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Effect of worker number and diapause duration on colony parameters of the bumblebee, Bombus terrestris (Hymenoptera: Apidae)
Journal of Asia-Pacific Entomology 14 (2011) 455–458
Contents lists available at ScienceDirect
Journal of Asia-Pacific Entomology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p e
Effect of worker number and diapause duration on colony parameters of the bumblebee, Bombus terrestris (Hymenoptera: Apidae) Md. Ruhul Amin ⁎, Yong Jung Kwon, Zin Mar Thet School of applied Biology and Chemistry, Kyungpook National University, Daegu, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 September 2010 Revised 8 June 2011 Accepted 8 June 2011 Available online 23 June 2011 Keywords: Bombus terrestris Colony Diapause Worker number
a b s t r a c t Bumblebee, Bombus terrestris queens undergo winter diapause and show a great difference in diapause duration in natural conditions. Queens emerged from diapause initiate colonies by producing a batch of diploid (fertilised) eggs that develop into workers. In this study we investigated the effects of both the duration of queen diapause (2, 3, 4, or 5 months) and colony size (artificially limited to 50, 100, 150, and 200 workers) on the number of sexuals (males or new queens — gynes) produced, when gynes are produced and the longevity of both the foundress queen and the colony. Both worker population and diapause duration showed significant effect on sexual gyne production, foundress queen longevity and colony longevity but their interaction effect was insignificant. The worker number and diapause duration, respectively showed significant effect on sexual male production and gyne emergence period, but their interaction effects were insignificant. © Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2011. Published by Elsevier B.V. All rights reserved.
Introduction Bumblebees are annual social insects and are predominantly distributed in the temperate regions of the world (Cameron et al., 2007; Williams, 2007). Only young, recently mated queens go into hibernation in late summer and survive winter in small cavities (Alford, 1969). The queens leave their hibernation in spring and initiate the colony by producing a batch of diploid (fertilised) eggs that will develop into workers. These workers assist the queen in caring for subsequent broods (Sladen, 1912; Free and Butler, 1959). The colonies produce more workers but sexual individuals (males and gynes) emerge later in the colony cycle. Although workers do not mate and cannot lay fertilised eggs (diploid eggs), they can produce haploid unfertilised eggs which develop into male offspring (Trivers and Hare, 1976). The workers start foraging only 2 or 3 days after their emergence. Not all workers leave the nest and forage; typically the smaller workers stay in the nest and perform ‘household duties’ (Heinrich, 1979; Goulson et al., 2002; Jandt et al., 2009; Couvillon and Dornhaus, 2010). The growth rate of the colony is related to the foraging and nursing efforts of workers (Schmid-Hempel and SchmidHempel, 1998; Queller and Strassmann, 1998; Pelletier and McNeil, 2003; Ings et al., 2006). ⁎ Corresponding author at: Department of Entomology, Hajee Mohammad Danesh Science and Technology University, Dinajpur 5200, Bangladesh. Tel.: + 88 01714260959(mobile); fax: + 88 0531 61311. E-mail address: [email protected] (M.R. Amin).
Final colony size is species specific: some species produce only about 30 offspring whereas Bombus terrestris can produce 400 (Free and Butler, 1959). Other factors influencing gyne production are the availability of food, the relative number of foraging and nursing workers, diapause duration of the queen, and the lifetime of colony (Beekman and Van Stratum, 2000; Lopez-Vaamonde et al., 2009). In a queen-right colony, the queen suppresses worker egg-laying and eats worker-laid eggs (Lopez-Vaamonde et al., 2003). As soon as the queen dies, the workers take over the role of egg-laying, defend batches of their own eggs, and eventually rear males (Paxton et al., 2001). Roseler and Roseler (1974) suggested that the production of males and gynes would depend on the number of workers or their density in the brood area. However, subsequent work demonstrated that different factors may inhibit worker ovarian development and oviposition (Duchateau and Velthuis, 1989). Colony productivity also depends on the timing of the initiation of the queen-worker competition point, which may be initiated by the changes in the pheromonal output of the queen (Van Honk et al., 1981) and determines the ratio of castes produced (Cnaani et al., 2000). In a mature colony, workers with developed ovaries have been observed to destroy queen-laid eggs and replace them with their own eggs. This kind of physical conflict between workers and queen may result in workers eventually killing the queen (Bourke, 1994). Diapause is a physiologically important state of bumblebee queens as it influences reproductive output, queen lifespan, and colony longevity. Beekman and Van Stratum (2000) observed that queen lifetime decreases with increasing diapause period. They also reported
1226-8615/$ – see front matter © Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2011. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aspen.2011.06.004
456
M.R. Amin et al. / Journal of Asia-Pacific Entomology 14 (2011) 455–458
that the total number of workers and colony lifetime increases when the length of diapause increases from 0 to 4 months. The length of diapause of B. terrestris queens may influence the intensity of reproductive competition with workers. We predict that relationships among bumblebee workers and queen reproductive physiology, aggression, and task performance are dynamic, and will change over the course of normal colony development and in response to changes in social structure. Therefore, the aim of this study was to determine the effect of colony worker force-size and the diapause duration of the foundress queen on the reproductive output (male and gyne production) and longevity of the colony.
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Materials and methods
Statistical analysis
Origin of insects and oviposition
Data were analyzed by general linear model (GLM) using SPSS (PASW Statistics 18).
Sixteen Bombus terrestris dalmatinus Dalla Torre (Hymenoptera: Apidae) colonies were imported from Koppert B. V., The Netherlands in 2004. Colonies were reared for mass culture in the Department of Agricultural Biology, Kyungpook National University, Daegu, Korea. For this experiment, 3rd generation queens of Koppert origin, ranging from 700 to 800 mg in body weight on the day of emergence, were mated with males ranging from 300 to 400 mg in body weight. Mated queens were artificially diapaused for 2, 3, 4, or 5 months in a refrigerator and then transferred into flying cages (40 × 30 × 30 cm) which were illuminated with a 220/240 V fluorescent white light (Philips MX 204 HF L36/18-860, 300 Hz), λ = 460 nm). The cages were provided with Actinidia arguta (Plench) pollen in a Petri dish and sugar solution (1:1.5 w/v) in a perforated tube. The cages were kept in a climate controlled room maintained at 25± 1 °C and 50± 5% RH for activation of the queens. After one week, the activated queens were moved to individual transparent plastic boxes measuring 16 × 11 × 7 cm. The boxes were provisioned with pollen and sugar solution (1:1.5 w/v) in a perforated tube. To stimulate oviposition, one frozen queen pupa attached with paraffin to hard drawing paper and two CO2 narcotized B. terrestris workers were supplied to each box.
Table 1 Number of males (mean ± SD) produced in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
4
5
50 100 150 200
156.0 ± 90.4 a 238.4 ± 130.4 a 120.4 ± 117.5 b 219.4 ± 51.9 a
157.4 ± 87.3 a 264.6 ± 127.9 a 224.4 ± 130.3 ab 218.0 ± 121.8 a
156.4 ± 130.2 a 253.4 ± 43.7 a 232.0 ± 91.7 ab 168.6 ± 155.6 a
150.8 ± 99.0 252.6 ± 79.3 281.6 ± 98.8 145.4 ± 95.4
a a a a
Results Male production Table 1 shows that worker number in the colony has a significant effect (GLM: F3, 64 = 2.67, p b 0.05) on the production of males, but the effect of foundress queen diapause duration (GLM: F3, 64 = 0.29, p = 0.82) and the interaction of worker number and diapause duration were insignificant (GLM: F9, 64 = 0.70, p = 0.71). Gyne production Number of gyne production in relation to worker number and diapause durations are shown in Table 2. Both the worker number in the colony (GLM: F3, 64 = 3.61, p 0.01) and foundress queen diapause duration (GLM: F3, 64 = 7.06, p 0.001) showed significant effects on gyne production, but their interaction did not (GLM: F9, 64 = 1.26, p = 0.27). Gyne emergence period
Colony development After oviposition, the workers were removed from the colony boxes and the date of first oviposition was recorded. After emergence of the workers of the first brood, the colonies were transferred to larger colony boxes measuring 27 × 18 × 13 cm. The colonies were provided with ample protein (prepared from 50% honey solution and pollen powder) and sugar solution (1:1.5 w/v). The colonies completed development in a climate controlled room maintained at 28± 1 °C, 50 ± 5% RH, and red light of 10 lx (Koninklijke Philips Electronics, Seoul, Korea) during the observation period. Treatments and observation Five colonies were observed for each diapause treatment (2, 3, 4 and 5 months) and worker population (50, 100, 150, and 200). The colonies were monitored daily and newly emerged workers were marked with white enamel paint on the thorax. When the colonies produced the target worker number of 50, 100, 150, or 200, the marking was stopped and subsequently emerging workers were removed from the colonies daily. The total number of sexuals produced, longevity of foundress queen, and longevity of the colony were recorded for each nest. Newly emerged males and gynes were removed daily and counts recorded. The longevity of colonies was calculated as the day after pupa supply for stimulation of oviposition to the day of colony destruction. Colonies were destroyed when they terminated production of male and gyne pupae.
There was a significant difference in the timing of emergence of gynes in relation to diapause duration of the foundress queen (GLM: F3, 64 = 7.62, p 0.001), but the effect of worker number in the colony (GLM: F3, 64 = 0.44, p = 0.72) and the interaction of worker number and diapause duration of the foundress queen (GLM: F9, 64 = 1.16, p = 0.33) were insignificant (Table 3). Foundress queen longevity Table 4 shows that both worker number in the colony (GLM: F3, 64 = 3.95, p 0.01) and foundress queen diapause duration (GLM: F3, 64 = 3.84, p 0.01) have significant effects on foundress queen longevity, but their interaction did not show significant difference (GLM: F9, 64 = 0.70, p = 0.71).
Table 2 Number of gynes (mean ± SD) produced in relation to worker number and diapause duration. Worker number
Diapause duration (month)
50 100 150 200
65.6 ± 38.3 43.0 ± 40.7 87.2 ± 21.9 46.0 ± 26.4
2
3 a b ab b
71.0 ± 18.1 84.6 ± 41.2 112.4 ± 21.9 89.8 ± 17.9
4 a ab a a
55.6 ± 14.4 104.0 ± 27.6 89.4 ± 28.8 80.8 ± 29.5
5 a a ab ab
50.0 ± 28.6 53.8 ± 28.6 65.0 ± 22.5 50.4 ± 27.4
a b b b
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
M.R. Amin et al. / Journal of Asia-Pacific Entomology 14 (2011) 455–458 Table 3 Gyne emergence duration in days (mean ± SD) in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
4
5
50 100 150 200
90.2 ± 9.4 a 91.8 ± 7.3 ab 95.8 ± 10.3 ab 87.6 ± 8.7 ab
93.2 ± 15.5 a 102.2 ± 9.8 a 102.0 ± 7.8 a 100.2 ± 7.5 a
97.6 ± 7.9 a 89.4 ± 8.2 b 88.4 ± 7.2 b 94.0 ± 12.1 a
88.6 ± 6.1 a 86.6 ± 7.9 b 87.8 ± 7.4 b 79.2 ± 9.4 b
457
Table 5 Colony longevity in days (mean ± SD) in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
4
5
50 100 150 200
121.4 ± 4.5 a 126.2 ± 17.3 b 126.4 ± 13.3 b 115.6 ± 7.4 b
126.0 ± 12.0 a 154.6 ± 12.0 a 159.0 ± 10.8 a 135.0 ± 9.2 a
122.8 ± 17.4 a 136.4 ± 20.5 ab 140.6 ± 16.3 ab 126.0 ± 8.9 ab
118.4 ± 12.0 130.4 ± 10.1 136.8 ± 18.2 124.2 ± 11.1
a b b ab
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Colony longevity
At the competition point, workers with developed ovaries lay haploid (male) eggs and destroy most of the queen-laid eggs. This physical conflict between workers and queens may result in workers killing the queen (Schmid-Hempel and Schmid-Hempel, 2000). Our study shows that founding queen longevity differed with the worker number in the colony and diapause duration of queen, but no significant difference was found by their interaction. In larger colonies, colony size sometimes buffers against mortality, as it is the case in the fire ant Solenopsis invicta (Adams and Tschinkel, 2001). However, colony size does not increase survival probability of established colonies in the ant species, Harpegnathos saltator (Liebig and Poethke, 2004). Known factors of colony mortality are density effects due to territoriality and colony competition (Adams and Tschinkel, 1995). In the present study, colony longevity was dependent on colony size and diapause duration of the queen. It may be due to the proper maintenance of the brood by the workers and the timing of the emergence of the sexuals. Post diapause survival duration of a queen is related to diapause duration which influences the production of sexuals (Holm, 1972). Therefore, the findings of this study indicate that the important factors on colony parameters of B. terrestris might be diapause duration, and proper worker population in the colony.
The effect of worker number in the colony (GLM: F3, 64 = 9.09, p 0.001) and diapause duration of the foundress queen (GLM: F3, 64 = 9.32, p 0.001) have significant effects on colony longevity, but their interaction (GLM: F9, 64 = 0.81, p = 0.60) was statistically insignificant (Table 5). Discussion In colonies of social insects, workers can be in conflict with their mother over both the number of sexual males and sexual females that are produced. Beekman and Van Stratum (1998) demonstrated that small colonies produce mainly males because of a smaller number of workers available to collect sufficient resources to rear more energydemanding female offspring. Additionally, larger colonies have lower rates of colony growth (Oster and Wilson, 1978). The generally cited concept concerning Bombus sex ratios is that, within a species, small colonies specialize in male production and large ones in gyne production (Muller and Schmid-Hempel, 1992a; Lopez-Vaamonde et al., 2009). In our study, the number of males produced was dependent on the number of workers in the colony. The foundress queen diapause duration and the interaction of worker number and foundress queen diapause duration were statistically insignificant. In B. lucorum, Muller and Schmid-Hempel (1992b) showed no significant correlation between colony size and numerical sex ratios. The present study indicates that worker number in the colony and foundress queen diapause duration have significant effect on the production of gynes but their interaction did not vary significantly, whereas Lopez-Vaamonde et al. (2003) showed that there was no evidence that worker aggression had a negative effect on gyne production. Tsuchida et al. (2003) showed that the frequency of workers' sons within queen right colonies did not increase with colony size. Our results suggest that the influence of worker density on the sex ratio outcome, and on the timing of gyne production is dependent on the diapause duration of the founding queen. It was suggested by Pomeroy and Plowright (1982) that the initiation of queen production is associated both with the presence of male brood and with the number of workers. Roseler (1970) reported that B. terrestris queens exerted pheromonal inhibition over gyne production in colony development.
Table 4 Foundress queen longevity in days (mean ± SD) in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
50 100 150 200
109.6 ± 14.4 a 105.8 ± 30.9 b 111.8 ± 15.5 a 85.2 ± 15.5 a
114.6 ± 15.6 152.8 ± 10.2 130.0 ± 21.8 110.2 ± 31.2
4 a a a a
111.2 ± 21.0 124.6 ± 24.7 121.0 ± 41.8 110.6 ± 17.5
5 a ab a a
109.4 ± 13.9 121.2 ± 13.1 115.6 ± 29.8 106.8 ± 15.4
a b a a
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Acknowledgments We thank two anonymous referees for their constructive comments on the manuscript. References Adams, E.S., Tschinkel, W.R., 1995. Density dependent competition in fire ants: effects on colony survivorship and size variation. J. Anim. Ecol. 64, 315–324. Adams, E.S., Tschinkel, W.R., 2001. Mechanisms of population regulation in the fire ant, Solenopsis invicta: an experimental study. J. Anim. Ecol. 70, 355–369. Alford, D.V., 1969. A study of the hibernation of bumblebees (Hymenoptera: Bombidae) in southern England. J. Anim. Ecol. 38, 149–170. Beekman, M., Van Stratum, P., 1998. Bumblebee sex ratios: why do bumblebees produce so many males? Proc. Royal Soc. Lond. 265, 1535–1543. Beekman, M., Van Stratum, P., 2000. Does the diapause experience of bumblebee queens Bombus terrestris affect colony characteristics? Ecol. Entomol. 25, 1–6. Bourke, A.F.G., 1994. Worker matricide in social bees and wasps. J. Theo. Biol. 167, 283–292. Cameron, S.A., Hines, H.M., Williams, P.H., 2007. A comprehensive phylogeny of the bumblebees (Bombus). Bio. J. Linn. Soc. 91, 161–188. Cnaani, J., Robinson, G.E., Hefetz, A., 2000. The critical period for caste determination in Bombus terrestris and its juvenile hormone correlates. J. Comp. Physiol. 186, 1089–1094. Couvillon, M.J., Dornhaus, A., 2010. Small worker bumblebees (Bombus impatiens) are hardier against starvation and than their larger sisters. Insect Soc. 57, 193–197. Duchateau, M.J., Velthuis, H.H.W., 1989. Ovarian development and egg laying in workers of Bombus terrestris. Ent. Exp. Appl. 51, 199–213. Free, J.B., Butler, C.G., 1959. Bumblebees. Collins, London. 208 pp. Goulson, D., Peat, J., Stout, J.C., Tucker, J., Darvill, B., Derwent, L.C., Hughes, W.O.H., 2002. Can alloethism in workers of the bumblebee, Bombus terrestris, be explained in terms of foraging efficiency? Anim. Behav. 64, 123–130. Heinrich, B., 1979. Bumble Bee Economics. Harvard University Press, Cambridge, Massachusetts and London. Holm, S.N., 1972. Weight and life length of hibernating bumblebee queens (Hymenoptera: Bombidae) under controlled conditions. Ent. Scand. 3, 313–320. Ings, T.C., Ward, N.L., Chittka, L., 2006. Can commercially imported bumblebees outcompete their native conspecifics? J. Appl. Ecol. 43, 940–948.
458
M.R. Amin et al. / Journal of Asia-Pacific Entomology 14 (2011) 455–458
Jandt, J.M., Huang, E., Dornhaus, A., 2009. Weak specialization of workers inside a bumblebee (Bombus impatiens) nest. Behav. Ecol. Sociobiol. 63, 1829–1836. Liebig, J., Poethke, H.J., 2004. Queen lifespan and colony longevity in the ant, Harpegnathos saltator. Ecol. Entomol. 29, 203–207. Lopez-Vaamonde, C., Koning, J.W., Jordan, W.C., Bourke, A.F.G., 2003. No evidence that reproductive bumblebee workers reduce the production of new queens. Anim. Behav. 66, 577–584. Lopez-Vaamonde, C., Raine, N.E., Koning, J.W., Brown, R.M., Pereboom, J.J.M., Ings, T.C., Ramos-Rodriguez, O., Jordan, W.C., Bourke, A.F.G., 2009. Lifetime reproductive success and longevity of queens in an annual social insect. J. Evol. Biol. 22, 983–996. Muller, C.B., Schmid-Hempel, P., 1992a. Correlates of reproductive success among field colonies of Bombus lucorum: the importance of growth and parasites. Ecol. Entomol. 17, 343–353. Muller, C.B., Schmid-Hempel, P., 1992b. Variation in life history pattern in relation to worker mortality in the bumblebee, Bombus lucorum. Funct. Ecol. 6, 48–56. Oster, G.F., Wilson, E.O., 1978. Caste and Ecology of Social Insects. Princeton Univesity Press, Princeton, New Jersey. Paxton, R.J., Thoren, P.A., Estoups, A., Tengo, J., 2001. Queen worker conflict over male production and the sex ratio in a facultatively polyandrous bumblebee, Bombus hypnorum: the consequences of nest usurpation. Mol. Ecol. 10, 2489–2498. Pelletier, L., McNeil, J.N., 2003. The effect of food supplementation on reproductive success in bumblebee field colonies. Oikos 103, 688–694. Pomeroy, N., Plowright, R.C., 1982. The relation between worker numbers and the production of males and queens in the bumblebees, Bombus perplexus. Can. J. Zool. 60, 954–957.
Queller, D.C., Strassmann, J.E., 1998. Kin selection and social insects. Bioscience 48, 165–1100. Roseler, P.F., 1970. Unterschiede in der Kastendetermination zwischen den Hummelarten Bombus hypnorum und Bombus terrestris. Zool. Naturwissenschaften. 2, 543–548. Roseler, P.F., Roseler, I., 1974. Morphological and physiological differentiation of the caste in the bumblebee species Bombus hypnorum (L.) and Bombus terrestris (L.). Zool. Jaharb Physiol. 78, 100–198. Schmid-Hempel, R., Schmid-Hempel, P., 1998. Colony performance and immunocompetence of a social insect, Bombus terrestris, in poor and variable environments. Funct. Ecol. 12, 22–30. Schmid-Hempel, R., Schmid-Hempel, P., 2000. Female mating frequencies in Bombus spp. from Central Europe. Insect. Soc. 47, 36–41. Sladen, F.W.L., 1912. The Bumblebee. McMillian Co. Ltd, London. Trivers, R.L., Hare, H., 1976. Haploidiploidy and the evolution of the social insects. Science 191, 249–263. Tsuchida, K., Saigo, T., Nagata, N., Takeuchi, K., Miyano, S., 2003. Queen-worker conflicts over male production and sex allocation in a primitively eusocial wasp. Evolution 10, 2365–2373. Van Honk, C.G.J., Roseler, P.F., Velthuis, H.H.W., Hoogeveen, J.C., 1981. Factors influencing the egg laying of workers in a captive Bombus terrestris colony. Behav. Ecol. Sociobiol. 9, 9–14. Williams, P., 2007. The distribution of bumblebee colour patterns worldwide: possible significance for thermoregulation, crypsis, and warning mimicry. Biol. J. Linn. Soc. 92, 97–118.
Contents lists available at ScienceDirect
Journal of Asia-Pacific Entomology j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p e
Effect of worker number and diapause duration on colony parameters of the bumblebee, Bombus terrestris (Hymenoptera: Apidae) Md. Ruhul Amin ⁎, Yong Jung Kwon, Zin Mar Thet School of applied Biology and Chemistry, Kyungpook National University, Daegu, Republic of Korea
a r t i c l e
i n f o
Article history: Received 23 September 2010 Revised 8 June 2011 Accepted 8 June 2011 Available online 23 June 2011 Keywords: Bombus terrestris Colony Diapause Worker number
a b s t r a c t Bumblebee, Bombus terrestris queens undergo winter diapause and show a great difference in diapause duration in natural conditions. Queens emerged from diapause initiate colonies by producing a batch of diploid (fertilised) eggs that develop into workers. In this study we investigated the effects of both the duration of queen diapause (2, 3, 4, or 5 months) and colony size (artificially limited to 50, 100, 150, and 200 workers) on the number of sexuals (males or new queens — gynes) produced, when gynes are produced and the longevity of both the foundress queen and the colony. Both worker population and diapause duration showed significant effect on sexual gyne production, foundress queen longevity and colony longevity but their interaction effect was insignificant. The worker number and diapause duration, respectively showed significant effect on sexual male production and gyne emergence period, but their interaction effects were insignificant. © Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2011. Published by Elsevier B.V. All rights reserved.
Introduction Bumblebees are annual social insects and are predominantly distributed in the temperate regions of the world (Cameron et al., 2007; Williams, 2007). Only young, recently mated queens go into hibernation in late summer and survive winter in small cavities (Alford, 1969). The queens leave their hibernation in spring and initiate the colony by producing a batch of diploid (fertilised) eggs that will develop into workers. These workers assist the queen in caring for subsequent broods (Sladen, 1912; Free and Butler, 1959). The colonies produce more workers but sexual individuals (males and gynes) emerge later in the colony cycle. Although workers do not mate and cannot lay fertilised eggs (diploid eggs), they can produce haploid unfertilised eggs which develop into male offspring (Trivers and Hare, 1976). The workers start foraging only 2 or 3 days after their emergence. Not all workers leave the nest and forage; typically the smaller workers stay in the nest and perform ‘household duties’ (Heinrich, 1979; Goulson et al., 2002; Jandt et al., 2009; Couvillon and Dornhaus, 2010). The growth rate of the colony is related to the foraging and nursing efforts of workers (Schmid-Hempel and SchmidHempel, 1998; Queller and Strassmann, 1998; Pelletier and McNeil, 2003; Ings et al., 2006). ⁎ Corresponding author at: Department of Entomology, Hajee Mohammad Danesh Science and Technology University, Dinajpur 5200, Bangladesh. Tel.: + 88 01714260959(mobile); fax: + 88 0531 61311. E-mail address: [email protected] (M.R. Amin).
Final colony size is species specific: some species produce only about 30 offspring whereas Bombus terrestris can produce 400 (Free and Butler, 1959). Other factors influencing gyne production are the availability of food, the relative number of foraging and nursing workers, diapause duration of the queen, and the lifetime of colony (Beekman and Van Stratum, 2000; Lopez-Vaamonde et al., 2009). In a queen-right colony, the queen suppresses worker egg-laying and eats worker-laid eggs (Lopez-Vaamonde et al., 2003). As soon as the queen dies, the workers take over the role of egg-laying, defend batches of their own eggs, and eventually rear males (Paxton et al., 2001). Roseler and Roseler (1974) suggested that the production of males and gynes would depend on the number of workers or their density in the brood area. However, subsequent work demonstrated that different factors may inhibit worker ovarian development and oviposition (Duchateau and Velthuis, 1989). Colony productivity also depends on the timing of the initiation of the queen-worker competition point, which may be initiated by the changes in the pheromonal output of the queen (Van Honk et al., 1981) and determines the ratio of castes produced (Cnaani et al., 2000). In a mature colony, workers with developed ovaries have been observed to destroy queen-laid eggs and replace them with their own eggs. This kind of physical conflict between workers and queen may result in workers eventually killing the queen (Bourke, 1994). Diapause is a physiologically important state of bumblebee queens as it influences reproductive output, queen lifespan, and colony longevity. Beekman and Van Stratum (2000) observed that queen lifetime decreases with increasing diapause period. They also reported
1226-8615/$ – see front matter © Korean Society of Applied Entomology, Taiwan Entomological Society and Malaysian Plant Protection Society, 2011. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.aspen.2011.06.004
456
M.R. Amin et al. / Journal of Asia-Pacific Entomology 14 (2011) 455–458
that the total number of workers and colony lifetime increases when the length of diapause increases from 0 to 4 months. The length of diapause of B. terrestris queens may influence the intensity of reproductive competition with workers. We predict that relationships among bumblebee workers and queen reproductive physiology, aggression, and task performance are dynamic, and will change over the course of normal colony development and in response to changes in social structure. Therefore, the aim of this study was to determine the effect of colony worker force-size and the diapause duration of the foundress queen on the reproductive output (male and gyne production) and longevity of the colony.
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Materials and methods
Statistical analysis
Origin of insects and oviposition
Data were analyzed by general linear model (GLM) using SPSS (PASW Statistics 18).
Sixteen Bombus terrestris dalmatinus Dalla Torre (Hymenoptera: Apidae) colonies were imported from Koppert B. V., The Netherlands in 2004. Colonies were reared for mass culture in the Department of Agricultural Biology, Kyungpook National University, Daegu, Korea. For this experiment, 3rd generation queens of Koppert origin, ranging from 700 to 800 mg in body weight on the day of emergence, were mated with males ranging from 300 to 400 mg in body weight. Mated queens were artificially diapaused for 2, 3, 4, or 5 months in a refrigerator and then transferred into flying cages (40 × 30 × 30 cm) which were illuminated with a 220/240 V fluorescent white light (Philips MX 204 HF L36/18-860, 300 Hz), λ = 460 nm). The cages were provided with Actinidia arguta (Plench) pollen in a Petri dish and sugar solution (1:1.5 w/v) in a perforated tube. The cages were kept in a climate controlled room maintained at 25± 1 °C and 50± 5% RH for activation of the queens. After one week, the activated queens were moved to individual transparent plastic boxes measuring 16 × 11 × 7 cm. The boxes were provisioned with pollen and sugar solution (1:1.5 w/v) in a perforated tube. To stimulate oviposition, one frozen queen pupa attached with paraffin to hard drawing paper and two CO2 narcotized B. terrestris workers were supplied to each box.
Table 1 Number of males (mean ± SD) produced in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
4
5
50 100 150 200
156.0 ± 90.4 a 238.4 ± 130.4 a 120.4 ± 117.5 b 219.4 ± 51.9 a
157.4 ± 87.3 a 264.6 ± 127.9 a 224.4 ± 130.3 ab 218.0 ± 121.8 a
156.4 ± 130.2 a 253.4 ± 43.7 a 232.0 ± 91.7 ab 168.6 ± 155.6 a
150.8 ± 99.0 252.6 ± 79.3 281.6 ± 98.8 145.4 ± 95.4
a a a a
Results Male production Table 1 shows that worker number in the colony has a significant effect (GLM: F3, 64 = 2.67, p b 0.05) on the production of males, but the effect of foundress queen diapause duration (GLM: F3, 64 = 0.29, p = 0.82) and the interaction of worker number and diapause duration were insignificant (GLM: F9, 64 = 0.70, p = 0.71). Gyne production Number of gyne production in relation to worker number and diapause durations are shown in Table 2. Both the worker number in the colony (GLM: F3, 64 = 3.61, p 0.01) and foundress queen diapause duration (GLM: F3, 64 = 7.06, p 0.001) showed significant effects on gyne production, but their interaction did not (GLM: F9, 64 = 1.26, p = 0.27). Gyne emergence period
Colony development After oviposition, the workers were removed from the colony boxes and the date of first oviposition was recorded. After emergence of the workers of the first brood, the colonies were transferred to larger colony boxes measuring 27 × 18 × 13 cm. The colonies were provided with ample protein (prepared from 50% honey solution and pollen powder) and sugar solution (1:1.5 w/v). The colonies completed development in a climate controlled room maintained at 28± 1 °C, 50 ± 5% RH, and red light of 10 lx (Koninklijke Philips Electronics, Seoul, Korea) during the observation period. Treatments and observation Five colonies were observed for each diapause treatment (2, 3, 4 and 5 months) and worker population (50, 100, 150, and 200). The colonies were monitored daily and newly emerged workers were marked with white enamel paint on the thorax. When the colonies produced the target worker number of 50, 100, 150, or 200, the marking was stopped and subsequently emerging workers were removed from the colonies daily. The total number of sexuals produced, longevity of foundress queen, and longevity of the colony were recorded for each nest. Newly emerged males and gynes were removed daily and counts recorded. The longevity of colonies was calculated as the day after pupa supply for stimulation of oviposition to the day of colony destruction. Colonies were destroyed when they terminated production of male and gyne pupae.
There was a significant difference in the timing of emergence of gynes in relation to diapause duration of the foundress queen (GLM: F3, 64 = 7.62, p 0.001), but the effect of worker number in the colony (GLM: F3, 64 = 0.44, p = 0.72) and the interaction of worker number and diapause duration of the foundress queen (GLM: F9, 64 = 1.16, p = 0.33) were insignificant (Table 3). Foundress queen longevity Table 4 shows that both worker number in the colony (GLM: F3, 64 = 3.95, p 0.01) and foundress queen diapause duration (GLM: F3, 64 = 3.84, p 0.01) have significant effects on foundress queen longevity, but their interaction did not show significant difference (GLM: F9, 64 = 0.70, p = 0.71).
Table 2 Number of gynes (mean ± SD) produced in relation to worker number and diapause duration. Worker number
Diapause duration (month)
50 100 150 200
65.6 ± 38.3 43.0 ± 40.7 87.2 ± 21.9 46.0 ± 26.4
2
3 a b ab b
71.0 ± 18.1 84.6 ± 41.2 112.4 ± 21.9 89.8 ± 17.9
4 a ab a a
55.6 ± 14.4 104.0 ± 27.6 89.4 ± 28.8 80.8 ± 29.5
5 a a ab ab
50.0 ± 28.6 53.8 ± 28.6 65.0 ± 22.5 50.4 ± 27.4
a b b b
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
M.R. Amin et al. / Journal of Asia-Pacific Entomology 14 (2011) 455–458 Table 3 Gyne emergence duration in days (mean ± SD) in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
4
5
50 100 150 200
90.2 ± 9.4 a 91.8 ± 7.3 ab 95.8 ± 10.3 ab 87.6 ± 8.7 ab
93.2 ± 15.5 a 102.2 ± 9.8 a 102.0 ± 7.8 a 100.2 ± 7.5 a
97.6 ± 7.9 a 89.4 ± 8.2 b 88.4 ± 7.2 b 94.0 ± 12.1 a
88.6 ± 6.1 a 86.6 ± 7.9 b 87.8 ± 7.4 b 79.2 ± 9.4 b
457
Table 5 Colony longevity in days (mean ± SD) in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
4
5
50 100 150 200
121.4 ± 4.5 a 126.2 ± 17.3 b 126.4 ± 13.3 b 115.6 ± 7.4 b
126.0 ± 12.0 a 154.6 ± 12.0 a 159.0 ± 10.8 a 135.0 ± 9.2 a
122.8 ± 17.4 a 136.4 ± 20.5 ab 140.6 ± 16.3 ab 126.0 ± 8.9 ab
118.4 ± 12.0 130.4 ± 10.1 136.8 ± 18.2 124.2 ± 11.1
a b b ab
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Colony longevity
At the competition point, workers with developed ovaries lay haploid (male) eggs and destroy most of the queen-laid eggs. This physical conflict between workers and queens may result in workers killing the queen (Schmid-Hempel and Schmid-Hempel, 2000). Our study shows that founding queen longevity differed with the worker number in the colony and diapause duration of queen, but no significant difference was found by their interaction. In larger colonies, colony size sometimes buffers against mortality, as it is the case in the fire ant Solenopsis invicta (Adams and Tschinkel, 2001). However, colony size does not increase survival probability of established colonies in the ant species, Harpegnathos saltator (Liebig and Poethke, 2004). Known factors of colony mortality are density effects due to territoriality and colony competition (Adams and Tschinkel, 1995). In the present study, colony longevity was dependent on colony size and diapause duration of the queen. It may be due to the proper maintenance of the brood by the workers and the timing of the emergence of the sexuals. Post diapause survival duration of a queen is related to diapause duration which influences the production of sexuals (Holm, 1972). Therefore, the findings of this study indicate that the important factors on colony parameters of B. terrestris might be diapause duration, and proper worker population in the colony.
The effect of worker number in the colony (GLM: F3, 64 = 9.09, p 0.001) and diapause duration of the foundress queen (GLM: F3, 64 = 9.32, p 0.001) have significant effects on colony longevity, but their interaction (GLM: F9, 64 = 0.81, p = 0.60) was statistically insignificant (Table 5). Discussion In colonies of social insects, workers can be in conflict with their mother over both the number of sexual males and sexual females that are produced. Beekman and Van Stratum (1998) demonstrated that small colonies produce mainly males because of a smaller number of workers available to collect sufficient resources to rear more energydemanding female offspring. Additionally, larger colonies have lower rates of colony growth (Oster and Wilson, 1978). The generally cited concept concerning Bombus sex ratios is that, within a species, small colonies specialize in male production and large ones in gyne production (Muller and Schmid-Hempel, 1992a; Lopez-Vaamonde et al., 2009). In our study, the number of males produced was dependent on the number of workers in the colony. The foundress queen diapause duration and the interaction of worker number and foundress queen diapause duration were statistically insignificant. In B. lucorum, Muller and Schmid-Hempel (1992b) showed no significant correlation between colony size and numerical sex ratios. The present study indicates that worker number in the colony and foundress queen diapause duration have significant effect on the production of gynes but their interaction did not vary significantly, whereas Lopez-Vaamonde et al. (2003) showed that there was no evidence that worker aggression had a negative effect on gyne production. Tsuchida et al. (2003) showed that the frequency of workers' sons within queen right colonies did not increase with colony size. Our results suggest that the influence of worker density on the sex ratio outcome, and on the timing of gyne production is dependent on the diapause duration of the founding queen. It was suggested by Pomeroy and Plowright (1982) that the initiation of queen production is associated both with the presence of male brood and with the number of workers. Roseler (1970) reported that B. terrestris queens exerted pheromonal inhibition over gyne production in colony development.
Table 4 Foundress queen longevity in days (mean ± SD) in relation to worker number and diapause duration. Worker number
Diapause duration (month) 2
3
50 100 150 200
109.6 ± 14.4 a 105.8 ± 30.9 b 111.8 ± 15.5 a 85.2 ± 15.5 a
114.6 ± 15.6 152.8 ± 10.2 130.0 ± 21.8 110.2 ± 31.2
4 a a a a
111.2 ± 21.0 124.6 ± 24.7 121.0 ± 41.8 110.6 ± 17.5
5 a ab a a
109.4 ± 13.9 121.2 ± 13.1 115.6 ± 29.8 106.8 ± 15.4
a b a a
Means within a row followed by same letter(s) are not significantly different (DMRT, p ≤ 0.05).
Acknowledgments We thank two anonymous referees for their constructive comments on the manuscript. References Adams, E.S., Tschinkel, W.R., 1995. Density dependent competition in fire ants: effects on colony survivorship and size variation. J. Anim. Ecol. 64, 315–324. Adams, E.S., Tschinkel, W.R., 2001. Mechanisms of population regulation in the fire ant, Solenopsis invicta: an experimental study. J. Anim. Ecol. 70, 355–369. Alford, D.V., 1969. A study of the hibernation of bumblebees (Hymenoptera: Bombidae) in southern England. J. Anim. Ecol. 38, 149–170. Beekman, M., Van Stratum, P., 1998. Bumblebee sex ratios: why do bumblebees produce so many males? Proc. Royal Soc. Lond. 265, 1535–1543. Beekman, M., Van Stratum, P., 2000. Does the diapause experience of bumblebee queens Bombus terrestris affect colony characteristics? Ecol. Entomol. 25, 1–6. Bourke, A.F.G., 1994. Worker matricide in social bees and wasps. J. Theo. Biol. 167, 283–292. Cameron, S.A., Hines, H.M., Williams, P.H., 2007. A comprehensive phylogeny of the bumblebees (Bombus). Bio. J. Linn. Soc. 91, 161–188. Cnaani, J., Robinson, G.E., Hefetz, A., 2000. The critical period for caste determination in Bombus terrestris and its juvenile hormone correlates. J. Comp. Physiol. 186, 1089–1094. Couvillon, M.J., Dornhaus, A., 2010. Small worker bumblebees (Bombus impatiens) are hardier against starvation and than their larger sisters. Insect Soc. 57, 193–197. Duchateau, M.J., Velthuis, H.H.W., 1989. Ovarian development and egg laying in workers of Bombus terrestris. Ent. Exp. Appl. 51, 199–213. Free, J.B., Butler, C.G., 1959. Bumblebees. Collins, London. 208 pp. Goulson, D., Peat, J., Stout, J.C., Tucker, J., Darvill, B., Derwent, L.C., Hughes, W.O.H., 2002. Can alloethism in workers of the bumblebee, Bombus terrestris, be explained in terms of foraging efficiency? Anim. Behav. 64, 123–130. Heinrich, B., 1979. Bumble Bee Economics. Harvard University Press, Cambridge, Massachusetts and London. Holm, S.N., 1972. Weight and life length of hibernating bumblebee queens (Hymenoptera: Bombidae) under controlled conditions. Ent. Scand. 3, 313–320. Ings, T.C., Ward, N.L., Chittka, L., 2006. Can commercially imported bumblebees outcompete their native conspecifics? J. Appl. Ecol. 43, 940–948.
458
M.R. Amin et al. / Journal of Asia-Pacific Entomology 14 (2011) 455–458
Jandt, J.M., Huang, E., Dornhaus, A., 2009. Weak specialization of workers inside a bumblebee (Bombus impatiens) nest. Behav. Ecol. Sociobiol. 63, 1829–1836. Liebig, J., Poethke, H.J., 2004. Queen lifespan and colony longevity in the ant, Harpegnathos saltator. Ecol. Entomol. 29, 203–207. Lopez-Vaamonde, C., Koning, J.W., Jordan, W.C., Bourke, A.F.G., 2003. No evidence that reproductive bumblebee workers reduce the production of new queens. Anim. Behav. 66, 577–584. Lopez-Vaamonde, C., Raine, N.E., Koning, J.W., Brown, R.M., Pereboom, J.J.M., Ings, T.C., Ramos-Rodriguez, O., Jordan, W.C., Bourke, A.F.G., 2009. Lifetime reproductive success and longevity of queens in an annual social insect. J. Evol. Biol. 22, 983–996. Muller, C.B., Schmid-Hempel, P., 1992a. Correlates of reproductive success among field colonies of Bombus lucorum: the importance of growth and parasites. Ecol. Entomol. 17, 343–353. Muller, C.B., Schmid-Hempel, P., 1992b. Variation in life history pattern in relation to worker mortality in the bumblebee, Bombus lucorum. Funct. Ecol. 6, 48–56. Oster, G.F., Wilson, E.O., 1978. Caste and Ecology of Social Insects. Princeton Univesity Press, Princeton, New Jersey. Paxton, R.J., Thoren, P.A., Estoups, A., Tengo, J., 2001. Queen worker conflict over male production and the sex ratio in a facultatively polyandrous bumblebee, Bombus hypnorum: the consequences of nest usurpation. Mol. Ecol. 10, 2489–2498. Pelletier, L., McNeil, J.N., 2003. The effect of food supplementation on reproductive success in bumblebee field colonies. Oikos 103, 688–694. Pomeroy, N., Plowright, R.C., 1982. The relation between worker numbers and the production of males and queens in the bumblebees, Bombus perplexus. Can. J. Zool. 60, 954–957.
Queller, D.C., Strassmann, J.E., 1998. Kin selection and social insects. Bioscience 48, 165–1100. Roseler, P.F., 1970. Unterschiede in der Kastendetermination zwischen den Hummelarten Bombus hypnorum und Bombus terrestris. Zool. Naturwissenschaften. 2, 543–548. Roseler, P.F., Roseler, I., 1974. Morphological and physiological differentiation of the caste in the bumblebee species Bombus hypnorum (L.) and Bombus terrestris (L.). Zool. Jaharb Physiol. 78, 100–198. Schmid-Hempel, R., Schmid-Hempel, P., 1998. Colony performance and immunocompetence of a social insect, Bombus terrestris, in poor and variable environments. Funct. Ecol. 12, 22–30. Schmid-Hempel, R., Schmid-Hempel, P., 2000. Female mating frequencies in Bombus spp. from Central Europe. Insect. Soc. 47, 36–41. Sladen, F.W.L., 1912. The Bumblebee. McMillian Co. Ltd, London. Trivers, R.L., Hare, H., 1976. Haploidiploidy and the evolution of the social insects. Science 191, 249–263. Tsuchida, K., Saigo, T., Nagata, N., Takeuchi, K., Miyano, S., 2003. Queen-worker conflicts over male production and sex allocation in a primitively eusocial wasp. Evolution 10, 2365–2373. Van Honk, C.G.J., Roseler, P.F., Velthuis, H.H.W., Hoogeveen, J.C., 1981. Factors influencing the egg laying of workers in a captive Bombus terrestris colony. Behav. Ecol. Sociobiol. 9, 9–14. Williams, P., 2007. The distribution of bumblebee colour patterns worldwide: possible significance for thermoregulation, crypsis, and warning mimicry. Biol. J. Linn. Soc. 92, 97–118.