I often question those who promote the haphazard introduction of plant
essential oils into hives.  Plants produce many of these secondary
metabolites due to their toxicity or repellency to insects.  Bees, are
insects.

I've experimented with quite a number for varroa control, since many have
shown promise in the lab.  And I've tested some off-the-shelf blends sold
by beekeepers.  Although lemongrass oil at low concentration mimics Nasanov
pheromone and is attractive to bees, it appears to me that bees in general
are stressed by essential oil treatments.  Brood may be killed, and the
colony will move the broodnest away from the vapors.  To date, the only
essential oil that I've been impressed with is thymol (actually sold as a
crystal at room temperature).

That said, we use thymol with great success every season for varroa
management.  We apply it during our prolonged nectar and pollen dearth in
August, at a time when colonies have already reduced broodrearing.  The
treatment further reduces broodrearing, thus exposing the remaining mites
to the product.  Once the treatment is removed from the hive, the colonies
quickly rebound, despite some residues of thymol remaining in the combs.

Bottom line: thymol is rough on the colony, but even rougher on varroa.  We
hit our hives hard with it at a time when there is little brood and the
weather is warm to hot, then allow the colony to recover.  The rest of the
season we allow the bees to make their own decisions as to which essential
oils they may wish to gather from local flora.


-- 
Randy Oliver
Grass Valley, CA
www.ScientificBeekeeping.com

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