Varroa and Tropilaelaps feed primarily on honey bee hemolymph when they reproduce
Until 2019, everyone knew varroa was a vampire that fed on honey bee hemolymph. But then Sammy Ramsey published his paradigm-altering study (Ramsey et al., 2019; highlighted in the April 2019 Notes from the Lab: 159(4):443-445), showing varroa was actually more like a werewolf since it fed primarily on the fat bodies of adult honey bees.
But this paradigm shift didn’t sit quite right with some researchers. Ramsey’s study was excellent, but some people felt the full feeding situation across all mite and bee life stages might be a bit more complex. Reproduction (i.e., making new mites) takes a lot of protein, which is present in hemolymph, while dispersal (i.e., finding new hosts) takes a lot of energy, which is present in fat. Because of this, a smart mite would probably feed on hemolymph and fat bodies, depending on whether it’s trying to reproduce or disperse.
So, is this the case? Does varroa feed primarily on fat bodies when it’s trying to disperse, but hemolymph when it’s trying to reproduce? And what about that other parasitic mite that still hasn’t arrived in the USA but every beekeeper is concerned about? What does Tropilaelaps mercedesae primarily feed on? These are the topics for the seventy-fourth Notes from the Lab, where I summarize “Life-history stage determines the diet of ectoparasitic mites on their honey bee hosts,” written by Bin Han and colleagues and published in the journal Nature Communications [2024].
For their study, Han and colleagues expanded on Sammy Ramsey’s groundbreaking 2019 study. To get us up to speed, let’s briefly review what Ramsey and colleagues found. The Ramsey study observed varroa feeding on adult bees, finding multiple lines of evidence that varroa feeds on fat bodies during this dispersal stage. In addition, they conducted an elegant bioassay with reproductive-stage mites, finding that they survived longer and were more likely to produce offspring when fed a greater proportion of honey bee fat body compared to hemolymph.
Han and colleagues have built on Ramsey and colleagues’ study in several ways. First, they observed varroa feeding on honey bee pupae (i.e., the reproductive stage of varroa) by introducing mites to 1,285 worker pupae and recording the specific area where feeding occurred. They used the slick fluorescent labeling methods pioneered by Ramsey and colleagues to differentiate whether varroa fed on hemolymph, which absorbs green Uranine stain, or the fat body, which absorbs Nile red stain. Both stains were fed to bees during their development, transferred into mites via feeding, and monitored via fluorescent imaging.
Next, Han and colleagues conducted a detailed proteomic and metabolomic analysis of reproductive- vs. dispersal-stage varroa. In other words, they extracted proteins and metabolites from mites that were either in the dispersal stage (mites obtained from adult worker bees) or reproductive stage (mites obtained from pupae), separated all the individual proteins and metabolites via chromatography, and determined their identity via mass spectrometry. The specific types of proteins and metabolites found in dispersal-stage mites were compared to those found in reproductive-stage mites.
Finally, the authors observed another parasitic mite, Tropilaelaps, feeding on honey bee pupae and conducted fluorescent staining experiments identical to those conducted with varroa.
So, what did they find? Does varroa feed primarily on hemolymph when it parasitizes honey bee pupae? Yes. As can be seen… (end of excerpt)
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