OXALIC ACID INFORMATION
Latest OA Dosage Application Research
Determining the dose of oxalic acid applied via vaporization needed for the control of the honey bee (Apis mellifera) pest Varroa destructor
Published 27 May 2021
Journal of Apicultural Research
Oxalic acid (OA) is a natural compound that has been used to control the honey bee (Apis mellifera) pest Varroa destructor. One method of OA application gaining popularity among beekeepers in the US involves vaporizing OA crystals with heat inside a closed hive. Herein, we tested different doses of OA applied via vaporization to determine the most effective amount of OA needed to reduce V. destructor populations below that of the negative controls. Forty experimental colonies were assigned to one of four treatment groups, with ten colonies composing each group. The four treatments were: (1) 1 g OA, (2) 2 g OA, (3) 4 g OA and (4) no OA (negative control). The OA was applied via vaporization once per week for three weeks. V. destructor infestation rate and colony strength assessments were estimated before, during, and after treatment applications. Colonies in the 4 g OA treatment group had significantly lower infestation rates than did those in the untreated control and 1 g OA treatment groups, but not those in the 2 g OA treatment group. The infestation rate of colonies treated three times with 1 g OA, which is the current legal limit for OA vaporization in the US, was not significantly different from that of colonies in the negative control or 2 g OA treatment groups. Colonies receiving the highest dose of OA were generally healthier than those treated at lower OA doses. Our results may lead to improved efficacy of OA vaporization, thus aiding beekeepers in their efforts to control V. destructor.
C. Jack: Dr. Cameron Jack, Assistant Professor Honey Bee Toxicology and teaches several courses related to honey bees and apiculture at the University of Florida. He is also responsible for coordinating the distance education efforts at the Entomology and Nematology Department.
E. van Santen: Edzard van Santen, PhD, Professor, Agronomy Department Director, Statistical Consulting Unit Institute of Food and Agricultural Sciences at the University of Florida
J. Ellis: Dr. Ellis is the Gahan Endowed Professor of Entomology in the Department of Entomology and Nematology at the University of Florida. At the University of Florida, Dr. Ellis has responsibilities in extension, instruction and research related to honey bees.
How Does Oxalic Acid Vapor Work?
Oxalic acid vapor is released into the beehive and spreads to all boxes inside it. It then cools down and remains attached to the surfaces of the hive. Mites come into contact with these surfaces as they move around the hive. The mites that have found their way onto the backs of bees also rub against the oxalic acid-coated hive surfaces as the bees move about. This way, mites suffer the effects of coming into contact with oxalic acid.
Using oxalic acid vapor generally results in a high death rate of mites within the first 2 days of application. After that, the numbers of dead mites collected in experiments and studies drop sharply. This may be due to reduced efficiency of the oxalic vapor and an already reduced mite population that leaves fewer mites available to die.
How Quickly does Oxalic Acid Vapor Lose its Effectiveness?
It is important that beekeepers note that oxalic acid from vaporization remains in the hive in doses that are high enough to kill mites for up to 3 days. After that, natural decay and hive cleaning activities makes the oxalic acid ineffective against mites. It is why a repeat treatment is required.
Oxalic Acid Vaporization – Single Treatment
Oxalic acid vaporization works on mites that are on the bees. Mites in capped brood are protected. Normally 2/3 of mites are in capped brood at any one time. The logic is therefore clear, the most effective time to treat is when there is the least amount of capped brood in the hive. That moment will vary depending where in the world you live, in the UK the ideal time would be just before new year. If the winter is mild a small amount of brood may be present even then. A single oxalic acid treatment at that time can knock back the varroa and give the hive a chance to go into the new season in good shape with minimum impact to the bees.
Oxalic Acid Vaporization- Multiple Treatments
Generally I support the approach of not treating bees but there are occasions when it makes no sense to simply let a hive collapse, having only one hive is just such a situation! Under these circumstances, oxalic acid vapor treatment can be very effective if the crisis arrives mid winter. But this is not normally the case, autumn is the time when varroa can get out of hand and to treat with OA at that time requires multiple treatments because there is still brood present.
Lets look at some bee & mite facts and do some maths! For the purpose of this exercise I will assume full cell size.
Brood is capped for between 12-14 days ( workers & drones ), less on small cell
Varroa mites stay in the phoretic state (on the bees) for on average 7 days between breeding cycles
Oxalic acid vaporization is considered 95% effective on phoretic varroa mites
Therefore 3 oxalic acid vaporization treatments, 5 days apart will ensure that every mite in the hive is exposed to the treatment. Treatments after this will be subject to the law of diminishing returns.
Treating with no (or little) brood
One approach to this is to apply oxalic acid at a time when there are very few Varroa under capped brood. Or, more accurately, when there is very little capped brood.
For this reason, oxalic acid is often applied in late fall or over the winter. Varroa present in the hive at this time are a metaphorical sitting duck for the treatment and we’ve a good chance of a highly effective application.
Estimates vary but most beekeepers would expect an effectiveness of 90% or more in wiping out exposed Varroa.
Treating with capped brood present
Oxalic acid can also be applied when capped brood is present. But now it becomes a tad more complicated and we’re back to our numbers game.
Let’s consider a hive with a mid-summer mite count we don’t like and we want to do something about that. We can consider the mites to be divided into two buckets – about 15% – 20% in the phoretic stage and the rest under cover of a capped cell. Let’s start the clock by applying oxalic acid (we’ll cover how to apply further down in this article).
It’s day 1.
The good news is that we just zapped 15% – 20% of the mites in the hive, with a degree of efficiency likely to be 90% or more. Those were our “phoretic sitting ducks”.
The bad news is that on “day 1” the majority of Varroa are in capped cells. But we want to get those too. So, we have to get to them when they exit the cell. Remember, each cell-embedded Varroa has 12 days of protection above their heads after they entered the cell.
We could just apply oxalic acid on days 2, 3 and so on, killing the Varroa exiting their cells on each subsequent day. That would absolutely have a really good impact on the Varroa load.
But as you have probably guessed, there’s a huge, showstopper issue, namely we’re going to massively over-treat and threaten the health of our bees. Don’t apply oxalic anywhere near that often!
So where’s the balance?
A common approach is to treat again 6 days later and then again another 6 days after that. Some beekeepers prefer a 5 day period but we’re getting a little subjective at that point (another beekeeper debate topic right there!).
Of course, there’s a “Varroa leak”, of sorts, here. We applied our treatment on day 1 and then wait till day 7. That still leaves the problem of Varroa exiting their capped cells on days 2, 3, 4 and so on. This means 5 days of Varroa leaving their cells and likely to find another cell about to be capped. The Varroa with fortunate timing can hide across multiple applications of oxalic acid.
But, despite this, a multi-stage application of oxalic acid can still be an effective approach, even when capped brood exists. It’s not a perfect way to zap Varroa but applying oxalic acid in this manner can have a seriously positive effect on getting the mite load under control.
Using oxalic acid vaporization when brood is present
A reader wrote that she was confused about scheduling oxalic acid vaporization treatments when brood is present. Some folks advised her to use three treatments five days apart, another advised three treatments seven days apart. She wants to know which is best.
I think the big question here is how long after a treatment does the oxalic acid continue to kill mites. As you know, oxalic acid vaporization sends a cloud of tiny crystals into the air which quickly attach to all the internal surfaces of the hive, including the combs, the woodenware, and the bees themselves. Although this substance is not particularly harmful to honey bees, it is deadly to varroa mites
Timing the treatments
In the rest of this discussion, I’m assuming that there is little or no residual activity after an oxalic treatment. A publication by Dadant states “The hive returns to pre-treated levels [of oxalic acid] shortly after treatment. Within days of vaporization, the bees will remove the residual OA crystals from the hive.” Is there enough oxalic acid in the hive during the removal stage to kill emerging mites? I don’t know.
What we do know is the varroa mites under capped brood cells are protected from the oxalic acid crystals, but the phoretic mites—those that are moving freely through the hive or riding on adult bees—will be killed by it.
Varroa under the cappings
Now let’s say you treat on July 1. Theoretically, all the phoretic mites will die in the next day or two. But you have approximately twelve days of varroa mites that are preparing to emerge. Why twelve? Because the brood develops in three stages. Brood is in the egg stage for 3 days, the larval stage for 6 days, and the pupal (capped) stage for 12. The rule of thumb here is that each stage is double the length of the previous one: 3+6+12=21.
Because someone is sure to correct me on this, I will point out that the brood cycle is actually a bit shorter because the larval stage is closer to 5.5 days. In the end, that makes the entire brood cycle closer to 20.5 days instead of 21. But because most people use 21 days as the brood cycle, and because there is some variability among populations, I will stick with that.
The egg and larval stages don’t count for much here because the mated female mite doesn’t climb into a brood cell until just before it’s capped. Basically, honey bee eggs and early-stage larvae are ignored by the mites.
Tracking the emerging mites
To reiterate, after the first treatment you have a shed load of dead mites and 12 days-worth of safely capped mites. During each of the next 12 days, some of the mites will emerge along with the brood. These emerging mites are mated and ready to go. Presumably, they attach themselves to a honey bee and ride around until they are taken to a ready-to-cap brood cell. They detect this, probably by scent, and scurry inside and bury themselves under the brood food.
You want to treat often enough to kill the emerging mites, but not often enough to injure the bees. Some reports say that the drones and workers are not damaged by repeated exposure to oxalic acid because they don’t live much longer than four to six weeks. However, the queen—a bee that can live a long time—is more apt to be damaged by repeated exposures. How many treatments she can withstand, I don’t know. But in any case, it makes sense to not treat more than necessary.
The standard choices
Let’s say you choose 5-day intervals. If you treat on day 1, and then five days later on day 6, and then five days later on day 11, you still have one more day of mites that will emerge on day 12 with no follow-up treatment. However, if you assume that the brood cycle is really a half-day shorter as I mentioned above, or if there is residual activity for a day or two after the treatment, then you should be okay.
On the other hand, if there is virtually no residual activity after treatment, or if your brood cycle is closer to 21 days, then maybe you are leaving a day’s worth of mites in the hive. It’s a crap shoot with many variables and several unknowns.
It seems like six-day intervals might be better than either five- or seven-day intervals because it evenly divides the capped period. But each day you wait gives the newly-emerged mites more time to find a soon-to-be capped brood cell to inhabit, which is why using oxalic acid vapor is never as effective when brood is present.
If I were doing this, I might consider using a five-day interval and then a six-day interval since there is no reason the intervals need to be equal. That means I would treat on day 1, then five days later on day 6, and then six days later on day 12. Anyway, it’s just an idea that may or may not make any difference.
This post is just a roundabout way of saying I don’t have a good answer to the question. But it shows how I would evaluate it if I was using oxalic acid vapor. I’m not a believer in one-size-fits-all answers, so you may have to experiment with different protocols to see which one works best for you.