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The role of the phoretic stage is unclear but must have some physiological role for the mites, since the mites cannot increase their population and they experience higher mortality during this stage by falling from the hosts or being groomed off by other workers.
The Varroa mite life cycle consists of two distinct phases: a phoretic phase during which the females stay on adult bees and feed on hemolymph (blood), and a reproductive phase taking place inside of capped bee brood cells. During the phoretic stage, the mites frequently switch among adult bees. The role of the phoretic stage is unclear but must have some physiological role for the mites, since the mites cannot increase their population and they experience higher mortality during this stage by falling from the hosts or being groomed off by other workers. Mites, however without a phoretic stage could reproduce up to eight cycles and the average number of offspring for the first five cycles would be four (De Ruijter 1987), both parameters are much higher than occurs naturally. However, no study has compared fitness of mites with phoretic stage to those without, to determine the contribution of nutrition, if any, obtained during the phoretic stage.
During the phoretic phase the mites clearly prefer nurse bees over foragers (Kraus 1993; Kuenen and Calderone 1997; Del Piccolo et al. 2010). However, all of these studies were conducted inside laboratories using either live or freshly frozen bees. For field studies, one study showed mite preference for .six and 12 day old bees (Kraus et al. 1986) but it appeared that they used only one colony. After six hours of the test runs, about 60% of the tested mites were situated on nurses, 35% on pollen collectors and only 5% on young bees which had just been uncapped. Tests using dead bees killed by freezing resulted in 55% of the mites being found on nurses, 35% on pollen collectors and 10% on young bees. The relative infestation of bees of different ages within the hive resulted in a two to three fold higher infestation of young bees compared to older bees and about a 20 fold higher infestation compared to pollen collectors. Differences were also found in the positions of the mites...