(Apis mellifera) QUEENS AND WORKERS DURING ONTOGENETIC ...

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RATE OF GROWTH AND DEVELOPMENT TIME OF AFRICANIZED HONEY BEE (Apis mellifera) QUEENS AND WORKERS DURING ONTOGENETIC DEVELOPMENT Patrícia Nunes-Silva1, Lionel Segui Gonçalves2, Tiago Maurício Francoy3 and David De Jong3 Department of Ecology, Institute of Biosciences, University of São Paulo (USP),São Paulo, SP, Brazil Department of Biology, Faculty of Philosophy, Sciences and Letters of Ribeirão Preto, University of São Paulo (USP), Ribeirão Preto, 3Department of Genetics, Faculty of Medicine of Ribeirão Preto, University of São Paulo (USP), Ribeirão Preto, SP, Brazil. 1

2

ABSTRACT Ontogenetic development is an important characteristic of the biology of bees. Until now there have been no studies about the rate of growth of Africanized honey bees (Apis mellifera) during ontogenetic development, even though developmental time is key to understanding many aspects of their biology. We examined the rate of growth and the developmental period of Africanized honey bee queens and workers. These two castes had similar growth curve patterns, gaining weight during the first nine days of development and losing weight from the ninth day until emergence. The developmental period of queens was 15 to 16 days, and the newly emerged queens weighed 201.59 mg ± 27.63 mg (n = 50). The workers completed their development 19.8 to 20.3 days after oviposition. The emerged Africanized workers weighed 84.40 mg ± 7.95 mg (n = 33). We confirm that the developmental period of Africanized workers is at least one day shorter than that of European workers; it is more similar to that of African bees. Key words: Africanized honey bees, ontogenetic development, queens, weight, workers

INTRODUCTION Apis mellifera is a broadly distributed species, and it has evolved into many subspecies, which have different morphological, behavioral and ecological characteristics [3,23,38]. In 1956, Dr. Warwick E. Kerr brought A. mellifera scutellata queens to Brazil [11]. In 1957, after the colonies had been moved to a eucalyptus forest, some of them swarmed, due to interference by a visiting beekeeper; these swarms became established in the wild and the new bees subsequently spread throughout the South American continent [12], both naturally and through transport by beekeepers [16]. As they spread, they crossed with the subspecies of A. mellifera that were introduced before A. mellifera scutellata, and the resultant poly-hybrids were called Africanized honey bees, because of the predominance of the morphological and behavioral characteristics of the African bees in these new hybrids [7]. _______________ Correspondence to: Dr. Lionel Segui Gonçalves Departamento de Biologia, Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto, Universidade de São Paulo (USP), Av. Bandeirantes, 3900, CEP: 14040-901, Ribeirão Preto, SP, Brasil. Tel: (55) (16) 3602-3806, Fax: (55) (16) 3633-6482. E-mail: [email protected]

The weight of the different subspecies of honey bees during development has been studied by several researchers, to determine the rate of growth [26,30], and to characterize larval stages [15,20]; the weight of emerged queens has also been used as a quality index [1,2,8,14,17,28,29,39,40]. The biology of Africanized honey bees has been widely studied, ever since these bees started causing problems for Brazilian apiculture. One of the most widely-studied aspects of honey bee biology is the life cycle. There are differences in the development period between subspecies [6,9,10,13,18,21,22,31,32,35,36], and as Africanized honey bees are poly-hybrids, it would be useful to know if their life cycle is more similar to that of African bees or of European bees. The former is expected, because of the predominance of other characters of African bees. Information about this trait (development period) could help answer many questions about the biology of Africanized honey bees, such as why they have high swarming and absconding rates [Otis GW, PhD thesis, University of Kansas, Lawrence, USA, [19,37], and why they are resistant to the parasitic mite Varroa destructor [5]. Tribe and Fletcher [33] supposed that one of the

Braz. J. morphol. Sci. (2006) 23(3-4), 325-332

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at 34ºC and 70% relative humidity. After weighing the individuals, we recorded their developmental stage. We compared the mean weights with the Student t test and we calculated correlations between age and weight; we also developed the respective regression equations, using Statistica [27]. The growth of queens and workers was compared with the Wilcoxon Signed Ranks test, using SPSS [25]. We determined the duration of ontogenetic development by observing the number of queens and workers that emerged during the 14th, 15th, 16th and 17th days, and the 19th, 20th and 21st days of development, respectively. This experiment was repeated three times. The queen cells and the two worker brood combs used for this purpose were kept in an incubator maintained at 34ºC and 70% relative humidity until all the bees had emerged.

MATERIAL AND METHODS

RESULTS The growth curves of Africanized honey bee queens and workers (Fig. 1) had a similar form with

Age-controlled samples of Africanized honey bee workers were obtained by confining a queen with a queen excluder-screen push-in cage (13 X 10 X 2 cm) on an empty brood comb during eight hours for egg laying. We used the same procedure (with a six hour interval) to obtain the larvae that we used to rear queens through a standard grafting technique. We used three colonies for these procedures. To analyze egg weight, we collected two sets of 50 eggs. To follow the rate of growth, we collected 10 individuals every day during the ontogenetic development of queens and workers; this was repeated five times, totaling 50 individuals per day of development. There were three exceptions: on the 20th day of development of workers we had a sample size of only 33 individuals, and on the 10th and 13th days of development of queens we had a sample size of 49 individuals. After they were collected, the bees were weighed with an electronic scale. This procedure was repeated every 24 hours throughout development. However, on the emergence day, we collected and weighed individuals during the entire day, as soon as each bee emerged. We kept the sealed worker brood combs and the queen cells with the individuals that were ready to emerge in an incubator maintained

Weight (mg)

reasons for the higher rate of swarming in the African subspecies A. mellifera adansonii, compared to European bees, is the shorter developmental period of the African workers. The development period of Africanized honey bee queens could be one of the reasons for the competitive advantage of Africanized virgin queens and could contribute to the spread of African alleles and the loss of European honey bee characteristics over time, because virgin queens that emerge early would have a greater chance to eliminate rivals that are within queen cells [4,24]. We examined the rate of growth of Africanized honey bee (A. mellifera) queens and workers during ontogenetic development, and we determined the duration of the brood period.

Age (days)

Figure 1. Rate of growth (mean weights) of Africanized honey bee (Apis mellifera) queens and workers during ontogenetic development. Queens: n=50 for all days, except days 10 and 13, with n=49. Workers: n=50 for all days, except day 20, with n=33.

Table 1. Regression equations and correlation coefficient (r) of weight during different periods of ontogenetic development of queens and workers of Africanized honey bees (Apis mellifera).

Queens

Workers

Development period 1 to 9 days 4 to 9 days 9 to 15 days

Regression equation y = 0.0003*exp(1.5929*x) y = -276.3 + 54.900*x y = 366.22 – 9.755*x

1 to 9 days 4 to 9 days 9 to 20 days

y= 0.0009*exp(1.3653*x) y = -115.9 + 23.352*x y = 134.85 – 2.042*x

y: weight (mg); x: age (days).


Correlation coefficient (r) 0.83 0.84 -0.47 0.87 0.89 -0.66

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Rate of growth of Africanized honey bees

Table 2. Mean minimum (Min) and maximum (Max) weights in mg of developmental stages of queens of Africanized honey bees (Apis mellifera) during ontogenetic development. Day of development 1, 2 and 3

0.10 (50 )

Min.–Max. (mg) -

4

0.15 ± 0.05 (50)

0.1 – 0.2

larva

5

0.52 ± 0.23 (50)

0.2 – 0.9

larva

6

4.49 ± 2.06 (50)

0.8 – 9.5

larva

Mean ± Standard deviation (n)

Developmental stage egg

7

29.19 ± 9.68 (50)

10.3 – 53.5

larva

8

173.20 ± 52.41 (50)

41.6 – 260.1

30% of cells operculated

9

275.89 ± 61.30 (50)

77.8 – 379.4


10

260.13 ± 38.32 (49)

137.0 – 326.5

4.1% larvae; 71.4% prepupae; 24.5% pupae with white eyes

11

264.14 ± 39.38 (50)

193.1 – 403.8

8% prepupae; 86% pupae with white eyes 6% pupae with pink eyes

12

248.29 ± 20.24 (50)

195.1 – 306.4

2% prepupae; 98% pupae with pink eyes

13

253.69 ± 18.20 (49)

207.4 – 282.8

2% pupae with pink eyes 98% pupae with brown eyes

14

240.56 ± 23.12 (50)

177.0 – 282.0

60% pupae with brown eyes; 40% pupae with brown eyes and body pigmentation

15

201.59 ± 27.63 (50)

increasing age, showing the same general pattern of growth; but they differed in intensity. As the worker eggs were collected during eight-hour intervals, we established the midpoint of this interval as time 0 for egg laying. Consequently the actual age would be within the interval of the indicated age ± four hours. In the case of the queen brood, the interval was ± three hours. During the first five days of development of queens and workers, there was moderate weight gain, 5.20 times and 8.44 times on average, respectively. However, from the 5th to the 9th day there was a much more accentuated weight gain, 530.56 times for queens and 153.81 times for workers, on average. Both queens and workers achieved their maximum weight on the 9th day of development. During the first nine days of development, queen and worker growth was exponential; they gained weight slowly during the first five days of development and

155.5 – 264.3

emerged

then rapidly until the 9th day, when the maximum weight was achieved (Fig. 1 and Table 1). During the entire period of development, the average weight gain of queens was 2,759 times, and in workers it was 1,299 times. There was a positive linear correlation between age and weight during the period of 4 to 9 days of development (larval stage) in both queens and workers (Table 1). Days 1-3 correspond to the egg stage. After the 9th day of development, the weight of queens became nearly constant, reaching a plateau at 13 days old, when it started to drop, while the weight of workers increased until 9 days old; when it started to drop (Fig. 1). There was a statistically significant difference between 9 and 10 days (P = 0.000), 14 and 15 days (P = 0.036) and 15 and 16 days (P = 0.023) of development of workers and between 17 and 18 days


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(P = 0.030), 18 and 19 days (P = 0.014) and 19 and 20 days (P = 0.000). In the queens, after the 9th day of development, there were significant differences only between 11 and 12 days (P = 0.006), 13 and 14 days (P = 0.004) and 14 and 15 days (P = 0.000). There was an accentuated loss of weight during the last days of development (queens: 13 to 15 days; workers: 19 to 20 days – Fig. 1). We found a negative linear correlation between age and weight during the period of 9 to 15 days of development of queens and during the period of 9 to 20 days of development of workers (Table 1). The growth curve patterns of queens and workers were similar, although the growth intensity was very different (Wilcoxon Signed Ranks test, P < 0.01). The queens were heavier (on average) than the workers at day 10 (t = 5.6; P < 0.05), though the greatest

difference between the mean weights of queens and workers was attained on the 9th day of development (t = 18.1; P < 0.05, Fig. 1). The large difference in weight observed between newly emerged queens and workers (t = 23.7; P < 0.05) is determined during the larval stage, when the mean weights of queens and workers were, respectively, 201.59 mg and 84.40 mg (Fig. 1 and Tables 2 and 3). The Africanized honey bee queens completed their ontogenetic development after 15 to 16 days of development (Fig. 2). We found great variability in the stages between the 10th and the 15th days of development (Table 2). The developmental period of Africanized honey bee workers lasted from 19.8 to 20.3 days (Fig. 3); there was considerable variability in the stages between the 9th and 19th days of development (Table 3).

Figure 2. Ontogenetic development of Africanized honey bee queens. The numbers indicate days after egg laying: egg (days 1 to 3); larva (days 5, 7 and 9); prepupa (day 10); pupa (days 11 to 14); adult (day 15).


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Table 3. Mean, minimum (Min) and maximum (Max) weights in mg for developmental stages of workers of Africanized honey bees (Apis mellifera) during ontogenetic development. Day of development 1, 2 and 3

0.09 (50)

Min–Max (mg) -

4

0.15 ± 0.06 (50)

0.1-0.3

larva

5

0.76 ± 0.19 (50)

0.4-1.3

larva

6

4.17 ± 1.06 (50)

1.9-6.7

larva

7

20.10 ± 6.17 (50)

9.1-34.2

larva

8

73.31 ± 16.59 (50)

34.8-111.0

larva

9

116.90 ± 10.06 (50)

81.5-130.1


10

110.27 ± 5.93 (50)

100.7-127.5

98% prepupae; 2% larvae

11

110.49 ± 7.31(50)

93.7-129.2

100% prepupae

12

110.30 ± 6.05 (50)

93.0-121.0

46% prepupae; 48% pupae with white eyes; 6% intermediate*.

13

109.65 ± 5.18 (50)

94.1-121.4

56% pupae with white eyes;

Mean ± Standard deviation (n)

Developmental stage egg

44% pupae with pink eyes 14

108.61 ± 6.16 (50)

92.1-122.3

100% pupae with pink eyes

15

106.06 ± 5.77 (50)

92.7-116.9

100% pupae with brown eyes

16

104.15 ± 7.17 (50)

85.8-121.5

100% pupae with brown eyes

17

103.78 ± 5.08 (50)

89.5-114.3

100% pupae with brown eyes and body pigmentation

18

99.80 ± 11.66 (50)

91.4-113.6


19

95.18 ± 6.36 (50)

81.4-106.4


20

84.40 ± 7.95 (33)

54.7-102.6

emerged

*intermediate: worker that has prepupal characteristics at the posterior extremity and pupal characteristics at the anterior extremity.

DISCUSSION The ontogenetic development of Africanized queens and workers can be visualized through the shape of their respective growth curves (Fig. 1). The period of weight gain (4th to 9th days of development) corresponds to the larval stage. After the 9th day of development, when the cells are operculated, no more food is provided to the individuals, so weight loss can be explained by the absence of feeding and the consumption of body reserves. Weights of workers increased faster then those of the queens during the first days of development. In general, this observation agrees with the findings of Stabe [26], Thrasyvoulou and Benton [29] and Wang

[34]. The reason for the slower growth rate of the queens remains unknown. It is possible that the variation in the stages found on the different days of development of Africanized queens and workers is a result of the methodology we used to obtain the individuals that we sampled. We confined the queens on the combs for eight or six hours; consequently, there would be a maximum difference in age of eight hours among the workers and of six hours among the queens. We obtained the individuals from several different colonies, so genetic variation could have influenced our results. Climatic conditions also influence development, and as we did this study along two years, some of the variation could be a result of climatic variables.


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Figure 3. Ontogenetic development of Africanized honey bee workers. The numbers indicate days after egg laying: egg (days 1 to 3); larva (days 4 to 9); prepupa (day 10 and 11); pupa (days 12 to 19); adult (day 20).



The Africanized honey bee queens were found to have an intermediate development time when we compare them with the development time of the subspecies that originated this poly-hybrid. Africanized queens had a development time of 15 to 16 days, while the queens of A. mellifera scutellata complete their development in 14 days [10] and European queens in 16 days [36]. Though the data on African and European queens was gathered in other regions of the world, with variations in climate and food availability conditions that could possibly have affected development times, we believe these figures to be fairly accurate based on personal experience of one of the authors (LSG) with such bees in Brazil soon after the African bees were introduced (unpublished data). The early emerging Africanized honey bee queens may have more opportunities to eliminate European rivals that are within queen cells, which may have contributed to the Africanization process (see [24]). The Africanized honey bee workers had a development time of 19.8 to 20.3 days, very similar to that of A. mellifera adansonii, an African bee subspecies, which has a development time of 18 days and 13.5 h, according to Tribe and Fletcher [32], or 19 to 20 days, according to Smith [21,22]. The development time of European workers is 21 days [36], which is longer than the development time of Africanized workers. ACKNOWLEDGMENTS We thank João José dos Santos for technical services and Dr. Adriana Backx Noronha Viana and Sabrina de Oliveira Fernandes Viana for statistical help. We also thank CNPq and CAPES for financial support.

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