Observations on oxalic acid drizzle and strip treatments

Technical Specialist Bees Report

Author: Elizabeth Frost, New South Wales Department of Primary Industries

 

Introduction

In this article I’ll focus on legal oxalic acid drizzle (oxalic solution) and non-legal oxalic acid and glycerin strips. A new Canadian study on both of these oxalic acid treatment types provides statistically significant results on summer treatment effectiveness and bee health impacts in Canada. While the Australian examples I’ll provide in this article are not statistically significant, they do provide some interesting early observations for winter treatment options.

In Australia we are lucky to have 7 different chemical miticide products available for Varroa management. You can read all about them here: www.varroa.org.au/chemicals. This variety gives us the opportunity to rotate chemicals with different modes of action. Chemical rotation is important so we can prevent varroa from developing resistance to a chemical miticide (especially the synthetics) for as long as possible. Api-Bioxal™ oxalic acid, either in solution (dribble method) or powder formulation (vaporisation method), is the most recent chemical product to be permitted for use in Australia.

Hopefully, in the near future, we’ll also have Aluén CAP oxalic acid slow-release strips approved for use. You can follow the company’s updates regarding Australian registration here: www.capproducts.com.au/. Over the past season, I’ve heard that some beekeepers have been trialling their own oxalic acid and glycerin strips, a chemical use which is not likely to ever be legal in Australia, due to a change in legislation being required. Making these strips correctly and safely is challenging, so I’m looking forward to when beekeepers have access to a legal oxalic acid slow-release strip product manufactured at a uniform, high standard.

 

Canadian study on oxalic acid dribble and strips by Thurston et al.

For beekeepers keen to learn more about oxalic acid varroa treatments, a new Canadian study tested oxalic acid and glycerin strips as well as oxalic dribble methods in the Canadian summer, when brood quantities are at their peak. The Canadian study estimated the efficacy, or effectiveness, of each treatment at killing varroa (Table 1).

 

Table 1. Varroa treatment effectiveness in a Canadian summer study. Adapted from Thurston et al. (2025).

Treatment type	Efficacy
Oxalic acid dribble	39.5 % (± 4.3%)
Oxalic acid and glycerin strips	55.8% (±3.2% )

Their results showed oxalic acid and glycerin strips had around 55.8% (±3.2% ) effectiveness at decreasing mite levels. This is considered a moderate level of control. They showed oxalic acid dribble had around a (39.5 ± 4.3%) effectiveness at decreasing mite levels. The low level of control with oxalic acid dribble during summer is to be expected, due to the high quantities of capped brood protecting mites from coming into contact with the oxalic acid liquid solution. Remember, oxalic acid solution, when dribbled between the top bars of frames covered with bees, is only effective at killing varroa mites on adult bees.

In regards to hive health, the Canadian study did not find any significant negative effects of either oxalic acid dribble or strip treatments on hive population, worker longevity, queen status or colony survival. Their application of oxalic acid and glycerin strips did temporarily result in a greater number of hives with spotty brood patterns early on in the treatment period. The spotty brood patterns recovered to normal after 45 days from the start of oxalic acid and glycerin treatment in the Canadian study.

Based on their results, the Canadian researchers and technicians found:

“Oxalic acid and glycerin strips might help control varroa mite populations, delaying their exponential growth and helping reduce economic losses for beekeepers, but this treatment should be considered as part of an integrated pest management strategy and not a stand-alone method for Varroa control.”

 

Oxalic acid drizzle without and with brood

Figure 1. 50 ml oxalic acid in sugar syrup solution drizzle 28 March 2025 in a totally broodless hive.

 

OA drizzle without brood: In most of Australia, we don’t have many if any periods without brood. There are, however, several ways to manipulate a hive to make it completely broodless. The hive I applied oxalic acid solution to in Figure 2 was a virgin queen bank I had set up for my Tocal College Queen Bee Insemination Course. Once the course was done, I let the hive go totally broodless. On 28 March this completely broodless hive had 21 mites in an alcohol wash. On the 28^th I added a new Apithor small hive beetle harbourage and treated the hive with 50ml of oxalic acid sugar syrup solution, made up according to the product label. 3 days later on 31 March I alcohol washed this hive again and it only had 1 mite in the alcohol wash. The hive had a ventilated bottomboard in place with a tray to look at mite drop as a result of the oxalic acid syrup solution treatment. 24 hours after treatment 471 mites had dropped dead onto the bottomboard tray as well as 100 small hive beetle (SHB) as shown in Figure 2. Oxalic acid solution via dribble/drizzle method was very effective at killing varroa, as all mites were on adult bees given there was no brood present.

Figure 2. Bottomboard tray mite and SHB drop 24 hours after oxalic acid drizzle and Apithor application.

 

OA drizzle with brood: If your hive has brood however, oxalic acid drizzle is less effective, as the Canadian study showed. On the 28^th of May I applied 50 ml of oxalic acid sugar syrup solution to a hive with a total of 12 frames of bees and a total brood area of 3 full depth frames. Plenty of bees, brood, varroa and small hive beetle at the end of May 2025 in the NSW Hunter Valley! This hive had a mite wash result of 32 mites on the 17^th of May and an average natural mite drop of 9.4 mites per day, monitored through the ventilated bottomboard between 17 May and treatment on 28 May with oxalic acid drizzle. I did not add an Apithor SHB harbourage to this hive as I was curious if OA drizzle alone would have any impact.

It was pleasing to see hundreds of mites fall dead through the ventilated bottomboard onto my counting tray in the 3 days after treatment (Table 2). However, on the 2^nd of June I did another alcohol wash to check how the oxalic acid drizzle had affected overall hive infestation and came up with 38 mites, which was higher than before treatment. This hive had a lot of adult small hive beetles, and I was keen to see if any would die with the use of oxalic acid drizzle alone. Not a single dead small hive beetle found during treatment with OA drizzle, however still plenty alive inside the hive!

 

Table 2. Mite counts in alcohol wash and ventilated bottomboard tray before and after oxalic acid drizzle treatment in a production hive with 3 frames of brood and 12 frames of bees.

Date	Alcohol wash # mites per 300 bees	Dead mites fallen onto bottomboard tray in 24 hrs
17/05/2025	32	Avg. 9.4
28/05/2025 (OA drizzle in)		Avg. 9.4
>29/05/2025		414
30/05/2025		229
31/05/2025		157
02/06/2025	38

After I got the 38 mites per 300 bees result in my alcohol wash, I prepared the hive for synthetic treatment which is not temperature sensitive. I consolidated my 12 frames of bees in their 8-frame full depth brood box with migratory lid, removed the ideal honey super and applied Apitraz strips (Figure 3). I chose Apitraz because the treatment I used prior to OA drizzle was Bayvarol in January 2025 and I needed to rotate to a chemical with a different mode of action to either Bayvarol or Apistan. I could’ve alternatively used Apivar which has the same mode of action to Apitraz. It made sense for me to use an amitraz product because I was able to consolidate this hive to the broodnest and remove all honey supers. This allowed me to use Apitraz, a product which should not be used with honey supers on which will be extracted for human consumption.

 

Figure 3. At the end of the 6-week Apitraz treatment my migratory lid was filled honeycomb that had to be disposed of according to Apivar permit.

 

Oxalic acid and glycerin strips

Disclaimer: Oxalic acid and glycerin strips are not legal for use in Australia currently, however, the company Aluén CAP is has applied to to have their oxalic acid slow-release strip product approved by the APVMA for use here. Many beekeepers overseas use oxalic strips and I’ve heard through the grapevine that Australian beekeepers have been trialling them this past season to rotate with the available chemical and mechanical control tools, all the while chasing as much honey as they can.

Pilot trial: Anecdotal evidence I’d heard of beekeepers using oxalic acid and glycerin strips during the 2024-2025 season included variable effectiveness on varroa control and hive impacts. I was keen to have a look for myself. I utilised Australian Pesticides & Veterinary Medicines Authority Permit 7250 which allows me in my role within the NSW Government to conduct a small-scale trial using an active chemical constituent on small numbers of livestock at a research station. In this case, the active was oxalic acid in glycerin mix applied via 3 cellulose strips per hive for 5 bee hives on NSW DPIRD research station Tocal Agricultural College. The strips were made according to the standard New Zealand 40:60 oxalic acid to glycerin ratio. A five-hive pilot trial will not yield statistically significant results. However, it might provide some valuable observations which could guide future trials that are properly planned, scaled, and include control hives without treatment for comparison. At the very least, I hope to provide some food for thought for beekeepers in the following section.

Figure 4. Hive 1 oxalic acid and glycerin strips hanging within the brood cluster in an 8-frame hive maximises bee and varroa contact with the strips.

 

Once the nucs had grown in population, I transferred them to 8-frame full depth brood boxes (Figure 4). It took 28 days within the Tocal mite-infested apiary for the 5 mite-free nucs to produce alcohol wash results yielding detectable mite infestations: 7, 0, 1, 5, and 3 mites per 300 bee sample respectively for the five hives. This speed of natural mite infestation was similar to the first nuc I brought back to Tocal early October 2023, shortly after the Eradication finished. In that case, the first nuc back onsite in October 2023 was the only managed hive at Tocal. It took 1 month for that hive to reach 5 mites in an alcohol wash which the NSWAA Executive was kind enough to conduct for me during their 2023 executive meeting at Tocal. I think the exercise caused a few nightmares!

Back to the 5 hives in the oxalic acid and glycerin strip trial. On 13 June 2025 I inserted 3 strips per colony, each overhanging on either side of a frame with brood. This is best practice for oxalic acid and glycerin cellulose strips, and in fact all synthetic strips legally available in Australia, as it results in the greatest amount of contact between the strips and the bees, where there is the most activity in the hive by bees that are also the most likely to have mites on them. Where the strips were tight, reducing bee space between the frame top bars and hive lid, some of the hives propolised the strip edges. None of the hives propolised strips which were hanging over brood between frames.

Some interesting things I noticed during this pilot trial were:

• how quickly the strongest hive chewed the strips (Figure 5)

• how some hives stopped raising brood under the strips (Figure 6)

• how the queen in the hive with the lowest population stopped laying, was seen being balled by her workers, and eventually disappeared (Figure 7)

• how poorly some hives removed strip debris from the bottomboard (Figure 8)
  

 

Figure 5. Chew-happy Hive 1 at 21 June 2025, 1 week after oxalic acid strips placed within the hive. By 20 July 2025 the only strip pieces left were a small square piece on the top bar.

 

Figure 6. Hive 1 brood at 28 June 2025. Note they've removed brood behind the strip and this Carniolan queen is full speed ahead laying drones in Hunter Valley NSW winter.

Figure 7. A queen being balled by her workers, forming an aggressively tight cluster around her, on 21 June 2025. She had just begun to lay again after a break which occurred at the start of treatment.

 

Figure 8. Chewed strip debris on ventilated bottomboard on 20 July 2025.

 

The most interesting thing of all of course, was assessing and comparing mite infestation between 13 June, the day I applied the strips and again 37 days later on 20 July 2025 (Table 3). The average treatment efficacy seems great at 80%, however the lowest efficacy at 40% is a possible concern. This hive had the strongest population, the biggest brood area, and chewed the strips to nothing the fastest. If a strong single hive of bees can do this in 37 days in winter, how fast are production hives in spring and summer chewing through them? A mite treatment can’t work if it doesn’t exist.

 

Table 3. Oxalic acid and glycerin strip treatment effectiveness range and average in a pilot trial at Tocal.

Hive ID	13 June 2025 - Alcohol wash # mites per 300 bees	20 July 2025 - Alcohol wash # mites per 300 bees	Treatment efficacy
1	15	9	40%
2	10	0	100%
3	14	1	93%
4	8	0	100%
5	6	2	67%
Average treatment efficacy			80%

 

Beekeepers should be cautious about relying on averages. If you average my 20 July alcohol washes it’s 2.4 mites per 300 bees. Seems okay right? However, if you look at individual hive results, Hive 1 has the highest at 9 mites per 300 bees and should be the first hive I alcohol wash next time to gauge if it’s time for another varroa treatment.

While Hive 1’s super strip-chewers were great at chewing away the strips, they were not as good at cleaning the debris out of the hive. I wonder, if this occurred in spring, summer, or autumn when our already high small hive beetle populations are actively reproducing, if juvenile hive beetle will enjoy this strip debris, bound to get nice and humid on a solid bottomboard.

Plenty for beekeepers to think about regarding oxalic acid sugar syrup drizzle and oxalic acid strips. In larger scale trials, oxalic acid and glycerin strips have yielded efficacies of 55.8% (±3.2% ) in the Canadian study mentioned at the start of this article, and over 95% in studies cited on Aluén CAP’s website. If the 55.8% efficacy shown in the Canadian study can be achieved in Australia and if the product is approved for use in Australia, oxalic acid strips will be a key future tool for Australia with its globally record-breaking long honey and brood production seasons. You can check out the Canadian study referenced in “Further Reading” and brush up on the currently legal oxalic acid and other treatment methods on the Varroa Program website.

In my next article I’ll focus on oxalic acid treatment using vaporisation. Until then, some further reading below and keep monitoring your mite levels!

 

Further reading

• National Varroa Mite Management Program. “Varroa chemical control options.” www.varroa.org.au/chemicals

• Thurston, D., Eccles, L., Kempers, M., Borges, D., Ducsharm, K., Ovinge, L., Stotesbury, D., Scarlett, R., Kozak, P., Petukhova, T., Guzman-Novoa, E., & Morfin, N. (2025). Efficacy and Safety of an Oxalic Acid and Glycerin Formulation for Varroa destructor Control in Honey Bee Colonies During Summer in a Northern Climate. Pathogens, 14(8), 724. https://www.mdpi.com/2076-0817/14/8/724