Chewin’ the fat

A little over a year ago reports started to circulate of a study showing that Varroa feed on the fat body of bees rather than on haemolymph.

Having worked in Glasgow through the early noughties the title of this post was a no-brainer and an outline draft was written in December 2017. However, the peer-reviewed paper wasn’t published until last month, so it’s only now we’ve got the chance to judge the study and consider its implications.

Varroa feed on hameolymph, right?

Historically this was the accepted dogma. However, the experimental data supporting this conclusion – based upon labelling bees with radioactive isotopes and seeing what the mites acquired after feeding – was really not definitive. The experiments had been done in the 1970’s and the specificity of the labelling was a bit dubious. In addition, during the intervening period scientists had determined that, unlike vertebrate blood which is rich in cells and nutrients 1, haemolymph has little of either and is actually a pretty lousy food source.

In addition, and somewhat more circumstantially, Varroa control using chemotherapeutics fed to bees (and subsequently taken up by the mite during feeding) had been relatively disappointing.

Perhaps these chemicals weren’t getting to the right tissues of the bee?

Perhaps Varroa don’t feed on haemolymph after all?

The Ramsey study

This new study reports three independent experiments that, together, indicate that Varroa actually feed on the fat body of bees, rather than on haemolymph. The paper is so-called ‘open access’, so anyone can access it and therefore I’ll just provide a synopsis of the important bits.

The questions Samual Ramsey and colleagues attempted to answer were:

  1. Where on the bee do mites feed? Is it primarily or exclusively near the fat body?
  2. When Varroa feeds, what host tissues are ingested?
  3. What sort of diet is required to maintain Varroa and allow their reproduction in vitro2.

Location, location, location

The authors counted phoretic mites on 104 bees. Over 95% of them were located on the underside of the body, predominantly on the left side of the bee, under the tergite or sternite3 on the third metasomal segment (i.e. the second visible segment of the abdomen).

Mite location on nurse bees

Mite location on nurse bees

This position is consistent with feeding on the fat body tissues which are most abundant under the inner ventral surface of the metasoma.

Seeing red

Bees were fed with Nile red, a lipophilic fluorescent stain that preferentially accumulates in the fat body. They co-fed bees with uranine, a differently coloured fluorophore that accumulates in the haemolymph. They then allowed mites to feed on the fluorescently labelled bees and subsequently photographed the mites under fluorescent light.

The rationale here was straightforward. If the mites fed on the fat body they would stain red due to taking up the Nile red stain.

Mites visualised after feeding on fluorescently labelled bees

Mites visualised after feeding on fluorescently labelled bees

Which they did.

It was notable that the red stain predominantly accumulated in the rectum and gut of the mite (image O above). The authors conducted all sorts of controls to confirm that the stains actually stained what they were supposed to – you can view these in the paper.

Babies!

In the final part of the study the authors maintained mites in vitro (in an incubator), feeding them on a diet containing increasing amounts of fat body or haemolymph. These are tricky experiments and in some way the least satisfactory part of the study.

Two results suggest that fat body was beneficial or essential to the mites. Firstly, only mites that had 50% or more fat body in the diet survived for 7 days. Secondly, there was a dose response to the amount of fat body in the diet and fecundity. Mites on a 100% fat body diet exhibited 40% fecundity, the highest level observed in the study.

What can we conclude from the Ramsey study

Of the three experiments presented, the Nile red fat body stain uptake by mites is reasonably compelling.

The feeding position study is essentially correlative, but there could be other interpretations of the data. For example, that location on the bee might be the least accessible to a ‘grooming’ bee. Perhaps it’s a survival mechanism?

Survival and fecundity in in vitro studies wasn’t great. However, in defence of the authors, fecundity of mites under natural conditions can be as low as 40% and is not higher than 80%. Not all mites have baby mites. Thankfully.

Only 20% of the mites survived one week under in vitro conditions, even on a 100% fat body diet. In contrast, mites fed haemolymph alone died within 48 hours. This poor level of survival was surprising and suggests other essential components of the diet were probably missing.

Other published studies have shown reasonable survival of Varroa for at least 3 days, with at least one report of mites surviving on flowers for up to 7 days. I’m also aware that other laboratories can maintain mites in vitro for longer than 7 days without using any honey bee-derived components in the diet.

Hang on … what is the fat body anyway?

The fat body is multi-functional. It has been compared to the vertebrate liver and adipose tissue. It acts as a major organ for nutrient storage, energy metabolism and detoxification of things like pesticides.

Vitellogenin made by and stored in the fat body reduces oxidative stress and is associated with extending the longevity of overwintering bees. The fat body also has critical roles in metamorphosis.

So, not only multi-functional, but also very important.

Significance of the results … is this a game changer?

This paper has been discussed online as a ‘game changer’. That’s probably a bit strong. Whilst the fluorescent stain uptake study is reasonably convincing it must be remembered that it was conducted on adult bees.

Do mites on pupae also feast on the fat body?

This will have to be determined in the future. It’s a more difficult experiment of course.

The other two studies, and a number of additional small observations I’ve not discussed here, are certainly supportive, but not alone hugely convincing. The in vitro study in particular will be interesting to compare with (currently unpublished) studies from other laboratories that do not use honey bee fat bodies in their mite feeding and maintenance diet.

Practical matters

Does it matter what part of the bee the mite feeds on?

Clearly it does for the mite, but what about the beekeeper?

I think this study is significant for the beekeeper for two reasons – the first will only be relevant if and when lipophilic miticides are developed, the second matters right now.

  1. Strategies are being developed to add highly specific miticides to the diet of bees which are then delivered to Varroa when the mite feeds. To date, these have been rather underwhelming in their performance. If Ramsey is right, modification of these miticides to make them lipophilic (like the Nile red fluorphore) will concentrate them in precisely the right place to ensure the mites get a lethal dose.
  2. A key product of the fat body is vitellogenin. The long-lived overwintering bees have high levels of vitellogenin. Mites feeding on, and depleting, the fat body would be expected to result in reduced vitellogenin levels in the bee 4. This would explain why high Varroa levels are associated with reduced longevity of winter bees and consequently increased overwintering colony losses.

The most important take home message

To prevent mites that feed on fat bodies from damaging vitellogenin production miticides have to be used early enough to protect the winter bees.

In the paper Ramsey makes the statement:

Simple reduction of mite loads late in the season to decrease the overwinter parasite load may not be enough, as it still allows for the mites to damage tissue critical to the process of overwintering …

Instead …

A treatment schedule that includes treatment in late summer or early fall before mites can significantly damage fat body in developing winter bees would likely be more effective.

Which is precisely the point I’ve made previously about treating early enough to protect winter bees.

What the Ramsey paper adds is the piece of the jigsaw possibly explaining why late summer treatment is so important.


Colophon

Chewin’ the Fat was a four-series Scottish comedy sketch show. It was broadcast from 1999 to 2002, with further Hogmanay specials until 2005. The show had a recurring cast of characters and sketches including The Big ManThe Banter BoysThe Lighthouse KeepersBallistic Bob and Taysiders in Space.

Gonna no' dae that

Gonna no’ dae that – The Lighthouse Keepers

Chewin’ the Fat was filmed in and around Glasgow (where I worked at the time) and the characters parodied a range of local ‘types’ … pretentious Kelvinsiders, Glaswegian gangsters, narcissistic golfers, The man from Kilmacolm, and shellsuit-wearing, chain-smoking, hard-drinking Glaswegian neds.

It was a bit rude and definitely an acquired taste. Without subtitles, some of the scenes would probably have been unintelligible south of the border.

Footnotes

  1. And is enthusiastically munched by a wide variety of ‘blood sucking’ parasites.
  2. By which I mean in the laboratory not in association with a bee.
  3. The overlapping abdominal plates.
  4. Of course, this will have to be demonstrated experimentally in due course.

14 thoughts on “Chewin’ the fat

  1. Rona McLeod

    Very interesting Dave! It’s wonderful getting an insight into some of the science going on to try and eliminate varroa destructor.

    I got the message in your recent post to treat for varroa no later than mid september, many thanks.

    Reply
    1. David Post author

      Hello Rona
      Timing depends upon season and climate. However, generally winter bees are being reared before many beekeepers have even thought of treating for mites.
      David

      Reply
  2. S Sharpe

    Hi,
    I am probably being daft but why would this be the case “Mites feeding on, and depleting, the fat body would be expected to have reduced vitellogenin levels:”
    Why wouldn’t the mites have increased vitellogenin and the bees reduced vitellogenin?

    Reply
    1. David Post author

      Oops … thank you. I’ll have my proofreader flogged.
      I’ve corrected it to Mites feeding on, and depleting, the fat body would be expected to result in reduced vitellogenin levels in the bee ….
      David

      Reply
  3. James Reid

    Gonna no dae that is the name of a horse that likes winning at Ayr racecourse. Like me, it’s getting on a bit but just occasionally has a good day. Nothing to do with bees, but named, I presume, after the aforementioned TV series.
    Great post and definitely seems to point to the efficacy of midwinter treatments.

    Reply
  4. Eric McArthur

    Interesting stuff! I don’t see any refs to the critical importance of monitoring mite drop! The question haemolymph/fat body nutrition for mite is ‘sideshow’. Beekeepers at present have the tools of the trade to stop the mite – irrespective of what it eats. Timing and judicious application of the organic acids is still ‘state of the art’. On an annual cycle: treat with formic acid around mid April – if mite drop is more than 2 mites/day. Treat again in mid June – if the mite drop warrants! Treat again with formic acid early September – to protect the brood/winter bees. Oxalic acid sublimation in late November if mite fall warrants. In late December, if mite fall is more than 1 mite per day apply the oxalic acid trickle. Monitoring constantly will keep the beekeeper ‘aware’. Always bearing in mind that re-infestation is a constant problem!!
    Eric McArthur

    Reply
    1. David Post author

      Hello Eric
      I’ve often discussed mite monitoring here, so regular readers should appreciate the importance. However, mite drop is notoriously inaccurate, being influenced by all sorts of things like the laying rate of the queen etc. Sugar or alcohol rolls are better.
      Where I am (in mid-Fife) there’s almost never any need for more than two treatments per season, assuming they’re timed properly. We’re reasonably remote and the local density of beekeepers means there’s not too much re-infestation. If you need up to five treatments per season there must be some heavily infested colonies near you.
      We monitor virus (DWV) loads in some colonies and, even with just a late August and late November or early December treatment, they are extremely low. My overwintering nucs this season are looking particularly good and haven’t dropped a mite since well before Christmas.
      Cheers
      David

      Reply
  5. Adam Darling

    Many thanks for another interesting article. And your reminder of treating bees for varroa early – in August -is worth repeating. Unfortunately the expression “autumn treatment” for varroa is out there. And it’s too late.

    Reply
    1. David Post author

      Hi Adam
      Late August/early September … but it’s worth repeating that this depends upon the season, the forage availability, the timing of the summer honey crop etc. It may be necessary to intervene in mid-season if mite counts justify it, or if you need to exploit a good flow from the balsam or ivy.
      Beekeepers generally just need to think a bit more, rather than following instructions by rote.
      Cheers
      David

      Reply
  6. Helen New

    Thank you for the you for the superb educational and entertaining posts. It is always a treat when the notification of a new one arrives on a Friday.
    Best wishes
    Helen New

    Reply
  7. Dafydd Jones

    Hi David,
    Do you think that the development of a lipophilic feeding inhibitor to deter the varrroa mites from feeding, rather than a miticide , might be less destructive/harmful to the bees?

    Reply
    1. David Post author

      Hello Dafydd
      It could work, but it would probably be more difficult to develop and test. The miticides we currently use are pretty good when you compare toxicity to bees vs. toxicity to mites. The ‘readout’ is also straightforward … you just ‘count the corpses’ (bees and mites). To develop an inhibitor of feeding you’d have to work out a way to determine whether it was working or not … you could wait until the mites starved, but that would probably take a long time. An additional problem would be to ensure that pupae received enough during their larval feeding stage to make them unpalatable to the mites within the capped cells.
      We have perfectly good mite control methods already … if they’re used correctly. I think more beekeepers should stop hoping for a ‘quick fix/magic bullet’ and instead concentrate on using the tools we currently have properly.
      Of course, this hasn’t stopped me thinking up all sorts of ways to develop and deliver novel mite control methods that will ‘save the bees’ AND make me a fortune 😉
      Some chance!
      Cheers
      David

      Reply

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