The effect of treatment against Varroa mite is determined by property of selected product applied at the rite time.
The number of mites usually increases slowly at the beginning of the season. Clinical signs may be seen at any time during the active season, although usually maximum numbers are reached late in the season (Figure 1), when the first clinical signs of infestation are obvious and can be recognized. The course of this parasitism is usually lethal, except in some areas, such as tropical Latin America (De Jong, 1997; Ritter et al., 1984). The life span of mites on larvae or adult bees depends on the temperature and humidity. Under practical conditions, the life span may vary from some days to a few months. In heavily infested bee colonies, clinical signs of varroosis can first be seen in the latter part of the season when the brood is reduced (Ritter et al., 1984). Heavy infestations are usually reached 3–4 years after the primary invasion, but can occur within weeks if infested by bees from nearby colonies that are collapsing.
Bees and their offspring that have been infected during the brood phase by only one parasitic mite show various ill effects, such as a shortened life span, changes in behaviour and disease susceptibility (De Jong & Goncalves, 1982). This is due to an increased susceptibility to deformed wing and acute paralysis virus, as well as to the infection of wounds and loss of haemolymph (Bailey, 1981; Ball, 1985).
Фиг.1. Population of bees and Varroa mites over 1 year: brood numbers (solid line) and mite numbers (broken line) – OIE, 2019
* OIE – Office International Epizootic
In order to avoid such harmful effects of mites on the bee colony, researchers recommend for Europe and some USA regions that we aim to keep the Varroa population below 4000-5000 mites as threshold levels in the hive. In high-density areas with heavily contaminated colonies, Varroa infestation rates can be extremely high and populations can accumulate harmful levels in a short time - sometimes a matter of weeks or months.
Reducing the population of mites depends on the effectiveness of the treatment. The mite population needs much more time to recover to the extent that it damages bees when continuous integrated pest management is applied - combining heavy chemical acaricides with soft natural substances and mechanical-biological methods. Understanding these principles is essential for the successful use of the Varroa destructor monitoring and control methods.
There is no one perfect time for acaricide treatment nationwide. However, for most beekeepers, the main treatment time is in the late summer to early fall - between harvesting honey and preparing colonies for the winter. During this period, the strength of the bee colony gradually decreases, while the level of infection continues to increase. The purpose of the treatment is to significantly reduce the population of mites in order to preserve the last few cycles of brood development, which will hatch young bees necessary for the successful survival of the colony in winter. If the treatment is delayed at that time, winter bees will receive greater load with pathogens - parasites and viruses. This would mean a shorter life, weakness and death of colonies in the winter, although the Varroa mite was controlled later.
It is most appropriate to treat at the end of July the cross point between the chart of brood development and the chart of development of mites (Fig. 1). But then, in most areas, bee honey is still harvested and usually the treatment is delayed until later. Exactly in this period the combination of essential oils CEO9 can be applied. The amount of essential oils that we bring into the hive is minimal (only 12 drops total of the three times the application of the strips) and the efficiency is over 80%. This means that there will be no harmful residues in the honey, but at the same time the mite population will be reduced significantly by the time the honey flow is over and it is allowed to apply a registered acaricidal veterinary medicinal product (VMP).
The essential oils applied by other methods - with sugar syrup or with different spongy material, cloth, corrugated board and other stuff that absorb them, cannot have the necessary effect. This is, on the one hand, to the inaccurate dosing through the sugar syrup and, on the other, to the absorption effect of the materials used. These methods reduce the required treatment dose and on mites act a small amount of active substances.
Colonies should also be treated in the spring - especially where mite monitoring reports that bees come out from the winter significantly infested. This will pose a serious risk to the colonies while waiting the end of the honey harvesting to apply the treatment in late summer.
The choice of VMP may indicate when to apply treatment. Some products (eg Apiguard) require warm weather for maximum efficiency. Others (oxalic and lactic acid) need periods without brood to be effective and are usually applied in late fall or early spring. Third (such as Varostop, Bayvarol and CheckMite +) can be used relatively safely in spring and autumn when there is no honey harvest). Beekeepers generally prefer to avoid chemical acaricides during the active season when honey boxes are on the hives to minimize the risk of residues in bee products.
The application of the essential oils combination, composite by Primavet-Sofia Ltd. - CEO, is a suitable tool throughout the active season. This is very useful for beekeepers - easy to use and dose, especially in severe infestations of bee colonies requiring emergency treatment during honey harvesting. CEO9 is a combination of essential oils that can meet the needs of conventional and especially organic beekeeping, where the use of acaricides is limited.