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Fogging Beehives with Oxalic Vapour to Control Varroa mites, also variously known as sublimation and oxalic vapour treatment


Fogging Beehives with Oxalic Vapour to Control Varroa mites
Oxalic acid is a normal part of many healthy foods we eat every day, but it kills mites!

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Monday October 10th 2011
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We had a leisurely morning.  Around eleven, Chris and  I went out and looked for mites.  This time, I found five on one board and one on another. What does that mean?  I don't know, but our goal was always to keep the number below ten a day, as I recall.  That was averaged over three days to a week.

I went out again mid-afternoon and found two mites on each of the six boards and one on another.  That is more what I would expect.  Click on the thumbnail for a high-def image clearly showing varroa and junk on a drop board. I wonder if the fact that the hives had been open on the bottom up until I inserted the drop boards somehow affected the drop for the first day.

I also noticed one of the hives on the scale being robbed, and I had deduced that from the scale readings while I was still in Muskoka.  I don't think I'll interfere with nature here, since a hive being robbed is not likely any good anyhow.  I'll put entrance reducers on all hives, though. The scale weight has dropped 13 pounds since yesterday and I assume that is from the robbing.

The pictures at left and right are of the top bars of the same hive (not the robbed hive), showing how well the foundation has been drawn.  It was my intent to move such boxes of new comb down, but I am now undecided.

Jean and family stayed for lunch, took a stroll downtown, then left for home.

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Tuesday October 11th 2011
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I went out after lunch and checked the hives.  This time, I am seeing mites: 2, 10, 8, 6, 12, and 16.  That is counting the tan ones, and I am seeing more than I am used to seeing of them. I assume that they are mites which did not quite manage to mature before the cells were opened.

Did I miss a lot of varroa in my counts on previous days?  I don't think so, but it could be.  My eyes were acting up a bit recently and they have been a little less acute than usual.  That condition is improving, but could account for missing mites.  I am also using tan-coloured dry drop boards and the mites are not as obvious on them as they were on the white plastic with oil that I used previously.

These counts are up where I am wondering if they are calling for immediate action.  I may do some alcohol washes as backups on several of the smaller hives.  I was not wanting to do that.

I was already planning to use oxalic drizzle later, but maybe some formic is called for, too.  It can't hurt since it reduces any tracheal mites I may have in my hives.

I looked back at October 17, 2002 and see that I wrote:

We're not seeing many mites from the natural drop counts we're doing. We leave the boards under the hives for several days and never find more than three or four mites. Most often, we count zero. Multiply that by what? 20, 30, 50? 100? I don't know, but counts this low should not be a threat.

On October 9, 2003, I wrote:

We picked up the varroa mite drop boards this morning and looked them over. The first batch of five sticky boards showed 10, 6, 2, 0 & 2 mites over three days of natural mite drop, and the second batch showed 0, 0, 1, 1 & 3. The third showed 0, 0, 1, 1, & 4. The boards had been in for three days and, therefore, each result should be divided by three then multiplied by 100 (the estimated average varroa lifespan ) to estimate total mite loads. Thus the worst hive could be assumed to have a 333 mite load. That is nowhere near serious. 350 mites as a total load, is very light, in fact. A visit to the varroa calculator gives a different perspective, but it also assume drone brood, which we no longer have at this time of year.

Counting is always difficult when there are so few mites, and we are always tempted to count the immatures -- the occasional mites we see in the board that were almost fully developed in a cell when the host bee emerged, but which die immediately of exposure, when deprived of the special conditions inside a cell. Such mites have the shape, and often the size of mature mites, but are pale and translucent. We know they did not live even one day after emerging, and were never part of the adult, reproducing, population. Since their lifespan is zero days, the 100 day estimated average lifespan multiplier does not apply, and I figure immatures should not be counted.

By this time of year, there should be very little brood in which mites can hide, so most varroa should be phoretic at this point. When the mites are phoretic -- on bees, and not hidden in brood -- they are at their most vulnerable, and have the highest mortality rates, so, even using a multiplier of 100, which could be high, we are seeing very low infestation rates. Over winter, the mites will be under even greater pressure, as they occasionally fall off bees and are exposed to the cold conditions at the hive floor, unless they are able to grab back onto another bee.

For some reason, varroa is not giving us much trouble. For the past several years, the only treatment we have used is a single Apistan™ strip placed in the centre of the cluster in the early spring and left for 42 days. Our tests always show very low levels of mites, much lower than when we used two strips in the fall plus several formic treatments. Granted, we had a dry year in 2002, and we spilt heavily this year, and both these factors tend to reduce varroa loads, but, nonetheless, we did not split all hives, and when everything is considered we still are seeing lower levels that we would expect and lower levels than we saw in the past.

I'm seeing much more than that today.  In those days my control was to use one strip of Apistan™  in March or April.

I have some Apistan™  around.  Maybe I should see if it works, but in the fall it is far less effective than in early spring.

I have been trying to upgrade the Galaxy Tab to the latest version of Android and Kies always stalled.  Tonight, there was a new version of Kies and I tried again.  The upgrade completed, but then the Tab just sat there and flashed.  Did I brick it?  I sure hope not.  Having the Tab has been a most liberating experience  It should be on warranty, but getting service may not be quick.

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Wednesday October 12th 2011
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This morning, Ellen and I drove to Rosebud to take in the dinner theatre.  After a buffet lunch, we saw Queen Millie of Galt.  The title and subject matter looked unpromising, but the writing and performance were excellent.

*    *    *    *    *    *    *

On return, I pulled drop boards and see that the amount of debris has increased over the days I have been watching. (I clean off the boards once a day.)

I'm wondering if I really know how to properly count mites on a natural drop board.  I did not see any the first day, then began seeing them.  I am now realizing that they look smaller to my eyes than they did a few years ago.  I've also become accustomed to seeing them in alcohol, where they are very obvious, and have become spoiled, I guess.   Surprisingly, becoming increasingly far-sighted makes nearby things appear smaller, when they can be seen at all. 

I am also finding mite colour to be an issue which I don't recall as being significant in previous years .  I remember noticing quite a few tan mites in alcohol washes in some yards when I was inspecting, and not in others.  We did not see as many light mites in the past, but then, we did not see many mites back then. (see yesterday's comments). 

If tan mites are non-viable due to early uncapping, then one would expect to see them in drops, but not washes, so I don't know what to think.   Today, we counted 23 mites on one board, but only 5 or 6 of them were  dark, and probably a third to a half of them were still translucent, but at least tan in colour.  Which ones should we count?

I'm off to do some research...

Bill says on his website:

24-Hour Prorated Natural Drop Test for the Varroa Mite:

Calculating the prorated 24-hour natural drop on a full size sticky board collected over a 3-5 day period provides the best indication of mite infestation levels.
The strength of the hive is important to obtain a reasonable indication of infestation. All data in our literature is for hives of approximately 30,000 bees, 10 frames of bees and 3-5 frames of brood in two deep boxes. In the early spring and late summer your hives will probably be this strength.
• Always count the entire board. Prorating and counting half the board is a big mistake.
Count only mature female mites. Concentrate on the size and shape. Be aware that mites can be of any shade of brown from light to dark to fully black and reportedly half black and half white.
Do not count mites of smaller size, w
hite, pearly white, or yellow. These mites are either males or immature mites, which cannot cause future damage.

Here is my answer (below) from Jean-Pierre Chapleau's web site.  I highly recommend checking out the site.  He provides the clearest and most detailed presentation on the topic I've seen.  I also very much like his Apinovar concept.

CAPA's website, on the other hand does not specify which mites to count, but does offer these handy conversion charts.

 (I think that the x-axis numbers need to multiplied by 100 to give the mites per 100).

And this advice:

Click above image to expand for easier reading

Jean-Pierre Chapleau also provided the following chart.  "Average natural drop in September vs. the strength in April". It is a bit hard to understand, but basically, the x-axis is the number of frames of bees in April and the bars show the 24 hour mite drops of the hives in each strength group the previous fall.  They are "Dead, 1-2, 3-4, 5-6, 7-8, and 9+"

We can see how he came up with 24 as a fall threshold.

At this point, it is getting to be late fall, and I think I'm in a safe zone for the hives I've been checking.  Nonetheless, I'll give them some formic for tracheal and get a few varroa in the process, then follow up with an oxalic drizzle.

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Thursday October 13th 2011
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This afternoon, my friends came by with their oxalic vaporizer and in about 3/4 of an hour, we treated my hives.  The unit is made by Cowen and costs about $10,000.  Before we were done, I already saw mite drop. The process took about 45 minutes start to finish, and we were not particularly efficient.  After some practice, I think the job would take about 2/3rds that much time.  Here are some pictures:




Click on each thumbnail to enlarge.
(What looks like fire in one shot is just a camera lens flare)

Bees covered with the dust seemed unharmed.  For bystanders, an occasional whiff is astringent, but not particular nasty, but a direct exposure could be quite unpleasant. IMO, the loading and activating mechanism could be improved a great deal with little work. Better design would eliminate the need for leaning over to load and activate the sublimation.

We smoked the hives minutes before applying the oxalic to loosen the clusters.  We did not block the entrances, but used probably about three grams per hive.  Recommendations (2g) are for  doubles and some of these are in four.  We blocked the auger holes, but maybe should have left the top one open to see when we had managed to fill the hive with the vapour.

For those who don't want to spend $10,000 on an evaporator, check this out. And this.

We had supper, and Meijers were on their way.

I checked the hive scale while I was at the hives and see a drop of 4 pounds since the tenth.  That is 4/3/4= 1/3 lb/day/hive.

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Friday October 14th 2011
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Joe Standing on the new EPS bee boxMeijers EPS boxes are now available from
Beaver Plastics Ltd
Head Office: 7-26318-TWP RD 531A,  Acheson, Alberta
Toll free in the U.S. and Canada: 1-888-453-5961
1 780 962 4433  Fax: 1 780 962 4640
E-mail: techsupport@beaverplastics.com

Price is $16.00 in lots of 500 or more FOB the factory, or delivered in truckloads of 1456 boxes.
Small quantities are $19.95 per box FOB the factory.

The boxes are high-density and cast in one piece.  Solid frame rests are built into the boxes during manufacture..

Around lunchtime, Joe and Oene dropped off a bucket loader and I spent the rest of the afternoon moving mulch.

24 hours after we treated with oxalic vapour, I pulled the drop boards and gave them a careful looking over.  The varroa look quite different today and the dead mites are almost black.  There are still some tan mites, but most are dark.  I am certain of the counts after the oxalic treatment, but less certain about which hive was which for the grayed-out previous results because at that time, I was less careful in marking which board came from which hive at that point. 

Here are the counts:

Oct 13-14
Mite Drop
After Oxalic Vapour
Oct 12-13
drop *
Oct 11th
drop **
1 43 3 2
2 220 17 10
3 29 8 8
4 119 19 6
5 23 48 12
6 81 11 16
Average 86 18 9


* Several of these natural drop boards may have been interchanged, but mites were carefully and accurately counted.

** Very casual reading and mites were probably missed

My earlier counts were very casual and I did not get careful about my counts until the count before and after the treatment. Once my friends showed up with the machine, I decide to get more professional about recording observations and confirming them.

I am thinking now that my initial conclusion that there was no drop and early counts last week may have been in error due to my inability to see mites well.  It seems my vision has changed over the years. 

My reports of the counts back in 2002 and 2003, however were accurate because  I always had the counts verified by Ellen and she is much more fastidious than I am about these matters.  As for right now, the average probably gives the best indication of how the oxalic is working.  The average drop increased by a factor of nearly five so far.

Now, I am using a magnifier with a built-in light and can see clearly.  I had Ellen verify my counts, too.

Some instructions mention sealing the hive.  Heilyser says 10 minutes.  We sealed up the auger holes, but left the entrances open.  Varrox recommends 15 minutes.  More.

From this article we have the following:

And from here, this...

If this chart holds for us, then mite kill should increase in the next two days to a cumulative 35%, then the steepest mite kill should be between day 3 and day 4, taking us to 60%, then 78% by day 6, leveling off after day 15 and be virtually complete by day 25.  We plan to repeat the treatment in a week or two, but we will see.

I figure at this time of year, average natural drop is very roughly 1% of the total mite population per day (others show 0.5%, but there is little practical difference) so the increase of about 5x we observed over the previous day's natural drop is right on track. 

I realise that I am working with a small sample and questionable data but I am less likely to question it strongly if it fits the previous observations.  Time will tell, since I'll have more and better data.

Increasingly, the oxalic links we used in the past and which were shown on my oxalic page and Randy's, too  have expired and information is getting harder to find.

I tried the wayback machine and recovered this article (here).  If I have time, I may dig up some other old articles.

Here are some more worthwhile links:

and a must-read:

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Saturday October 15th 2011
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After it warmed up, this morning I went back out to spread some more mulch.  A quick peek at the drop boards shows I have some counting to do at suppertime when the next 24 hours are up.  The mites are much easier to spot with the naked eye after an oxalic fogging than before, as they are now noticeably darker in colour, and much more numerous, amid less debris

I couldn't  wait, and pulled out the drop boards and counted the mites (in white below) after 20 hours, instead of 24.  That should not be a problem, as the numbers can be adjusted, but I did not bother to do so.  I just put down what I got.  This is not rocket science, after all.


Mite Drop
After Oxalic Vapour
Natural 24-Hr
Oct 14-15
Oct 13-14
Oct 12-13 Oct 11th
1 46 43 3 2
2 173 220 17 10
3 20 29 8 8
4 162 119 19 6
5 13 23 48 12
6 95 81 11 16
Average 85 86 18 9

* The pre-treatment numbers are somewhat sketchy, since we
were not being as careful before, as after the treatment.

One reader wrote the following and included a screenshot:

Hi Allen,  Just to let you know that when I do a mite drop count, I take a Digital photo of the board then bring it up on the computer monitor, overlay a grid at about 200 pixels in a contrasting colour from the view menu, then magnify it and it is really easy to count without even taking the boards away from the hives. Chris.

He adds, later:

I rescue colonies from various places, as well as swarms. the rescued colonies I cut out the brood combs and place them into frames with mesh fitted to one side which have had the wires cut and bent to form spikes which secure the comb to enable the frames to be inspected after the bees have built brace comb to secure the comb into the frame, Photos attached.

The mite loads from these rescued colonies are always very high, normally 1000 plus so I treat them as soon as I am able with Apiguard gell ( Thymol )

Liskeard, Cornwall, UK ( PL14 6RN )


I've played with the digital photo idea for mite counting in the past, but never really found it suited me, but today I took another look.  I am set up with grid lines on the drop boards and like that very well, when combined with a magnifying lamp.  I still count all squares, but the lines make double-counting or missing mites unlikely.  My view with the lighted magnifying lamp is excellent and I can do more than just count.  I can examine the items on the drop board from various angles as well as the top.

This is not a dismissal of the photographic approach, though.  I think it can make the job much easier and cleaner -- and provide permanent record incase anomalies show up later.

I notice his image is about 165 KB.  I took some snapshots of a drop board with my camera and found that images of that size are poor when blown up for purposes other than counting, and even a bit sketchy for accurate counting.  I seldom use the full ability of my camera due to the size of high-res files, so I decided to shoot at 12 megapixels and the fine setting just to see how much better they are. The raw result is here, thumbnail shown at left.  Warning: the file is about 6 MB.  After some cleaning up, I present the same image at right.  (Also large). 

I tried various programs for looking at the images for detail.  Microsoft Office Picture Manager turned out to have the best anti-aliasing and thus the most recognizable rendering of varroa in the programs I have tried so far.

Actually, now, looking at them both in the browser, from the Internet, not graphic software locally, I am not sure the second picture is an improvement!

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Sunday October 16th 2011
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I'm now collecting and counting the boards at 2 PM +/-, and for sake of consistency and tabulating in Excel, I adjusted the 20-hour reading from the other day to a 24-hr number.

We've now completed Day 3 and are dropping 105 dead varroa a day vs. 18 or so a day before the oxalic, but at 320 total dead per hive (est.), we are still a long way from killing 100%, which I estimate very roughly at around 1400 per hive, average. Comparing to the charts above, we seem a little behind their progress, with 20% kill, or so, whereas they show about 30-50% at this point. Of course, they know what their total load is an we don't - yet.  At this point, I am making wild guesses.  The next two days are predicted to give some of the best drops, according to the literature I've seen.  The curve should steepen.  We'll see. 

On the chart at right, which is also  shown yesterday, we are only along about as far as shown by the blue square.  (We're the pink line on the lower right of each chart).  The line is only halfway up to where it flattens.  Faster methods give the steeper curves on the left, but they all get to about the same end result -- eventually.

Otherwise, the day was spent doing some web work and spreading the mulch.  We had a neighbour lad come over for a few hours to help Ellen rake it out.  She was occupied with a studio visitor.

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Monday October 17th 2011
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The excitement mounts.  How heavy will the varroa drop be today?  I can hardly wait. 

One thing, though, is that the more I try to accurately record experiments in progress, the more I become aware of the many things that can go wrong and how they must happen and how they must often be covered up in the studies we read and many people believe.  When watching a slick PowerPoint presentation, I seldom, if ever hear much about he glitches.  The audience listens politely and usually any conclusions that are reached and not questioned too much.

I'm no scientist, but here are some of the things that happen in a simple observation of mites dropping after an oxalic acid application by evaporation. (BTW, I think we should have closed the entrances more.  We'll do that next time).

  • I sometimes mixed up the drop boards and had to guess which one was which (not important to the average)

  • I did not record the drops at exact time intervals (affects the graphing)

  • wind sometimes blows the mites around and possibly off.  (could result in errors)

  • wasps get in and could conceivably carry off some mites or eat them on the spot  (could result in errors)

  • ants could do the same if they were around (could result in errors)

  • I talk about the counts as if they were 100% of the drops in the first place, but in fact, these screens have a bar across the middle.  One would assume that the mites that land on it have no place to go but down, but bees could conceivably carry them out instead.  (could result in errors)

  • there is also a board at the front, so the actual screen area is less than 100% by more than a bit.  How many go out the door?  I don't know. (could result in errors)

  • there are up to four brood chambers and the frames do not necessarily hang above one another.  That, with burr and ladder comb, may result in mites not reaching the drop board.  (could result in errors)

  • when I count, and the number gets high, do I always remember whether I am at 69 or do I then say 60 again? (could result in errors)

  • do I always follow the same pattern around the board?  Do I miss a line sometimes? (could result in errors)

  • what happens if boards get dropped on the way to counting? (could result in errors)

  • is the counting assigned to someone whose accuracy, or commitment cannot be measured? (could result in errors)

Let me be the first to say that I make mistakes, and the more I make in spite of my best planning and efforts, the more it is obvious to me that everyone does.  How, in an elaborate experiment taking place over time with a number of people involved can we assume that no serious errors were made?

My answer is:  We can't. 

Question Everything.

*    *    *    *    *    *    *    *   

At 2, I went out and got the boards.  I did a count of one board with just my reading glasses and then re-counted with the lighted magnifying glass.  I got 84 the first time and 91 the second, 8% more, FWIW.  With the light and the magnification, I am counting light-coloured, marginal, and damaged mites which are questionable to the unaided eye.

Click to enlarge

I see the drop is reduced from yesterday.  It will be interesting if the warm day tomorrow brings the counts back up. Maybe the dose is wearing off.  I wondered if we should have closed the entrances. At any rate, 30% of the 1400 (average) estimated varroa load is now killed, according to my rough estimates.  That compares to 60% kill on the benchmark chart I am using.  Today, we are falling behind the expected result.

There should be very little brood in the hives now.  I see the occasional pupa being carried out, and the drones were being kicked out the day that I first saw the mites.

Today I will finish spreading the mulch and go to Three Hills.  At least that is my plan.

*    *    *    *    *    *    *    *   

I did spread the mulch, but never did get to town.  Probably tomorrow. 

After I counted mites,  It occurred to me to grease the drop boards, so I got a roller and covered the boards with a thin film of Vaseline.  The small foam paint roller worked well, and the Vaseline I used was actually Equate brand, which is a bit thinner than the real stuff. Now the masonite boards are protected from water.  I noticed that a few which had been used and gotten wet had warped. 

Now, we'll see if I have to be as careful to keep them covered when I pull them out to protect from breezes.  Maybe the grease will prevent the debris from shifting around. 

The boards we used formerly had screens built in and were greased.  They were not full-width, though and these are.  We often used sheets of white Permadent with screens to fit and a pull tab of duct tape for drop boards.  Sometime we used Coroplast.  The Permadent held the debris nicely in the little cups formed by the cell bases.   Using full-width boards, it is interesting to note that the mites sometime fall in concentrated areas, and sometimes fairly evenly across the boards.

The hive scale shows a loss of 6-1/2 lbs since the 13th.  That's 3/8 lb each /day.

*    *    *    *    *    *    *    *   

People ask if I am applying for the Commercial Beekeeping Instructor job posted at right.  No, I am not.  Maybe 5 or 6 years ago, I might have considered it, but plain and simple, I don't need or want the work.   I've done quite a bit of teaching over the years, including running the Computer Camp at Olds College and a beekeeping course at Red Deer College, but that is in the past. Also, Fairview is a good eight-hour drive from my home.  How could that work?

So, I hope they find someone suitable.  If they have problems and need someone to fill in, I could be available for a spell, and enjoy it, but anyone who takes this job on should plan to stay more than a short while and plan to work long hours.  This project is not just a typical college classroom course.  There is plenty of classroom time, but the job, done right, requires plenty of off-site and after-hours work.   Oddly, the job advertisement does not cover much in the way of detail that an applicant really needs to know.  I hope they get the right person, but they have not left themselves much time.  Maybe they have their person picked out and the application process is just a formality.  I hope so.

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Tuesday October 18th 2011
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Today is turning out to be one of those days.  The phone rang numerous times, Ellen is off to town and I should be too, but I have three jobs to do this morning and so it goes. Today look s like a good day to get the bees ready for winter since it is the warmest predicted for a while. Saturday may be OK, too, but who knows?  The forecasts keep changing.

The counts today are lower.  It's plain to see that the effect of the oxalic is wearing off.  I'm guessing that closing the entrances makes a difference.  We didn't.

Click to enlarge

I see that compared to our benchmark (right), we are at 39% of estimated load and the benchmark is at 70%.  Of course we won't know until we drop all the mites, one way or another.

Using the Vaseline makes the boards easier to look at and applying with a roller leaves a texture on the Vaseline that holds the mites and other junk.  Cleaning the boards will be more work and messier and I have to roll them again, adding several minutes to the job. 

Scraping and re-applying Vaseline turned out easier than expected, and adds very little extra work, but does make the drops stay in place so that the position of the dropping mites is more obvious than if they can slide around in transit.

The scale gained a pound since yesterday.  Robbing, I guess, or moisture changes.

*    *    *    *    *    *    *    *   

I was busy finishing the mulching and leveling some ashes, but I did check the hives quickly and see that 15 of the 26 (one is a goner, as suspected) need to have the top box removed and placed on the bottom since they have too much new comb.  Most are full frames of newly drawn and capped full combs, but I am reluctant to have the bees finish the winter on new comb.  In my experience the survive much better on comb that has been in service longer.

*    *    *    *    *    *    *    *   

Joe reminded me tonight that Albert Robertson did some formic and oxalic treatments and reported them at saskatraz.com.

See http://saskatraz.com/articles/SBA2011.pdf for the entire presentation.  Only excerpts are shown here.

Click to enlarge

Figure 3. Mean varroa drop per day from June 18 to November 23, 2008 at Saskatraz-Q.

Formic acid treatments (mite wipe pads, 60% formic) were made on September 30th and October 17th . Oxalic vapour treatment was performed on October 25th.

In the fall of 2008 we treated Saskatraz-Q (Figure 3) with formic acid (mite wipes, 60% formic) on September 30 th and October 17 th . The varroa drop rate showed a decline, until the second treatment on October 17 th , where an increased drop rate was observed. The colonies were winterized in insulated wraps at the same time as the second formic treatment.

On October 25 th , the colonies were treated with oxalic vapour. The outside temperature was 4°C. A massive increase in varroa drop rate occurred over the next few days falling back to pretreatment levels within 14 days.

In the spring of 2009, only three colonies out of the 12 survived. The three surviving colonies had mite infestation levels below the apiary mean % adult bee infestation of 20%, but three colonies with the lowest mite levels (SAT-96,7.5 %; SAT 93, 4.5%; SAT-84, 5.3% also died). All colonies with mite levels above the apiary mean, 25% died.

Scanning electron microscopy of dead bees from colonies treated with the formic-oxalic treatments showed reduced hair coats when compared to untreated bees (data not shown). Oxalic treated bees showed oxalic crystals on their hair coat. Oxalic fall treatment at 4°C outdoor temperatures was very effective at killing varroa on adult bees. However, the combined formic-oxalic treatment may have caused excessive stress to the colonies, adding to colony mortality.

Figure 6. Effects of one and two consecutive oxalic treatments 3 weeks apart.

The blue bars give percent varroa infestations on adult bees prior to treatment, red bars 54 days after first treatment. The green bars show that oxalic sublimation is very effective at removing varroa mites from adult bees, with two applications removing over ninety percent of the phoretic varroa from the adult bee population.

Analyses of varroa drop on sticky boards indicated the varroa were all dead. It has been suggested that oxalic crystals condense from the hive vapors on to varroa mites and honey bees. We have observed the crystals on bee hairs by scanning electron microscopy after fan driven oxalic sublimation and have experiments in progress looking at the effect of sublimation on the honey bees external surface.

Oxalic acid is thought to kill varroa mites because of the dehydration effects of the attached crystals. In addition, depending on sublimation conditions and temperature of the heating pan some formic acid may be released

Click to enlarge

Figure 7. Varroa drop over time after oxalic sublimation (2 grams/colony)
 Treated (red); untreated (blue).

Figure 7 shows the varroa drop over time after oxalic sublimation experiments. The blue line shows varroa drop in untreated colonies, red in oxalic treated.

The highest kill rate occurs in the first 6 or 7 days, decreasing between 7 and 14 days and leveling off after three weeks. Our second oxalic treatment was based on these observations.

Preliminary investigations into the effects of oxalic sublimation on varroa mites in sealed brood indicated some varroa kill on pre-emergent adults. The efficacy of these treatments will not be known until spring.

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Wednesday October 19th 2011
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I see the drops are leveling out already, at about half the expected total, so either my estimate of total mites is high, or it is looking as if the treatment will be only 50% effective.  Time will tell.

It is interesting to note that although mites fall randomly under the cluster, mites also tend to be found concentrated in specific small areas on the bottom. This suggests to me that the mites are not at all evenly distributed throughout the cluster and that the alcohol wash we take for granted as being the Gold Standard may well be quite variable.

It seems also that there is brood in the hives still, some of them at least, since I am seeing some pale coloured mites drop.   Randy explains something about that below on Bee-L:


> > (AD) A question for Randy: >

(RO) I'll give it a shot, Allen, but I'm just in from a very long day of pulling honey and putting on pollen supp, and the data is no longer fresh in my mind.

 > > > (RO)... In an experiment I recently ran, I dusted a single-story colony with > powdered sugar, collected the mite drop for 1 hour, then killed all the > bees, washed the mites from them, and counted ‘em all. During that hour, 34% > of the phoretic mites fell onto the sticky—in one hour, I collected as many > mites as I would have in nearly 6 days of “natural” mite drop >

(RO) With a short-term powdered sugar accelerated drop, most of the mites that drop are phoretic. In a natural drop, if brood is present, about half of the mites that drop are ones coming out of emerging brood, that would not survive for more than a few hours (Lobb 1997 Mortality of Varroa jacobsoni Oudemans during or soon after the emergence of worker and drone honeybees, Apidologie 28:367).  (Emphasis added)

(RO) So the two methods are not directly comparable. What I was saying was that the accelerated drop caused as many mites to fall, as natural drop would in 6 days. This wasn't meant to imply that 34% of actual, viable phoretic mites drop in 6 days.

> (AD) I read several other pages on your site and and still could not figure out > exactly what procedure you used to apply the sugar. >

(RO) Wow, you are right! I've given ppt presentations with step by step photos so many times, that I thought that I had put them into an article. I will try to get them onto my site (I'm sending you a ppt of them).

> > > (AD) Makes the sugar, however you did it, look pretty good!

> (RO) I don't use it any more. I find the other natural miticides to do an adequate job.

> > > (AD) At this point, I am trying to guess mite populations from a natural drop. > Using a > 100 multiplier

(RO) This is one of the problems with natural drop--in order to use it to predict the total mite population, you need to use a multiplier, which varies greatly depending upon the amount of brood and the season. That is why I attempted to work such a conversion into my Excel spreadsheet, by scouring the literature for actual data in which natural drop was compared to measured total mite populations. You may find it useful...

(RO) I'd like to clarify the 34% drop figure.  Of the total number of phoretic mites at that moment, 34% were dislodged from the bees within the first hour of dusting. Over the course of 24 hrs, with a really good dusting, you will cause the drop of close to 50%. Unfortunately, if there is much brood, up to 70% of the total mite population can be in the brood, and will emerge over the course of the next 12 days.

(RO) You can see the result of these mathematics in my "Brass Tacks" article--sugar dusting can be effective, provided that it is done thoroughly (frames stacked, humidity not too high), and frequently. Dusting broodless colonies daily for 5 days will eliminate most mites.

(RO) If colonies contain brood, then the break even point with dusting (at which the mites reach zero population growth) occurs at roughly weekly dustings.  If one wishes to actually drop the total mite population, you'd have to dust more frequently.

(RO) Even though I no longer dust, it is a very effective method for estimating mite populations, and a mite management tool that can be used by recreational beekeepers, provided that they realize its limitations.

(RO) I know of plenty of recreational beekeepers who keep resistant stock, and simply dust several times when mite populations climb, and manage to maintain healthy colonies. Commercially, though, I don't find it to be the most cost effective method of mite control.

> > >  (AD) Even if half that 6% were non-viable mites emerging during the six days, > that > would still require a daily natural drop of around 3% of the total mite > population. > That seems high to me under most conditions. What am I missing?

(RO) What I was saying that the 1-hr dusting drop equaled the natural drop that I had counted for the previous 6 days. It was merely an observation of the data, not theoretical. But let's do the math to see whether it was a fluke.

(RO) Say that there are 100 total mites in the hive, 2/3 in the brood (although likely half at some times of the season). So 33 are phoretic. If dusting causes 34% of these to drop, then about 11 would drop.

 (RO)  Let's compare that figure to see if it would equal the expected natural drop for 6 days: If 2/3 of the mites are in the brood, that makes 67 of the hundred in the brood. They will all emerge within 12 days, so about 6 would be emerging per day. If 50% of those drop naturally, that is about 3 per day.

(RO) So we are in the same ballpark. Does that help?

You can read more of Randy's thoughts and  experiments at http://www.scientificbeekeeping.com

Later, I wrote this:

> (RO) The results of this study also indicate that mite-sampling data can be >highly variable. Mite numbers from sticky board samples were found to vary >by as much as 250% in as little as two weeks.

I've seen alcohol wash vary by as much as 850% _in one hive_ and my recent observations seem to indicate that in lightly infested hives, at any rate, mature mites tend to drop in patterns, rather than randomly under the cluster.

That is tentative, of course since I don't know exactly where the cluster is in these heavy hives right now, and any patches of brood -- and I have not looked,  but the debris on the board and experience makes me think that there are mite "hot" spots in the cluster. (Of course, that is why we try to sample from an area of brood, but it may not be that simple).

Moreover, in a three and four storey hive like mine, some falling mites could well be getting hung up in burr comb on the way down, as my frames do not all hang perfectly scraped and exactly above one another the way they do in most BEE-L reader's and all researchers' hives. When I do this, it is not an exact process, but rather a medley of errors which hopefully cancel and do not obscure the signal.

> (RO) These data make it difficult to set mite number thresholds for beekeepers to > use when making management decisions for colony treatment.

That is why single measurements or even single studies are not very reliable, and I believe that the useful thresholds are set based on large samples with outliers thrown out and with a margin of safety (grain of salt) added.

Some published thresholds IMO skate close to the edge and beekeepers believe them at their peril. Prudent beekeepers add an additional margin of safety.

That said, I am starting to conclude that my mite loads were not as high as I thought going in, judging by the mite mortality. I had a poor record in the days before fogging with oxalic because I was just beginning to monitor when the machine showed up and we blasted them.

Since then, I have been digging and writing and pondering, and recording my findings and thoughts at http://www.honeybeeworld.com/diary

Comments and criticisms are welcome.

Talk back here

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