Varroosis is a parasitic disease that attacks all individuals of the hive: larvae, nymphs and adults, both workers and drones. It is a very serious disease caused by the development and multiplication of the ectoparasite mite Varroa destructor, discovered in the bee Apis cerana, by Oudemans, Indonesia (1904, Java Island). Being a long time present in this bee, a balance was established between A. Cerana and V. destructor. From 2000 on, two species of distinct virulence were differentiated: V. jacobsoni and V. destructor.
This disease enters a healthy hive through the bees that walk in the field. A champion laborer visits several flowers during the gathering of nectar and pollen, coming in contact with bees from other apiaries. Another form of contagion occurs when a beehive weakened by Varroose can not defend itself and begins to be plundered, the bees of another hive invade the sick hive, come in direct contact with the varroa that circulates freely in search of uninfected bees. On the way home, the bee that has been plundered takes the varroa with it to the bosom of a healthy hive.
The balance between Apis cerana and Varroa destructor witnesses a relationship that prepares the host and ensures the life of the parasite.
The extent of parasitosis in Southeast Asia is the result of many factors, including:
– the replacement of A. Cerana by A. Mellifica to improve bee yields in those regions;
– the dispersion of A. Cerana-favorable biotypes, urbanization and changes in agricultural methods.
In the mid-1960s, varroa was found in Apis mellifica. But the passage from Apis ceranapara to Apis mellifica seems to go back many years before. Unfortunately, its pathogenic role was ignored. The first clinical signs appeared three or four years after the onset of the infestation. Since then, the parasite spread very rapidly and was fatal. The varroa does not
found in the new host no obstacle to its development. But it had, however, a displacement of the ecological niche because during the evolution it had a long time to realize it.
If we compare the biology and physiology of Apis mellifica to that of Apis cerana, this fulminating propagation can be explained. It is caused by the absence, in Apis cerana, of the phenomena that keep the pressure of the parasite to a level that is bearable for the colony:
– In Apis cerana (duration of the worker’s evolutionary cycle: 18 to 20 days, duration of the male’s evolutionary cycle: 21 to 24 days), varroa does not multiply in the male egg position. The operative time of the worker’s posture is too short to allow a complete development cycle of the parasite. However, female varroa may be imprisoned.
– In Apis cerana, when climatic conditions are favorable, there are four or five swarms per year, which limits the number of parasites in the colony.
– In Apis cerana, parasite cleaning behaviors are essential and may be either self-cleaning, the bee removes the parasites themselves, or mutual cleaning, the bee, after a dance of request for help, is aided by the other bees. In 15 minutes, according to hundreds of authors, Apis cerana releases 99% of the parasites when, similarly, the elimination would be insignificant (0.3%) in Apis mellifica. Apis cerana would clear 90% of the eggs posture parasitized in four days.
Not all authors agree on the importance of parasite hunting and clearing colony infestation. The cleaning behavior would simply be responsible for a host change but the parasite could disturb its reproduction.
– The poor regulation of the posture nest temperature would be unfavorable to the multiplication of varroa destructor, whose ideal breeding temperature is between 31.3 ºC and 34.2 ºC, according to the ambient temperature.
– The rate of juvenile hormone (bee hormone involved in maintaining the larval state, controlling the development of the genitals and sexual behavior and controlling respiration and nutrition), weaker in Apis cerana than in Apis mellifica, would limit the multiplication of the parasite. This hypothesis is controversial since it can be proved that this hormone has no effect.
– Apis cerana alveoli, which are smaller, may also be a limiting factor for reproduction rate.
These are the different points which, taken as a whole, explain the tolerance of Varroa destructor for Apis cerana. The others are perhaps still to be discovered.
As of 1964, parasitosis, which spread to the whole world, leaves only nowadays some intact areas and causes the death of a considerable number of colonies. The example of France illustrates this spread. Varroase appeared in the Alsace region in November 1982 and spread throughout the territory.
The causative agent is a mite: Varroa destructor Oudemans. It presents a bimorphism (existence of two distinct forms in a species animal or vegetal) very marked sexual in the adult state. There is, therefore, the female that is the form of dissemination of the disease; the male; the larval forms and the nymphs, still immature. The female lives in the adult bee and in the egg laying, the male only in the posture.
Female varroa feeds on the hemolymph of the adult bee, the hemolymph of the bees’ nymphs and sometimes on the larvae. Protoninfa, deutoninfa, male and their immature forms feed on hemolymph of bees’ nymphs.
After being closed in the alveolus, the female (founding female) that will give birth to future varroas feeds according to the age of the parasitized individual. Generally, the female founder pierces the nymph’s cuticle in the fifth segment. To do this, if necessary, the female or female founders push away the bee nymph’s feet by pushing them to increase the space. This feeding hole will be used by all offspring and even by other founding females.
Several authors are about to determine the amount of hemolymph absorbed by the adult female. This value varies according to the physiological state of the mite according to the time of year. Female varroa (weight: 0.3mg) feeds every two hours and carries 0.1 mg hemolymph.
Linked to food is excretion, which should also be mentioned. The feces are deposited in the wall of the alveolus even though many founding females are present. The excretion activity is estimated to be up to 15 times for 24 hours. The movement between the feeding zone and the excretion zone is very frequent.
Location of varroa
Within the colony, the female varroa is situated:
– In the adult bee, with more than two days, preferably in the nursing bees, in places where hemolymph puncture is possible, ie where the intersegmental membrane is: abdominal, ventral and dorsal rings, thoracic-abdominal joints (petiole) and head-thorax (neck), leg joints. There may be one or more parasites. This choice allows them to be closer to the posture. The parasite distinguishes indoor bees from harvester bees. It is estimated that about 20% of the parasites present in the bees harvester, which ensures the spread of the disease. The secreted pheromone in the Nasanof gland, whose production depends on the age of the bees, can intervene in the choice. Geraniol, composed of this pheromone, has a repulsive role. When females do not feed, they are located between the abdominal rings or the thorax.
– In the maneuvered posture of male or female, but evenly.
There is a preference for male posture that will be made due to the size of the alveoli and the greater secretion of attractive substances related to the size of the male larvae. In the same way, the real cells, although little attractive, can attract the female varroas. In case of heavy infestation, the parasites may be in the open posture, with more than three days, as well as in the real cells (2 to 3% of the cases). A female, recognized by its light color, is present in the head of the bee. Male varroa is found only in the operculated posture of male or female worker.
The development cycle of Varroa destructor occurs at the same time as the development cycle of the working bee or male during the posture phase.
Female fertilized varroa will enter the alveolus prior to the operation (about 20 hours earlier for infestation of working cells, about 40 hours earlier for male cells) and allowed to attach, which assumes the influence of call factors. These factors can be chemical, called kairomonas. Kairomonas are the chemical substances that the larvae emanate and that stimulate their predators and parasites and that also trigger the operation. These factors may also be mechanical and depend on the distance between the larva and the cell opening.
The female varroa moves between the wall of the alveolus and the larva and penetrates into the royal jelly but does not feed on it. Thus, it is beyond the reach of the workers. After the operation, the breeding female attaches to the larva and feeds on hemolymph to stimulate ovogenesis. The posture begins after the fifth change of the bee larva, that is, 60 to 74 hours after the operation. Two to nine eggs are put, one for every 30 hours. The first egg will give one male, the other females. The first egg is positioned in the anterior part of the cell (after the operculum), which avoids the risks of crushing during the pre-nymph movements during its development. The other eggs are positioned inside the alveolus after the exuvias and the bee’s ejections.
The coupling takes place in the alveolus, most of the time in the fecal accumulation zone. A female often accepts the male and the males of other offspring in case of multi-infested cells, which limits the phenomena of consanguinity. These are the spermatophores and not properly the sperm that are injected into the female genitalia. About two days later they are already in the spermatheca. The male dies when the bee is born.
The duration of the cycle of female varroa (ontogeny) from posture to adulthood is variable like that of the bee (about eight to nine days). The duration of the male varroa cycle is six to seven days. If these durations are compared to those of the operation, taking into account that the eggs are laid every 30 hours and that the laying does not begin before spinning the cocoon (about 3 days after the operation), a close to reality, which may be subject to several oscillations.
These are data of a single creative female that penetrates the alveolus. But many females may at the same time be trapped, but the number of offspring decreases significantly as the number of mites increases. In fact, food loses quality (larvae and nymph are depleted), lack of space in the alveolus, larval and varroal chemical secretions may play a negative role.
The same creative female can perform many cycles, but usually fewer than two. Some make up to seven cycles and lay up to 30 eggs. All the creative females that are allowed to be enclosed do not produce progeny. In 20% of cases, for example, it has been observed that males die before ensuring coupling. Other factors, such as the season, the feeding, the bee race … may also come into play.
Finally, females fertilized before beginning a breeding cycle should spend about five days in adult bees (forensic phase) to mature sexually. This period is very variable and depends on the quality of your food.
Endurance, life span
The life span of female varroa in a colony is two to three months in summer and six months in winter. Far from the host, this duration varies depending on the weather and would be from seven to nine days, in good weather days. In the posture left at room temperature, the female varroas are still alive fifteen days later. After this isolation, your reproductive vitality can improve.
Varroasis is one of the diseases in which the favorable causes have a less determinant role than for the other diseases. In fact, the presence of the parasite suffices for itself because the disease develops in an irremediable way. At this time, Apis mellifica has not yet developed a specific adaptation that allows it to tolerate varroas.
The rapidity with which the infestation will increase and cause the loss of elements of the colony depends on many factors that may be internal or external to the colony. The main ones are:
– bee and parasite: considerable presence of posture, blockage of posture, swarming, importance of male posture during the year, duration of the operation – variable in relation to bee strain or climatic conditions (food logo), relatively strong attraction of posture, cleaning behavior;
– the environment of the colonies: season, climatic conditions, diseases and / or intoxications of adult bees that will break the bee / posture balance and bee environment, source of contamination.
In a different context, it is necessary to add to this list the quality of the treatments, whose capacity varies greatly.
The pathogenic action of Varroa destructor is exerted on the adult bee and on the postura.Sobre the adult bee is more evident:
– a mechanical action: the presence of one or more parasites will obstruct the movements of the bee during flight and, in general, all its activity within the colony.
– a scavenging action: periodically, the female varroa will get hemolymph, which will weaken the bee and disrupt its metabolism. It is estimated that they take at each meal 0.1 or 0.2% of the volume of hemolymph of an adult worker. If the total life of the bee is considered, 40% of the hemolymph volume is taken. There is a decrease in the number of hemocytes and a change in the rate of hemolymph proteins. The importance of this predation depends, of course, on the number of parasites, but also on the pressure exerted by them, that is, the number of varroas in relation to the number of adult bees. When the pressure of the parasite is strong, the functioning of the mandibular and hypopharyngeal glands is affected and, therefore, the quality of the royal jelly also decreases. Another consequence will be the weakening of the fat body, so important in bee metabolism.
– a vector action: the perforation performed by varroa chelicerae will lead to the inoculation of pathogenic germs and viruses in the host.
In general, the adult bee shows a reduced activity, shows a disturbed metabolism and, consequently, will have a shorter life span.
If the life span of the groups of non-parasitized bees is studied in laboratory conditions, it is verified that, after 25 days, 50% of the non-parasitized bees survive and 25% of the parasitized bees die.
At the level of posture, there are the same main actions:
– a traumatic action: the population of varroa, inside the alveolus, will contribute to damage the plaques that are the origin of future appendages. This may translate into the birth of slightly deformed bees. The lack of space is one of the elements (with the spolishing action) that will obstruct the harmonious development of the future bee: 6% of bees born parasitized show a smaller abdomen. Certain larvae, very parasitized even before the operation, will eventually come out of the alveoli and fall into the tray of the hive. Inside the alveolus, oxygen consumption increases because of the agitation of the larva.
– a spoiling action: losses, as a function of parasitism during the nymphal phase, would be 15 to 40% of the haemolymph volume. This action is reflected in the metabolism and causes the birth of bees less vigorous, condemned to a shorter life, with the atrophied hypopharyngeal and mandibular glands. With a parasitism of one to three mites, the volume of the hypopharyngeal glands decreases to 13.5%. The weight loss of adult insects parasitized by more than three mites would be 30%.
– a vector action: certain diseases of the posture will develop: European foulbrood, American foulbrood, saciform stance. The action of certain viruses that were previously present in the colony without pathogenic role, increases rapidly. Acute paralytic virus (APV), injected directly into the hemolymph by varroa bites, causes the larvae to die. The deformed wing virus (DWV) is at the origin of atrophied bees. The exact mode of contamination of this virus is yet to be determined.
The bee that will be born may then be morphologically and physiologically deficient. It will suffer a loss of weight (about 21.6%), a result of the intensity of parasitism.
Following the rate of infestation, symptoms will appear with greater or lesser severity due to many factors such as the time of year, climatic conditions, beekeeping techniques, contaminations.
One of the difficulties inherent in this parasite is the absence of any sign of disease, even to a critical point where the colony would hardly be recoverable. The beekeeper, even the most forewarned, may be surprised. Only a screening can show that pathogen exists in greater quantity or evidence some warning signs, such as atrophied bees and varroas on adult bees.
Later, when the pressure of the parasite increases, the symptoms stand out:
At the level of adult bees and before the colony:
– Dead bees;
– Atrophied bees and nymphs;
– Newborn larvae present on the flight board;
– Bees that crawl in the sun, walking in disorderly directions;
– Bees with deformed wings, sometimes black.
– Bees with distant and asymmetrical wings;
– Packs of bees in the vicinity of the colonies, in the sun or hanging on the branches (desertion).
At the level of posture, and the colony:
– Decreased posture of the queen;
– Mosaic posture;
– Nymphs alive, but atrophied under the operculum;
– Nymphs dead in the operculum, in their normal position of evolution (tongue out, legs delayed).
– Larvae on the wall of the alveolus, light brown to dark brown, with a pastey consistency, sometimes filing.
– Relatively corroded nymphs, partly out of the alveoli. This shows cannibalism provoked by the search for proteins and can be interpreted as a means of self-protection.
– depopulation of the colony;
– Presence of honey and pollen reserves disproportionate to the bunch. The bees become prematurely harvester because of the atrophy of the hypopharyngeal glands caused by the hemolymph taking by the parasite. The colonies are found without bees and with strong supplies of honey.
At the level of the posture, symptoms similar to those of European Loque (LE), American Loque (LS) and saciforme (CS) posture may appear. They happen thanks to the development of the virus of acute paralysis (APV).
These diseases deserve greater prominence. They take advantage of the general weakening of the colony, the bees and the posture to act. Once a colony is reached by varroase it also develops the true American Loch, the European Loch, the Ringworm. The set of these symptoms is not always related to the rate of infestation.
The symptoms described should never, in theory, be found. Careful handling of the colonies should prevent them because:
In this state, the colony is hardly recoverable
In particular, two warning signs must be taken into account:
– bees with atrophied wings;
– Varroas on top of adult bees.
Finally, there is an increase in cases of tubular posture or bald posture. This comes groups of small moths glued to the base of the alveoli, which force the larva of the bee to climb to the handle, preventing the construction of the operculum. Observed later, this anomaly could be caused by the weakening of the colony caused by Varroa destructor.
Varroase spreads very quickly and inexorably. Nothing can stop her. Different factors, both natural and apicultural, are at the base of this state.
Among the natural factors are:
– the flight of the males, their change of colony and hive;
– plunder: certain compounds of the alarm pheromone have an important role in the passage of varroa from the dead bee to the attacking one;
– drift of harvesters;
– desertions. Desertions means the departure of bees from their hives carrying the parasites. Desertion is a behavioral response to a very high onset of parasitism. In 1900, this phenomenon has already been described for acariasis of trachea.
– transport of the parasite by other insects, in particular wasps.
Among other factors inherent to the beekeeper:
– the concentration of colonies in the same region;
– transporting bees with bees during collection;
– the sale of swarms;
– the trade of queens.