OAV Temperature Follow-up, a Call to Action, and an Update on Thymol Blocks
June 17, 2026
Contents
HOW IMPORTANT IS A VAPORIZER’S SETPOINT TEMPERATURE?. 3
OAV Temperature Follow-up, a Call to Action, and an Update on Thymol Blocks
Randy Oliver
ScientificBeekeeping.com
First Published in ABJ February 2026
I’ve previously written about oxalic vaporizer setpoint temperatures; a recent dataset got me to revisit the subject. But first it’s time for us to call out the troops!
A CALL TO ACTION!
When invited to speak in France a few years ago during late summer, I visited a beekeeper whose apiary was getting mobbed by the “Asian hornet,” aka the yellow-legged hornet. His hives’ foragers were afraid to fly, and his colonies were dwindling from the predation pressure by a mob of ravenous wasps. He had buckets of sugar syrup set around his hives as traps, each with several inches of drowned hornets in them. It was an absolutely ugly thing to see, and he didn’t know whether his colonies were going to be able to survive.
Later in the trip, I photographed some of the hornets scouting one of John Kefuss’ hives (Figure 1). Then in 2023, at the National Honey Show, I watched a presentation on its invasion of England [[1]], where they were desperately trying to stop it. One problem was that the hornets readily hitch rides on trucks and boats — meaning that it could quickly spread to other mild-winter areas of the country if we don’t stop it now.
Fig. 1 The yellow-legged hornet is not a “potential” threat to honey bees, our native pollinators, and American beekeepers — it’s already here and starting to spread!
This hornet has become a huge problem for beekeepers in Europe, is wiping out native pollinators, and disrupting ecosystems.
WE NEED TO ERADICATE THIS DANGEROUS PREDATOR BEFORE IT SPREADS FURTHER!
Our federal agencies in charge of invasives, APHIS and ICE, have screwed up this invasion even worse than Australia did with varroa. Instead of recognizing it as a national threat to beekeepers, native pollinators, and any managed crops requiring insect pollination, APHIS handed the job of exterminating this national threat over to the underfunded state of Georgia. This is by no means a criticism of anyone in Georgia, but we taxpayers are currently spending close to $200 billion for immigration enforcement — how about redirecting some of that money to locate and eliminate a true national threat?
We beekeepers need to contact our representatives and demand federal action! And ask our farmers and nature lovers to do the same. The feds could engage Georgia residents to help by offering a $250 reward for nest sightings (and perhaps a bounty for destroying nests on one’s own property). EPA should allow emergency use of any insecticides necessary. And we should call out the National Guard to use the military’s remote-controlled drones to locate and kill any nests in inaccessible areas with a shot of fipronil.
If we don’t stop the yellow-legged hornet now, it will be something we regret for the rest of our lives. You can help by asking your local and national associations to contact your congressmen. And you can sign the BeeCause petition in Charlie Linder’s article in this issue (and ask your honey purchasers, and local farmers and environmentalists to do so too). https://beecausealliance.org/tropilaelaps-petition/
HOW IMPORTANT IS A VAPORIZER’S SETPOINT TEMPERATURE?
Earlier in this series [[2]], János and I discussed setpoint temperatures for vaporizers. I recently received a data set from a beekeeper who had applied thirteen 6-gram OAVs (oxalic acid vaporizations) at 5-day intervals August through October, but recorded no decrease in the mite drop counts that he took every other day from his stickyboards (until the colonies went broodless).
Practical application: This strongly suggested that the efficacy of his treatments was quite low, and I couldn’t figure out why.
So I asked him the setpoint temperature of the vaporizer he was using — it was below 190°C. That raised my curiosity, since I’d gotten poor oxalic output at that temperature when I’d tested other vaporizers. I’d also been previously forwarded a study from Greece [[3]] that showed far greater efficacy from a setpoint temperature of 240°C compared to 187°C. And I notice that when I set a vaporizer to 190°C, the cloud of vapor does not come out nearly as vigorously as it does at 240°C, making me wonder whether there is both reduced OA (oxalic acid) output at the lower setpoint temp, perhaps along with less efficient penetration of the cluster?
To answer that question, since I’ve already titrated what sort of OA residues are on bees following 4-gram OAVs at 230°C from other vaporizers, I set an InstantVap to 190°C, and vaped two doubles (each with 16 combs of bees), with the same 4-gram dosage, along with an adjacent similar hive with the setpoint at 230°C. Two hours later we pulled combs from the upper box of each hive to pluck bees from three different combs for titration (Figure 2).
Fig. 2 One thing that we noticed on the center comb of the upper box of the 230°C hive, was that even two hours after application, there were scattered bees with white-dusted wings (which I tried to avoid when plucking bees for titration). Why there were so many dusted bees in the center comb of the upper box, I don’t know, but when I intentionally dropped some into other test tubes, they immediately changed the dye color from blue-green to orange-yellow, indicating OA residues in the very high 200µg range.
The results of our titrations were surprising — and apparently associated with such bees (Figure 3)!
Fig. 3 We titrated 7 bees from each comb. Those from the hives vaped at 190°C (the upper two charts) carried barely detectable OA residues, whereas those from the hive vaped at 230°C, had markedly higher residue levels (with three high-residue outliers, perhaps from moving to the center after being hit elsewhere). I only ran one control hive, since I’ve already published data from a number of other hives vaporized at 230°C that also consistently showed higher residues.
Practical application: A common conjecture is that keeping a vaporizer’s setpoint temp at 190°C would result in less acid thermal decomposition, resulting in more acid getting onto the bees. But this may not be the case!
When I drop a 4-gram oxalic acid dose into the vaporizer set at 190°C, the temperature quickly drops to 170°C, then slowly warms back up as the OA boils off (taking longer to vaporize than it does at 230°C). Although I had already observed that a large cloud of fog comes out of a vaporizer set at 190°C, we didn’t observe any of that cooler fog escaping either of the hives vaporized at that temp, whereas there was clearly leakage out of the blocked entrance of the 230°C hive.
Practical application: Could it be that the more rapid and “vigorous” OAV cloud at 230°C (or higher) results in better penetration of the cluster than one at a lower temperature?
We clearly need much more applied research on this widely-used method of mite control!
UPDATE ON THYMOL BLOCKS
I wrote about an experimental new application method for thymol in 2024 [[4]]. To see whether I could replicate the excellent results, this last summer I followed up by testing it in a number of yards, and with different arrangements of the blocks (Figure 4).
Fig. 4 In my original trial, run during 90°F (32°C) weather, the thymol blocks gave incredible results in every hive. But last summer when I tried to replicate those results in cooler weather, or in hives that were in the shade, the degree of mite reduction was disappointing.
So to determine whether the method required hot sun on our dark plywood hive covers (to vaporize the thymol), I tried again in warmer weather, covering some hive covers with white sheets, and left others dark.
Results: There was poorer mite reduction under white covers. This method of thymol application apparently requires hot sun on dark hive covers above the rims!
So I continued testing in cool weather this fall, by placing the blocks in the middle of the cluster between the brood chambers. Again, poor efficacy! The thymol apparently just gets released too slowly.
This method has great promise, but don’t count on it yet. More experimentation needed!
CITATIONS AND NOTES
[1] https://www.youtube.com/watch?v=lHNype6Xb8M
[2] The How and Why of Oxalic Vaporization Part 2, ABJ November 2025
[3] Milioti Vaia, Arambatzakis Vassilis, Goras George (2022) Results from the use of oxalic acid by sublimation for varroa control during the summer months. Melissokomiki Epitheorisi 36(1): 21-27.
[4] Extended-Release Thymol Blocks, ABJ April 2024 https://scientificbeekeeping.com/extended-release-thymol-blocks/
