Hygroscopic larval provisions of bees absorb soil water vapor and release liquefied nutrients

Authors: CANE, JAMES - Retired ARS Employee; Love, Byron

Submitted to: Apidologie
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/21/2021
Publication Date: 10/1/2021
Citation: Cane, J.H., Love, B.G. 2021. Hygroscopic larval provisions of bees absorb soil water vapor and release liquefied nutrients. Apidologie. https://doi.org/10.1007/s13592-021-00883-5.
DOI: https://doi.org/10.1007/s13592-021-00883-5

Interpretive Summary: Many alfalfa seed growers manage populous aggregations of the alkali bee, Nomia melanderi, to pollinate their valuable seed crop. Growers invest in subirrigation systems to satisfy this native bee’s need for damp soils in which to nest, but why? We show that the mass provisions of alfalfa pollen and nectar that the mother bees assemble for each offspring are strongly hygroscopic. Although she waterproofs each nest cell’s walls, she leaves the nest cell plug uncoated, permitting air exchange with the saturated soil atmosphere. We experimentally show daily hygroscopic uptake of water by nest cell provisions. This added water explains our finding that mature larvae weigh 60% more than the provision that they ate because mature larvae contain 3x more bodily water than what they consumed as nectar. The accumulating free water around an uneaten provision is a nutritious broth of sugars and free amino acids. Because such free water was not seen around provisions bearing a feeding larva, we conclude that young bee larvae drink it away as it forms. Hygroscopic provisions are expected to contribute substantial dietary water to most ground-nesting bees, which is the predominant nesting habit of bees. Provisions of two cavity-nesting Osmia bees also proved to be hygroscopic at soil humidities, but not so in the deadwood tunnels where they nest. How larvae of such cavity-nesting bees acquire sufficient dietary water awaits investigation.

Technical Abstract: Larvae of most bee species feed individually on a mass provision of pollen and nectar cached by the nesting female. Nectar, like any sugar-water solution, is hygroscopic, absorbing water vapor at a rate defined by nectar osmolarity and relative humidity. In these experiments, we show that larval provisions of a representative ground-nesting bee, Nomia melanderi, are strongly hygroscopic, absorbing substantial water vapor from this bee’s preferred nesting soil. Liquid accumulating around the isolated provision is a nutritious broth of nectar sugars as well as amino acids leaching from the pollen. After consuming their provisions, larvae weighed 65% more than their original provision because hygroscopy greatly augmented dietary water available in the provision. Hygroscopy was most intense during the egg and early larval stages, but provision liquefaction (and possible drowning) is to some degree offset by rapid hydration of collected pollen, which was found to usually double its weight in absorbed free water. Larval provisions of two cavity-nesting Osmia species also readily absorbed water vapor from soil, but they gained little or no moisture at relative humidities measured in tunnels of their natural deadwood nesting substrates. Consequently, their mature larvae weighed less than the provision that they ate. These new insights explain some nesting traits of many ground-nesting bees, such as why females waterproof all but the earthen cell caps of their nest cells, and why many colletids cache liquid provisions for their larvae. Progressive hygroscopy and so sugar dilution likely mediates the succession of microbial mutualists and pathogens in provision masses of ground-nesting bees.