Tag Archives: brood break

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.


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.


Convenience or laziness?

It’s cold and dark and all is quiet in the apiary. Hives appear somnolent. Colonies are clustered 1 and, other than the odd corpse or two on the landing board, I’ve not seen a bee for at least a fortnight.

The apiary in winter ...

The apiary in winter …

Based upon previous experience I suspect colonies are – or very soon will be – broodless. I usually reckon that the first extended (2-3 weeks) period of cold weather 2 in the winter is the most likely time for the colony to be broodless.

In 2016/17 this was the first week in December.

In 2017/18 it was just a day or two later.

In both instances, when the hives were checked, they had no brood.

What’s all this about being broodless?

If a colony is broodless there are no capped cells in which the Varroa mite can ‘hide’. As a consequence it’s an ideal time to apply a miticide like a trickled solution of Api-Bioxal 3.

There are very good reasons why a midwinter OA treatment is necessary, particularly if you treated early enough in the autumn to protect the overwintering workers from the ravages of Deformed Wing Virus (DWV). High DWV levels reduce the lifespan of bees and contribute to many (possibly most) winter colony losses. I’ve even suggested here that “isolation starvation” might actually be due to Varroa-transmitted viral disease.

Time of treatment and mite numbers

Time of treatment and mite numbers

Early autumn treatment protects the winter bees but also leaves the long autumn for the residual mites to continue replicating.

And there will be residual mites. No treatment is 100% effective.

So, paradoxically, if you treated early enough in the autumn to really help protect the winter bees, your mite levels will be higher at the end of the year.

Which also means they’ll be higher at the beginning of next year.

Not a good start to the 2019 season 🙁

Convenience or laziness?

Many beekeepers, for convenience, laziness or historical precedent, choose to apply the winter OA treatment between Christmas and New Year. I suspect that this is often too late. If the queen starts laying again around the winter solstice there will be sealed brood – and therefore unreachable Varroa – by the end of the month.

I’d prefer to have a cold and damp afternoon in the apiary slaughtering Varroa now than the convenience of treating them less effectively during the Christmas holiday period.

The latter might be more convenient … the office will be closed, I’ll be replete with turkey and sprouts and it will be a good excuse to ‘escape’ visiting relatives and yet more mince pies 4.

But is it the best time for your bees?

We have the technology

We have a couple of hives with Arnia hive monitors fitted 5. These have a temperature probe inserted into the brood nest. Brood rearing temperature is around 34°C. Here is a trace of one colony over the last month.

Arnia hive monitor temperature

Arnia hive monitor temperature

The colony temperature was pretty stable (around 33-35°C) until about the 19th of November and has dropped about 10°C since then. Although I’ve not opened the colony I think that this is additional evidence that the colony is broodless 6.

Beekeeping by numbers

Keeping bees properly involves being aware of the seasons, the available forage and the state of the colony. This varies from month to month and year to year 7.

You can’t mechanically (‘by the numbers’) add supers on the 5th of May and harvest honey on the 15th of June. Sure, it might work some years, but is it the best time to do it?

Similarly, you can’t optimally treat a colony for Varroa on the 30th of December unless the climatic conditions and state of the colony coincide to make that the best time to treat.

It might be, but I suspect that generally it’s a bit late if there is a brood break.

If you’re going to the trouble of preparing the OA treatment, donning the beesuit and disturbing the colony you might as well do it at the right time for the bees.

I’ll be treating in between the predicted sleet showers and sunny periods this weekend.

Time to treat

Time to treat

Isn’t evolution a wonderful thing? This post started with a working title of Know your enemy” and was on a different topic altogether. I’ll save that for next week.


The above was written at the beginning of the week. Now the weekend is closer it’s clear the weather is going to be cold with heavy snow predicted. Unless the forecast is wrong (and how often does that happen?!) I’ll hold off treating until a) it’s over 5°C, and b) the roads are safe.