Med. Weter. 2013, 69 (12) 741
Praca oryginalna Original paper
Woyke (23) recommended breeding honey bees resistant to Varroa. He described both the existence of variations in the resistance to Varroa in honey bees as well the mechanism of the resistance. Harbo and Harris recommend (9) breeding bees resistant to V. destructor from colonies showing a low percentage of Varroa destructor mites in broods as well as a low coefficient of parasite population growth. Studies indicate several factors that can have an effect on the degree of colony infestation by this mite, such as: bee species and subspecies, mite genotype, but also local climate conditions (1, 4-8, 10-12, 14, 15). Guzman et al. (7) showed that the parasite population growth was lower in the Primorski Russian bees than in Italian bees. Assessing the degree of brood infestation and the coefficient of Varroa destructor population growth seems to be a necessary procedure when selecting honey bees for increased resistance to infestation by this parasite (18, 21).
The aim of the study was to assess the effect of the honey bee subspecies on Varroa destructor population growth and the extensiveness of brood infestation by the parasite.
Material and methods
The study was carried out in 2009 and 2010 at the Api-culture Division of the University of Warmia and Mazury
in Olsztyn. The following subspecies of the honey bee were used:
– Carniolan bees (Apis mellifera carnica) – represented by two lines: Kortówka (CarK) and Dobra (CarD),
– Caucasian bees (A. m. caucasica) of the Woźnica line (Cau),
– Central European bees (A. m. mellifera) of the Augu-stowska line (Mel),
– crossbreeds of A. m. capensis with A. m. carnica drones, not constituting a line yet, characterized by a short post-capping period (PCP), bred at the Apiculture Division in Olsztyn (18-22).
Five experimental groups were created, each one for each subspecies, and marked with specific symbols (bold in brackets). The first year, 40 colonies (8 colonies per group), and the next year 35 colonies were created (7 colonies per group).The study was conducted in accordance with the method developed by Koeniger et al. (13).
Tests were carried out in trapezoid top bar polystyrene mating nuclei. The nuclei were colonized with bees at the beginning of May each year. Virgin queens from each ex-perimental group were introduced to the newly established colonies. The queens were then instrumentally inseminated with the semen of drones (8 µl) appropriate to a given line. After the replacement of worker bees, all experimental colonies were treated with an anti-varroa preparation (Api-warol with amitraz) to eliminate any possible presence of
V. destructor. At the beginning of capping, 50 mites placed
Effect of the honey bee subspecies on Varroa
destructor population growth and brood infestation
BEATA BĄK, JERZY WILDE, MACIEJ SIUDA
Apiculture Division, Faculty of Animal Bioengineering, University of Warmia and Mazury, ul. Słoneczna 48, 10-710 Olsztyn, Poland
Bąk B., Wilde J., Siuda M.
Effect of the honey bee subspecies on Varroa destructor population growth and brood infestation Summary
The aim of the study was to check the effect of the honey bee subspecies on Varroa destructor population growth and on brood infestation by the parasite. The studies were carried out in 2009-2010. The following five subspecies of bees were tested: Carniolan bees represented by two lines, Kortówka and Dobra; Caucasian bees of the Woźnica line; Central European bees of the Augustowska line; crossbreeds achieved by the absorptive crossing of A. m. capensis with A. m. carnica drones selected for a short post capping period (PCP), bred at the Apiculture Division in Olsztyn. The coefficient of V. destructor population growth was similar in all groups, ranging from 1.3 to 1.5. The level of infestation of broods by Varroa destructor in all groups was also similar (17.2 to 19.6%) and no statistically significant differences were noted. We conclude that the honey bee subspecies has no effect on parasite population growth.
Keywords: Apis mellifera carnica, Apis mellifera caucasica, Apis mellifera mellifera, Apis mellifera capensis, PCP line, Varroa destructor, brood infestation
Med. Weter. 2013, 69 (12) 742
on living worker bees were introduced to each nucleus. In total, 3750 V. destructor females were introduced.
Greased sheets were placed on the bottom of the nuclei, under a metal grid with mesh small enough to keep bees away, so that all falling mites would get stuck to it. We also recorded the number of mites found on the bottom. This was important because these mites were unable to reproduce, and they were not taken into account in estimating of parasite population growth.
Just before the emergence of worker bees from the cells, brood combs were placed in an incubator. The worker bees that emerged in the incubator were killed by freezing. The degree of the bees’ infestation by the parasite was then de-termined by the flotation method, where the sample of bees are stirred in the container with alcohol for 10 min, and then mites are separating from bees by pouring the alcohol over a sieve with a mesh size of approximately 2-3 mm.
The level of brood infestation was estimated by deter- mining the number of mature parasite females found in 100 cells (16), according to the formula:
BI = V.d.f × 100 C BI – level of brood infestation
V.d.f – number of mature mites found on the emerging bees from a comb (final population of V. destructor) C – number of all cells with brood (number of all emerg-
ing worker bees).
Also a coefficient of parasite population growth was estimated according to the formula:
PG = …….V.d.f…… V.d.i – V.d.d
PG – a coefficient of parasite population growth V.d.i – 50 mites (initial population of V. destructor) V.d.d – number of mites found on the bottom (debris of
V. destructor)
V.d.f – number of mature mites found on the emerging bees from a comb (final population of V. destructor). The data collected were analyzed statistically. The signi-ficance of differences among the groups was assessed using the analysis of variance (Fisher’s test).
Results
Number of mites dropped onto the nuclei bottom.
Some of the fifty V. destructor females introduced to the nuclei dropped onto the nuclei bottom. The biggest
V. destructor debris was found in CarD – an average 5.5; the lowest number of parasites dropped onto the bottom sheets in Cau – an average 2.8. In the remaining groups, the average number of V. destructor females that came off the bees was: 3.4 for CarK, 4.3 for Mel, and 4.8 for PCP. No statistically significant differences were noted among the groups (F74 = 1.234, P = 0.3045, Tab. 1).
Level of brood infestation by the parasite. The
highest degree of brood infestation by V. destructor was observed in broods from groups PCP and CarK, where 19.6 and 19.2 mites, respectively. Cau was in-fested at a medium degree, and Mel and CarD in the lowest degree. The level of infestation in these groups was similar: i.e. 17.2% and 17.3%, respectively. No statistically significant differences were noted among the values (F74 = 0.4879, P = 0.7445, Fig. 1).
Coefficient of V. destructor population growth.
The coefficient of V. destructor population growth was similar in all groups and ranged between 1.3 in Tab. 1. The average number of V. destructor mites in artificially infested colonies, and the coefficient of parasite population growth
Group No of tested colonies
Average No of mites
Coefficient of parasite population growth
Standard deviation of the coefficient of parasite
population growth On the bottom After the assay in the brood (on the emerging bees)
CarK 15 3.4 65.2 1.4 0.8
Mel 15 4.3 68.6 1.5 1.1
CarD 15 5.5 62.3 1.4 0.9
Cau 15 2.8 70.8 1.5 0.5
PCP 15 4.8 58.8 1.3 0.8
Explanations: CarK – Apis mellifera carnica (Kortówka line); Mel – A. m. mellifera (Augustowska line); CarD – A. m. carnica (Dobra line); Cau – A. m. caucasica (Woźnica line); PCP – crossbreeds of two subspecies: A. m. carnica × A. m. capensis
Fig. 1. The mean levels of infestation of broods by V. destructor at the end experiment with s.e.
Explanations: CarK – Apis mellifera carnica (Kortówka line), Mel – A. m. mellifera (Augustowska line), CarD – A. m. carnica (Dobra line), Cau – A. m. caucasica (Woźnica line), PCP – crossbreeds of two subspecies: A. m. carnica × A. m. capensis
CarK Mel CarD Cau PCP
15 16 17 18 19 20 21 22
The mean levels of infestation of the
brood
by
(%)
V
Med. Weter. 2013, 69 (12) 743 PCP to 1.5 in Mel and Cau. No statistically significant
differences were noted among the groups (F74 = 0.5096, P = 0.7288, Tab. 1)
Discussion
The growth of the V. destructor population in the groups, measured with the coefficient of parasite popu- lation growth, did not differ significantly, and ranged between 1.25 and 1.49. Woyke (24) examined the dy-namic of V. jacobsoni population growth in Vietnam. He found that it varied according to the region. The ratios of brood: worker infestation was only 0.8 in the hot south while it ranged between 2.9 and 8.2 in the cool north. The author suggests that the mother mites remain outside sealed brood for shorter time in a cool climate than in a hot one. Consequently the infestation rate as well as population grow is higher in cool regions than in hot ones. The coefficients of Varroa population growth calculated in the discussed assay were lower (from 1.3 for PCP to 1.5 for Mel and Cau) than those achieved by Schultz (17) for Carniolan bees – 2.3, and higher than those achieved by Wilde (21) – 1.2.
The degrees of the brood infestation by mites in Mexico varied (39.8-43.4%) These levels were not different (2). In the present study, the intensity of in-festation by V. destructor was highest in PCP and CarK (over 19%), and lowest in Mel and CarD (over 17%) but no significant differences were noted. In the both experiments it was proved that we cannot claim that the bee subspecies affects the coefficient of V. destructor population growth or the intensity of brood infestation by the parasite. The obtained results in the owners research confirms that of previous studies conducted by Bąk (3). They showed that the attractiveness of brood of different bee subspecies kept in Poland to V. destructor does not differ.
When studying V. destructor population growth, we assessed the number of parasites that had dropped on the bottom of the nuclei. The total mite population able to reproduce was thus smaller by the number of mites found on the bottom, which had its effect on the final result. The highest number of mites on the bottom sheets was found in CarD (5.5, on average), and the lowest – in Cau (2.8, on average). The result proves the good condition of the mites used to infest the colonies, as in the study carried out under similar conditions by Siuda and Wilde (20), the average number of mites found in the debris was higher 8.8.
Conclusion
The coefficient of V. destructor population growth and the extensiveness of honey bee brood infestation by the mite for individual groups did not show sta-tistical differences. Therefore, we can conclude that the honey bee subspecies has no effect on parasite population growth.
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Corresponding author: dr Beata Bąk, ul. Słoneczna 48, 10-710 Olsztyn; e-mail: beciabak@wp.pl
