gained little weight via hygroscopy. Consequently, their mature larvae weighed less, not more, than the provision that they ate. These new insights explain some nesting traits shared by many ground-nesting bees, such as why females do not waterproof the earthen cell caps of their nest cells, or why many colletids cache liquid provisions. Progressive hygroscopy and resulting sugar dilution may also mediate succession of microbial mutualists and pathogens in provision masses of ground-nesting bees.
Highlights
The diets of solitary bee larvae generally consist of a mass of pollen mixed with nectar gathered and cached for each offspring by their mother
Water vapor uptake by fresh provision masses of the wood-nesting bees O. lignaria and O. sanrafaelae were measured for two contexts, that of N. melanderi nesting soil (Sect. 2.2) and over a saturated NaCl solution whose humidity (75%) approximates what we measured for sealed tunnels in suitable deadwood (Sect. 3.4)
Provisions of the ground-nesting bee N. melanderi were strongly hygroscopic when exposed to the atmosphere of their nesting soils (Figures 1, 3)
Summary
Humidity of the nesting substrate is significant because the sugary mass provisions of bees. Decades after Malyshev, studies of three different ground-nesting halictid bee species produced unexpected evidence consistent with a hypothesis that some of the bodily water in a mature bee larva must come from nectar but from the surrounding soil as well (Knerer 1969; Batra and Bohart 1970; May 1972). To characterize the nutritional value of any accumulating “broth” for the developing larva, free water from around some hydrating provisions was removed and analyzed for both sugar and amino acid content arising from the nectar and leached pollen, respectively Unlike these ground-nesting halictids, mature larvae of a cavity nesting bee (Osmia cornuta) weighed less, not more, than their initial provision (Bosch and Vicens 2002). Hygroscopic larval provisions of bees absorb soil water vapor and release liquefied nutrients
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