[data from the 'Managing Varroa' DEFRA handbook, 2005]
"In the UK researchers agree that it is wise to aim to keep the varroa population below about 1,000 mites; above this level the risk of damage from the mites... can quickly become very significant."
The table above shows at what point a dangerous level of infestation is reached, with the number of mites per day found on the varroa floor plotted against the month. Anything over 9 mites/day, no matter what time of the year, is considered a 'severe risk' of total colony collapse (this is, of course, when there is no treatment being applied - I hope to find considerably more dead mites than that now that I've applied the varroacide treatment).
As this table depicting varroa population increase shows [number of mites in the infestation plotted against number of days], anything but a very effective varroa treatment is almost pointless - if I only remove 80% of the infestation it will be right back at exactly the same danger level in two and a half months. This is why I have to use a number of treatments, and not just now but continually.
