This is the main reason for feeding light syrup.
Water....
http://jeb.biologists.org/cgi/content/full/212/3/429
Honeybee colonies collect water for two reasons, related to different types of weather: for cooling of the brood area by evaporation on hot days, and for feeding the larval brood when foraging is limited on cool days (Lindauer, 1955Go; Seeley, 1995Go). The classic studies of Lindauer showed how bees regulate the hive temperature in hot conditions (Lindauer, 1955Go). Water is collected by water foragers, then distributed around the hive and in cells containing eggs and larvae; fanning accelerates its evaporation, as does regurgitation and evaporation on the tongue (Lindauer, 1955Go). Visscher and colleagues measured mean water loads of 44 mg in honeybees collecting water under desert conditions (Visscher et al., 1996Go). Paper wasps and hornets also use water for cooling their nests, but the highly social stingless bees do not (Jones and Oldroyd, 2007Go; Roubik, 2006Go).
The second need for water – for consumption by nurse bees when feeding the brood – is an aspect of water use by honeybees that tends to be underestimated (Johansson and Johansson, 1978Go). Nurse bees feed young larvae a secretion from their hypopharyngeal glands; for worker larvae after the third day this jelly is supplemented with honey and pollen (Crailsheim, 1998Go). As already mentioned, the water content of royal jelly is high, so nurse bees have a great need for water when brood rearing is intensive; this water cannot always be obtained from nectar.
The regulation of water collection in honeybees is discussed in detail by Seeley (Seeley, 1995Go). In essence, the rate of unloading of water foragers indicates the colony demand for water (i.e. the feedback system is similar to that for nectar). In this way the balance between collection and consumption of water is maintained. Importantly, water collection does not interfere with the collection of concentrated nectar by the colony (Kuhnholz and Seeley, 1997Go). The first bees to start water collection may be stimulated by the collective increase in crop sugar concentration of all bees in the nest, due to trophallaxis (Lindauer, 1955Go; Seeley, 1995Go), or possibly by the collective increase in haemolymph osmolality. Apart from environmental factors, the tendency of honeybee foragers to collect water, nectar or pollen has a genetic component (Hunt et al., 1995Go). Workers with the lowest sucrose response thresholds, i.e. those able to distinguish low sucrose concentrations from water in proboscis extension response tests, become water foragers (Pankiw and Page, 2000Go).
Water foraging is regulated according to current demand and water is not stored in combs by temperate honeybee colonies: this is because nectar availability fluctuates widely and water sources usually do not (Seeley, 1995Go
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If swarming pre your main flow is your concern is it not better to get it over and done with so you have fresh queen laying up vigorously than to have them swarm during the flow?
It's certainly how I want to manage affairs.
PH