Pedantically not phoresy

The life cycle of the ectoparasitic mite Varroa destructor essentially consists of two stages. The first is within the capped cell, where reproduction takes place. The second occurs outside the capped cell when the recently-mated female progeny mites matures while riding around the colony attached to a nurse bee.

Almost without exception this second stage is termed the phoretic phase.

It isn’t.

Phoresy

Phoretic is an adjective of the word phoresy. Phoresy is derived from the French phorésie which, in turn, has its etymological origins in the Ancient Greek word φορησις.

And φορησις means being carried.

Which partly explains why the correct definition of the word phoresy is:

An association between two organisms in which one is carried on the body of the other, without being a parasite [OED]

Phoresy has been in use for about a century, with the word phoretic first being recorded in the Annals of the Entomological Society of America (25:79) in 1932:

It is possible, as suggested by Banks (1915), that such young mites are phoretic, being carried about from place to place on the host’s surfaces.

And, no, they weren’t discussing Varroa.

“Without being a parasite”

These are the critical words in the dictionary definition of phoresy which makes the use of the word phoretic incorrect when referring to mites on nurse bees.

Because mites on nurse bees are feeding – or at least a significant proportion 1 of them are.

They are therefore being parasitic and so shouldn’t be described as phoretic.

Om, nom, nom 2

Last week I discussed the recent Samual Ramsey paper presenting studies supporting the feasting of Varroa on the fat body of bees.

In the study they harvested bees from a heavily mite-infested hive and recorded the location on the bee to which the mite was attached.

The majority were attached to the left underside of the abdomen. More specifically, the mite was wedged underneath the third abdominal tergite 3.

What were they doing there? Hiding?

Yes … but let’s have a closer look.

Ramsey and colleagues removed some of the mites and used a scanning electron microscope to examine the attachment point on the bee. Underneath the tergite there is a soft membrane. The imprint of the body of the mite was clearly visible on the membrane.

Varroa feeding location on adult bee

Scanning EM of Varroa feeding location on adult bee

The footpads of the mite were left attached to the membrane (left image, white arrows), straddling an obvious wound where the mouthparts had pierced the membrane (black arrow). Between them, the inverted W shape is presumably the imprint of the lower carapace of the mite.

The close-up image on the right even shows grooves at the wound site consistent with the mouthparts of the mite.

These mites were feeding.

Extraoral digestion

Varroa belongs to the order (a level of classification) Mesostigmata. Most mesostigmatids feed using a process termed extraoral digestion.

Extraoral digestion has also been termed ‘solid-to-liquid’ feeding. It involves the injection of potent hydrolytic enzymes which digest solid tissue, converting it to a semi-solid that can be easily ingested. It can reduce the time needed to feed and it increases the nutrient density of the consumed food.

If Varroa fed on haemolymph it wouldn’t need to use extraoral digestion. Instead it would need all sorts of adaptations to a high volume, low nutrient diet. Varroa doesn’t have these. It has a simple tube-like gut parts of which lack enzymatic activity … implying that digestion occurs elsewhere.

A picture is worth a thousand words

Do the images of feeding mites support the use of extraoral digestion?

EM cross-section of Varroa feeding

EM cross-section of Varroa feeding

The image above 4 shows the cross-section of a Varroa (V), wedged under the tergite (Te), feeding through a hole (arrow in the enlargement on the right) in the membrane (M). The fat body (FB) is immediately underneath the membrane. The scale bar is incorrectly labelled 5.

A close-up of the wound site shows further evidence for extraoral digestion.

Feeding wound at higher magnification

Feeding wound at higher magnification

Beneath the wound site (C, arrow) are remnants of fat body cells (white arrow) and bacteria (black arrow; of two types, shown in D). A closer look still at the remnants of the fat body (E and F) shows cell nuclear debris (blue arrows) and lipid droplets (red arrows).

These images are entirely consistent with extraoral digestion of fat body tissue by feeding Varroa. The presence of bacteria near the wound suggests that bacterial infection may result from Varroa feeding, possibly further contributing to disease in bees.

So, pedantically it’s not phoresy

So-called phoretic mites, unless they’re on the thorax or head of the bee, are not really phoretic. They are being carried about, but they are also likely feeding. By definition that excludes them from being phoretic.

Instead they are ectoparasites of adult bees.

What are the chances that beekeepers will stop using the term phoretic?

Slim to none I’d predict 6.

And, of course, it doesn’t really matter what the correct term for them is.

What’s more important is that beekeepers remember that it’s at this stage that mites are susceptible to all miticides.

The June gap

But it’s also worth thinking about the potential impact of brood breaks.

During brood breaks all the mites in the colony must be ‘phoretic’.

Generally, the majority of the mites in a hive are in capped cells. Depending upon the stage of the season, the egg-laying rate of the queen and other factors, up to 90% of the mites are associated with developing pupae.

But as the laying rate dwindles more and more mites are released from cells and become ‘phoretic’, unable to find a suitable late-stage larva to infest.

And which bees do the mites associate with?

Nurse bees primarily, for reasons I’ll discuss in the future. But – spoiler alert – one of the reasons is likely to be that they have a larger fat body.

So, a mid-season brood break (e.g. the ‘June gap’) is likely to result in lots more nurse bees becoming both the carriers and the dinner of the mite population.

Some or many of the nurse bee cohort may perish, perhaps from damage to the fat body or from the viruses acquired from the mite. However, bees exhibit phenotypic plasticity, meaning that older bees can revert to being nurse bees when the queen starts laying again.

Late season brood breaks

In late summer mite levels are usually at their highest in the hive. A brood break occurring now will release a very large number of mites to parasitise the adult bee population.

Presumably these mites select the bees best able to support them 7.

And which bees are these? The nurse bees of course. But it’s also worth remembering that there are key physiological similarities between nurse bees and winter bees. Both have low levels of juvenile hormone and high levels of vitellogenin (stored in the fat body).

So I’d bet that the ‘phoretic’ mites during a late season brood break would also preferentially associate with any early-produced winter bees.

Furthermore, once the queen starts laying again – perhaps in early/mid-autumn – the winter bees being produced would be subjected to the double-whammy of high levels of mite infestation and potential damage from ‘phoretic’ mites.

Practical considerations

More work is required to model or actually measure the impact of late season brood breaks, high levels of ‘phoretic’ mites, nurse bee numbers and winter bee development.

Compare two colonies of a similar size with a similar mite load, treated at the same time in early autumn with an appropriate miticide. If one of them experienced a late summer brood break (pre-treatment) and consequent high levels of ‘phoretic’ mites, does this reduce the chances of the colony surviving overwinter?

Who knows? Lots and lots of variables …

Fundamentally, it remains important to treat colonies early enough to protect the winter bee population. You’ve heard this from me before and you’ll hear it again.

However, it’s something to think about and I can see ways in which it might influence the strategy and timing of mite control used. I’ll return to this sometime in the future.


 

Footnotes

  1. >95% of the mites on adult bees in the recent study by Ramsey et al., 2019.
  2. Where nom is an onomatopoeic adjective, first used by the Cookie Monster on Sesame Street, to indicate the sound of ravenous eating. Nom was defined in 2004, though the Cookie Monster is a whole lot older, and is also precisely the noise Varroa makes … if you listen very, very carefully.
  3. The sclerotized plate over the abdominal segment.
  4. It’s a longitudinal section through the bee. The head end is to the left side of the image, but it’s only showing a narrow slice of the abdomen.
  5. If it isn’t the Varroa is about 2 feet long!
  6. And I’ll struggle as well … I’m going to try and use quotes to enclose the word ‘phoretic’ in the remainder of this and future posts.
  7. Evolution tends to arrange these sort of things rather neatly.

10 thoughts on “Pedantically not phoresy

  1. John

    Always thought provoking thanks, reading this it sounds like a brood break is a bad thing where the nurse bees may be overwhelmed. I’ve been using snelgrove boards for a few years and thought I noticed a reduction in varroa number
    In these hives come Autumn again I thought due to the brood break. I contacted someone who has been using the boards an awful lot longer than me and he leant towards that idea to. Not very scientific I know, but curious?

    Reply
    1. David Post author

      I think the answer is that we don’t yet know John. There will presumably be a balance between the ability of adult bees to maintain the mite population with the ability of the bees to get rid of the mites through grooming etc. It will also depend upon the mite load in the hive. If it’s low, then it’ll be largely irrelevant.
      I agree that it’s thought provoking …
      Cheers
      David

      Reply
  2. Finlay Bennet

    Presumably varroa on larvae in enclosed cells are also ectoparasitic? In which case is there a word we can correctly use to differentiate between varroa larvae and varroa on bees? Juvenile and adult mites?

    Reply
  3. Emily

    I always thought brood breaks were good for keeping varroa levels down as it’s a break in varroa reproduction too. Are you suggesting a late season brood break could actually be worse for the colony’s health if the mites then switch to feeding on the adult future winter bees?

    I can’t recall many times seeing a brood break in July or August, but perhaps it depends what area of the country you’re in and how much forage is about.

    Reply
    1. David Post author

      Hi Emily
      I think it’s complicated … or might be. Yes, you’re absolutely correct in saying that a brood break is beneficial as it interrupts the replication cycle of the mite. That’s definitely ‘a good thing’. What this paper makes clear is that the ‘phoretic’ mites are likely feeding on the adult bees, primarily the nurse bees and possibly the (in some regards) physiologically-similar overwinter bees. The impact on either of these populations will presumably depend upon the relative population sizes of the bees and the mites, coupled with the duration of the brood break.
      All a bit complicated.
      I’ve seen brood breaks in most months of the season at one time or another. July/August isn’t all that unusual … it all depends upon the local forage which in turn is influenced by the weather.
      Without knowing anything more I think I’d be concerned about the impact of late summer brood breaks in heavily infested hives. Perhaps this would be a justification for early intervention?
      I’m going to have to think about this a bit more 🙂
      Cheers
      David

      Reply
  4. Steve

    A few thoughts:-
    1) With mites continuing to feed outside of their cells, does this extend their lifespan?
    2) Do mites feed so readily on older bees, when they take the place of nurse bees post a brood break?
    3) The research illustrates the importance of finding bees with grooming behaviours – the hunt is on!
    Regards
    Steve

    Reply
    1. David Post author

      Hello Steve
      I don’t think we yet know the answer to the first two questions. I’m pretty certain that Varroa can live (‘phoretically’) for weeks or months during broodless periods, for example during long Canadian or North American winters. Presumably they’re feeding some of this time. I guess we could expect that mites that feed would live longer than those that don’t …
      Clearly the potential detrimental consequences of broodless periods would be reduced by bees able to groom more effectively.
      Cheers
      David

      Reply

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