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Extended Release Thymol Blocks


My Preliminary Findings 1

Preparation. 2

Application. 4

Colony Response to the Treatment 6

Final Inspection. 13

Effect Upon Varroa. 15

Is it Worth Preparing Your Own?. 15

Remaining Questions to Answer 15

There’s a Reason that EPA Registers Miticides! 16

Citations and Notes 16

Extended-Release Thymol Blocks

Randy Oliver


First published in ABJ April 2024


Since varroa mites spend roughly 70% of their time under the cappings, miticides are most efficacious if applied in an extended-release formulation —which exposes the mites to the treatment while they are in their dispersal phase.  With regard to thymol, slow release is achieved by dissolving it in oil, or embedding it in absorbent strips or aqueous gel.  But all the currently-registered thymol products release their vapors fairly quickly, and thus require repeated applications.

In 2022 I published the results of my experimentation (under permit) of a novel way of applying this biopesticide [[1]].  Since the EPA has now stated that it does not restrict “own use” of generic thymol, we beekeepers may be able convince our State Lead Agencies to follow suit and allow us to use this method.

My Preliminary Findings

I had previously determined colony tolerance of slow-release thymol, as well as a dose/response curve  (Figure 1).

Fig. 1. My preliminary research suggested that the optimal dose of thymol to apply by this method would be around 40 grams –– less in total than the amount allowed by the label of Apiguard (up to four weekly applications of 12.5 grams).  So that’s what I tested last summer.


I had originally experimented with Homasote fiberboard (made from recycled newspaper [[2]]), but had trouble finding it in California, so also tested an available acoustic soundboard made from wood fiber [[3]].  I dissolved USP-grade thymol [[4]] in denatured alcohol at the ratio of 1 gram of thymol crystals to 1 mL of alcohol (the thymol dissolves quickly if the alcohol is slightly warmed and stirred).  This creates a nearly-saturated solution that contains ~0.6 grams of thymol per milliliter of solution.


Let’s say that you wanted to treat five hives, so you’d need 4 blocks per hive x 5 hives = 20 blocks in total.

Step 1: Use a table saw to cut twenty 1” x 4” blocks from ½”-thick wood soundboard (1” x 4¾” for Homasote) and place them upright on end, side-by-side in a container.  Important note: these dimensions are for thymol dissolved in denatured alcohol as above (see the note below).

Step 2: Then mix up your thymol solution to match the number of blocks.  For 20 blocks, each to hold 10 g of thymol (40 g per hive), dissolve 20 x 10 g = 200 g of thymol crystals into an equal number of milliliters (200 mL) of alcohol.

Step 3: Pour the solution into the container so that the blocks evenly absorb it (Figure 2).

Fig. 2 Because you made the blocks longer than necessary for full absorption, they will completely suck up all the solution.  Since you know how much total thymol you added, you then know how much thymol (on average) each block absorbed.  Simple!

Note:  I used denatured alcohol (ethanol typically with 10% methanol).  Because ethanol is naturally produced during the fermentation of beebread (and since bees like ethanol), and since I allowed the alcohol to evaporate off prior to placing the blocks into hives, I wasn’t concerned about contamination of my combs. However, ethanol is not on the Minimal Risk Inerts list, so while writing this article, I purchased some 99.9% isopropyl alcohol (which is on the List), assuming that it would dissolve thymol similarly to denatured alcohol.  Once again, so much for assumptions!

I pulled out the scale and graduated cylinders, and went to the lab (my kitchen) to compare the two:

Saturated solutions (updated)

I’ve tried at least two brands of denatured alcohol and one brand of 99.9% isopropyl alcohol.  Each appeared to have a different ability to dissolve thymol!

In general, 1000 grams of thymol to 1000 mL of denatured alcohol makes ~2070 mL of nearly-saturated solution.

So ~21 mL solution/10g of thymol

Difference if you’re using 99.9% isopropyl alcohol:  thymol does not appear to be as soluble in isopropyl alcohol as it is in ethanol or methanol!  In my single test, 50 g thymol + 100 mL 99.9% isopropyl alcohol (heated)  = 128 mL slightly supersaturated solution.  If you use isopropyl, you’ll need to cut your blocks at least 50% longer.


1 g thymol to 1 mL denatured alcohol to make a saturated solution.
Add 20 mL of the saturated solution to each block, which will leave ~10 g of thymol per block.
So for a 40 g dose per hive, place four ½” x 1” x 4” blocks into a pan or jar and add 84 mL (4 x 21) of solution.  The blocks will suck it all up.

Once the blocks have absorbed the solution, spread them out and allow the alcohol to flash off. For quick evaporation, spread out the blocks in a single layer over a screen –– in the sun or a warm ventilated place.  It often takes less than an hour for the alcohol to evaporate off.

Apply 2 – 4 blocks per hive, dependent upon the ambient temperature, and where placed (between the brood chambers or in a rim on top).

A tip:  If you write down the starting weight of a block before absorption, you can reweigh them as the alcohol evaporates, until the blocks weigh only slightly over 10 grams more than they started at (due to the added weight of the thymol) –– it’s not necessary to completely evaporate the alcohol.

A common misconception:  It’s a common misconception that thymol is highly volatile.  It isn’t!  At 77°F (25°C), the vapor pressure of thymol is only 0.016 mm of mercury, whereas the vapor pressure of ethanol is 59 mm Hg).  Virtually all the alcohol will evaporate off before a measurable amount of thymol does!  Once most of the alcohol has evaporated, the blocks can be stored in a sealed plastic bag or other airtight container (Figure 3).

Fig. 3 The blocks (Homasote on the left, wood soundboard to the right) are here packed for storage after drying.


Application to The Hives

We started the field trial near the end of July, in hot, dry weather (Figure 4).

Fig. 4 In the California foothills, our honey flow is generally over by the end of July, providing us a window of opportunity to eliminate varroa before we start feeding pollen sub to stimulate brood rearing in mid-September.  I wanted to see how a single application of thymol blocks compared to the two rounds of Apiguard that we’ve used in the past (which really disrupted brood rearing in hot weather).

Unfortunately, due to my sons’ successful mite management to date, we needed to perform a lot of mite washes to find enough hives (34) with mite counts high enough to use for the trial (so the experiment was divided between five different yards, providing replication).  All the test colonies were in double deeps, generally with brood in both boxes.  We used a randomized block design, blocking by the starting mite counts for each yard, to randomly assign whether a colony would receive Homasote or wood fiber blocks.  Four colonies had excessively high mite counts, so I applied blocks containing a total of 48 grams of thymol to them to see how they’d handle the higher dose.

We used a 1½-inch rim atop each hive, and placed four thymol blocks in the corners (Figure 5).


Fig. 5 We replaced our dark plywood migratory covers over the rims.  Midday temperatures of the tops of the covers reached 160°F (120°F on the undersides), so the thymol received plenty of heat to drive evaporation.

Colony Response to the Treatment

During the 21 days of treatment (slightly over one full bee brood cycle [[5]]), we inspected the hives regularly.  There was considerable variation in how each colony reacted to the treatment (Figures 6-12).

Fig. 6 Most colonies ceased rearing young brood in the upper chamber, but continued unabated below.

Fig. 7 And then they started storing nectar from the light flow (or from jars of syrup in some yards) in the former brood cells (you can just catch the glisten in this photo).  I liked this, since we want the box above the cluster to be filled with honey (natural or from syrup), prior to winter.

Fig. 8 In many of the hives, the queen ignored the fumes and filled empty cells in the upper chamber with eggs, but we often saw them not progress to the larval stage (we couldn’t tell whether the eggs died, or the nurses cannibalized them).


Fig. 9 Not to be outdone, a couple of colonies completely ignored the fumes, built fresh comb in the rim, and continually reared brood right up to the blocks!


Fig. 10 The bees generally left the thymol blocks alone, only building propolis “shells” around a few.  Only after three weeks did any begin chewing them.


Fig. 11 Our most surprising observation was that every colony maintained continual brood rearing during the 21-day treatment — most of them mainly in the lower box.

Fig. 12 Unlike as with Apiguard –– the particles of which the bees carry down through the brood area, greatly disrupting brood rearing –– there was always plenty of brood of all ages in every hive at every inspection.

We ended the trial after 21 days, by which time the thymol had largely evaporated from the blocks, and most colonies had begun brood rearing in the upper chamber again (Figure 13).

Final Inspection

Fig. 13 Corrine Jones helped me with final inspections.  The sealed brood in this frame is at least 12 days old — indicating that the nurses were accepting and feeding larvae in the upper box halfway through the treatment.  We were surprised by the amount of brood rearing that took place during the hot, dry August pollen dearth (compared to our long experience with Apiguard).

We didn’t observe any queen losses or other notable adverse effects upon the colonies.

Effect Upon Varroa

The colonies fared surprisingly well; I can’t say the same for the unfortunate mites (Figure 14).

Fig. 14 It didn’t take long to count the mites in the final mite washes! We counted 851 at the start, and only 14 at the end –– a 98% reduction.  The results were consistent from yard to yard (not shown).

Practical application:  This is one of the most efficacious [[6]] treatments I’ve ever seen for colonies with brood, and it only took a single application and three weeks duration, leaving the colonies robust and thriving.

I tracked the colonies for the rest of the season.  There was no bounce-back of mites, and the colonies went into winter in great condition.

Is it Worth Preparing Your Own?

There is a labor saving from being able to only apply a single application for a three-week treatment with few adverse effects, but it takes some time and effort to prepare the blocks.  Here’s a breakdown of estimated costs (not including shipping), using prices from the internet and Wintersun Chemical (a large commercial supplier of USP-grade industrial chemicals that we find easy to work with)(Table 2).

Table 2.  Unfortunately, off-the-shelf thymol is expensive in small quantities. The cost of the soundboard is negligible in quantity, so I didn’t include it, nor did I include any shipping costs.

A Remaining Question

I wondered what difference it would make if I applied the treatment later in the season, in cooler weather, while we were trying to grow weak colonies by feeding syrup and sub in late September?

So I waited until the weather cooled, and treated two very weak colonies with moderate mite counts, located in the shade.  They took syrup and pollen sub eagerly during treatment, but they didn’t grow much (but no colonies were growing much at that time).  The degree of mite reduction wasn’t great.  The thymol blocks (likely at a lower dose) may need to be placed between the brood chambers in cooler weather.

Practical application:  These blocks can indeed be placed between the brood chambers, but I have only preliminary observations from a few test colonies, and only during the summer. They appeared to tolerate up to 36 grams of thymol well.  Please let me know your results if you try different placements, time of year, or application methods!

There’s a Reason that EPA Registers Miticides!

An adverse effect report:  I just heard today from a southeastern beekeeper who applied 36 grams of thymol to singles in warm springtime weather, and observed some queen kills –– likely due to that being too strong a dose for singles.

In the registration process of any miticide, the EPA pays great attention to the formulation and the label, to ensure that application of a registered miticide by the end user does not pose an unreasonable risk to the public, the environment, the bee colony itself, or to the applicator (you).  Keep in mind that plants create oxalic acid, formic acid, thymol, and essential oils as poisons — you can’t just go about applying them willy-nilly.

Practical application: Thymol applied at too high a rate of release, relative to temperature, hive size, and colony strength, can kill bees and brood — details are important!  My test was on double deeps, and with the 40-gram dose in a rim on top.  If your state allows use of unregistered miticides, follow tested methods of application, or experiment very carefully on only a few hives.

Citations and Notes

[1] Thymol — A new application method? Part 2.  American Bee Journal December 2022  https://scientificbeekeeping.com/thymol-a-new-application-method-part-2/

[2] Homasote 440 Medium Density Fiber Board; (94-98% paper cellulose, 1-6% paraffin wax, <0.1%  copper metaborate (Homasote was approved by EPA for the Miteaway II pads).

[3] ½-inch Sound deadening panel, Building Products of Canada Corp.  “produced from non-toxic organic material and natural wood fibres that are wax impregnated.”

[4] From Wintersun Chemical www.wintersunchem.com

[5] A bee brood cycle averages ~20 days in hot weather), whereas a varroa reproductive cycle is typically around 17 days during active broodrearing.

[6] Technically I couldn’t calculate efficacy, since I didn’t run a control group.