> the concept that low mite levels will be followed inevitably by rapid increases leading to damage and therefore early interventions are warranted. 

Funny you should mention... the Covid situation helps to illustrate that control of the potential carriers  (varroa or humans) is not an optimal way to control viruses.  

Here I sit in the very aptly-named in Madrid airport frequent-flyer lounge named "Dali" (as the experience is suddenly very surreal!).  We were vaccinated a while ago, so we have our vaccination certificates, we also got negative PCR tests before packing, and we had our Honeywell microfiber masks with 2.5 micron replaceable inserts, and our Iberia points card and biz-class tickets, so they let us in.  But our masks were unfamiliar to the staff, and despite the small US Navy emblem on each, and the "2.5µ" marking on the filters, we were asked to don masks that were more familiar to the lounge staff, even if they were likely far less "protective" Chinese knockoffs of Korean "k95" masks.   Regardless of mask quality, we are far safer separated from the majority of the travelers, and my obedience can be easily bought with free drinks and excellent food.  We've been grounded for a solid year, so I'm at my most gracious, even to crazy people who want to defy rules or make up  new ones on the fly.  

For varroa and viruses, this is a good example:
http://dx.doi.org/10.1371/journal.pone.0057540

> Some of us think a much higher level can be tolerated: 

This seems to be akin to not reefing sails on a sailboat in high winds, and risking a knock-down.  There's a trade-off between sailing faster, and the time spent recovering from a knock-down, most of it being the time taken for the lecture from one's significant other about how the carefully-organized supplies for 3 weeks of island-hopping are now in need of total reorganization.  Another analogy would be "drifting" a car around curves - dirt-track racers seem to only travel sideways around the track, but a trivial error here ends one's day, and requires repairs.  

If we had antivirals, we'd be able to react in a more traditional manner, but for now, all we can do is kill varroa to slow viral spread. 

Several quote from the paper cited: 

"Despite the application of acaricide, varroa mites and viral titres continued to rise while remaining below the levels observed in the untreated group. In many cases, the treatment only helped to control varroa loads and viral titres to a limited extend, and led to no significant reduction in the prevalence of virus-sick bees. In part, this may be explained by the recent findings [47], [48], that show negative impact of acaricides on honey bee immunity."

"Adding to this effect of acaricides, the observed failure of the treatment to reduce varroa numbers in several colonies might have led to immuno-suppression syndrome as observed in other studies" [This is a reference to pyrethroid effects].

"DWV has also been suggested to lead to colony losses independent of varroa infestation possibly in synergy with an uncharacterised stress factor [4]. Thus, we hypothesise that the effect of treatment may constitute this stress factor."

So, in this context,"IPM" seems best served by keeping the virus load low with  "softer" treatments (Oxalic/Formic), as it seems to work as well as pyrethroids, without the collateral damage to bee immune systems.

> at moderate levels some compensation from colonies reacting to the higher levels can occur, and actual measurable economic injury does not occur until higher levels are achieved.

But one cannot maintain a "moderate level of infestation" for long.  Isn't more accurate to say that one is gaining a limited benefit from a colony that is on the path to death by PMS? Yes, I can make splits, never treat, and stay ahead, but recent splits won't rapidly  turn into colonies that grade well for pollination, and can't make much honey anywhere South of places that enjoy 18-hour daylight when blooms are blooming.

> According to one of the principles of crop IPM, some economic injury can be accepted as long as its cost is lower than the cost of control. 

But with viruses, the economic injury is very soon "total", if not addressed early and often.   (Beekeepers see a deadout, travelers wander airports with very few shops or services open, and we could have played stickball in  the middle of the street in  Times Square from April 2020 to only recently.   All these are the same thing.)

> Mathematically, the economic threshold for treatment is the level of the pest where the lines for economic injury and the cost of controlling the pest intersect.  

But the ability to control at all is limited to points when infestation is low, so these models do not address a "animal viral" scenario well.  Plants don't move around, so  plant viruses are easier to treat.

> reliably counted levels of mites?  Or quantified overall viral loads 

See the cited paper, as it is one example of exactly this effort.

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