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.
Some of the early reports claimed that the coated surfaces continued to kill mites for up to three weeks. But if this were true, mites would continue to be killed as they emerged from the brood cells. We know this isn’t the case. More recent reports say that levels of oxalic acid in the hive quickly return to normal after a vaporization treatment, which makes more sense.
In deciding how much time to leave between treatments, you have to decide how long the chemical stays active in the hive. My assumption here is that there is little or no residual effect.
A word about safety
Before I continue, I want to make it clear that I am not a fan of oxalic acid vaporization. As it happens, I have a love affair with my lungs, a co-dependency you might say. I need them and they need me. Stories and photos of haze-filled apiaries with coughing, hacking, half-blind beekeepers make me shudder. I know, I know. I’m silly and overcautious, right? Fine, I’ll cop to that. But just because “everybody” does something, doesn’t mean it’s a good idea.
Once you decide to vaporize, I strongly suggest you read the EPA label for using oxalic acid in a bee hive. Also read the Material Safety Data Sheet (MSDS) for oxalic acid, and read the instructions that came with your respirator which you undoubtedly purchased and plan to wear. Practice fitting the respirator to your face and check for leaks, and make sure you purchased the right cartridge. If you can smell the acid, the respirator isn’t working.
I think beekeepers are way too cavalier about breathing OA vapor, and while most will get away with it, some may not. In my most humble opinion, anyone who thinks that standing upwind of the vapor will protect him, doesn’t have enough IQ to be a beekeeper.
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.
Honey Bee Suite