The Shift Toward Bioacaricides

Contents

SYNTHETIC MITICIDES. 2

BIOPESTICIDES. 3

PEST RESISTANCE MANAGEMENT. 4

SHIFTING TO “NATURAL” TREATMENTS. 5

MONITORING THE DEGREE OF MITE INFESTATION.. 6

TREATMENT THRESHOLDS. 7

PHYSICAL, MECHANICAL, AND BIOTECHNICAL METHODS. 10

PUTTING IT ALL TOGETHER. 13

BIOLOGICALLY RESISTANT STOCK. 14

NOTES AND CITATIONS. 15

The Shift Toward Bioacaricides

Randy Oliver

ScientificBeekeeping.com

First Published in ABJ July 2025

  Our industry has long depended upon synthetic chemicals to control varroa, but as the mite evolves resistance to these “silver bullets,” beekeepers worldwide are forced to shift towards other ways to control this pest.

In my last article I explained that the aim of Pesticide Resistance Management (PRM) is to extend the useful life of a pesticide by slowing down or preventing the development of resistance to it by its target (in our case the development of resistance to synthetic acaricides by our beloved varroa mite).  Unfortunately, our industry has largely failed at this — largely because of the limited number of registered choices available to us over the course of time.

Practical application:  PRM uses strategies such as integrating different pest control methods, reducing reliance on a single pesticide, and using pesticides with different modes of action.

This brings us to the subject of Integrated Pest Management (IPM) — a holistic stepwise approach to managing pests, not completely dependent upon pesticides, as illustrated in Figure 1.

Fig. 1 The IPM pyramid for varroa, giving the most weight to the portions at the bottom, and stepping up only if necessary.  But in most countries, beekeepers skipped the lower steps entirely and jumped straight to the top (currently the surprising governmental approach in Australia).  So let’s start there and work down.

SYNTHETIC MITICIDES

U.S. beekeepers today have only a few registered synthetic (aka “conventional”) miticides at their disposal (Table 1).

Table 1.  The EPA maintains a current list of registered products at [[1]].  As you can see, we have a limited selection of synthetics at our disposal.  Although they do have three different modes of action, not all were brought to market soon enough to allow us to utilize rotation, which led to the sequential development of resistance to each one by the mite.

BIOPESTICIDES

Let’s go down one step in the IPM Pyramid.  Rather than skipping straight to conventional synthetic chemicals to control varroa, the IPM strategy would have been to see whether we could control varroa by using biopesticides (biological substances or organisms that damage, kill, or repel organisms seen as pests) that specifically target mites (technically called “bioacaricides”).  As explained in a recent review of the use of bioacaricides in agriculture [[2]]:

Bioacaricides (biological acaricides) are pesticides of biological origin used to protect crops from mite pests. There are three types of bioacaricides. Microbial acaricides are based on microorganisms (fungi and bacteria) that are pathogenic to mites. Other bioacaricides are manufactured from natural active substances of microbial, plant or animal origin and control mites by toxic action (biochemicals).

Twenty-three years ago I was excited when U.C. Davis’ Christine Peng presented her findings that the fungus Hirsutella was pathogenic to varroa [[3]].  Since then a number of researchers have experimented with “directed evolution” of various parasitic fungi that are safe for bees (and can tolerate cluster temperature), but unfortunately none have yet been brought to market [[4]].

That said, I’ll focus upon the use of biochemicals, which are strongly favored by the Environmental Protection Agency over synthetic pesticides (xenochemicals [[5]]), (so long as they are registered products).  The reasons that the EPA favors biochemicals is that they readily degrade in the environment, and in many cases are things that we already consume in our diet.

Natural mite control:  Fortunately for us beekeepers, plants have also been fighting mites for nearly 400 million years, so they’ve had a long time to develop “secondary metabolites” [[6]] that repel or kill the critters.  Oxalic acid, formic acid, and thymol all fall into this category, as do the fragrant essential oils (which may smell good to humans at low concentrations, but at higher concentrations are repellent to bees and lethal to mites).

Some bioacaricide formulated products are already registered by the EPA for sale or distribution (Table 2).

Table 2  The biopesticides currently registered by the EPA for varroa control.  For our purposes, you can likely assume that each active ingredient has a different mode of action, and can thus be used for rotation.

Practical application:  The EPA has left restrictions on “own use” of unregistered active ingredients up to each state – read my summary at [[7]].

There are also additional natural phytochemicals on the EPA’s Minimal Risk list with potential for varroa control, that do not have restrictions against their use (Table 3).

Table 3.  Plant oils (and an acid) on the EPA’s Minimal Risk list [[8]] that have shown potential as varroacides.  These off-the-shelf substances can be legally applied to hives for mite control, but no one as of yet has put a well-tested formulated product based upon any of them on the shelf.

Practical application: As far as the Minimal Risk phytochemicals in Table 3, they may test well in the lab, but it’s tricky to figure out how to apply them efficaciously to hives, without unduly disrupting the colony.  In the EPA’s snail-paced world of pesticide regulation (and lag in updating the Minimal Risk list), other food-grade aromatic oils, such as oregano oil, would be legal to apply your hive as aromatherapy, but not if your intent were to use them for pesticidal purposes. 

PEST RESISTANCE MANAGEMENT

As I pointed out in my previous article, when applying miticides, use them in rotation to avoid breeding resistant mites.  In addition, it may be best for us to aim for moderate efficacy rather than 99% kills, in order to reduce the selective pressure towards resistant mites (Figure 2).

Fig. 2  In the simulation above, three moderate (85%)-efficacy treatments (with the first proactively applied in early spring) resulted in sustainable mite control (the ending mite count did not increase above the starting count).

Practical application:  In the above simulation, I proactively reduced the mite wash count to zero in early March (which makes all the difference in the world for mite management), and used only moderate-efficacy treatments.  I did not specify the products use for the treatments, but to avoid selecting for mites resistant to a particular miticide, it’s best to rotate modes of action  [[9]]). Using low-efficacy treatments and rotation decreases the selective pressure towards mites developing resistance

SHIFTING TO “NATURAL” TREATMENTS

Because the bioacaricides are found in Nature, they are often referred to as “natural” treatments.  Each has appropriate times of the season (and colony condition) when they are most appropriate (Figure 3).  For example, you might not want to use a strong formic treatment as the colony is going into winter, due to potential queen loss.  And although oxalic dribble is cheap, quick, and efficacious when a colony is broodless, it’s not appropriate when there’s a lot of brood present (due to minimal efficacy and slowing of colony growth).

Fig. 3  Weigh your treatment options relative to time of season, colony condition, amount of brood, concerns about setback or queen issues, and whether honey for harvest is on the hive.  One can just treat by the calendar, but it’s more reasonable to follow the IPM practice of monitoring the pest pressure.

MONITORING THE DEGREE OF MITE INFESTATION

Moving further down the IPM Pyramid, we come to monitoring the degree of the pest infestation rate.

Practical application:  If you’re not monitoring the mite infestation rates of your hives, you’re working blind!  The more hives that you monitor, the better you can appraise your overall status regarding the varroa/virus complex, as well as identify any high-mite “varroa factories” in your operation.

The quickest and most accurate method for monitoring that I’ve found is a mite wash of a half cup of bees, taken from a comb adjacent to the broodnest (but not containing open brood), after allowing the older bees to fly off [[10]].  I’ve got a video of me doing one at [[11]], although I now use Dawn detergent (1 Tbl per half gallon of water) rather than alcohol.  With either Dawn or 90% alcohol, no shaking is necessary (after waiting 60 seconds for the mites to release) – only a gentle swirling for around 30 seconds.

There are a number of mite wash cups on the market, but I prefer my own, made from 16-oz Solo cups (Figure 4).

Fig. 4  I’ve improved the design of my original mite wash cup (the commercial ones on the market were based upon mine, but are awkward to use).  I’ll publish how to make your own in an upcoming article, but for now you can order them from my helper Jacob McBride.  I told Jake that I’d pitch his product if he were willing to sell them cheaply enough for clubs to pass them out to their members ($20 for a 10-pack).  You can order from him at forbeessake@gmail.com.

TREATMENT THRESHOLDS

Rather than wasting money on unnecessary treatments, IPM uses monitoring to determine whether the pest population is exceeding the “economic threshold” — the pest population density at which control measures should be initiated to prevent the pest population from reaching the “economic injury level” (EIL). The EIL is the point where the cost of pest control equals the economic losses caused by the pest.

Practical application: “Treatment threshold” tells you whether treatment for varroa is indicated or even worth it.  In our own operation (having a substantial proportion of mite-resistant colonies), our savings from not applying unnecessary treatments more than pays for the cost of monitoring.

Back in 2006, the recommended treatment thresholds were all over the place [[12]] (and in my opinion generally too high).  See Figure 5 for my personal observations on the effects of the mite infestation rate upon a colony.

 Fig. 5  The effects of the varroa/DWV complex upon a colony, relative to mite wash counts of ½ level cup of bees.

Practical application:  Don’t be misled by low springtime wash counts! Take a look at the bottom row in Fig. 5 – during swarm season, nearly 4 out of 5 mites are in the brood, and will not show up in a mite wash count.  So even though the mite count of the adult bees is very low, the varroa population of the hive is invisibly growing exponentially. 

Even though you don’t see much effect upon the colony from mite counts up to 6, by that time varroa is about to explode, since the mite population is increasing at a scary daily rate (Figure 6), along with the viruses that it transmits as the percentage of pupae get parasitized.

Fig. 6  Understand exponential growth!   The varroa population increases exponentially faster than does the mite wash count.  The more mites in a hive, the greater net increase in additional mites per day (and the steeper their growth curve).  Starting with a daily increase of only a few mites per day, by mid-May (with a mite count of only 3), the varroa population is growing at a net increase of ~50 mites per day.  A month later (at a count of 6) it’s growing by ~100 mites per day, and in another month (at a count of 13) by nearly 200 additional mites per day!  By that time your hive has been converted from a honey producer to a varroa factory!

Practical application:  Treat proactively rather than reactively – it’s easier to maintain a low mite level than to bring a high mite level down! Your colonies will be healthier and more productive if you reduce the mite wash count to zero early in the spring, and then keep the count low for the rest of the season!

Since we began our selective breeding program, for which we take a wash count from every single hive before our first summer treatment (in order to identify any potential breeders), it’s really opened our eyes!  We can skip that treatment for any colonies exhibiting counts of zero.  On the other hand, roughly 5% of our hives turn out (for whatever reason) to be outliers with counts way above the average infestation rate.  Giving these “mite factories” special attention (and requeening) makes overall mite management  much easier.

PHYSICAL, MECHANICAL, AND BIOTECHNICAL METHODS

Stepping down the IPM Pyramid yet again, before you rely on chemical treatments, you first try biotechnical methods to deal with the pest.  I’ve tried drone trapping, open-mesh floors, sugar dusting, small cell, allowing colonies to swarm, and thermal treatment.  Some helped a bit, but by far the most cost-effective biotechnical method I’ve tried is an induced brood break coupled with an oxalic dribble (in IPM you can combine methods).

The combination of an induced brood break (by caging the queen) and oxalic acid is now widely used in Europe, typically in summer, and often also in the winter.  An alternative is to temporarily confine the queen to a single comb (Figure 7).

Fig. 7  You can use commercial isolation cages or division boards made with queen excluder sides.  It may help to isolate her on drone comb, since mites preferentially enter cells with mature drone larvae).

Practical tips:  Forget using a syringe to apply oxalic dribble (Figure 8).  And we find that using a weak glycerin solution rather than sugar syrup works better [[13]].

Fig. 8  An oxalic dribble works great on colonies without brood (and is cheaper, quicker, and safer than vaporization).  For those with a few hives, oxalic dribble is most easily applied by using a 250mL wash bottle, with 1mm cut off the tip (which then creates an opening that applies 5mL per second with a gentle squeeze).  For larger operations, apply it with a garden sprayer set to a 5mL/sec stream.

Another way is to create a brood break is to temporarily split the hive, separating the combs containing any sealed brood into a separate hive (and using oxalic or formic acid (Figure 9)).

Fig. 9  Splitting a hive and separating the open brood from the young brood allows you to kill most all the mites by applying oxalic at different time points for each division.  Alternatively, you could blast the portion containing sealed brood with formic acid (without risk of killing the queen), and then quickly return the treated sealed brood back to the hive (some variation of this may be useful against tropilaelaps, should it ever get here).

The above method can be used to kill several birds with one stone:

1. Prevent swarming in the spring.
2. Controlling varroa.
3. Requeening the hive, and…
4. Solving the “marriage problem” (when your spouse yet again sees you doubling your number of hives to avoid swarming, and complains “I thought this was going to be a hobby!”). Just recombine the divisions once the honey flow begins, and make a huge honey crop with the amplified colonies).

Practical application:  Finding the queen can be a pain, but it’s not necessary to do so.  By simply shaking the bees off the combs containing sealed brood, you can make the split without ever spotting the queen [[14]].

PUTTING IT ALL TOGETHER

We’ve successfully managed 1500-2000 hives since the year 2001, using only natural treatments.  The breakthrough for us was to reduce the mite populations of our colonies in early spring, by creating induced brood breaks (which also allows us to control swarming, and sell a thousand nucs each spring (Figure 10).

Fig. 10  After returning from almond pollination, our colonies are bursting with bees and brood.  So just before they swarm, at each holding yard we set up a portable table and create a nuc assembly line.  We typically take three 4-frame nucs out of each hive, leaving the queen back in the original hive, but without any sealed brood.  This allows us to immediately apply an oxalic dribble before we close the hive back up.

We immediately move the nucs to out outyards (so that the older bees don’t return to their parent hive), and install ripe queen cells the next day.  Then on Day 18 or 19 after making the nucs, we check for queenrightness, combine any queenless ones with queenright ones, and apply an oxalic dribble, which due to the induced brood break, is highly efficacious (explained at [[15]]).

Practical application:  Our income is largely from nuc sales (at the expense of maximal honey production).  By instead simply splitting each hive in half, you could prevent swarming, make up for winter losses, control mites, and recombine as many splits as you want at the start of the honey flow.

We then follow with the treatment rotation shown in Figure 11.

Fig. 11  Critical for this mite management strategy is the proactive control of varroa early in the season by the induced brood break created by splitting and inserting queen cells.  Those who live where the honey flow is extended would need to come up with their own management strategy.

BIOLOGICALLY RESISTANT STOCK

The foundation of IPM is to make your life easier by keeping plant or animal strains that are naturally resistant to pests [[16]].  I do this in my orchard and vineyard by keeping only those cultivars that are naturally resistant to common diseases.  We’ve also been deeply engaged in doing so with our bees – requeening all of our hives each year with the daughters of queens whose colonies required zero mite management over the course of the previous year (Figure 12).

Fig. 12  The mite wash count tags from some of our breeder queens this spring, started from nucs last spring. None received any varroa treatments whatsoever, despite being exposed to mite immigration, and going to almonds. This our goal for the final solution to The Varroa Problem, which I’ll be covering in my next articles.

NOTES AND CITATIONS

[1] https://www.epa.gov/pollinator-protection/epa-registered-pesticide-products-approved-use-against-varroa-mites-bee-hives

[2] Marčić, D, et al (2025) Bioacaricides in crop protection—What Is the state of play? Insects 16(1): 95.  https://www.mdpi.com/2075-4450/16/1/95

[3] Peng, C, et al (2002) Virulence and site of infection of the fungus, Hirsutella thompsonii, to the honey bee ectoparasitic mite, Varroa destructor. Journal of Invertebrate Pathology 81(3): 185-195.

[4] Han, J, et al (2021) Directed evolution of Metarhizium fungus improves its biocontrol efficacy against Varroa mites in honey bee colonies. Scientific Reports 11(1): 10582.

[5] A chemical compound that is foreign to a biological system or the natural environment.

[6] Secondary metabolites are organic compounds produced by plants, bacteria, fungi, or animals that are not essential for their basic growth, development, or reproduction, but may have roles in defense, interactions with the environment, or other specific functions.

[7] https://scientificbeekeeping.com/instructions-for-extended-release-oxalic-acid/

[8] https://www.ecfr.gov/current/title-40/chapter-I/subchapter-E/part-152/subpart-B/section-152.25

[9] As I suggested in laying out a strategy in an article in this Journal nearly 20 years ago:  IPM 1 Fighting Varroa : The Silver Bullet, or Brass Knuckles? https://scientificbeekeeping.com/fighting-varroa-the-silver-bullet-or-brass-knuckles-2/

[10] Smokin’-Hot Mite Washin’ ABJ September 2017 https://scientificbeekeeping.com/the-varroa-problem-part-10/

[11] https://scientificbeekeeping.com/how-to-perform-an-alcohol-wash/

[12] https://scientificbeekeeping.com/fighting-varroa-reconnaissance-mite-sampling/

[13] https://scientificbeekeeping.com/oxalic-acid-treatment-table/

[14] https://www.youtube.com/watch?v=-HIxryJo2PE&t=312s

[15] https://scientificbeekeeping.com/simple-early-treatment-of-nucs-against-varroa/

1. [16] Breeding for parasite resistance saves farmers millions of dollars