The assertion was made that screened bottoms were *never* meant to be
used alone but with other treatments. If you look at this history of
it, however, it was claimed that the bottom board could *eliminate*
chemical treatments.

The history of bottom screens:

Original idea, posted by Jean-Pierre Le Pabic at
http://www.apiculture.com/plateau-anti-varroas/index_us.htm

quote:

The principle of the anti-varroa bottom board has come from the fact
that: wild colonies of honey bees can be found, free of varroa, in
highly infested areas. The assumption is that many varroa mites fall
from the colony and are eliminated in this way, having no means to
climb back. On the contrary, in a hive, they do not have any
difficulty joining up with their initial environment from the classic
bottom boards. The Legris anti-varroa bottom board is designed so
that they fall through the tubes under the hive.

According to my experience on my colonies, *this bottom board
eliminates any need for chemical treatment*. The varroa mites are not
fully eliminated, but they remain in such a small number that they
are no longer harmful. In the worst case, only one treatment per year
might have to be done, which is enough to economically justify it.

-- end quote


quote from Apis:

Drs. J. Pettis and H. Shimanuki of the USDA-ARS Bee Research Lab
published their results using especially designed bottom boards to
reduce Varroa populations (American Bee Journal, Vol. 139, No. 6,
pp.471-473, June 1999). The good news is that in the 30 colonies
fitted with modified bottom boards, there was an approximately 14 and
28 percent lower mite fall when compared with normal bottom boards in
June and July. The bad news is that the results were not
statistically significant, and by September mite levels in all
colonies (experimental and controls) reached damaging levels. Thus,
it appears that the modified bottom boards slowed Varroa population
development, but cannot be relied on as a single, effective treatment
for these mites. The authors remain optimistic that physically
separating the bee nest from the hive floor by use of a wire mesh (#8
hardware cloth) used in conjunction with resistant stock, smoke,
dusts or other control agents should provide a more integrated
approach to Varroa control, reducing use of conventional pesticides
in the bargain.

The above conclusions coincide with those of Troy Hart and Dr. R.
Nabors at the University of Missouri, who studied pollen traps to
control Varroa (American Bee Journal, Vol. 139, No. 5, pp. 366-367,
May 1999). The principal is the same. The bee's nest is separated
from the trapped pollen by a screen mesh.

A byproduct of Drs. Pettis and Shimanuki's investigation was that
colonies on modified bottom boards produced significantly more brood
seven weeks after being established with packages than those with
conventional bottom boards. The authors state that this result
appears to correlate with those obtained using a slatted rack. In the
same issue of American Bee Journal, pp. 747-476, Dr. Keith Delaplane
at the University of Georgia reports on a three-year study using the
slatted rack. In this investigation, the modification did produce
more brood near the hive entrance, but did not result in a general
brood production increase when compared to controls.

-- end quote


Note the following:
        * by September mite levels in all colonies (experimental and
controls) reached damaging levels
        * more brood near the hive entrance ... not ... a general
brood production increase when compared to controls
        * The authors remain optimistic

(Optimism is a good thing but not when it obscures the facts.)

 From Hoosier Buzz

quote:
[Rinderer] tested an IPM program of Screened bottoms, formic acid,
both, and no treatment. Results were as follow for Russian stock - 1%
to 8% using all four. Using both formic acid and screen did have
lowest mite population, no treatment had highest mites but still low.
Domestic stock results were, 9% for both, 21% with formic acid, 69%
with screen, and 70% with no treatment. It was also found that the
Russians are more hygienic then domestic stock.
-- end quote

This study seems to point to an increase in effectiveness of formic
acid, but this small increase pales before the effectiveness of
requeening with Russian stock.

        Russians with formic and traps: 1% infection rate
        Russian controls: 8%
        Domestic with formic and traps: 9%
        Domestic controls: 70% infection rate

--

Peter Borst
Ithaca, NY
[log in to unmask]
http://www.people.cornell.edu/pages/plb6