So often we hear of beekeepers losing a hive late summer or fall. The symptoms are often similar -
- the colony was strong
- they couldn’t see anything wrong when they last looked
- plenty of honey stores
- very few bees now, where did they all go
- a few dead bees on the bottom of the hive
- beekeeper saw no sign of mites
The last, “no sign of mites”, is the red flag.
Mites are not easily visible. You have to test for them. If you are in the USA then you can be sure you have mites, the question is not if, but how many.
Some prefer the sugar roll, as it doesn’t kill any bees. It is not as effective at counting mites, so be aware of this if you use it. Alcohol wash is more accurate, but kills a cupful of bees.
Both these tests only test for the phoretic mites (those mites attached to a bee). It doesn’t test for the mites that are in a cell, safely capped under the brood.
I read that the varroa population in a hive triples every month. I did a mathematical model to verify this. I was able to confirm that varroa do indeed triple every month.
So one mite in the hive at the beginning of March,
will be 3 in April.
9 in May,
27 in June
81 in July
243 in August
729 in September
2187 in October
and 6561 in November.
This is the total number of mites in the hive. You may have often read that 80 to 85% of the mites are not phoretic. My model showed that this percentage varied a lot while there are only a few mites. Anywhere from 0% to 100%. But later in the season, when the numbers are high, I found around 80% to 95% under the capped brood, and only 5% to 20% phoretic.
That is assuming you only had one mite in March. Mites can survive the winter inside the hive, residing in the cluster with the bees. If you have two survive winter, then these numbers would be double. If your hive picks up mites from either foraging alongside bees with mites or robbing a mite infested neighbor’s hive, then the numbers can increase dramatically.
Our bee club participates in a research project with Univ of Maryland. They issued us with a report which you can view here http://aabees.org/ebooks/S16_SABF_6.pdf
If you don’t want to read the whole report, then skip to the top of page 3 and look at the graph. You can see how our varroa population exploded in August.
A report for the results across the nation is available here http://aabees.org/ebooks/SAAL_FinalReport_2015.pdf
Again, you can skip to the graph at the top of page 3. The mite count first went over the “red line” in August, and had exploded by October. This is when many hives collapse due to heavy mite loads.
When I hear beekeepers say they saw no sign of mites, it is entirely possible at that stage, the mites were there, but in low numbers, leaving little evidence and most were capped under the brood.
If you don’t test you won’t know.
If you do either a sugar roll or alcohol wash, remember that this is only the phoretic mites. These tests don’t tell you how many mites are under the capped brood. You can assume the phoretic mites were about 15% of the total in your hive, but this number might not be entirely accurate.
Oxalic acid vaporizing is a popular method for treating as it is not harmful to the bees and doesn’t get absorbed into the wax. After a few days, the OA crystals are removed by the bees. It is very effective at killing mites that it physically gets in contact with, but doesn’t kill any that are capped under the brood as it is not able to penetrate the wax capping. For this reason it is often recommended to only use OA when the hive is broodless.
If you use OAV when the hive has brood, then you need to take the life cycle of the mite into account. After emerging, a new mite is thought to be about 5 days phoretic. I have seen numbers much longer and shorter, but five days is thought to be average. After this they enter a cell and are capped under brood. Since OAV is only able to kill phoretic mites these 5 days are crucial.
Personally I treat with OAV as soon as I remove the honey supers, which is usually around the end of June since that is when our main nectar flow is over. At this time, the hive has plenty of capped brood, so a once off treatment is not effective. In order to knock back the mite population, without harming the bees, I treat three times, five days apart in early July. This won’t remove all the mites from the hive, but the population will be knocked back substantially. I repeat this regimen in September and then once between Thanksgiving and Christmas, when we have a brief broodless period.
I hope this helps and gives you some guidance on dealing with the mites without harming your bees.
More articles
https://beeinformed.org/2016/03/08/why-did-my-honey-bees-die/
http://scientificbeekeeping.com/mite-washer-still-improving/
http://scientificbeekeeping.com/simulation-of-sugar-dusting-against-varroa/
http://www.bushfarms.com/beesmath.htm
http://oxavap.com/information/
Might as well have some fun while we are waiting for the inevitable… 
