MORTALITY OF Varroa destructor IN BROODLESS AFRICANIZED AND CARNICA HONEY BEE (Apis mellifera L.) COLONIES

Geraldo Moretto

Geraldo Moretto. Biologist. Doctor in Genetics, Universidade Regional de Blumenau (FURB). Professor of Genetics, Departamento de Ciências Naturais, FURB. Address: Centro de Ciências Exatas e Naturais, Universidade Regional de Blumenau, 89.010-971 Blumenau-SC, Brazil. e-mail: gmoretto@furb.br

Summary

Varroa destructor infestation varies with climate conditions and race of Apis mellifera bees. Africanized bees show greater tolerance to varroa compared to bees of European races. Reproductive ability of female mites, hygienic behavior and grooming behavior are important factors in population dynamics of this parasite. The present study shows the mortality rate of the V. destructor mite in Africanized and Carnica bee colonies in Southern Brazil. The daily proportion of dead and live mite fallen on the bottom of the hive was determined when the total mite population was of adult bees. In Africanized bee colonies the daily proportion of dead mite was 6.30%, while in Carnica bee colonies was 2.11%. The daily proportion of live mite on the bottom of the hive was 2.45% and 0.82% in Africanized and Carnica bee colonies, respectively.

KEYWORDS / Africanized Bees / Apis mellifera / Carnica Bees / Varroa Mortality / Varroa destructor /

Resumen

El grado de infestación alcanzado por el ácaro Varroa destructor varía con las condiciones climáticas y la raza de abejas Apis mellifera. Las abejas africanizadas presentan mayor tolerancia al parásito cuando se comparan con las abejas de razas europeas. La habilidad reproductiva, el comportamiento higiénico y el comportamiento de "grooming" son importantes factores en la dinámica poblacional del ácaro. El presente trabajo há evaluado la tasa de mortalidad diaria del ácaro V. destructor en las colonias de abejas africanizadas y cárnicas en el sur de Brasil. La proporción diaria de varroas muertas y vivas en el fondo de las colmenas fue determinada cuando las colmenas tenían solamente abejas adultas (colmenas sin crías). La proporción diaria de ácaros muertos fue 6,3% y 2,11% en las colonias africanizadas y cárnicas, respectivamente. La proporción de ácaros vivos encontrados en el fondo de la colmena fue 2,45% en las abejas africanizadas y 0,82% en las cárnicas.

Resumo

Os níveis de infestação alcançados pelo ácaro Varroa destructor variam com as condições climáticas e a raça de abelhas Apis mellifera. As abelhas africanizadas apresentam maior tolerância a esse parasita quando comparadas com abelhas das raças européias. A capacidade reprodutiva, o comportamento higiênico e o comportamento de "grooming" são importantes fatores na dinâmica populacional desse ácaro. O presente trabalho avaliou a taxa de mortalidade diária do ácaro Varroa destructor em colônias de abelhas africanizadas e carnicas. A proporção diária de varroas mortas e vivas encontradas no fundo da colmeia foi determinado em colônias de abelhas contendo apenas operárias adultas(ausência total de cria). A proporção diária de acaros mortos foi de 6,30% e 2,11%, nas colônias africanizadas e carnicas, respectivamente. Quanto a proporção de ácaros vivos encontrados no fundo da colmeia, foi de 2,45% nas abelhas africanizadas e de 0,82% nas abelhas carnicas.

Received: 08/09/2002. Modified: 11/05/2002. Accepted: 11/12/2002

Introduction

The mite Varroa destructor (Anderson and Trueman, 2000), after attacking Apis mellifera bees, rapidly dispersed itself throughout the world, causing serious losses to apiculture (De Jong, 1990).

The dispersal of the V. destructor in various parts of the world is characterized by different population dynamics at locations where the parasite became established. The degree of infestation varies from region to region, with climate and race of A. mellifera being factors that affect the development of the parasite (De Jong et al., 1984; Moretto et al., 1991). Thus, under the conditions of Europe and other regions of temperate climate the mite has hindered the practice of apiculture without the use of acaricides, while in other regions of the world a perfect parasite-host relationship has become established between the mite and the bees (Gonçalves, 1987).

Among the various races of A. mellifera bees, Africanized bees show greater tolerance for the varroa than bees of European races. Reproductive ability of female mites is known to be lower among Africanized bees than that of European bees (Camazine, 1986). However, the different reproductive ability of varroa among different A. mellifera subspecies seems not to be the only factor responsible for the different virulence of the pest. Hygienic behavior of bee colonies is also an important factor acting on the population dynamics of this parasite (Spivak and Gilliam, 1998; Spivak and Reuter, 2001).

Moretto et al. (1993) obtained a 40% rate of Africanized bees that were able to rid themselves of the parasite among 20 colonies of artificially infested Africanized worker bees. When European and Africanized bees are reared together in the same hive, European bees are found to be more heavily infested (Gúzman-Novoa et al., 1996; Moretto and Mello, 1999).

Correa-Marques et al. (2000), in a study of mites collected from the bottom of Africanized bee hives, observed the occurrence of mutilation of various body parts, indicating possible attacks by the workers. Similar results were also obtained with other A. mellifera subspecies (Bienefield et al., 1999; Zaitoun et al., 2001).

The objective of the present study was to assess the mortality rate of the V. destructor mite in Africanized and Carnica bee colonies.

Material and Methods

The study was conducted in the experimental apiary of the Department of Natural Sciences, Regional University of Blumenau, Municipality of Blumenau, Santa Catarina state, Brazil. The experiments were conducted using 4 colonies with 4 frames covered by Africanized bees collected from natural swarms in the Blumenau region, and 4 colonies (nuclei) of Carnica bees whose mated queens were imported from the experimental apiary of the University of Tübingen, Germany. When Carnica queens were introduced in Africanized bee colonies, varroa infestation on adult bees was similar (approximately 2%) to the colonies with Africanized queens. Experiments started five months after introduction of the queens, when the population consisted exclusively of Carnica bees.

The daily proportion of dead and live mites fallen on the bottom of the hive was determined when the total mite population was concentrated on adult bees. Thus, the experiments were carried out when the 8 bee colonies (4 Africanized and 4 Carnica colonies) had only adult bees. Colonies containing only adult workers were obtained by imprisoning the queens in introduction cages. When each colony was fully devoid of brood, the live and dead mites found on the bottom of the hive were counted daily for 6 consecutive days. To prevent bee contact with the varroas on the bottom of the hive, a Nº 8 mesh net was placed between the bottom and the nest.

After the 6 days of counts, all bee colonies were treated with the Bayvarol® acaricide to kill the mites present in the colonies. As each bee colony had only adult bees, only one stripe of the acaricide was placed between the combs. Varroas (live and dead) found on the bottom of the hive were counted from the first day after treatment up to the third consecutive day, when no mites were detected. Thus, the number of varroas collected during the 6 days preceding treatment with Bayvarol, summed to the number of varroas killed by the treatment was considered to be the total varroa population of each bee hive during the experimental period. Daily rates of live varroas fallen on the bottom of the hive and daily mortality rates (only dead varroas) were determined. The mortality rates and the rates of fallen live mites were determined by dividing the daily number of dead and live varroas collected daily on the bottom of the hive respectively by the total number of varroas in the hive. Both daily percentages were calculated only with actual number of mites in the colony. In order to determine the daily percentages, the number of dead and live mites collected on the previous day was excluded from total population.

The Student test was used to determine the differences in mortality rates and in the rates of falling live varroas at the hives of Africanized and Carnica bees.

Results and Discussion

The mean mite population was 182 (91-416) varroas in colonies of Africanized bees and 501 (248-1081) varroas in Carnica bee colonies.

Tables I and II show the daily rates of dead and live mites registered on the bottom of the Africanized and Carnica bee hives. The average daily percentage of dead varroas detected in the present study was 6.30% in Africanized bee colonies and 2.10% in Carnica bee colonies (t= 3.36; P= 0.00) and the average daily percentage of live varroas found on the bottom of the hives was 2.45% for Africanized bees and 0.82% for Carnica bees (t= 5.58; P= 0.00).

In general, the degree of infestation reached by the varroa parasite is lower in Africanized bees than in bees of European races (De Jong, et al., 1984; Gonçalves, 1987). The reproductive success of varroa females in worker brood cells is considered to be an important factor in the population dynamics of the mite in different bee races (Camazine, 1986; Medina and Martin, 1999).

However, the different reproductive ability can not be the unique factor involved in the different levels of infestation reached by the varroa pest. In bees of European races, under temperate climate conditions, mathematical models have estimated a daily mortality rate of the mite V. destructor of the order 0.2 to 0.4% during winter, a period with little or no brood present in bee colonies (Fries et al., 1994; Martin, 1998; Calis et al., 1999; Fries and Pérez-Escala, 2001).The higher mortality of V. destructor in Africanized bee colonies could act as a limiting factor in the development of the pest. In the present study the daily mortality and daily proportion of live mites found on the bottom of Africanized bee colonies was about three times those found in Carnica bee colonies.

Among the total number of dead mites found in the two races of bees in the present study, a part must have been related to the natural mortality of the varroa. However, the different daily rates of dead varroas observed for the two groups of bees suggest the presence of mechanisms that reduce the viability of this parasite among Africanized bees. The grooming mechanism first described for Apis cerana bees (Peng et al., 1987) and later for A. mellifera (Boecking and Ritter, 1993; Moretto et al., 1993) could be responsible, at least in part, for the different mortality rates of V. destructor among Africanized and Carnica bees.

During the total absence of worker and drone brood the daily mortality rate of varroa among Africanized bees was over 6%. Thus, we may estimate that the population of the mite could be eliminated within a period of 15 to 20 days of absence of brood in bee colonies. In contrast, since the mortality of varroas among Carnica bees is approximately 2% under the same conditions, the mite population would be eliminated within a period of approximately 50 days. This estimate could be higher if the living mites are considered. Although part of these varroas could be able to produce a re-invasion in natural conditions, some of them probably would die in a short time.

Under tropical climate conditions, as is the case for Brazil, there are no prolonged periods with the absence of brood in bee colonies, as observed in regions of temperate climate. However, several periods of low food flow usually occur through the year, due to draught or excess rain, leading to a drastic reduction of brood in bee hives. Therefore, during these periods of low brood presence, large part of the varroa population is concentrated on adult bees, a fact that possibly reduces the mite population to low levels.

The varroa mortality among Carnica bees detected in the present study was at least 20 times higher than the mortality detected in European bees under the climatic conditions of Europe (Fries et al., 1994; Martin, 1998; Calis et al. 1999; Fries and Pérez-Escala, 2001). Under tropical climate conditions, the levels of infestation reached by the varroa pest are usually lower than those observed in regions of temperate climate (Moretto et al., 1991; De Jong and Soares, 1999). Thus, the climatic conditions seem not to be only related to the reproduction of the varroa parasite, but also to its mortality.

ACKNOWLEDGMENTS

The author thanks Anne Zillikens for providing the Carnica queens, and Denilson Bittencourt and Arnor Bublitz Filho for their assistance during the study.

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