> Bill , can you explain more about the "oregano" source. I couldn't open the
> ars link.
>

 Quote from the study- The compounds used were origanum, 2-heptanone, thymol
and cinnamon oil. The main component of origanum is carvacrol

When I searched for carvacrol it came back as a component of oregano
essential oil. Only thymol and carvacrol were found in the larva.

 The recommended dose was 0.5 ml per litre of syrup, which is much more than
> the drops you mention.
>
>
Using my converter program, about 15-16 drops equals one ml so we are in the
area of 6 drops per liter (7.5 to 8) and well within the larger range found
on the internet. Plus, one drop (.065 ml) is within the eyeball  range of
error. So .5 ml is probably just as good as 6 drops since eyedroppers are
less calibrated than something used to measure .5 ml. Also, remember that
thymol as an essential oil is already at 75% so .5 ml is fine.

It sounds like .5 ml was arrived at just because it is an easily remember
amount and much better than six drops (.34ml) or 7 ( .46 ml) or 8 (.52).

Dave Cushman's site

http://www.dave-cushman.net/bee/naturaloils.html

has some Ld-50  figures for essential oils. Thymol was100 ppm and the max
amount fond in the study for larva/pupa was 50 ppm so it is 1/2 the Ld-50
(granted that Dave may be off, but his research is uniformly excellent). The
paper summary is a bit confusing as you cannot tell which oil was 1 million
ppb (1 ppThousand, which would be 1ml in 1 liter). Thymol was found at
50ppm, but without knowing the initial feed, it is difficult to know what is
going on.

Right now, based on many beekeepers who seem to enjoy some success from
Thymol, the 6 drops per liter seems the safe way to go. .5 ml is a step
closer to Dave's LD-50, but seems relatively safe because it is diluted oil.
I have no idea what the concentration of oregano should be since most who
use Thymol add lemon grass and spearmint, not oregano. My guess is the
origional essential oil feed was the latter two and adding thymol later is
what really gives any control. Sort of like FGMO which never worked until
thymol was added and then it worked better. It was not the FGMO but thymol.
Might be the same here, overkill but harmless. If I try it out, it will be
thymol all by itself.

Oregano essential oil is much cheaper, but the study only noted that
concentrations found in larva should be enough to control Varroa but there
was no follow up (to my knowledge) to confirm the guess. Thymol is a known
as far as Varroa control. BTW, one oz of Thymol essential oil would treat a
bit below 60 liters of syrup (at 8 drop application rate). One oz runs about
$7-16.

Bill Truesdell
Bath, Maine

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