Varroa Destructor: Origin, Spread and Consequences

Originally, Varroa was a parasite of Apis cerana, the Asian honeybee, which had developed special defence mechanisms and was able to tolerate the mite. The introduction of specimens of Apis mellifera in south-east Asia during the 1940s allowed the mite to make the host jump, parasitising the Apis mellifera as well and spreading throughout the East and, subsequently, to the West. To date, only Australia and Madagascar seem not to have suffered from the arrival of this deadly mite. In Italy, the first case was recorded in 1981, in the province of Gorizia. Since then, the mite has spread like wildfire, making beehive infestation a common problem for everyone.

The Varroa destructor exhibits a strong sexual dimorphism, i.e. the male and female display very different morphological features. The female is larger than the male, reddish-brown in colour, with four pairs of legs placed on the front side of the body, which has an ellipsoidal shape. The males, on the other hand, are smaller, whitish in colour and are mainly found in the cells, where they are born, breed and then die. It is therefore the female mite that is responsible for the parasitic action against bees. They feed on the haemolymph of adult bees and brood, preferring the male.

The biological development cycle of Varroa follows that of bees. In the absence of brood, the Varroa remains firmly anchored to the body of the workers, waiting for the queen bee to resume oviposition in order to reproduce in turn. The Varroa only reproduces within the brood. It recognises the pheromones emitted by the larvae close to operculation and therefore manages to penetrate the cell before it is sealed. The mother Varroa takes refuge at the bottom of the cell, in the middle of the royal jelly, until the operculation is complete. It then attaches itself to the larva and starts sucking its haemolymph.

About 2½ days after the operculum closes, the varroa begins to lay its eggs. The first of these is usually male, the subsequent female. It can lay between 4 and 6 eggs per reproductive cycle. The longer the period of larva and pupa development, and thus the duration of operculation, the more eggs it can lay. This may be one of the reasons why varroa prefers male brood, as it has a longer developmental period and thus allows more sexually mature varroa to develop.

Fertilisation:the male reaches sexual maturity about 6½ days after laying, while the female takes a day less. This causes the male and female to be fertile at the same time, allowing the mite to fertilise. Maturity: once the bee is fully developed and ready to emerge from the cell, varroa that have reached maturity will cling to the bee's body, escaping with it. Immature females and males, on the other hand, will remain in the cell and die. The fertile varroa will only leave the body of the bee at the appropriate time to return to the cell and resume the reproductive cycle. In general, it has been studied that they can perform an average of 2-3 reproductive cycles during their entire existence.

Varroa has profoundly negative effects on the health of its host. It has been observed that infested bees are born with a lower weight and lower carbohydrate and protein concentration. In addition, they very frequently present malformations of various types: reduction in size, underdeveloped abdomen and malformations of sting, wings and legs. The immune system of the bee is also particularly compromised, lowering its defence capabilities and increasing the chances of attack by other pathogens, such as bacteria and fungi. It has been shown that an infested bee during its foraging phase covers shorter distances in flight and has lower harvesting performance than non-parasitised foragers. Varroa is also a vector of several viroses, such as deformed wing virus (DWV) and acute paralysis virus (ABPV).

In the case of Varroa, it makes no sense to speak of contagion, but rather of 'reinfestation'. A treated hive may still find itself heavily infested with Varroa as a result of the drift and drift phenomena, which bring together bees with Varroa in their foretic state and 'healthy' bees. The beekeeper may also become a vector of Varroa when he moves swarms or brood combs from one hive to another. A beekeeper who does not administer treatments correctly may also spread the mite more widely, affecting neighbouring hives.

One of the main revealing signs is, certainly, the presence of uneven and irregular brood, a sign of increased mortality. The presence of misplaced or liquefied, or brown (but not slimy) larvae is also a sign of infestation. In addition, the bees are in lower numbers than normal, are small, less developed, with deformed wings, and have difficulty flying. Other symptoms may be a change of queen, a state of orphanhood and abandonment of the hive. When one can see the parasite anchored to the adult bee's body with the naked eye, one is in the presence of a heavily parasitised family. In less severe cases of infestation, one can assess the number of mites present in the hive either by taking a sample of adult or brood bees, counting Varroa, or one can periodically inspect the bottom of the hives, looking for the number of fallen mites.

The only way to contain Varroa infestation and preserve the health of the colony is to administer treatments correctly. These are generally carried out, in temperate climates, between the end of July and the beginning of August and in November, in the absence of broods and before the families go into winter. We have already addressed the subject of summer buffer treatments, which are necessary to allow the bees to reach autumn in the best possible conditions. The treatments are of various kinds, involving the use of chemical means, which can, however, create drug-resistance phenomena, and products based on active ingredients of natural origin, which are also permitted for those who practice organic farming. Some also resort to mechanical pest control tools, such as the use of the Campero frame to eliminate the male brood.

The key, however, is the correct administration of the treatment, with the right timing and care. This is the only way to reduce the level of infestation and allow the family to survive. Knowing when and how to intervene, therefore, is essential to the success of these treatments. Using monitoring systems can allow you to be timely and maximise the effectiveness of treatment. Discover the benefits of 3bee monitoring systems!