In field crop IPM, we have two different terms: treatment threshold and
economic injury level.

The economic injury level is the level of pest infestation where the
monetary loss due to damage equals the monetary cost of treatment. There is
no point in spending a bunch of money on controlling a pest if the cost of
control is greater than the damage due to the pest.

However, sometimes you have to control a pest before it reaches the
economic injury level, because if you wait, it will be too late (for one
reason or another). For example, sometimes one life stage of the pest is
easier to control than another. Varroa is a good example. It is easier to
control varroa in the fall or early spring because (1) not much brood is
present, and so most of the mites will be out where we can get to them with
chemicals, fumigants, or microbials (it's very hard to kill mites that are
in capped brood cells) and (2) you are not in the middle of honey
production, when you have to be careful about what kind of things you put
in the hive.

Thus, the concept of a treatment threshold is important for varroa control.

Also, in crop IPM, it is generally accepted that a pest which vectors a
disease has a very low economic threshold. That is, a few pests can cause a
lot of damage.

I have to agree with what a few have expressed. A true economic threshold
has not been established for varroa. Keith Delaplane and Michael Hood did a
very nice study on the levels of varroa that will cause colony decline.
However, they didn't actually do an economic analysis to show the cost of
damage (nor did they show what impact varroa had on honey production).
Their numbers are probably a good indicator of what your treatment
thesholds should be, though.

I think we are still learing about the viruses. Brenda Ball said at a
recent insect pathology meeting, "I think the viruses have always been
there, they just were never a problem until varroa showed up and began
acting as a vector." In otherwords, the viruses didn't spread much until
they had a vector. So far, the easiest way to slow the spread of the
viruses in a hive is to control varroa.

I am glad to see beekeepers discussing IPM. If nothing else, more careful
use of pesticides in the hive can go a long way to reducing the incidence
of pesticide-resistance in varroa. In the long run, it usually is worth the
extra effort.

Rosalind James

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