There are several highly efficient treatments against varroa mites which are based on substances from natural origin. They are less prone for resistance issues or residues in bee products than synthetic miticides. On the downside, they need a little bit more attention and knowledge to achieve best results. Thymol is one of the natural substances that are successfully used against the parasitic mite. There are two main products with thymol as main ingredient registered in Canada: Thymovar and ApiLife Var. The latter is the latest registration in Canada, dating only from October 2020. In the US, two main products with thymol are registered: Apiguard and ApiLife Var.
However, these products are not the same: Apiguard is a gel with thymol, while the other two products are strips, saturated with this substance. ApiLifeVar is a blend of thymol with eucalyptol, menthol and camphor, while Apiguard and Thymovar rely on pure thymol. These differences are important for understanding which product to choose in different conditions.
Thymol acts by its vapours, it must evaporate to kill the varroa mite. The bees distribute the substance in the hive by their activities, like ventilation or removal of the product, thus supporting the evaporation. For this process, the external temperature is important: the highest efficacy is achieved when the temperature ranges between 15-30°C and never falls under 12°C. The ideal range for thymol treatments is 20-25°C. This makes it clear why products based on thymol are summer treatments: They need temperatures seldomly reached in the colder months.
Comparison of the three products under different environmental conditions
Not many studies are available comparing the three products by means of their efficacy under different environmental conditions. The bee institute in Liebefeld (Switzerland) compared Thymovar and ApiLifeVar, finding high and similar efficacy for both products (around 90%).
A first extensive comparison of all three products was made in Italy: the scientists performed a study at three different locations in Northern, Central and Southern Italy. In the Centre and the South, all three products showed high efficacy, killing more than 90% of the varroa mites in the colonies. In the North, however, Apiguard was less reliable. The efficacy dropped to 66.9%, which is insufficient for protecting bee colonies until the winter treatment. The researchers attributed this low efficacy to the lower temperatures and low activity of the bees: under these circumstances, the workers did not “work” on the gel to remove it. By this, the gel surface dried, impeding the evaporation. The other two products did not have this problem.
In Germany, the same occurred: all three products were tested in Hesse (with cooler climate) and in the Rhine region (with milder climate). Apiguard in this trial had the lowest efficacy on both sites, with 43.1% in Hesse and 71.5% in the Rhine region. Thymovar worked better, though this product was also less efficient at the cooler site in Hesse. In the Italian study, Thymovar in addition demonstrated some problems in tolerability: the bees removed brood and honey from beneath the strips. In Northern Italy, the colonies decreased significantly in strength. At one of the two apiaries in this region, the study had to be interrupted because of the severity of the effects. This may have been due to a higher dose of thymol in Thymovar than in the other products and may have been released too fast. No such adverse effects occurred in Germany, however. ApiLifeVar finally, was equally efficient at all sites and no problem with the safety for the bees occurred. In the table below, you can find a summary of the results of these studies.
Table 1: Mean efficacy of registered thymol products for varroa treatment in Italy and Germany.
Apiguard
Thymovar
ApiLifeVar
Northern Italy
66.9%
93.6%
93.7%
Central Italy
94.3%
99.5%
94.5%
Southern Italy
96.5%
97.5%
96.7%
Hesse, Germany
43.1%
86.5%
95.0%
Rhine valley, Germany
71.5%
92.6%
95.9%
Product characteristics influence efficacy
In addition to the final efficacy, both studies measured the kinetics of the efficacy during the treatment period, i.e. how fast the products kill most of the mites. Again, under the warm conditions in Central and Southern Italy, all three products reached many mites already in the first week. Under the cooler conditions in Northern Italy and Germany though, Apiguard started only very slowly and killed only about 10% of the varroa mites within the first week of the treatment. Thymovar and ApiLifeVar, in this short period, already killed 30-35% of the mites in Italy and 30-40% in Germany.
In this context, it is important to understand the conditions for treatment success with thymol. As already mentioned, this substance acts by its vapours. The concentration in the hive air must be high enough to kill the varroa mites, but low enough not to harm the bees. This concentration ranges between 5-15 µg/l hive air. Tests in Switzerland showed that Apiguard, under cooler conditions, did not reach this therapeutic concentration. It remained under 4µg/l hive air, which explains the only low efficacy on cooler sites. Thymovar and ApiLifeVar on the other hand reached the therapeutic concentration already in the first week of the treatment. Interestingly, the hive air concentration decreased during this week in treatments with Thymovar, while it slightly increased using ApiLifeVar.
Of all three products, ApiLifeVar was the most independent of environmental conditions. The efficacy remained over 90% at all sites. This may be due to the different composition of this product: unlike the other two, it is a blend of thymol with menthol, eucalyptol and camphor. An interesting property of this blend is that it remains liquid at colder temperatures. Pure thymol is solid until a temperature of 49-51°C, therefore also under hive conditions of 35°C. Mixing thymol with other aromatic substances decreases its melting point (i.e. when it becomes liquid). Both solid and liquid thymol can pass to the gaseous stage, which is necessary to reach the varroa mites on the bees. However, the step from liquid to gaseous (evaporation) is much more constant and reliable than the step from solid to gaseous (sublimation), especially under cooler temperatures. This may be the explanation for the more consistently high efficacy of ApiLifeVar under cooler climatic conditions.
Risks and side-effects of the treatment
Every medicinal product – like varroa treatments – has risks and side effects. Despite being a “natural” substance, thymol can be toxic for bees when the concentrations in the hive air are too high. This could happen, for instance, using DIY preparations with thymol crystals. The formulation of registered products helps to avoid this risk, if the label instructions are respected. On the other hand, underdosing the treatments for cost reasons may result in insufficient efficacy and subsequent colony losses.
Thymol is a fatty substance. Therefore, it may form residues in wax and enter honey by small wax particles. A recent study in Spain showed that the thymol concentrations increase significantly during the treatment, mainly in wax and honey. This is not a risk for the consumer: Thymol has a FAO GRAS status, meaning that it is “generally recognized as safe”. However, higher concentrations may change the taste of the honey. During the treatments, the concentration in honey could surpass the sensory threshold. Sensitive test participants noticed the taste even three months after treatment. These results confirm the recommendation to never treat with honey supers present. In addition, it is advisable to not mix the combs from the brood nest with those from the honey super.
It is important to note that these risks are mainly a consequence of wrong applications or of using DIY applications like pure thymol crystals or in presence of the honey super. If registered products are used according to the label instructions, the benefit of reducing the mite load clearly prevails.
Dr. Claudia Garrido
BeeSafe – Solutions for Bee Health and Pollination
www.bee-safe.eu
ALV-10-CA-N01-12/21