Don’t you love it when someone validates your opinions? Especially someone who knows something? Here’s what happened. Last week I wrote a post called Mitekeeping for everyone, in which I say that the most important thing a beekeeper can learn is the Varroa mite life cycle and how it relates to the honey bee life cycle.
Then, the very next day, I discovered an article in the new Bee Culture (October 2012, Volume 140, Issue 10, p. 22) called “Varroa Mite Reproductive Biology.” In it, Michigan State University bee researcher Zachary Huang writes, “The Varroa mite is currently the most severe pest of managed honey bees worldwide. Understanding the Varroa mite’s reproductive biology will therefore allow us to better manage this important pest.” Exactly right.
In the article Zachary explains many of the factors that affect mite reproduction and life cycle, including the brood caste, the age of the host larvae, details of the phoretic stage, the host species (Apis mellifera vs A. cerana), cell size, etc. But the thing that most caught my attention was a short paragraph on hive humidity.
According to the article, mite reproduction drops significantly when the relative humidity is high. In one study (Kraus and Velthuis, 1997) at 59-68% relative humidity, 53% of mites produced offspring, whereas at 79-85% relative humidity only 2% of the mites produced offspring. Zachary goes on to explain that, he too, has had experience with this phenomenon in his lab.
Right away I contacted Zachary to ask him if anything is known about how this works and whether it is wise for beekeepers to continue lowering hive humidity in the winter. I was really worried about this because hive moisture is my favorite “bad guy.”
He responded that 1) nothing is known about how high humidity affects Varroa reproduction and 2) it is still good to control hive humidity in winter for other reasons. And since there isn’t much brood being raised at that time, the effect on mite reproduction is probably minimal.
Okay, I can relax for the moment. But seriously, read the article. It is filled with tidbits that will improve your Varroa mite i.q. It certainly improved mine.
Rusty: This article confuses me. Mr. Huang says that high humidity reduces mite production and then tells you that it is not known if humidity effects the mite production or not but that it is wise to control the humidity anyway.
My question is, should the humidity be kept high or low during the winter months? I am now setting up my hives for winter and plan to keep the humidity as low as I can. Do you think this is the right way to go.
Mr. Huang says that high humidity reduces mite production for sure, but they don’t know how or why it does. In other words, at this time they don’t understand what it is about high humidity that reduces mite production.
When I asked him if keeping hives dry in winter was the wrong approach, he said no. The reason being that so very little brood is raised in winter that mite reproduction is low anyway.
There are many other reasons that hives should be kept dry all year long. High moisture in a hive promotes many diseases, and it can chill the brood and the bees in the winter. In summer, too much moisture reduces honey production. Also, other parasites such as small hive beetle thrive in high moisture.
As with any scientific inquiry, we learn things in bits and pieces. This report of high humidity being detrimental to Varroa mites is a clue that something else is going on, but we don’t know what it is. It may turn out to be not the moisture directly, but maybe a mite disease that thrives in that environment . . . or something else we don’t know about. Until we know the mechanism, we don’t know how to apply that knowledge to beekeeping.
So, at least for now, the same old recommendation applies: keep your hive as dry as possible all year long but especially in the winter.
The other aspect to hive humidity is that SHB reproduces at high humidity levels. Between 40-45% humidity is the threshold for SHB reproduction. Anything lower and the larvae can’t pupate. Properly ventilated colonies will experience better results for both varroa and SHB, as well as better winter performance.
When mold and mildew build up inside of hive bodies, it creates a very unhealthy atmosphere for the colony and opens up a whole new host of problems including brood diseases. It is a good idea for beekeepers to make summer splits after harvests to ensure their apiary’s survival. Besides that, raising splits can also help proliferate local survivor bee genetics and can also be sold in local clubs very easily!!
Have a great fall!!!
This is a fascinating finding, and we must research it more.
We did notice that hives that we had the quilt levels had lower mite counts coming out of our winter, if the hive is more ventilated, I would have thought the humity would have been lower. Hence more mites, but we got less? Maybe if the hive is nice and dry the bees can deal with the mites better?
Please keep us informed Rusty, if you hear anymore.
By the way we mentioned this article in our podcast at http://kiwimana.co.nz/10
Thank you for the shout-out. I agree that the finding is fascinating, especially since it doesn’t seem to make sense with other things we know about Varroa. And yes, I usually have lower mite counts in dry hives as well. So I think something is else is going on, or two or more variables are acting in tandem. I will keep bugging Zachary and make sure he tells me the latest . . . which I will pass on.
You’re kidding. I’ve got 78% humidity at the moment. Maybe I’ll never have a mite problem (Wouldn’t that be nice?)
Interesting, of course it does not make sense to try and knock mites in winter. Perhaps raising humidity for 3 weeks in spring and again in fall?
What we really need to research more is the pseudo scorpion (book scorpion) Penn State has been finding some cool stuff about this natural predator.
Indeed pseudo scorpions on paper sound like an idea, but I don’t think people have managed to use them to effectively to control varroa mites.
We interviewed a research technician doing a study on them a while back; it’s some interesting work and worth a listen. But she is having trouble keeping the mites inside the bee colonies.
I guess time will tell with these little guys.
This is an older topic, and there may well be further research on this matter that I am not aware of. I followed the link to Mr. Huang’s article, and have not found any more up to date information on that site.
While I am new to beekeeping, my area of interest is building sciences. One thing that comes up often in regards to indoor health is the topic of mites. One apparently generally accepted factor in terms of mite health in homes is the humidity level. It has been suggested on several occasions that both high humidity and low humidity are detrimental to mite welfare, for the following reasons:
At lower than equilibrium humidity (equilibrium humidity being the lowest humidity level the mite can live in) the mite will simply desiccate given enough time in those conditions.
At higher than equilibrium humidity, the mite will gain excess moisture from it’s environment, reducing it’s ability and desire to feed off a host. At a high relative humidity in general, mold growth has also been shown to slow mite development.
Keep in mind, this teaching was about entirely different mites. Presumably, all mites would operate the same way (even if only because their minute size makes them highly dependent on localized conditions), however mite tolerances could potentially be very different! A quick Google search reveals a book entitled Encyclopedia of Pest Management by David Pimentel, which gives us an idea about the role temperature plays in this. It states that for one species of mite, the equilibrium humidity varies from 55% at 15 degrees Celsius to 75% at 35 degrees Celsius.
This makes perfect sense, of course. Higher temperature would result in quicker desiccation were it not for higher humidity. Unfortunately for us, it would appear as though we must wage two potentially conflicting battles. It would seem the best bet all around would be to maintain the hive at the highest temperature easily tolerable by the bee, while maintaining the lowest humidity tolerable by the bee. According to the COLOSS Honey Bee Research Association (though I’m not personal aware of their credibility as a source), the humidity should ideally be maintained at 60-70% for honey bees – right in the varroa sweet spot.
I expect that Varroa mites, which spend much of their lives inside the confines of a brood cell, could absorb as much moisture as they needed from the larval and pupal stages of the bee. Even phoretic adults often burrow beneath the abdominal tergites, which would afford them some protection from desiccation. But I’m not surprised to hear that the ideal humidity for Varroa is the same as for honey bees. Since the mites co-evolved with the honey bee (albeit Apis cerana, not Apis mellifera) it makes perfect sense.
I was just talking to my friend, beekeeper from Syria with 25 years experience who kept about 200 hives. I was saying that my mite levels are particularly high this year (2017) despite having done some aggressive splitting. He said that it is common knowledge back home in Syria that more rain and humidity cause higher mite levels. Haven’t found much online but was led to this article. I’m curious about this. I’m heading to Apimondia in Turkey 2017 and I will be meeting beekeepers from around the world. I will ask around to see what others think or if there is other research done elsewhere.
This is a great paper, thanks!