Prevent resistance to miticides: rotate your treatments
Last week I reminded readers that an oxalic acid dribble was a viable alternative to vaporization. I also said that, personally, I was unwilling to purchase and store all the paraphernalia that is required, such as the vaporizer, 12-volt battery, and respirator. After all, I keep a small apiary that never exceeds 15 hives. For me, the dribble method is fast, easy, economical, and effective.
However, after a series of communications from various readers, something occurred to me. Many of the people who purchased the equipment were defending their choice by saying it would last a long time, and in any case, now they had everything they needed for reliable mite treatment and they wouldn’t have to purchase anything else.
That’s when it hit me: Many of those people have no intention of using anything but their vaporizer for Varroa control. Mites can be checked off the list. Problem solved.
Or is it?
What some beekeepers are forgetting is that oxalic acid, no matter how it is applied, needs to be rotated with other mite control products. This simple step is the only thing that can delay the inevitable resistance that mites will develop to oxalic acid or any other product.
Living things vary in their ability to develop resistance to chemicals, diseases, and environmental conditions. Some, like cockroaches and mosquitoes seem to evolve overnight, and the pesticides that worked wonders last year won’t touch them this year. Other species—and honey bees are a good example—change much more slowly. The speed and efficiency of these changes is genetically controlled, and it is the same basic phenomenon that gives us antibiotic-resistant diseases such as MRSA.
From what we’ve seen so far, Varroa mites are more like roaches than honey bees. One by one, they have become resistant to various chemical controls, and resistance to the newest treatments will come in time. Oxalic acid is no exception.
After a treatment of nearly any poisonous substance, a few survivors remain. These resistant survivors reproduce and raise resistant or partially-resistant offspring. After a few more treatments of the same substance and and few more generations, all the offspring will be resistant to that substance. Only by killing those resistant individuals with other chemicals can we break the chain of resistance. Simply put, to preserve the ability of pesticides to work in the future, they have to be alternated today.
EPA advice on oxalic acid and resistance to miticides
The Environmental Protection Agency (EPA) label for the use of oxalic acid dihydrate in honey bee colonies is very specific about resistance. It reads as follows:
To delay resistance:
- When possible, rotate the use of miticides to reduce selection pressure as compared to repeatedly using the same product, mode or action or chemical class. If multiple applications are required, use a different mode of action each time before returning to a previously-used one.
- Base miticide use on Integrated Pest Management (IPM). This includes proper pest identification, monitoring for locality specific economic threshold and economic injury levels, record keeping, and utilizing all available control practices (cultural, biological and chemical).
- Maximize efficacy by following all label instructions including dosage and timing of application.
Tips from Randy Oliver
In the section of the EPA label titled “Directions for Use” we are told to “Consult state guidelines and local extension experts for monitoring protocols and thresholds for treatment.”
Following that advice, I went to the Washington State University Extension website and it directed me to Randy Oliver’s site for tips about using oxalic acid. Randy is even more blunt than the EPA about the necessity of rotating mite treatments. He writes:
No one even knows for sure what the mode of action of OA is against varroa, nor how it is absorbed. And no matter, I can assure you that some mites will be more resistant than others, which implies that some degree of resistance is possible. Remember, there is only a small margin of safety between the dose that kills mites, and the dose that kills bees. That means that varroa only needs to develop a slight degree of resistance until OA is as toxic to the bees as it is to the mites. Rotate treatments!
In other words, as mites becomes resistant to oxalic acid, we cannot simply apply greater and greater dosages. If we do, we will soon reach the dosage that takes down the bees as well as the mites.
Modes of action
Both the EPA and Randy mention the mode of action as being a factor in determining chemical rotations. According to Wikipedia, “A mode of action describes a functional or anatomical change, at the cellular level, resulting from the exposure of a living organism to a substance.” So basically, the mode of action is the way in which a chemical pesticide kills the mite.
The directive in the EPA label is to rotate between pesticides that have different modes of action. When I read this, I immediately wondered how a typical beekeeper, myself included, could easily identify the mode of action of any particular substance.
I passed this question on to Randy. Although he didn’t elaborate, I gathered from his answer that he favors alternating those that are organic acids with those that are not. So even if we don’t know for sure the modes of action for any particular substance, we can assume that acids behave similarly to each other, as do members of other groups, such as the phenols.
For example, oxalic acid and formic acid are both acids so we can assume they behave similarly in the cell. Perhaps the same is true for hop beta acids, but little is known. There may indeed be additional modes of action within these substances but, at present, we don’t know that either. Thymol, on the other hand, is not an acid but a phenol. So in Randy’s practice, he alternates between oxalic acid and a thymol-based product. He also suggested one could use Apivar (a synthetic miticide) as a third alternative if he or she were not opposed to such products.
Randy’s take on this makes eminently good sense, and I was dumbstruck to learn that I was doing it wrong. All along I’ve been meticulously alternating between different products, but they were products with the same (as far as we know) mode of action.
It’s up to us to treat responsibly
I urge all of you to take the subject of mite resistance seriously and never use the same treatment two times in a row. If you treat twice a year, for example, you need to use two different treatments having two different modes of action. It is fine to use your vaporizer as long as you alternate its use with another product.
An oxalic acid vaporizer is the “gotta have it” piece of equipment right now. But if beekeepers begin using it for every consecutive treatment, it won’t be long before it becomes as useless as a tanging pan.
A final note
The above analysis is based on my own interpretation of the official EPA label and Randy’s reporting at ScientificBeekeeping.com. At the very least, everyone who uses oxalic acid in a beehive should be thoroughly familiar with the official label. In addition, a careful study of Randy’s work will clarify most oxalic acid questions. Please take the time to read both.
Honey Bee Suite