From: "James Fischer" > The key point about "IPM" is that one must know pest
population
> in terms of how much damage the pests will do.  The dead-giveaway
> with varroa is that their population growth starts to "go exponential"
> or "spike", and the resulting increased population of mites is what
> "overpowers" a colony."

Is this strictly true as written?
European researchers seem to say it is not generally the mites that cause
colony collapse (bees are weakened but not killed ) but the associated
increase in viruses.  These do maim or debilitate individual bees and as the
PROPORTION  of affected bees increases colony life becomes unviable - there
are too many under-performers to be carried by the healthy bees.
The proportion of affected bees increase as the mite population increase BUT
ALSO  as the bee population decreases in autumn.  So a colony may can carry
say 2000 mites amongst say 60000 bees in summer (very broardly 3%
infestation), and still survive when the mites have increased to 10,000 (15%
infestation) , but is likely to collapse when the bees have declined
naturally to 15,000 in autumn ( raising the infestaion level to broardly
60%).  [All numbers only illustrative].
It has been posted that mite populations are larger in large colonies. Is
that true?  2,000 summer mites have ample breeding opportunity in even a
small colony, so competition for breeding sites/brood cells will not create
a check until the proportion of mites to brood has reached very high levels.
The queen in a small colony can still be laying say 1,500 eggs a day - and a
mite population of 2,000 needs at most say 2000/21=95 brood cells being
sealed per day - brood production in a larger colony  may be a bit higher
but that would not confer any benefit to the mites.
The concepts of economic threshold and economic injury level are interesting
but are these the most helpful for beekeepers?  Economic Injury level
suggests we wait until the reduction in injury exceeds the cost of
treatment - but that could apply when injury has reached a high absolute
level - so it is good in a business sense but overall production could be
lowered.  Do we not need to consider the ' most cost-effective
intervention'.  This was expressed as 'a stitch in time saves nine', in the
old days.  The principle of IPM as I understand it is to steadily weaken the
mite population so that it is never brought to very low levels but also
never grows to crisis point.  So, drone culling in spring, perhaps formic
acid or a thymol treatment in late summer and oxalic acid in late autumn.
To do any of this we have to monitor the fall of mites say 3 times a year -
in each colony.  Mesh floors/sticky boards are simple to use - but involve 2
visits close together.  That is generally ok for hobbyists but may be
impractical I realise for large operators with large numbers of hives in
distant locations.  They presumably have to use immediate sampling of just a
few hives, say ether roll, and accept the lower accuracy with regard to the
apiary as a whole.

Robin Dartington

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