A load of old rhubarb

From Simon's beekeeping blog. Comments are welcome via email.


Varroa mites are a fact of beekeeping life. Varroa is a parasite which feeds on the pupa of the honey bee. It replicates on these host pupae and, by trasmitting virii like the deformed wing virus, interfere with its development into an adult bee. A newly-emerged Varroa hitches a ride on young bees, looking for a cell with a larva which is about to pupate, goes in it, and the infestation produces up to two new mites. This hitchhiking mite is known as phoretic. So, when there's a lot of brood developing, the mites can really multiply. When the queen isn't laying, these phoretic mites don't succeed in finding any brood and just hang around.

In midwinter, it's cold, dark, the days are short and there's not much nectar or pollen, and the queen will slow down her laying. The mites which are in the colony will be phoretic, and this is the time to strike with a miticide. The beekeepers in Somerset talk about the longest day — the winter solstice — as the time to strike. This year it's the 22nd of December, at least in the northern hemisphere. Nature, however, doesn't work to a calendar, and in reality the first long cold spell is a good time to choose.

We have been studying our bottom boards quite closely all year and have concluded there's not really a Varroa problem in either hive, but we've both agreed for some time that a midwinter treatment would be a good idea, because we have seen mites and it's now that they will be phoretic. We treated twice with Apiguard on 29 September and 15 October, but we left it rather late. Autumn came quickly and Apiguard is not as effective in the colder months.

The midwinter treatment is oxalic acid. It's present in rhubarb leaves at a 0.5% concentration. There are two main administration methods — sublimation or trickling. Spraying is also done elsewhere but not approved here in the UK. Trickling is the easier method and involves using a specialised trickle bottle (or syringe) with a treatment like Api-Bioxal, though if you go with this method you should note that the manufacturer's recommended concentration is higher than conventionally used in the UK.

The alternative to dribbling is sublimation. Sublimation is the chemical term for a substance which goes from the solid to the gas phase without passing through the liquid phase. When oxalic acid powder — specifically, its dihydrate — is heated on a tray inside the hive entrance, a vapour results, spreading throughout the hive and killing the mites. Overheating the acid produces formic acid and carbon monoxide and so getting the temperature right is critical. This is the method we used when I volunteered at Walworth Garden. We put oxalic acid crystals into the tray on the vaporiser, pushed it through the entrance to the hive, and connected the vaporiser to a car battery for a minute or two. To avoid the leakage of the gaseous oxalic acid, we put a tray under the open mesh floor and an old blanket over the top of the hive.

This afternoon, Thomas hosted a demonstration of oxalic acid sublimation at the apiary on his farm, on National and WBC hives and nucs. His hives are all fairly traditional and wooden, but I expect it's necessary to be a bit more careful with a hot tray like that around a poly hive or Correx floors. Anyway, as usual seeing a different beekeeper performing the same process, in subtley different ways, led to new understanding. For example, no blanket is needed, at least for the bees' sake. It only serves to limit the spread of the vapour, which has already escaped and condensed.

We also learned that oxalic acid treatments meant that no Apiguard would be needed that year. Also, earlier I reported that the Apiguard tray was only half-full when we took it away. This isn't apparently a cause for concern — the vapour is the thing that does the trick, and the Apiguard appears reduced because the bees try to get rid of it.

Having only a couple of hives, we thought we would try out the economy vaporiser from Simon the Beekeeper. Following Thomas's demo, we will wait for a cold day when the bees are all inside, fit a stop along the vaporiser's length so it stops about five or six frames in from the entrance, put 2.5 to 3g of oxalic acid dihydrate into the tray, insert it and connect it to something like a motorbike battery. Especially in the case of our colony, which loves to produce tons of burr and brace comb, the tray might get stuck on the way in, but we shall have to cross that bridge when we come to it.

The great thing about sublimation is — ordinarily — that the hive doesn't need to be opened. Even in summer, opening the hive disturbs the colony for days, but with sublimation, the hive is left intact during the (often bitter) weather. It's also believed that there's less toxicity to the adult bees and brood. Some beekeepers advocate opening up the hive and digging out all the extant larvae, so that all mites are phoretic. It was said at the demonstration that this would lead to a 99.6% rate of destruction of the mites. I very much doubt this is necessary in our case or even beneficial.

In general, too, sublimation is seen as superior to trickling as a treatment, but the main disadvantage is the exposure of oxalic acid to the beekeeper. Oxalic acid may be found in rhubarb and other plants, but this doesn't mean it's benign, as many people seem to think. Experts say that the vapour recondenses into an aerosol of tiny crystals, less dangerous than a vapour, and that the oxalic acid quickly recondenses in the hive, coating everything in sight, and any leakage will be in the form of these tiny crystals. However, it still stands that in the concentrations that are found in treatments, it is poisonous to humans, with a lethal oral dose of between 15g and 30g causing kidney failure. It should be treated with respect and the beekeeper working with it should do so safely. At a minimum, a respirator with suitable filters is required, along with acid-resistant gloves, wellies and goggles, if the respirator only covers the nose and mouth.

With all this in mind, those paper dust masks are not going to cut it, even though many beekeepers seem to use them. There's a lot of confusion about what is necessary, so I went to 3M and consulted their filter selection guide. The filter type for oxalic acid (also known as ethanediocic acid) and its dihydrate is A/P. This refers to certain organic gases and vapours (A), with a particulate filter (P) for the acid's solid phase — the tiny crystals. Also, for A/P combinations, the guide recommends that 'a gas vapour filter be used in addition to the traditionally accepted particulate filter'.

Other sources (e.g. funnybugbees and Backyard Bees NC) recommend considering a filter that can handle acid gases too (type E), that is, a combination organic vapour and acid gas filter, even though the guide doesn't specify this. I guess the guide doesn't specify it because the acid gas almost immediately turns to fine particulate crystal. In the USA, 3M's pink and yellow filters (US order code 60923, à la Breaking Bad) fit the bill, covering OV and AG with an integrated particulate filter. Of course, the range is different in the UK, but as far as I can tell, the gas, vapour and particulate filters available to us include order codes 6096 with acid gases, and both 6091 and 6095 without, since the melting point of oxalic acid is around 189.5°C (and 101.5°C for the dihydrate).

I'll be the first to admit that most winters, I'm a bit sickly, and having been wheezing and coughing with a maddening chest infection for what now seems like nearly two months, for the demonstration I decided to pop on a Draper Expert 13500 — actually a rebranded (and more expensive) B-Brand BB3000 with A1P2 filters. I was able to stand right next to the demonstrator — as well as several hundred rather alarmed bees — and get a good look without any whiff of the oxalic acid. So, it was instructive, but also absurd as I was chatting, muffled, to the other attendees with it on. I think next time I'd go for one of the 3M ones, since the B-Brand one seemed to feel quite laboured to breathe through, despite the favourable reviews on Amazon.

So now, it's a case of waiting for a nice cold morning when the bees are inside and hunkered down, and we can go down with either a little 7Ah battery or one of dad's tractor batteries, and try it out.

Under the influence of Night Nurse, I had a dream over the festive period that the BBC had started doing a quiz programme on BBC2 solely on the topic of beekeeping. The set was a standard panel show set, but made up entirely of orange and yellow hexagonal design elements. Paul Daniels was one of the contestants, which was confusing, because (a) he's not a famous beekeeper and (b) he died in 2016. The dream suggests that my interest in beekeeping has now permeated my subconcious. To be honest I'm rather missing real beekeeping as opposed to things like fumigating, but at least it makes a change from feeding. Roll on Spring, when we can get back to the real stuff.

The other bit of Christmas news is that Ⓑ opened the present from Dave and Ⓓ tonight. It was really generous — a lidded bucket to take away burr comb, wax dust, and other bits from an inspection. Ben hates the smell of cardboard in the smoker, so they had put in the bucket smoker fuel — both cotton fuel and lavender wood chips. On top of all this, there was a bee gym, a station with notches and plastic blades which bees can use to scratch and groom themselves in order to control varroa mites. Finally, there was a cool silicone mould to make tablets of beeswax from the various bits of wax we recover from the hive. This is all really exciting, and I'll report back on our experiences with all of these things in the new year. Many thanks, Dave and Ⓓ!