I have posted this in the past, usually at that time of the year when folks are asking 'why did my bees abscond in [September / October / November] timeframes.
Anatomy of a mite crash
To understand the anatomy of a mite crash (often mistaken for a late season abscond by inexperienced new beekeepers), it's important to first understand the biology of the honeybee, as well as the biology of the varroa mite, and more importantly, get an understanding of how the two life cycles interact.
I have read extensively on the biology of both the honeybee, and the varroa mite. For this summary, I will forgo the tedious process of finding and quoting all of the references, but summarize what is now my understanding of how the two life cycles interact, along with some basic arithmetic to show the interaction. Over the years I have always equated the progression of a bee colony to the brood cycle of the bees, and just assumed the progression of the mite population would follow the same cycle since the mites are raised in the bee brood. This is a huge mistake.
As beekeepers, we should all understand the life cycle of a honeybee during the summer season already. An egg is laid, it emerges as a larvae 3.5 days later. A worker is capped 6 days later (day 9) and emerges as an adult bee 11 days later (day 20). The drone is different, capped on day 10 to emerge on day 24, spending 3 more days under cappings than the worker. Most of us think of a 'brood cycle' in terms of 3 weeks because it is a timeframe that is easily remembered, easily transferred to a calendar, and closely approximates the progression from egg to adult bee of a worker bee, the vast majority of the population in a colony of honeybees.
The varroa mite has a completely different life cycle. A fertile female varroa mite has an average lifespan of 27 days during the summer season. The female varroa mite will enter a cell shortly before it's capped to do her reproductive magic in that cell while it is capped. For a worker cell the capped phase is 11 days, and during that time the female will produce one male offspring and averages something like 1.5 female offspring. Since it's not possible to produce half an offspring, for this discussion we will assume the lower bound, and it's one viable offspring. The end result then becomes this. The female varroa goes into the cell, to emerge 11 days later along with one viable offspring. The average length of the phoretic phase is 4.5 days according to much reading on the subject, at which time we will have two viable mites entering cells to reproduce. Both of these mites will produce one viable offspring, but the original foundress mite will be reaching end of life, so at the conclusion of this mite brooding round we will have 3 viable mites in the colony as offspring derived from the original foundress mite. Accounting for 4.5 days of phoretic behaviour before these 3 enter cells, we are now 31 days from the start of the cycle, and have 3 viable mites in the hive. So the simplified way of looking at this, the mite population will triple in 31 days, about once a month, during a period when the mites are propogating in worker cells.
Things change when drone brood is present. The drone brood is preferred by the mites because of the longer capped period. After 4 days of phoretic behaviour a foundress mite will enter a drone cell that will be capped for 14 days instead of 11. Literature suggests that the average success rate for offspring in drone brood is 2.5, so again, simplify the numbers and conservatively call this 2 viable daughters for a mite that propogates in a drone cell. After the capped period, we have 3 viable mites emerging, which spend 4.5 days phoretic then enter drone brood which is capped for 14 days. When the 14 days are up, we have 9 mites in those cells, one of which is the original foundress and dies from age, leaving 8 viable mites. This process took 36 days to grow from 1 to 8. An increase by a factor of 8 over 36 days equates to doubling the population of mites every 12 days, for easy comparison, lets call that 2 weeks.
So, in a vastly simplified and somewhat conservative set of estimates, we can say the mite population will triple in a month where only worker brood is present, and it will double every two weeks when drone brood is present. Keep in mind, I have ignored the 'half' part of the averages, so this is an extremely conservative description of mite population growth thru the season.
Now we look at a honeybee colony that has a stable population after building up. The queen is laying 1500 eggs a day, so there are 1500 bees emerging each day, and another 1500 dieing off. If you do the math on population size, there will be roughly 30,000 house bees, and an equal number of foragers, this is your proverbial 'booming' hive with about 60,000 bees in total, managing on the order of another 30,000 brood cells in various stages from egg to emerging bee. On July 15 we do a mite wash and count 1%. We washed house bees, and, will make another conservative assumption. Mites prefer house bees, so all the mites are on the house bees, foragers are clean. 1% on 30,000 bees is 300 mites (it would be 600 if we include foragers in our population estimate). Keep in mind, this is just the phoretic population, for every phoretic mite, there are 3 more under cappings, so, the actual mite population is 300 phoretic and 900 under cappings, for a total of 1200 mites. There is drone brood present till Aug 1, so for another 2 weeks. Two weeks later on Aug 1 the total mite population is 2400 mites, but we have reached the point where new drone brood is no longer present, so the population of mites will no longer double in two weeks, it triples in a month. This brings us up to 7200 mites on Sept 1, and left unchecked, that mite population will grow to 21,000 mites by Oct 1.
Now lets look back at our bee population. On July 15 we had a booming hive with 60,000 bees, but the laying rate of the queen is already starting to reduce and by mid September she is only laying 500 or so eggs a day. The bee population still looks huge as we still have roughly 40,000 bees in the colony, but, the dieoff rate of foragers from age now far exceeds the rate of replacement bees being raised. By early October we are down to 30,000 bees total in the colony as they reduce population going into winter. But we have long reached a crossover point by now, 30,000 bees and 20,000 mites, the infestation rate is more than 50%. We have the queen laying 500 eggs a day, so only 500 worker cells available for mites to go into, and we have a few thousand mites looking for a cell to enter. Every cell ready for capping has a mite, many of them more than one mite.
The net result of all this, is very predictable. Timeframes vary by climate, but you can basically set your clock based on when the bees stop raising drones in your area. At this time, the queen rate of laying eggs is reducing, and the mite population triples over the next month, while the bee population decreases and the brood rate cuts in half. By the end of the second bee brood cycle without drone brood present, the mite population is large enough to infest every worker cell that is developing. This results in the perfect storm of bee deaths. We have a generation of foragers dieing off due to natural aging. At the same time, we have a generation of house bees that should be graduating to the forager role, but, many of them were compromised by mites during development, so they are not really healthy and many dieing off prematurely due to various mite related virus issues. At this same time, we have a generation of new bees emerging, all of whom are totally compromised and much of this population is to sick to be of use in the colony. The population is now dwindling so quickly that there aren't enough bees to incubate what brood is left in the hive, so the next generation (which should be your long lived winter bees) are dieing in the cells, chilled. The timing of this rapid decline will correspond with the 3rd brood cycle after they stop raising drone brood in the average case.
A 1% infestation based on a wash or sugar roll in mid July left unchecked, is a dead hive in October or November, they just dont know it yet. Ofc, these numbers are based on averages, so, there will be outliers in both directions. Yes, there will be colonies that survive unchecked with this level of mites, and yes, there will be other colonies that dont make it this far into the cycle. But the averages suggest, you can set your clock starting at the time your bees stop raising drones. Count ahead 2 brood cycles, and the colony will look strong, lots of bees coming and going, nothing to worry about. But that's exactly the time the perfect storm of bee deaths due to mite infestation starts to accelerate and manifest itself in the form of a hive that crashes from 'looks strong, going to be a good cluster for the winter' into 'no bees left' just two or three weeks later. An autopsy of the colony will show virtually no bees left, brood frames with a fair amount of spotty capped brood, now dead, probably a few with heads sticking out as they tried to emerge but didn't succeed. For those who have never seen it happen before, these symptoms must add up to 'they absconded' because it doesn't seem realistic for that many bees to die off so quickly. Reality is, they died, and that many bees did die off that quickly.
I started this thread today because we have reached that point in the season where we need to be thinking about our winter bees, and how the colony is preparing for them. Beekeeping is not an instant gratification exercise, everything you do has it's effect on the colony 6 weeks down the road. An egg laid today will be part of your forager population by the middle of August. If you want your winter bees raised in a healthy colony, the time to act is now.
I will once again posit as clearly as I can possibly state it. A 1% mite infestation on July 15, left unchecked, will result in a dead colony by October.
We see this every year here on beesource, starts mid to late September with a post titled 'why did my bees abscond'. Invariably it comes from folks that didn't heed the 'left unchecked' part of the mite equations statement. This year I am doing a pre-emptive strike, posting my mite tirade well in advance, and come October when the 'why did my bees abscond' thread inevitably shows up, I will post a link back to this posting.
How to deal with the 'left unchecked' clause is a topic for a different thread, we have hundreds of threads with an equal number of opinions on the subject. I would rather that discussion does not come into this thread, it means different things to different people and it is invariably a heated topic that results in screaming matches. My point is, do the check. Count them, and know what you are dealing with. Lets leave the 'what to do about it' to the other threads, this is about understanding how the populations can grow, and why it's important to CHECK.