New research shows that applying oxalic acid vapor in the brooding season is a preventative measure, at best — if you stick to the label rate
Since its approval for use in Canada in 2005 and the U.S. in 2015, oxalic acid has become a standard method of varroa management. Though widely known to be most effective in broodless colonies, some beekeepers might feel squeezed in the summer by high mite levels, high temperatures, and maybe a dash of poor planning. Hopguard is approved for summer use, though efficacy is purportedly dubious. Formic acid is also approved for use while honey supers are on, but it is not advised for small or weak colonies. Some beekeepers report problems with queen loss after formic treatment, especially in hot weather, leaving them looking for other options.
In 2021, the Environmental Protection Agency announced that the oxalic acid product label, which dictates legal usage, now accommodates application while honey supers are on. Typically, when supers are on, there’s also brood, and in practice many beekeepers have already been applying oxalic acid to colonies when there is ample brood present.
This is done despite knowledge that the treatment does not penetrate capped cells, where most of the mites reside. The rationale is that repeating applications on intervals of five or six days should expose all the mites to the treatment eventually, thus achieving an effective mite kill even during the brooding season. Right?
Wrong. A new paper by Jennifer Berry and colleagues1 shows that oxalic acid treatment during the summer at the label rate does not lower mite levels compared to pre-treatment infestation, even when the treatment was repeated seven times on five-day intervals. Oxalic acid may be cheap, but that is a lot of apiary visits for a method that does not even bring mite loads down below the treatment threshold, at a critical time of year. “At best, it held the pre-experiment mite levels static,” say Berry, Geoff Williams, and Lewis Bartlett, who are coauthors on the study, in a joint statement..
Season matters
Berry and colleagues applied oxalic acid using the vaporization method, which is considered by some to be easier on the bees. To do this, a small amount of powdered oxalic acid (one gram per brood chamber, according to the American label) is placed in the head of vaporizer, which is then inserted in the bottom entrance of the colony. The device heats up, first melting, then vaporizing the crystals, which quickly solidify again as they cool in the air. This creates a crystalline mist of oxalic acid that coats the bees and hive surfaces.
In their study, Berry and colleagues tracked mite loads of almost 100 colonies in Alabama and Georgia during the summer. Half of those colonies received the oxalic regimen, lasting a total of 30 days, and the other half received no mite treatments. Five days after the last application, the researchers assessed varroa loads in all colonies again, and compared them to pre-treatment levels.
“It is well known that oxalic acid vapour does not penetrate the capping of brood cells, where bees and varroa mites develop,” the authors say. “However, we wanted to investigate if this procedure could still be incorporated into the beekeepers’ toolbox, especially during spring and summer, to keep varroa levels stable until other products could be used.”
Although they found that mite levels in colonies receiving oxalic acid were unchanged compared to pre-treatment loads, the mite population in control colonies increased by over 4% (four mites per 100 bees) during the study. Summer oxalic acid treatments did not make the mite population shrink, but did prevent it from growing.
This means that at the tested dose, while there are brood present in the hive, oxalic acid might buy you some time, but it isn’t sufficient to relieve your bees of varroa on its own.
“The literature suggests that varroa remain in the dispersal phase — on adult bees — anywhere from one to ten days or more,” Berry and colleagues say. “So, it could be that our five-day application intervals missed a lot of dispersing varroa mites.” The authors stress that it was likely the presence of brood, and not the higher temperatures in the summer, that influenced the outcome of the treatment.
Some companies selling vaporizers claim that late summer and early fall are “great” times to use repeated oxalic vapor applications. They even include instructions stating that treatments should be repeated on five-day intervals for four weeks during the brooding period. In light of Berry’s work, these instructions should be updated to indicate that while this method prevents mite population growth, it does not actually reduce mite loads.
Risks of queen caging
Oxalic acid works much better in the absence of brood, and it may be tempting to artificially induce broodlessness by caging the queen. But research conducted by Cameron Jack, Edzard van Santen, and ABJ’s own Jamie Ellis in Florida shows that this can be detrimental to colony health.2
Using ten colonies per group, the researchers tested the efficacy of single oxalic vapor applications, three repeated applications (eight days apart), and vapor treatments combined with caging the queen to induce a brood break. Their results show that, despite all queens being alive after the 24-day caging period, colonies with caged queens were 21 times more likely to perish within the next six weeks.
It is not clear exactly why the colonies with caged queens had such low survival, but Jack, an assistant professor at the University of Florida and the lead author of the study, speculates that it may have been related to the time of year. “We applied our brood breaks in late summer,” he explains, “and although it still felt like summer on the thermometer, our queens were slowing their laying in preparation for winter.”
The researchers did not expect caging at this time to be so detrimental, since winters in Florida are mild, and brood production continues late into the season. Jack notes that the poor success may have also been related to the colonies already being crippled by high varroa loads. Regardless of the exact cause, based on the high probability of colony death, the authors recommend that brood interruption “only be attempted during the [early] summer months or possibly not at all.”
Previous studies in the U.S.3,4 and Italy5,6 did not document such detrimental outcomes of queen caging, and integrating summer brood breaks into the varroa management plan is recommended in Europe.7 Why Jack’s study differed is not clear, but it is enough to warrant caution.
More evidence that repeated treatments are ineffective
Jack and colleagues also report that three vaporization treatments are not better than one — both application strategies (one gram per brood box applied once or thrice) yielded similar mite falls to colonies receiving no treatment — and while combining oxalic treatment with brood interruption did improve efficacy, in terms of mite kill, those colonies were still more likely to die.
Combined with Berry’s study, these data tell us that that three treatments eight days apart is not enough, and even seven treatments five days apart only prevents mite population growth, rather than knocking the mites to their knees.
“As scientists, we have to be careful not to overstate the findings of our research,” Jack cautions, adding that the studies should be ….
