Would only apply oxalic once a year at most.
Some research on more.
Efficacy of repeated trickle applications of oxalic acid in syrup for varroosis control in Apis mellifera: Influence of meteorological conditions and presence of brood
Nicolaos Bacandritsosa, Corresponding author contact information, E-mail the corresponding author,
Iosif Papanastasioua,
Costas Saitanisb,
Antonio Nanettic,
Erifylli Roiniotia
Abstract
Oxalic acid field trails for the control of varroosis (Varroa destructor) were carried out in an apiary located on the Mt. Imittos (Attica, Greece). The colonies received four successive applications (approximately one every 16 days) with 4.2% oxalic acid (OA) and 60% sugar solution by trickling method with two alternative types of syringes (an automatic self-filling dosing and a single-use) from the broodright to broodless period. The results indicate that the first three applications (from 6th October to 25th November—broodright period) resulted in 65.3% cumulative mite mortality, while only the last application (after the 26th November—broodless period) resulted in 77.3% mite mortality. Very low outern temperatures reduce to the minimum the bee movability, which may result into a slower development of the OA efficacy. No poor colony growth or queen loss were observed even if the bee colonies were received the four successive OA applications with the last one taken place at a very low outern temperature (6.2 °C). The trickling method using an automatic-filling syringe seems to be a very quick way for applying oxalic acid in large apiaries (approximately 150 hives/h).
http://www.sciencedirect.com/science/article/pii/S0304401707002750
Indirect effects of oxalic acid administered
by trickling method on honey bee brood
F HATJINA,* L HARISTOS
The effects of oxalic acid administered by the trickling method on brood development of honey bee colonies were evaluated (a) by observing the development of marked cells of young (< 3 days old) and old (> 3 days old) larvae, and (b) by measuring the area of open brood for several weeks post application. Oxalic acid, dissolved in a 50% sugar solution, with an end concentration of 3% w/v oxalic acid, was applied twice by the trickling method during summer to 10 colonies. A high percentage of young (12.6% and 9.5%) and old honey bee larvae (10.6% and 5.6%) were removed from their cells after the first and second oxalic acid applications, respectively.
The surface of the open brood area was also reduced by 17.5% after the two oxalic acid applications and
stayed low for about two months. For the same period of time the open brood area in 10 control colonies increased by 34.5%. The two oxalic acid applications removed 60 ± 12% of varroa mites adhering to adult honey bees, while the natural fall of mites measured in control colonies (for a period of 40 days) was 32 ± 4%.
Combining the detrimental effect on brood development with the low relative effectiveness on varroa removal, oxalic acid application by the trickling method when open brood is present is not as safe as has been regarded in the past. Consideration needs to be given to the use of different sugar and oxalic acid concentrations in the treatment solution in order to minimize its adverse effects on open honey bee brood.
http://www.ibra.org.uk/articles/20080612_119/print
