Recent discussions on economic thresholds for treatment have shown 
widely different views on economic injury.  The view that it happens at 
a very low level appears to be based on the concept that low mite 
levels will be followed inevitably by rapid increases leading to damage 
and therefore early interventions are warranted.  Some of us think a 
much higher level can be tolerated: 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.

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.  
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.  A hypothetical modeled higher level of 
the pest in the future is not a concept used in crop IPM because many 
things can happen to a pest (and its host) from point A to point B in 
the course of a growing season.  Beekeeping differs in many ways from 
row crops, but some of the concepts still have something in common.  It 
may be valuable to consider some of these concepts to avert rapid loss 
of efficacy of control products, contamination of end products, 
unintended consequences that have led to the adoption of IPM in crops.  
Some of the reports of almost constant treatments with amitraz to keep 
mites from taking off in some operations sound a lot like the weekly 
spray planes over cotton in the days before IPM.

Other than the perception that low to moderate levels of mites in 
colonies have to be affecting their performance, production, growth 
because mites eat fat bodies and because mites vector horrible viruses, 
is there any hard data out there?  Has anyone measured colony 
parameters at different reliably counted levels of mites?  Or 
quantified overall viral loads (not just presence or absence, or 
detection vs. no detection) in colonies with different varroa loads?

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