The varroa mite lives as a parasite on honeybees. It grows and reproduces in the capped brood in the beehive. In one developing cycle a female Varroa mite lives phoretically on the bee for about one of three weeks. The damage of honeybees by Varroa mites happens in several ways. Larvae and bees are weakened by the sucking out of blood and fat and are susceptible to diseases and less resistant to environmental pollution. In addition, bacteria and viruses are transmitted by the mites and can cause serious damage to a colony.
Natural defensive behaviour of honeybees
Some bee colonies show natural defensive behaviour against Varroa mites. Sometimes brood cells are opened by the bees and partly or completely cleaned. This behaviour (VSH – Varroa sensitive hygiene) inhibits or hinders the reproduction of the Varroa. There are many efforts to promote this behaviour through breeding, e.g. by “Arbeitsgemeinschaft Toleranzzucht“.
Furthermore, in some colonies Varroa are bitten and fatally injured by bees. Where these injuries are inflicted on them is not clear, perhaps during social grooming.
Resistant bees in Avignon, Le Mans, Arnot Forest and Gotland
Without fully understanding the mechanisms, we can conclude that allowing natural selection leads to a balance between varroa and bee. If the colonies are left to themselves for a long time, an adaptation between host and parasite takes place. Whether the bees evolve defence mechanisms, whether the varroa become less aggressive, or perhaps both, is not clear. The balance between host and parasite has been demonstrated in three different studies by different renowned scientists and institutes from different countries (you find the studies under Downloads).
In addition to the studies listed below, the Gwynedd Region in Wales also confirms this adaptation between host and parasite in practice.
Varroa resistant bees on Gotland
A joint project of the University of Agricultural Sciences (Uppsala, Sweden), the Centre for Bee Research (Bern, Switzerland) and the State Institute of Apiculture (University of Hohenheim, Stuttgart) started a trial on the island of Gotland in 1999. 150 colonies with Varroa infected were left to their own devices without any intervention by the beekeeper. The total population has shrunk to only 7 colonies in 4.5 years, including 6 original colonies. Overall, the stock has shrunk to 4.7% of the original stock.
In the following 2 years, a balance between host and parasite establishes. Mortality is declining much, with swarms predominating. At the end of 2005, the number of colonies increased to 13 (8.7%). The authors think that an adaptation must have taken place, which allowed the survival of the host and parasite. They think further attempts are necessary.
The only source of the project’s progress is the doctoral thesis of Barbara Locke from Sweden (University of Uppsala): In 2010 the number of colonies has increased to 23 again! The number of untreated colonies has tripled within 6 years. Conventional comparative populations were also not dealt with acids/chemistries in 2010. The reproduction rate of female mites is much higher among these colonies. This author also comes to the conclusion that an adaptation must have taken place.
Resistant bees in Avignon, Le Mans
Researchers from France observed colonies who have survived for at least 2 years without any interventions of bee keepers. They recorded Varroa infestation, mortality rate, colony strength, honeyproduction, swarms, etc. They carried out the same measurements on conventionally held bee colonies. In 1994 they started observing 12 untreated colonies, in 1998 they added another 42 untreated colonies and in 1999 another 28 untreated colonies.
The authors do not set up any significant differences between the mortality rates of the two experimental groups. Here, too, an adaptation between host and parasite is presumed.
On average, honey production is 1.7 times lower than that of the conventionally held colonies. . This, of course, makes us bee keepers uncomfortable. However, we should bear in mind that the bee colonies we work with have been bred for most honey production since decades. Natural selection and reproduction of the western honey bee is almost eliminated. Therefore it is not an unnaturally low honey production of the untreated colonies, but an unnaturally high production of the treated colonies!
The authors of the study see in the results a tolerance of the bees opposite the Varroa mites. They believe that beekeeping in France is possible with integrated Varroa management. Beekeeping, which interferes with the relationship between host and parasite only with the aid of soft biological or biotechnical agents.
Varroa resistant bees in Arnot Forest (New York State)
Thomas D. Seeley, one of the two authors of the study “The nest of the honey bee” from 1976, carried out a study in Arnot Forest between 2002 and 2005 on eight wild bee colonies in tree cavities. The set of wildlife found there corresponded to the set of colonies the author had found a quarter of a century earlier by using the same methods (before Varroa came to North America). This study also reveals the viability of bee colonies infected with Varroa. The author suspects that the mite has adapted to its host due to the experiments carried out.