Insectes Soc. 50 (2003) 59– 61 0020-1812/03/010059-03 © Birkhäuser Verlag, Basel, 2003
Insectes Sociaux
Research article
Do policing honeybee (Apis mellifera) workers target eggs in drone comb? L. A. Halling and B.P. Oldroyd* School of Biological Sciences A12, University of Sydney, NSW 2006, Australia, e-mail:
[email protected] Received 14 March 2002; revised 15 August 2002; accepted 11 September 2002.
Summary. Honey bee (Apis mellifera) workers police each other’s reproduction by eating worker-laid eggs while preferentially leaving queen-laid eggs. Policing effort is presumably associated with biological costs, including those of search time and of errors of identification of eggs so that the wrong eggs are retained or eaten. Selection should act to minimise these costs. One way costs can be reduced is to focus policing effort on eggs that are most likely to be worker laid. We show here, in accordance with this prediction, that policing effort is focussed on drone cells where workers are more likely to lay eggs, rather than on worker cells which are more likely to contain queen-laid eggs. Key words: Policing, drone cells, costs, honeybee, Apis mellifera.
Introduction Worker policing is any behaviour of workers that has the effect of suppressing another worker’s reproduction. Worker policing can evolve because in polyandrous colonies, workers are more related to male offspring of the queen (r = 0.25) than to the male offspring of half sisters (r = 0.125) (Ratnieks, 1988). Worker policing via oophagy of worker-laid eggs has been documented in three species of Apis: A. mellifera (Ratnieks and Visscher, 1989), A. florea (Halling et al., 2001) and A. cerana (Oldroyd et al., 2001), in some vespine wasps (Foster and Ratnieks, 2000; Foster and Ratnieks, 2001a; Foster and Ratnieks, 2001b) and some ants (Kikuta and Tsuji, 1999; Monnin and Peeters 1997). In all these species, bio-assays show that worker-laid eggs are removed by police workers more quickly than queen-laid eggs which are generally retained. Worker policing by egg removal is expected to be associated with biological costs of two kinds. First, policing workers need to spend time and effort searching combs for work* Author for correspondence.
er-laid eggs (Wattanachaiyingcharoen et al., 2002). Second, there are costs associated with any mistakes that are made. Mistakes are of two possible kinds. First, if queen-laid eggs are destroyed through miss-identification, then this lowers colony-level fitness as resources are wasted. Second (and conversely), if worker-laid eggs are retained in error, then this reduces the inclusive fitness of workers and the fitness of the colony as a whole (Wattanachaiyingcharoen et al., 2002). Wattanachaiyingcharoen et al. (2002) suggested that selection should act to minimise the costs of policing. Costs can be minimised by directing policing effort at those eggs that are more likely to be worker laid, as this behavior both reduces errors of identification and reduces search time. A consequence of the inability of workers to mate is that all worker-laid eggs are haploid, whereas most queen-laid eggs are diploid. Thus, policing need only involve haploid eggs. Workers could target haploid eggs by either evaluating the ploidy of eggs through direct or indirect cues, or by targeting drone comb for policing effort (Wattanachaiyingcharoen et al., 2002). Oldroyd and Ratnieks (2000) showed that A. mellifera workers do not appear to distinguish between queen-laid fertilized eggs and queen-laid unfertilized eggs in policing assays, suggesting that workers may not react to differences in ploidy, at least for young eggs. Here we show that worker-laid eggs placed in drone cells are policed more quickly than eggs placed in worker cells.
Materials and methods This experiment was conducted in December 1999 at the University of Sydney. We used standard worker policing assays for A. mellifera (Oldroyd and Ratnieks, 2000; Ratnieks and Visscher, 1989) to test differences in rates of policing for worker-laid eggs placed in drone cells versus the rate for worker-laid eggs placed in worker cells. To obtain worker-laid eggs we dequeened three nucleus colonies, and removed all combs containing eggs and larvae. One week later we added clean drone combs to the queenless colonies to obtain fresh worker-laid eggs. We added a replacement drone comb each time we removed the drone comb so that all eggs used in these assays were less than 24 hours old. Worker-laid eggs were removed from their cells using modified forceps (Taber, 1961) and placed on microscope slides. Twenty eggs were
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L.A. Halling and B.P. Oldroyd
Do policing honeybee (Apis mellifera) workers target eggs in drone comb?
then transferred from the microscope slide into a row of worker-sized cells. Twenty others were transferred into a row of drone-sized cells in the same comb. The rows were marked with coloured pins. To avoid effects of position, each loaded row was approximately equidistant from the edge of the comb. A third row was marked to be monitored for any eggs laid by workers of the discriminator colony. One loaded comb was then placed into the middle of the upper chamber of each of two two-story, queen-right discriminator colonies. The queens of the discriminator colonies were confined to the bottom chamber by queen excluders. The experiment was repeated on each of four days using the same pair of discriminator colonies. Transferred eggs were counted at intervals: 30, 60, 120 and 180 minutes after the loaded combs were transferred to the discriminator colonies.
Results The proportion of the 20 eggs remaining at each observation was transformed to normalize the data using an arcsine √x transformation (Steel and Torrie, 1980). A repeated-measures ANOVA indicated that eggs placed in drone comb were removed significantly faster than eggs in worker comb (Table 1, Fig. 1). The two colonies discriminated at different rates, but in the same direction as indicated by the non-significant interaction between comb type and discriminator colony (Table 1). Comparing least-square means revealed that after one hour, 1/3 fewer eggs transferred to drone cells remained as a proportion of eggs remaining in worker cells. During the experiment, no eggs were observed in the rows of empty cells showing that (as expected) egg laying in the test combs was rare.
Discussion Our study has shown that in A. mellifera, worker policing targets eggs laid in drone comb. Since laying workers preferentially lay in drone comb, targeting drone comb probably increases the efficiency of worker policing by reducing the costs of search time and potentially reducing errors (Wattanachaiyingcharoen et al., 2002). Obviously, a system in which worker policing only targets worker-laid eggs placed in drone cells is vulnerable as laying workers could avoid policing by exclusively ovipositing in
Figure 1. Proportion of 20 worker-laid eggs removed from drone cells and worker sized cells in a policing bioassay. Bars indicate the standard errors of the means, but are too small to resolve for some points. Data are untransformed
worker cells. Thus policing still occurs even in worker-sized cells. However, workers show a strong (though not exclusive) preference for laying in drone cells (Free and Williams, 1974; Miller and Ratnieks, 2001; Oldroyd et al., 1999; Page and Erickson, 1988; Page and Metcalf, 1984; Ratnieks, 1993; Robinson et al., 1990). This is probably adaptive because drones that are reared in the smaller worker cells have smaller body sizes and are less reproductively successful than drones reared in the larger drone cells (Berg et al., 1997). Nevertheless, we show here that virtually all worker-laid eggs are removed within 4 hours, even in the worker-sized cells. This suggests that workers still police worker comb, but at a lower rate than drone-sized cells. This therefore suggests that selection has indeed acted to target policing effort where workers are most likely to lay eggs or that workers are more permissive of eggs in worker cells, adopting a higher threshold before egg rejection. A potential bias in these experiments is that drone cells are approximately 1.6 times larger than worker cells. Hence the number of policing bees per cell may be higher on drone cells than on worker cells purely because of crowding reduc-
Table 1. Repeated measures ANOVA of the proportions of worker laid eggs remaining (arcsine √x transformed data) Source
M.S.
d.f.
F
P
Between treatments Comb type (C) Discriminator colony (D) C*D Error
5612.0 2002.5 445.6 133.6
1 1 1 30
42.0 15.0 3.3
< 0.001 0.001 0.08
Times within treatments Time (T) T*C T*C*D Error
4292.6 922.9 62.3 6052.3
3 3 3 90
63.8 13.7 0.9
< 0.001 < 0.001 0.4
Insectes Soc.
Vol. 50, 2003
ing the number of policing bees per cell. However, we suggest that this is an unlikely cause of our results as it is doubtful that the number of workers available for policing is the limiting factor for the rate of policing.
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