The need for new Varroa treatments
The parasitic mite Varroa destructor is the main pest of the honeybee Apis mellifera. Although small in size, it has produced enormous losses to the apiculture and pollination industries in addition to the almost extinction of wild honeybees throughout the world. Varroa is an ectoparasite (ecto = external) that affects both larvae and adult bees by piercing their cuticle (skin) and feeding on their internal tissues . This leads to the transmission of viruses and a decrease in hive viability. Without human intervention to reduce the effects of the mite, most beehives infected with Varroa die within one year. Even with human intervention, Varroa causes significant losses. In the US, beekeepers have reported annual hive loss rates of 30% and often reaching 40% or more since 2007 .
The Varroa mite has been a problem since the 1950’s when Varroa jumped hosts from Apis cerana, the Asian honeybee, to Apis mellifera, the European honeybee. Varroa mite was found in Japan in the 1950’s; China and India in the 1960’s; in Eastern Europe, South America, and parts of Africa in the 1970’s; in Western Europe and North America in the 1980’s; in the UK in the 1990’s; and in New Zealand and Hawaii in the 2000’s –. While A. cerana evolved tolerance to the mite, A. mellifera had not been exposed to it which led to the devastating effects. One of the main differences is that while in A. cerana the Varroa mite reproduces only in drone brood, in A. mellifera the mite also reproduces in worker brood . As the Varroa mite is a vector for viruses, relatively non-virulent viruses such as Deformed Wing Virus (DWV) have become highly prevalent. It is the effect of these viruses that actually produces the colony death .
Imagine the situation for beekeepers when facing the Varroa mite challenge for the first time back in the 1960’s, 1970’s, or 1980’s. A common initial reaction was to try to eradicate the mite. In Japan, for example, Varroa mite was first found in 1955, had spread to most of the south by 1958, and was in all areas within 10 years. Initially, beekeepers would burn the infected bee stocks so as to burn any mites, and sterilise the hives with caustic soda. Later on, beekeepers started looking for chemical treatments . However, in the first few decades there were no Varroa mite treatments available in the market. Nearly every country affected initiated some efficacy studies with various acaricide sprays, fumigants, and powders . Beekeepers adapted the use of other pesticides for beekeeping. Unfortunately, there were no studies showing that these treatments were effective, whether they were safe for the bees, and whether they contaminated the honey. In hindsight, perhaps one would think that the best would have been for beekeepers not to interfere and let the bees evolve tolerance against the mite. Some proponents argued not to use pesticides inside the hives . The problem is that this would have meant that most of the colonies would have died and no beekeeper really likes that idea. A search of old research papers shows a number of different chemicals tested and cumbersome ways of applying them. Most of these chemicals are no longer used , –.
There are several challenges in finding chemical treatments that are suitable for Varroa mite control. First, they have to be able to affect the mites without affecting the bees. This is easier said than done given that mites and bees are relatively closely related as both are arthropods. Second, they don’t have to contaminate the honey as this could be toxic for human consumption. Third, they have to be practical for the beekeeper to apply. Initial chemical interventions produced undesired effects such as high bee and queen mortality and potential to produce honey contamination. There were also reports of mites developing resistance to the treatments . Given these constraints, it may be considered a blessing that after a few decades of studies plus trial and error we have a few reliable chemical treatments available. These are divided into synthetic chemical treatments and organic chemical treatments.
Synthetic chemical treatments are the preferred option for beekeepers. The most common active ingredients are the formamidine amitraz, the pyrethroyds flumethrin/tau-fluvalinate, and the organophosphate coumaphos (coumaphos is not approved for Varroa mite control in New Zealand). Unfortunately, Varroa mites are developing resistance to these synthetic treatments, in particular the pyrethroids and coumaphos. This is a significant issue in North America, South America and Europe and is a developing problem in New Zealand. Amitraz is the only synthetic chemical treatment that remains effective in most areas throughout the world.
Alternatively, beekeepers can utilise organic chemicals, such as oxalic acid, formic acid, and thymol. There are commercial formulations or can be purchased in bulk. Given their mode of action, development of mite resistance to these treatments is unlikely. However, their application is labour intensive, they are not as safe for bees and beekeepers as the synthetic treatments, and are somewhat unreliable as their effectiveness can vary with weather. As mite resistance to synthetic chemicals increases, many beekeepers follow up with an organic treatment as a backup.
Treatments are usually applied at least twice a year, in spring and autumn, before and after the honey season and are usually alternated to reduce the development of treatment resistance in mites. The fact that amitraz is the only active ingredient that remains effective in some areas may lead to overuse and subsequent mite resistance. This creates the need for new chemical treatments to enable treatment alternation and to provide alternatives in order to delay or cope with mite resistance to other treatments. Pheromite has been working on developing new chemical treatments against the Varroa mite for more than 3 years and in future posts we will discuss some of the challenges in more detail. S. D. Ramsey et al., “Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph,” Proc. Natl. Acad. Sci., vol. 116, no. 5, pp. 1792–1801, 2019.  Bee Informed Partnership, “Honey Bee Colony Losses 2017-2018: Preliminary Results,” 2018. [Online]. Available: https://beeinformed.org/results/honey-bee-colony-losses-2017-2018-preliminary-results/. [Accessed: 07-Jun-2019].  “Varroa destructor,” Wikipedia, 2019. [Online]. Available: https://en.wikipedia.org/wiki/Varroa_destructor. [Accessed: 07-Jun-2019].  A. Arzone, “The danger of using pesticides to control Varroa jacobsoni oud.,” Apiacta, vol. 2, pp. 1–3, 1984.  E. Crane, “The Varroa mite,” Bee World, vol. 59, no. 4, pp. 164–167, 1978.  D. De Jong, R. A. Morse, and G. C. Eickwort, “Mite Pests of Honey Bees,” Annu. Rev. Entomol., vol. 27, pp. 229–252, 1982.  A. C. Stort, L. S. Goncalves, O. Malaspine, and F. A. Moura Duarte, “Study on Sineacar Effectiveness in Controlling Varroa Jacobsoni,” Apidologie, vol. 12, no. 3, pp. 289–297, 1981.  W. Rath, “Co-adaptation of Apis cerana Fabr. and Varroa jacobsoni Oud,” Apidologie, vol. 30, no. 2–3, pp. 97–110, 1999.  P. Rosenkranz, P. Aumeier, and B. Ziegelmann, “Biology and control of Varroa destructor,” J. Invertebr. Pathol., vol. 103, pp. S96–S119, 2010.  E. Crane, “Selective annotated bibliography of the Varroa mite and its control in honeybee colonies,” no. 37. The Eva Crane Library of the Internation Bee Research Association, 1985.  S. Bogdanov, V. Kilchenmann, and A. Imdorf, “Acaricide Residues in Beeswax and Honey,” Swiss Bee Research Centre. 1999.  O. F. Grobov, “Varroa disease in honey bees,” Apiacta, vol. 4, pp. 3–5, 1976.