Varroa in Africa – a Serious Threat

VARROA IN AFRICA – A SERIOUS THREAT Mike Allsopp ARC-Plant Protection Research Institute, Private Bag X5017, Stellenbosch 7599, South Africa. Tel: ++27 21 8874690; Fax: ++27 21 8875096; Email: [email protected] INTRODUCTION The most serious pest or disease 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 recently 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 no longer a relatively benign pest, and the result in most cases is the death of the 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 (Krause & Page 1995; Finley et al. 1996) 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 (Allsopp et al. 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 immediately convened a workshop during which it was concluded, on the basis on 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 (de Jong et al. 1982), with rapid spread in areas of commercial beekeeping activity and more gradual spread elsewhere. What was less certain is what effect the varroa mite would have on the honeybees of Africa. It is known that the virulence and effect of the mite is significantly affected by environmental factors (Moretto et al. 1991; Marcangeli et al. 1992), and there were also other variables to consider. Most important was the general belief (Medina 1998; Erickson et al. 1998; but see Page 1998) that African honeybees would be tolerant to the varroa mite, and that varroa would have little impact on the bees of Africa. Recent evidence suggests that at least three different aspects should be considered when estimating the impact of the varroa mite on African honeybees. 1. 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 (Moretto et al. 1991), and their defensiveness (Moretto et al. 1991), 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 (Ducos de Lahitte et al. 1998), from Brazil where varroa has not been destructive in Africanized bees (Moretto et al. 1991; Rosenkranz & Engels 1994), and from early work with Cape honeybees (Apis mellifera capensis) which suggested that these bees Proceedings of the 37th International Apicultural Congress, 28 October – 1 November 2001, Durban, South Africa APIMONDIA 2001 To be referenced as: Proc. 37th Int. Apic. Congr., 28 Oct – 1 Nov 2001, Durban, South Africa ISBN: 0-620-27768-8 Produced by: Document Transformation Technologies Organised by: Conference Planners would be tolerant to varroa (Moritz & Jordan 1992). 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. 2. 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 (Anderson 2000), 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. 3. 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 (Ball 1997; Bowen-Walker et al. 1998). 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 nothing 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. VARROA RESEARCH PROGRAMME Value of honeybees 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. Source of the varroa It has been found that the varroa mite, Varroa jacobsoni, 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 (Anderson 2000). 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 (Anderson 2000). This species has been called Varroa destructor, and this is the type found in South Africa (Anderson 2000). Table 1: Estimated value of honeybees in commercial crop pollination in South Africa Annual Annual Honeybee added Crop Hectares production value value (tons) (R million) (R million) Deciduous fruit 171 630 2 358 106 3 716.64 1 296.52 Berries 750 4 682 21.82 8.16 Nuts 14 500 6 565 43.90 11.58 Tropical Fruit 114 509 1 391 154 2 103.45 672.70 Grain crops 322 600 402 840 594.96 86.08 Oilseed crops 845 000 1 133 477 969.67 523.80 Vegetables 48 300 892 907 1 172.26 293.95 Seed production ? ? 127.69 102.15 Other ? ? 1 019.85 210.30 TOTALS 1 517 289 7 707 020 9 770.24 3 205.24 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 regions. Varroa mites have also spread into the wild honeybee population, including the Kruger National Park, Cape Peninsular National Park, Tsitsikamma National Park and the Cedarberg. Impact of varroa The comprehensive monitoring of mite levels and colony condition in >300 commercial colonies belonging to Cape beekeepers indicates that varroa numbers are strongly negatively correlated with colony size, brood production, and pollen storage (Pearson Correlation Coefficients; p = 0.0001) Hence, as varroa numbers in a colony increase, the colony weakens, and often dies. There is, 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 non- treated colonies, however, indicate massive differences in colony survival and productivity, in at least some situations (Table 2). Table 2: Comparison between varroacide-treated and control colonies in the Western Cape and Kwazulu-Natal (KZN) Treatment Colonies Control Colonies Original Surviving Average Original Surviving Average number of colonies honey number of colonies honey colonies after 6 production colonies after 6 production months in months in surviving surviving colonies colonies Western 22 21 13.6kg 20 7 2.8kg Cape KZN 14 13 22.9kg 10 0 0.0kg In colonies that have not succumbed to the mite in the short-term, tremendous (as much as 95%) levels of brood mortality are being found, resulting in the gradual collapse of these colonies.

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