Usually they don't recommend more than one treatment.
I am not sure who "they" are.
There seems to be a bit of internet lore out there with most of the issues seeming to be climate dependent and some at a much higher % then is used in modern times.
When I am forced to I do a round of 3 weekly treatments as a rescue to knock back levels post harvest of hives that have high counts and are in danger of bombing out(replace queen in the spring). I am sure its a bit harsh on them, but they bounce back and winter well( to be fair I would NOT do that to winter bees) Its hot here in Aug, options are limited if I want to stay away for systentnics.
4 treatments
Bacandritsos, Nicolaos & Papanastasiou, Iosif & Saitanis, Costas & Nanetti, Antonio & Roinioti, Erifylli. (2007). Efficacy of repeated trickle applications of oxalic acid in syrup for varroosis control in Apis mellifera: Influence of meteorological conditions and presence of brood. Veterinary parasitology. 148. 174-8. 10.1016/j.vetpar.2007.06.001.
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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 degrees C)."
4 treatments
Acaricidal effect of oxalic acid in honeybe(apis mellifera) colonies
Aleš Gregorc, Ivo Planinc
Apidologie 32 (4) 333-340 (2001)
DOI: 10.1051/apido:2001133
Treatments were applied to twenty-fourcolonies on August 12 as follows: (1)six colonies received a 3.4% OA and 47.6%sucrose in water solution (w/w) using oxalicacid dihydrate (Riedel-de Haën), sucrose(sugar) and de-mineralised water, (2) sevencolonies received 3.7% OA and 26.1%sucrose (w/w) (Nanetti, 1999), (3) sevencolonies received 2.9% OA and 31.9%sucrose (w/w), (4) four-control coloniesreceived de-mineralised water only. Treat-ments were repeated on August 21, August30, when the brood was present, and on October 28 during a broodless period
In OA treated colonies no queenlessnesswas recorded and worker bee mortality(3.25 ± 1.25 bees/day) five days after Augusttreatments were not significantly differentfrom control colonies (2.5 ± 0.58 bees/day,P> 0.1). Mean bee mortality after OctoberA. Gregorc, I. Planinc336Figure 1. Natural mite mortality per day observed in three pre-treatment periods (groups A, B, C), mitemortality following three oxalic acid (OA) applications in August (group D), post treatment mitemortality per day (group E), one 3.7% OA/26.1% sucrose application on October 28 (group F) andmite mortality following two Perizin applications to all colonies (group G). Groups D and F give themite mortality per 4 days and group G the total mite mortality. Bars indicate standard deviations.
OA treatment was on average 1.63 (± 1.32)bees/day and was significantly lower thanthat observed after Perizin treatments (11.54± 3.49 bees/day). Over-wintering coloniesand early spring development in the year2000 were normal without visible change
4 treatments
Gregorc A, Planinc I. (2012) Use of thymol formulations, amitraz, and oxalic acid for the control of the varroa mite in honey bee (Apis mellifera carnica) colonies. J Agric Sci. 56(2): 123–129.
Experiments were conducted in three apiaries to assess the comparative effi cacy of: Thymovar (Andermatt BioVet AG); Apiguard (Vita Europe Ltd., UK); an oxalic acid solution (OA) which consisted of 2.9% oxalic acid and 31.9% sugar in water; and amitraz fumigation, for controlling the honey bee mite Varroa destructor. Mite mortality increased signifi cantly (p<0.001) in the Thymovar, Apiguard, OA or amitraz treated colonies. The relative mite mortality after: four OA applications, two Thymovar or two Apiguard applications during August and September in the Senično apiary was 41.80% (±14.31), 14.35% (±10.71), and 18.93% (±13.56), respectively.
8 treatments a year for 8 years
Maggi, Matías & Damiani, Natalia & Brasesco, Constanza & Szawarski, Nicolas & Mitton, Giulia & Mariani, Fernando & Sammataro, Diana & Quintana, Silvina & Eguaras, Martin. (2016). The susceptibility of Varroa destructor against oxalic acid: A study case. Bulletin of Insectology. 70.
A commercial apiary composed by 54 colonies of a re-gional ecotype of A. mellifera located at Federal (30°57'4.42"S 58°47'55.78"W: Entre Rios province, Ar-gentina) was selected as biological model for the assays. In this apiary, the OA was topically applied as the sole option for Varroa control during eight consecutive years (2000-2008). Thus, this V. destructor population was considered as the „focal‟ population in our study. An average of eight treatment applications (± one) per year was made
Here is the treatment chart for Varromed, a pre mixed OA dribble aproved in the EU
It suggests up to 5 treatments in a row and a max of 9 per year