Oxalic acid is a very effective treatment for Varroa but only during broodless periods when the kill rate can be above 90%. When brood is present the kill rate is closer to 30% as most of the Varroa are in the brood where this acid cannot reach them. Click here for more on oxalic acid.
For this reason oxalic acid tends to be used during the mid-winter broodless period – if there is one!
However, winter is not the only time bees are broodless and oxalic acid can also be used during summer broodless periods when the Varroa are phoretic (out and about on the bees).
Imagine a swarm…
Technically, that swarm is Varroa-free. Note I say ‘technically’ – because there are of course a few Varroa present attached to the flying bees but most are left behind sealed in the brood or creeping about on the comb and on the young bees.
This point accepted then you can deal with the other half when all the brood has hatched three weeks later. If you treat with oxalic acid at this time, before the new queen starts to lay, you can expect a spectacular knock-down of mites.
These conditions occur each time you allow a swarm to get away or when you perform an artificial swarm. An oxalic treatment can thus be incorporated into your artificial swarm routine at, or shortly after day 21. Click here for detailed artificial swarm instructions.
However, regard must be given to the possibility of honey contamination.
Oxalic acid is a natural constituent of honey. The concentration varies depending on the botanical origin of the honey but it is at levels that cause no harm to humans and experimental work has shown there is no significant increase in honey due to spring or summer oxalic acid Varroa treatment (Rademacher &Harz). And they must have had their supers on.
Now imagine your way through an artificial swarm…
After you have moved the original box to the opposite side and it loses that second cohort of flying bees it simply doesn’t have the ‘man-power’ to be putting anything into supers. It doesn’t need supers so move them to the parent stand at least for a couple of weeks until the new queen starts to lay. If it is short of stores in the meantime – feed it.
Because oxalic acid has hydrophylic or water-loving properties, it is thought unlikely to accumulate in wax either (Rademacher &Harz).
Trickle or Vaporise/ Sublimate?
I prefer the trickling method, especially for summer, as this allows you to tailor the dose according to the strength of the colony ie only trickle active seams of bees and if they are particularly weak give only 2mls of a 3.2% solution per seam instead of 5mls if they are strong. Click here for oxalic acid recipe.
It is less easy to tailor the dose if you are vaporising.
If, like me, you use Snelgrove boards then oxalic acid can be incorporated into methods 1 or 2 at day 21 as above but it might be an idea to temporarily cover the metal grille throughout the treatment.
Aliano, N. An Investigation of Techniques for using Oxalic Acid to reduce Varroa mite populations in Honey Bee Colonies and Package Bees. (2008) University of Nebraska
Nanetti, A., R. Büchler, J.D. Charrière, I. Fries, S. Helland, A. Imdorf, S. Korpela, and P. Kristiansen. Oxalic acid treatments forVarroa control (Review). (2003) Apiacta 38: 81-87
Rademacher, E.R & Harz, M. Oxalic acid for the control of Varroosis in honey bee colonies – a review. (2006) Apidologie 37: 98–12
Rashid, M, Wagchoure, E.S., Mohsin, A.U., Raja, S., Sarwar, G. Control of Ectoparasitic Mite Varroa destructor in Honey Bee (Apis mellifera. L.) colonies by using different concentrations of Oxalic acid. (2012) Journal of Animal and Plant Sciences, 22(1): 72-76
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