The most serious parasite of honeybees in the 20th century has undoubtedly been the ectoparasitic mite, Varroa destructor (formerly Varroa jacobsoni). Relatively harmless on its natural host, the Eastern honeybee, Apis cerana, the varroa mite has crossed onto the Western honeybee, Apis mellifera, and spread from its Asian origins throughout most of the world. On the commercially important Apis mellifera the varroa mite is not a benign pest, resulting in most cases in the death of the parasitised honeybee colony. In regions of the world where the varroa mite is well established, such as Europe and the USA, wild honeybee populations have all but disappeared as a result of varroa mortality and commercial beekeeping is only possible with the liberal use of anti-varroa pesticides.
Introduction
The most serious parasite of honeybees in the 20th century has undoubtedly been the ectoparasitic mite, Varroa destructor (formerly Varroa jacobsoni). Relatively harmless on its natural host, the Eastern honeybee, Apis cerana, the varroa mite has crossed onto the Western honeybee, Apis mellifera, and spread from its Asian origins throughout most of the world. On the commercially important Apis mellifera the varroa mite is not a benign pest, resulting in most cases in the death of the parasitised honeybee colony. In regions of the world where the varroa mite is well established, such as Europe and the USA, wild honeybee populations have all but disappeared as a result of varroa mortality and commercial beekeeping is only possible with the liberal use of anti-varroa pesticides.
Varroa destructor was first found in South Africa in August 1997, the first report of this mite in sub-Saharan Africa. An immediate survey revealed that the mite was common and widespread in both commercial and wild honeybee populations in the Western Cape, but absent from the rest of the country. The South African National Department of Agriculture convened a workshop during which it was concluded, on the basis of international evidence, that there was no prospect of containing the spread of the mite, nor was there a biocontrol agent available that could be used to eliminate varroa. It was accepted that varroa would eventually spread throughout South Africa, and probably throughout sub-Saharan Africa. The time span for this spread in South Africa was estimated to be between 2-7 years, with rapid spread in areas of commercial beekeeping activity and more gradual spread elsewhere. What effect the varroa mite would have on the honeybees of Africa was less certain. The general belief that the African honeybee would be tolerant to the varroa mite as a result of environmental factors or other variables, and that varroa would have little impact on the bees of Africa had to be tested.
At least three different aspects should be considered when estimating the impact of the varroa mite on African honeybees.
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The general belief that African honeybees, perhaps by virtue of their short post-capping time in brood development which could result in large numbers of unfertilized daughter mites, their hygienic behaviour, and their defensiveness, would prevent varroa from increasing to dangerous levels in the colonies, and hence would be tolerant to the presence of the mite. Support for this view comes from data from North Africa where varroa has seemingly been of little importance, from Brazil where varroa has not been destructive in Africanized bees, and from early work with Cape honeybees (Apis mellifera capensis) which suggested that these bees would be tolerant to varroa. This view would predict that varroa would spread throughout the African honeybee population, but would be little more than an additional arbitrary pest present in the colonies.
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It has also been suggested that what has made the Africanized honeybees of South America tolerant to the varroa mite is not some behavioural attribute of these bees, but rather that there are a number of different species and populations of mite, and that the one present in South America is not particularly virulent. This view predicts that if the more virulent strain of mite is present in South Africa, then it will result in the type of destruction witnessed in North America and Europe.
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A third possibility to consider is that not only are the race of honeybee and the strain of varroa mite important in predicting the outcome of honeybee-mite interactions, but also what viruses are present in the honeybee population. There is considerable evidence that colonies infected with varroa eventually collapse as a result of secondary infections, and of these, viruses activated by the presence of the mites are most important. The outcome of this scenario is impossible to predict, as very little is known about the honeybee viruses of South Africa.
In both of the last two scenarios it would be predicted that resistance or tolerance in the honeybee population would develop, but only after the collapse of the majority of the population. In such a case the resistance developed could potentially be masked by the use of chemical treatment by beekeepers to sustain susceptible colonies and the resistance might not be expected to spread through the population.
Although it remains to be determined what effect the mite will have on honeybee populations of Africa, the threat was considered to be sufficient to establish a Varroa Working Group comprising of researchers, beekeepers, users of honeybee pollination, and Department of Agriculture officials. This Working Group instituted a Varroa Research Programme to monitor and investigate the mite in South Africa, the preliminary results of which are presented here.
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Source of the varroa
It has been found that the varroa mite that has caused devastation to honeybee populations almost throughout the world for the past thirty years is not a single species, but rather a species complex, consisting of at least 18 types of mite. Of these different types and species, only two are able to reproduce on Apis mellifera, and only one, the Korean-Russian type, is responsible for the extreme damage as seen in Europe and the USA. This species has been called Varroa destructor, and this is the type found in South Africa. Circumstantial evidence suggests that the varroa entered South Africa at Simonstad harbor, probably on a swarm onboard a cargo-ship from Europe.
Distribution
In 1997 the varroa mite was to be found only in the Western Cape, but as expected the mite has spread rapidly throughout South Africa, almost entirely as a result of migratory beekeeping activities, and is now present in commercial honeybee colonies in all provinces.
Varroa mites have also been found in wild honeybee colonies where no beekeeping takes place, including the Kruger National Park, Cape Peninsular National Park, Tsitsikamma National Park and the Cedarberg.
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Impact of varroa
The comprehensive monitoring of mite levels and colony condition in more than 300 commercial colonies belonging to Cape beekeepers indicated that varroa numbers were strongly negatively correlated with colony size, brood production, and pollen storage. Hence, as varroa numbers in a colony increased, the colony weakened.
There was, however, no clear-cut relationship between varroa infestation rate and colony mortality. Many colonies severely infested with varroa mites have not died during the course of the study, and it is still not known how acutely the mites will impact on the honeybee population of South Africa.
Comparisons between varroacide-treated and nontreated colonies, however, indicate massive differences in colony survival and productivity, in at least some situations.
In colonies that did not succumb in the short-term, high levels of brood mortality was found (as much as 95%), resulting in the gradual collapse of those colonies.
As the parasites spread, colonies with as many as 50 phoretic mites per 100 bees were not uncommon. This represents some 30 000 mites in large colonies, and clearly indicates that the prediction, that certain behavioural attributes of African honeybees would limit varroa population growth has not taken place. However, after three years of varroa mites having been present in a region, mite numbers were greatly reduced. Whether this was because of mite-tolerance developing in the bees, or because the colonies were too weak and with such high levels of brood mortality they could no longer sustain mite population growth, remains to be determined.
It is too early to draw firm conclusions about the impact of the varroa mites on African honeybees. Clearly, a large percentage of colonies are dying, but only time will tell if the African honeybee populations will collapse on the scale witnessed in Europe and North America.
In South Africa the value added to crop production by the commercial pollination of honeybees has been estimated to be in the order of R3.2 billion per annum (Table 1). It is also worth noting that this agricultural output sustains some 250 000 jobs. However, and in contrast to the Americas, perhaps the greatest threat of varroa in Africa is to the wild honeybee populations that pollinate as many as 40-70% of indigenous flowering plants. Should South Africa and the rest of Africa suffer the loss of wild bees witnessed in other parts of the world, this could have significant implications for floral conservation and biodiversity.
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Effect on pollination efficiency
A 14% reduction in pollination efficiency was found in colonies that were heavily varroa infested in contrast to varroa-free control colonies, in the pollination of pumpkins. This was despite there being more foraging activity in the varroa-infected colonies, perhaps in an effort to compensate for the reduced efficiency of foraging workers. These results need to be confirmed on other crops to proof general significance.
Secondary bee diseases
Colonies infested with high numbers of varroa exhibit additional problems with other diseases and pests. Poor brood patterns are common in these colonies. Small hive beetles, chalkbrood and Braula coeca appear to be greatly increased in varroa-infected colonies. Chalkbrood, which was previously rarely reported in South Africa, is now widespread and almost ubiquitous. In addition, at least two viruses (Black Queen Cell Virus and Acute Paralysis Virus) have been found to be contributing to honeybee and colony mortality in varroa-infested colonies. There appears to be no correlation between varroa and tracheal mite levels, and tracheal mites remains uncommon.
Colony collapse in the summer rainfall region of the country is extremely rapid, probably due to the combined contributions of varroa and the Cape Honeybee Problem. The relative importance of these two factors must, however, still be determined.
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Chemical control
Synthetic varroacides have been found to be extremely effective in the control of mites (>98%) whilst alternative chemical controls (e.g. formic acid) have been found to be less effective ( killing only 70%). Two commercial varroacides (Bayvarol® and Apivar®) have been registered for use in South Africa. Most beekeepers, who originally were against the use of any chemicals in their colonies for the control of varroa are now using some varroacide to protect their colonies. Wild honeybees can obviously not be treated with varroacides, and there is great concern amongst beekeepers that the catching of honeybee swarms, the lifeblood of their industry, is on the wane.
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Dwarf and deformed workers is the result of varroa parasitism |
Mite reproduction
Varroa mites are found to successfully reproduce in both worker brood and drone brood in Cape honeybees, with mites being found in 6% of worker cells and 24% of drone cells (sample size 22 000 cells). The reproductive rate in worker brood is calculated to be 1.4 (that is, 0.4 daughter mites produced per cell), and 1.9 in drone brood. Most significantly only one mature mite is present in 56% of varroa-infected cells with emerging worker bees and 27% in drone brood. This mean that reproduction has not been successfully completed, either because the short post-capping period of Cape bees has prevented completion of the mite reproductive cycle, male mite mortality, or the foundress was infertile. |
The data suggest a significant percentage (>27%) of infertile female mites in the population. These infertile mites are probably the result of incomplete reproduction due to the shortened post-capping period found in the worker brood of Cape honeybees. The extremely high numbers of varroa mites found in Cape honeybees (>30 000 in some colonies) indicates however, that although the short post-capping period of Cape bees must limit mite population growth to some extent, it is insufficient to prevent mite levels increasing to harmful levels. This data also indicates that the general presence of drone brood for much of the year is crucial to mite population growth.
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Reproduction of Varroa destructor in Cape honeybees
| |
White-Eye Worker Pupae |
White-Eye Drone Pupae |
Emerging Worker Brood |
Emerging Drone Brood |
| Cells Examined |
8846 |
3283 |
6104 |
1118 |
| Cells with Varroa |
5.98%
(0.0 - 42.55) |
24.0%
(0.0 - 74.87) |
4.83%
(0.0 - 49.33) |
33.81%
(0.0 - 84.72) |
| Number of Adult Mites per Infested Cell |
1.24
(0 - 4) |
1.83
(0 - 10) |
1.74
(0 - 6) |
3.48
(0 - 21) |
| Cells with Only a Single Adult Mite |
85% |
73% |
56% |
27% |
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Hygienic behaviour
A small population of selected Cape honeybee colonies have been tested for hygienic behaviour, as a possible basis for resistance to mites and hence the basis for selection and breeding of varroa resistant Cape honeybees. Hygienic behaviour in these colonies has been found to be extremely variable, both between colonies, and over time, but the Cape honeybee appear to be more hygienic than European races with 100% of dead brood being removed by this unselected population within 48 hours. This hygienic trait, however, seemed ineffectual against varroa mite infestation, and all but 2 of the 20 colonies died within 18 months. At present, there seems to be little correlation between the hygienic behaviour of Cape honeybees and their tolerance to varroa mites and natural resistance to the mites does not appear to be a common trait.
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Conclusions
South Africa has the varroa mite that has caused widespread collapse of honeybee colonies throughout the world, and nothing has emerged during the Varroa Research Programme to suggest that the South African situation will be any different. The mite has spread all over the country, including the wild honeybee population, and will eventually be found in all honeybee colonies in a matter of only a few years. Severe colony damage and loss is being witnessed due to the mite and associated secondary diseases.
Left to their own devices African honeybees may be able to accommodate the mite as they appear to have done with other honeybee diseases. It is expected that large numbers of African honeybee colonies will die as a result of varroa, both in the wild and managed bee populations, but thereafter, resistance to the mite is expected to develop rapidly in these populations. As varroa-resistant bees would produce more swarms and drones, the resistance should spread through the population and simply allowing natural selection to take its course should result in African honeybees becoming tolerant to the varroa mite. The economic demand for commercial honeybee colonies will, however, dictate that beekeepers treat colonies with varroacides should honeybee losses become considerable. This will artificially sustain the susceptible honeybee population, and will retard the development and spread of a naturally-selected varroa-resistant population.
Hence, a comprehensive response to the varroa threat is required, involving Integrated Pest Management (IPM) strategies, further research, and regional, governmental and legal strategic actions. Included in this strategy are:
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The development of mechanisms or legislation for the regional control and rotation of varroacides with different modes of action, to slow down the development of resistance in the mite population and to prolong effective chemical control.
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The development of guidelines for the use of non-regulated chemical products presently being used against the varroa mite.
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Mechanisms to ensure the responsible use of chemical measures.
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The development of cultural (non-chemical) control measures against varroa, to supplement chemical control.
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The active development of natural resistance to the varroa mite in the wild honeybee population, by restricting the use of chemical control in certain regions, to facilitate the development of tolerance by natural selection.
The presence of the varroa mite in Africa clearly represents a severe threat to the beekeeping industry, to agriculture dependent on honeybees for commercial pollination, to the wild honeybee population, and to the conservation of indigenous flora relying on honeybees for pollination. Only time will tell how severe the threat is.
