di Claudia Garrido, BeeSafe –
Beekeepers are in a pretty happy position compared to other animal husbandry sectors. Of course, there are plenty of problems and challenges, one of them being the Varroamite. But there is one thing that is unique: treatments with natural substances against this parasite are more efficient than synthetic acaricides.
There are three natural substances mainly used for varroa treatments: formic acid, oxalic acid and thymol. The latter is an aromatic compount of the essential oils of some aromatic plants like thyme. For varroa treatment, it has to evaporate and the bees distribute it in the hive by their activity (ventilation or removal of the product). The external temperature is important: the efficacy is highest when the temperature ranges between 15-30°C and never falls under 12°C. The ideal range for thymol treatments is 20-25°C. Therefore, the temperature is the first – and very important – parameter to consider when treating with thymol.
Registered products with thymol – same active substance but different formulations
In Europe, there are three registered products with thymol as main active ingredient: Apiguard, ApiLifeVar and Thymovar. However, these products are not exactly the same: Apiguard is a gel with thymol, while the other two products are strips saturated with it. ApiLifeVar is a blend of thymol with eucalyptol, mentol and camphor, while Apiguard and Thymovar rely on pure thymol.
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 also this product was less efficient at the cooler site in Hesse. In the Italian study, Thymovar in addition also showed 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 happened.
Product characteristics and efficacy
Both studies measured the course 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 slow 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.
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.
From all three products, ApiLifeVar was the most independent from environmental conditions. The efficacy remained over 90% at all sites. This may be due to the different composition of this product: differently to 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.
As for every treatment, thymol has to be used in the right conditions. On the one hand, there is the lucky condition of high efficacy of natural products against the varroa mite. On the other hand, to achieve this some deeper knowledge and observation skills are necessary. In this context it is crucial to use only registered products. Only these give you sufficient information how and when to use it. Most importantly, the dose is always the same. Home-made preparations do not give this security: if the dose is too low, the efficacy is insufficient, if too high, it may harm the bees. Thymol is a natural substance, but it is a medicinal product. As such, it has to be handled with care and responsibility.
