Mites spike in late August-September in my yards. The spike is fast, exponential population increase, and is accompanied by DWV. Unless treated the hives will dwindle and die by November-December.
In past years, I used the proprietary formic pads (MAQS) to knock down the spike. Cannot get them this year. Planning on going to Oxalic vapor (tested on two colonies with an early May erruption of mite). In previous years, I killed a favorite colony with Oxalic -- so the lethal dose vs. effective dose needs careful monitoring.
A very observant beekeeper I know thinks the main mode of formic was that it disrupted the queen and hive, forcing a brood break.
I know Finnski advocates Oxalic drizzle; however in my mild climate, I don't get a true winter cluster and brood is usually present year-round. I've read Dutch research that indicates that the syrup/Oxalic mix is topical. The syrup increases the viscosity, so the oxalic stays on the bee's hair longer-- it is not appropriate for them to feed. They tried other thickners to get high viscosity.
Commercial beekeepers used Taktic (off-label) for years. Taktic is now off the over-the-counter market, but its active ingredient (Amitraz) is in the Apistan strips now licensed in many states. One big operator I spoke with used Taktic mixed in water on hot summer hives, rather than the oil formulations used by others. He felt the water solution contaminated less. Commercial operators managed for pollination (NOT honey), so have many opportunities with supers off the hive.
Michael Bush and industrial operators are converging on a brood-break strategy (though both are likely loath to admit the convergence). The economics of "8 covered frames" hives for $200 pollination contracts are pushing operators to split in April and again in July-August. An overwintered colony yields 6-8 daughter colonies by the next winter. (Typically requeened in April, and a walk-away split in July). This means colonies are brood-breaked several times per year (November, April, August), and the mites are kept down to low levels by the constant colony churn.
Hobby beekeepers manage for colony longevity-- and this means mites build up the second year. A typical pattern is good nearly-mite-free thrift the first year (off a late spring nuc or package), and the fall of the second year the colony goes to hell. Unfortunately, the novice beekeeper extrapolates that his management caused the first year health, and becomes convinced that his particular management hocus-pocus (Warre hives, no-treatment, sage smudges) is a panacea.
I want to make a critical comment on the no-treatment, survival colony strategy. This works where the keep can control the total breeding population of drones (via isolation or saturation). I suspect this is the case of MB's yards. Hobby beekeepers cannot emulate this, because the key feature (isolation and saturation) is not present in a backyard freely-mating colony.
Queens are **Obligate** out-crossers for mating. This means the genetics will always revert to the norm. This has huge adaptive advantages for bees-- and lets the species maintain stability (for millions of years). Creating and maintaining genetics that are mite resistant is just not an option for the small-time keepers that dominate this forum -- the whole inertia of the honey bee evolutionary strategy is against it. (By refusing to "evolve", the honey bees pressure the flowers to adapt to their requirements (nectar composition, etc)). On the other end of the spectrum are specialist native bees (say orchid bees) that are highly adapted to a single species of flowering plant-- these are fragile species, as if the plant succumbs so does the insect.
Honey bees are adapting to varroa pressure-- these are called AHB. AHB traits -- swarming several times per year, smaller cell size, high temperature (ie rapid pupation) are all anti-varroa behaviors. The more manageable selected varroa traits (Minnesota hygenic or VSH) may or may not have any role in the survivor genepool where I keep bees.
In short, there is a lot of mystification about "survivor" colonies and mites. Individual hobbyists could not select this option unless they live on an "island" where they can control the genetics. The "survivor" traits may not have any relationship to VSH, there are multiple ways for bees to adapt to Varroa, and the easiest path (for my region) is AHB traits-- and these are bad for husbandry.
I advocate clearly separating the "no treatment" and the "brood-break" approaches. Brood-breaks, either chemically induced (viz. formic) or by "walk-away" splits in the summer, will substantially reduce mite population, but it is a (behavioral) treatment and must be recognized as such. Brood-breaks imply their own minimum requirements to manage successfully.