Bob Harrison wrote:

> In fact David Vanderdusen told us at
>the ABF convention that he recommended a miteaway treatment every four
>months in migratory operation with intense brood rearing.

This has been known for 20 years. The following is from June 1988:

Frank Robinson, chairman of Florida's Varroa Task Force and secretary-
treasurer of the American Beekeeping Federation, suggested:

A. Varroa will eventually spread throughout this country.

B. Eradication isn't possible and the best that can be hoped for is to
maybe slow advance of the pest.

C. Chemical treatments are not going to achieve a "mite free" status
so keeping populations of mites below some economic threshold level
should be the goal.

D. The longest possible interval between treatments and the least
numbers of colonies that are treated should be the goal of any control
program.

E. Residues in honey and/or wax may be unavoidable, but will almost be
assured if regulations require or even encourage "preventative"
treatments.

F. As far as possible, the normal movement of colonies, queens and
package bees must be a goal of any program.

* * *

> Dr. Eric Mussen in his January/February 1992 issue of From the UC Apiaries published a piece called "Varroa Getting Nasty." It seems many beekeepers in California got a surprise when their colonies collapsed last fall. The symptoms at first seemed to be classical for tracheal mites: 1. rapid loss of adults; 2. tiny clusters of bees with a queen; and 3. abundance of stored honey and pollen. Not characteristic was varying amounts of capped brood. The latter revealed that something else was going on; the adults were not being replaced. Developing pupae were killed in their capped cells by mites and never emerged.

> To prevent colony collapse, Dr. Mussen suggests checking bees for Varroa two to four times per year. Finding a mite or two doesn't mean the colony is in immediate jeopardy, but it will require treatment sooner or later. And if another check, not too much later, turns up a lot of mites, then you are the unlucky recipientof someone else's failure to detect a problem.

* * *

Rob Currie wrote in the Canadian Entomologist, 2006

> Honey production was improved by spring acaricide treatments. When the mean abundance of V. destructor was 2 mites per 100 bees in mid-April, honey production increased from 66 kg per colony in untreated colonies to up to 116 per colony in colonies treated with acaricide. When V. destructor levels were 21 mites per 100 bees in mid-May, spring acaricide treatments increased honey production from 1.3  kg per untreated colony to up to 48  kg per acaricide-treated colony. For the prairie region of Canada, producers will need to assess colonies in both spring and fall and treat when the mean abundance of V. destructor is more than 2 mites per 100 bees in spring to prevent losses in honey production. Producers should treat when the mite level is greater than 4 mites per 100 bees in late August to early September to prevent fall or winter colony loss. In this study, colonies with varroa mite levels of more than 17 mites per 100 bees in late fall experienced significant winter loss.


Sources:

http://entomology.ifas.ufl.edu/sanford/apis/apis88/APjun88.HTM

http://entomology.ifas.ufl.edu/sanford/apis/apis92/APAPR92.HTM

http://pubs.nrc-cnrc.gc.ca/ent/n05-024.html

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