Varroa mite

2007 October 6

I hate these soooo much . . .

This is one that, as a hobbyist beekeeper, I am all too familiar with:


This is a varroa mite, Varroa destructor[1]. They are a parasite of honey bees, derived from a parasite of the asiatic honey bee Apis cerana. The asiatic honey bee has established a stable host/parasite relationship with their strain of varroa mites, so the mites don’t wipe out their colonies. Unfortunately, the western honey bee Apis mellifera, the one that most of modern beekeeping is based on, has not established such a relationship, and if left alone these mites will wipe out a normal western honey bee colony within 1-3 years, depending on the climate. These mites got into North America (probably on imported bees, but nobody is quite sure when or where), in the early 1980s, and have come close to wiping out both commercial and hobbyist beekeeping in the US several times since then.

When you look at a varroa mite from above, mostly you see their hard, more or less disc-shaped shell. When they hang on to a bee, this shell serves as a shield with edges that fit tight against the bee’s body, making it hard for the bees to pick them off [2]. Their legs are visible when you flip them over (being arachnids, they have eight legs):


These little buggers are surprisingly nimble, they scurry along at a pretty brisk pace and scamper from bee to bee when they get a chance.

What they do to the bees is this:

(1) An adult female in a beehive finds a cell in the comb where a honeybee larva is nearly ready to be capped, and she crawls into it.

(2) She lays four eggs at roughly one-day intervals. The first egg hatches into a male, and the later ones are females. They all feed on the hemolymph[3] of the bee larva as they grow. This weakens the bee larva and frequently causes it to grow up deformed[4], although if there was only a single mite originally infesting the cell the bee usually isn’t killed outright.

(3) Just before the bee larva emerges from the cell, the male mite mates with his sisters and dies [5].

(4) The mites then emerge with the bee, and run through the colony looking for a new cell to infest. During this time, they spend a lot of time riding around on adult bees, who sometimes carry them to other hives[6].

As long as there are only a few mites, it is no big deal. The problem is, it doesn’t stay at just a few mites: they can double their numbers every couple of weeks during the time that the bees are raising their larvae, and the exponential growth can quickly overwhelm the bee population. A particular problem is that, in the fall, the queen bee stops laying eggs right at the time that the mites have built up a huge population. The mites then spend all their time sucking hemolymph from adult bees, shortening their lives. As the number of bees gets smaller, the mites then start getting to be two or three to a bee, killing them faster, and fairly quickly *poof*, no more live bees.

So, what can be done about these little monsters? The first really effective control measure was “Apistan”, which are plastic strips impregnated with fluvalinate (a contact miticide). As the bees brush against the strips, they pick up the miticide, which kills any mites either on them, or on other bees that they brush against. This worked really well at first, although it only affects adult mites that aren’t in the cells with bee larva. The strips therefore have to stay in the hives for 4-6 weeks to make sure that all the mites have a chance to come out and be exposed to the miticide. Unfortunately, while the fluvalinate originally killed 95-98% of the mites, the survivors included mites that had mutations that made them resistant to the chemical [9]. These resistant mites quickly spread throughout the country, and now the Apistan is unreliable.

The same thing happened with the second chemical (Coumaphos, an organophosphate), which had the added problem that it was more toxic to humans than the fluvalinate was.

So, there are currently a huge profusion of different mite control methods under development, some of which work, some of which don’t, and some of which people argue incessantly about. Many of them are not applicable to our climate here in northern Michigan, either because (a) they require fuming the hive with organic acids or essential oils, and only work if the outdoor temperature gets above about 80 degrees F for at least a month[11]; or (b) they require the use of strains of honeybee that are “resistant” to the mites [12], but that are poorly adapted to the rather harsh climate we have here.

What I am currently doing is this: For six weeks at a time, I go out once a week, open up each hive, and use a flour sifter to completely cover all the bees with powdered sugar. This does two things: it makes the bees go crazy trying to groom all this powder off themselves, and it coats the adhesive pads on the mites’ feet so that they have a hard time hanging on to the bees. The mites almost immediately fall or are knocked off of the bees. If this was all that happened, nothing would be accomplished, but I have *also* put screens in the bottom of all the hives that are large enough for the mites to fall through, but small enough that the bees can’t get through. The mites then fall through to either a “sticky board” that I can pull out to see how many there were, or fall down to the ground, but either way they never get back in with the bees, and die. Horribly, I hope. The nice thing about this method is that I can do it any time without having to worry about maybe contaminating honey or beeswax with chemicals, or about the outside temperature, or about whether the interval is just right, and it gives me an immediate check on just how bad the infestation is in each hive at the time. I expect that the mites will adapt to this, but it will buy time for the bees to adapt and evolve better grooming behavior too, so in the long run this is something that should continue to work, although I will still obviously have hive losses from the bees that *don’t* groom themselves. Then again, that’s what selective breeding is all about. Evolution is a harsh mistress.

Sorry about going on about this at such length, but of all the arthropods I’ve shown so far, this is the one that I have spent the most time and effort battling, by a huge margin, so there was a lot to say.

[1] Up until a few years ago, they were all known as Varroa jacobsoni, but then based on genetic studies (and, as it turns out, appearance – “a” and “b” in this picture are V. jacobsoni, while “c” and “d” are V. destructor) it was decided that the ones infesting the domestic western honeybee was (a) sufficiently different, and (b) completely reproductively isolated from the parent species, so that it was useful to consider them to be a new species. Some people seem to want to claim that there were two species of varroa mite all along, and that one of them was able to survive on the western honeybee while the other was not. I think that’s probably nonsense — I think it is much more likely that what we have seen is the evolution of a new species of mite right before our eyes, prompted by the presence of western honeybees alongside of asiatic honeybees[8]. This gave the mites opportunities to hop from one species of host to the other, and they kept it up until one of them survived the hop. This one then reproduced on the western honeybees, her offspring adapted further to take advantage of the new host, and quickly diverged into a new species.

[2] Putting size relationships into perspective: If we had a parasite that was as big compared to us as a varroa mite is to a bee (about 1/10 of our body length), it would be about 8 inches across, or about the size of a very large rat, a guinea pig, or a smallish chihuahua.

[3] Insects don’t have what we would generally recognize as blood, but they do have a fluid that moves through their bodies to carry nutrients and remove waste products. It just isn’t the primary means for moving oxygen through their bodies, and so it has no hemoglobin in it.

[4] This may be due to physical damage to the growing larva, but is more often due to viral infections like “Deformed Wing Virus” that is carried by the mites from one bee to another. Basically, bees have the same sort of disease issues with mites, as we have with mosquitos and tsetse flies.

[5] As you might imagine, this brother/sister mating leads to pretty substantial inbreeding, to the point that some beekeepers claim that the mites are basically cloning themselves. However, this is neglecting the fact that you sometimes get two different mites infesting the same cell, and in that case their male offspring have a chance to crossbreed with each others’ sisters. So, they do get some of the advantages of sexual reproduction, which is why they have not simply gone completely parthenogenic and eliminated the males altogether.

[6] Honeybees that go out to forage for pollen and nectar usually come back to their home hives, but many times they get confused and come back to the wrong hive. The other hive would kick them back out if they tried to just sneak in unladen[7], but if they are carrying a load of nectar or pollen they are allowed in unmolested. If they happen to be carrying a mite on them, the mite then gets in too. Also, the male bees (drones) basically spend their lives flying around from hive to hive, waiting for a chance to mate with a young queen. They are therefore a perfect vector for carrying around the mites, which could be considered a sexually transmitted disease of honeybees. Kind of like pubic lice in humans.

[7] An unladen bee that tries to get into a different hive probably isn’t lost at all, she’s probably trying to sneak in and steal honey (referred to as “robbing”). Bees try to do this to each other all the time. If a hive is weakened enough by mite infestation that bees from other hives can sneak in and rob them, then the mites can hop onto the robbing bees and be carried back home with them. This is why, if one hive collapses from high mite numbers, the mites get carried back to the other nearby hives, and this sudden increase in mite numbers can often make them collapse, too. This happened to me one year, I lost seven out of eight hives after one of them collapsed and the others robbed it out. Then, that spring a bear knocked apart the surviving hive . . . but this is getting into another story altogether.

[8] The problem is that, while the asiatic honeybee does produce honey and can be kept and domesticated in hives, their colonies are not as large as those of the western honey bee and produce significantly less honey. So, in the late 1800s, the western honeybee was introduced into Asia to try to make their beekeeping more productive. Sounds like a good idea, but this is what we got from it.

[9] The resistance problem is particularly bad here, because it is really hard to find miticides that will kill the mites without also killing bees[10]. The problem is this: Imagine that humans are infested with small, parasitic chihuahuas, which for some reason we can’t just pick off and throw away. Now, dogs are more easily poisoned by chocolate than humans are, so we start dousing the infested humans with huge quantities of chocolate. This works at first, but since dogs aren’t all that distantly related to us, it doesn’t take much adaptation for them to become as tolerant to the chocolate as we are. Suddenly, the chocolate doesn’t work, so then we have to go to a second chemical that is more toxic to them than to us: Onions. That works for a while, but then the same thing happens, so we use a third chemical (grapes) which isn’t even that effective to start with, and stops working even quicker . . . and now the chihuahuas can tolerate practically everything that we can tolerate, and its back to trying to get people to pick them off by hand. The situation with the bees and mites is similar: since they are both arthropods, they have similar metabolisms and biochemistries, and so it is hard to find “ideal” pesticides that are fatal to the mite and harmless to the bee. We mostly end up with things that are more toxic to the mite than to the bee, but will kill the bees too if the dosage gets too high, and that the mites can adapt to with relatively little change to their metabolisms.

[10] The beekeeping industry is full of people pushing their current miracle cure for mites (mineral oil fogging, oxalic acid, formic acid, “essential oils”, you name it), and a lot of them will say something that I find to be spectacularly stupid: “It is impossible for the mites to become resistant to my {insert miracle cure here}”. That is complete and utter hogwash. Sure, it is impossible for the mites to, say, become able to survive being dunked in concentrated sulfuric acid, but they don’t have to. They just have to be *at least as capable of surviving it as the honeybees are*. I can kill every mite in my hives anytime I want, but the methods for doing this will kill all the bees too, which kind of defeats the purpose. And, if the *bees* can survive a treatment, it is at least theoretically possible for the *mites* to be able to survive. Even mechanical methods, that just try to knock the mites off of the bees by inducing the bees to groom more intensively, will eventually lead to mite “resistance” in the form of mites that can hang on better or spend less time outside of brood cells where they can be knocked off.

[11] 80 degrees F? For a month? In *Houghton*? HA! HA! HA!

[12] Ah, mite resistant honeybees. What a can of worms *that* opens up. It seems like everybody’s got their pet strain of bees that is “resistant”, but more often than not, they don’t turn out to be resistant when anybody else uses them. And then there are the “small cell” beekeepers, like Dee Lusby in Arizona, who are “regressing” their bees to a smaller size by using honeycombs with smaller cells, basically constricting the bees. They claim this keeps the mites from reproducing either by making the bees mature faster (giving less time for the mites to mature in the cell), or by mechanically reducing the room for the mites in the cell. The problem with this is that some people try “small cell” and claim it works and is the ideal solution to all of our mite problems and anybody who says different must not be doing it right, while many other people try it too, and say it doesn’t work and their bees all died and it’s all lies, lies I tell you! The thing that I find interesting is that the people who say it works seem to be in areas where the “africanized honey bee” has come in from South America (Arizona, where Dee Lusby is, has been considered “africanized” for some time now). The africanized bees are well known to be able to tolerate the varroa mites much better than the bees of European descent. So the question I have is this: Are the “successful” small cell beekeepers successful because they made their bees smaller, or are they successful because they have unknowingly switched from keeping European-descended bees to keeping African-descended bees (and in the process have bred strains of africanized bees that don’t do the whole “stinging casual bystanders to death” thing that they are notorious for)? The problem from my point of view is that, if their success is because their bees are africanized, then this method is no help to me because the africanized bees don’t survive through the winter in harsh climates like this.

5 Responses
  1. October 9, 2007

    Thankyou. Our fence estimates weren’t too terrible, so I think we’ll be getting bees come spring. I was wondering if there were any non-poisonous ways to control the mites, but I hadn’t started looking yet. I’m not particularly “green,” but not having to worry about poisoning the bees or the honey is a lot simpler than just using pesticides.

    When I read your comment about the mites becoming resistant to the powdered sugar treatment I was about to ask how, but then I finished reading. 🙂 Makes perfect sense. I find it odd that the bees don’t *notice* the mites and just groom them off ASAP.

    Can I pick your brain and ask what type of bees you have? I think I’m waffling between Italians and Carnolians, but that’s just from book-learning. Winters here in the Chicago ‘burbs aren’t as bad as you get, but there’s still usually a vicious cold streak in January.

  2. October 9, 2007

    I usually run Carniolans. They have done well for the overwintering, are not too inclined to sting, and while they may not produce as much honey as Italians, they produce enough for a hobbyist. Of course, it probably isn’t all that much difference, I’d recommend just using whatever strain the other beekeepers in your area are using.

    As far as getting started, there is a beekeeper just outside of Madison (Eugene Woller, Gentle Breeze Honey) that will sell you “nucleus hives” if you ask nicely. I like using “nucs”, they are an excellent way to get started if you don’t already have hives with drawn comb. I had to get some bees from him one year after all the beekeepers closer to me had been knocked back so far that nobody could spare any nucs. He’s a very nice fellow, and can probably give you lots better advice about beekeeping than I can.

  3. March 19, 2009

    Dear Beekeeping friend,
    We have dusted our bees with icing sugar and yesterday we counted 250 mites off one hive so we did it again today the same and after about five minutes took off another tray of 207 mites! The weather is unusually dry here in Devon for March days so we will make the most of it and powder every day ’till we get the numbers down. There is not much brood, a small area on two frames, so today we made the awful decision to destroy it and hope that with fewer mites the varroa will not affect the brood quite so badly. We used the thymol treatment, Apigard, at the end of last summer but it has not got the bees through the winter very well. We have used the icing sugar method before but never on consecutive days and never so early in the year. It seems that the bees dont mind too much. They eat up the sugar that doesn’t fall down with the mites. How are your bees surviving? Best Wishes, Jill Beagley.

  4. March 19, 2009

    Well, since I wrote this page, my beekeeping activities have been kind of rocky. The mites got out of control again in the fall of 2007, and that winter all of my hives were wiped out by about February by the mites. I took a break from beekeeping for the last year to try to figure out what I should do next. One of the things that has come out in the meantime in a number of studies, is that the sugar dusting does help to keep the mite numbers from increasing fast, but doesn’t actually *reduce* the mite numbers in most cases. It is a good diagnostic test to find out roughly how many mites you have, and it is a good delaying tactic to keep the mites from getting out of hand until you can get some other treatment on them, but the powdered sugar is not sufficient as a sole mite control method.

    I’ve been asking around for suggestions from other beekeepers as to what I should do, taking into account my specific issues with the climate. The approach that sounds most promising is:

    1. Use the “Russian” bees, which were imported by the USDA from the Primorsky region about ten years ago. These bees have been infested by varroa mites for almost a century, and have evolved some degree of mite resistance. The are also adapted to cold climates.

    2. Around the middle of the summer, it is recommended that I take *all* of the hives in a yard, and make splits and produce as many small nucleus hives as possible. This both produces a break in the brood cycle (giving the bees a chance to clear off a lot of the mites, maybe with the assistance of powdered sugar or other mite treatments), and also ensures that there is no single hive with a huge reservoir of mites, that can collapse and dump a huge mite load into all the other hives, producing the chain reaction that knocks out the whole apiary.

    So, that’s what I’m going to try this year. I have some packages of Russian bees on order for mid-May, and we’ll see how it goes.

  5. Pete permalink
    August 7, 2010

    Thanks v. much for the concise info on varroa – very useful to a newbie as I am.
    Found 3 in my varroa tray today – not too worried yet as it had been in for a couple of days.
    Will try the icing sugar soon to see what collects afterwards.
    BTW – it’s great fun using a magnifying glass to see the little B*****s – then you just home in with the glass in the sunshine and …….**POP** …they go off like a little firecracker.


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