Conflict over male production in Vespinae wasps.

Abstract

Insect societies are well known for their cooperation. However, a number of conflicts do occur within their colonies. The eusocial Hymenoptera (bees, ants and wasps) are no exception, with several reproductive conflicts affecting their societies. One of these conflicts is over male production. This conflict is caused by the fact that the workers in most species, though unable to mate, possess functional ovaries and retain the capacity to lay unfertilized, male eggs. In addition, since both the mother queen and the workers are each most related to their own sons, they would be expected to strongly compete over male production. One important mechanism that can resolve this conflict is policing, whereby workers are prevented to successfully reproduce, thereby maintaining the reproductive primacy of the queen. Policing can be carried out by both the mother queen (queen policing), and by the workers (worker policing), and can occur either via aggression or selective egg eating. In this thesis, we examined the ultimate causes and underlying proximate mechanism that enable worker policing by egg eating in Vespinae wasps.In a first set of studies, we looked at the importance of the relatedness among workers as an ultimate cause for the selection of worker policing in three species of Vespinae wasps, the German wasp Vespula germanica, the Norwegian wasp Dolichovespula norwegica and the Saxon wasp Dolichovespula saxonica. Our data show that patterns of policing can be surprisingly complex. In particular, different evolutionary factors, including relatedness, as well as effects on colony productivity and the sex ratio, are not mutually exclusive and need to be considered together to explain prevailing patterns of worker policing in eusocial Hymenoptera. In addition, we provide evidence for the occurrence of selfish worker policing in D. norwegica. In this type of policing, workers carrying out the policing also occasionally lay eggs themselves, thereby acquiring direct fitness benefits. This finding supports selfish worker policing to be an important mechanism that can greatly facilitate the evolution of worker policing. Inclusive fitness theory and empirical data suggest that in eusocial hymenopteran species where colonies are either headed by a single mated queen or a multiple mated queen, workers should police other workers' eggs only in colonies headed by a multiple mated queen. Despite earlier evidence for facultative worker policing in Dolichovespula saxonica by Foster and Ratnieks (2000), we found no evidence for facultative worker policing but show that seasonal progression of the nest and the location in the nest where males were reared explain the patterns of worker reproduction. Furthermore, we showthat chemical variability in cuticular hydrocarbon profiles among the workers was not significantly correlated with queen mating frequency and, therefore, is likely not used by workers to facultatively police according to the queen mating frequency. Our results suggest that the earlier evidence for facultative worker policing in D. saxonica by Foster and Ratnieks (2000) may have been due to accidental correlations with certain confounding variables, or, alternatively, that there are large interpopulation differences in the expression of worker policing.In another study, we looked at the proximate mechanism that enables workers to selectively police worker-laid eggs. We demonstrate that in the common wasp Vespula vulgaris, surface hydrocarbon profiles of queen-laid eggs and worker-laid eggs can contain the necessary information for workers to effectively police worker-laid eggs.Finally, in a last chapter, our data suggest that the beetle, Metoecus paradoxus, which is known to parasitize Vespula vulgaris colonies, can elude host recognition in the adult stage by mimicking the wasps‟ cuticular hydrocarbon profiles. This study is the first to show chemical mimicry by a non-social insect parasitizing social wasp colonies.Acknowledgments I Summary III Samenvatting V Contents VII Introduction IX Conflicts in insect societies IX Queen-worker conflict over the sex ratio X Conflict over male production XI The basis for the conflict XI Policing as a conflict-reducing mechanism XII Proximate mechanisms involved in policing XIV Chemical mimicry by social parasites in wasp colonies XVI Study organisms XVII Structure and outline of the thesis XXII Chapter 1 1 Worker policing in the German wasp Vespula germanica Chapter 2 17 Co-occurrence of three types of egg policing in the Norwegian wasp Dolichovespula norwegica Chapter 3 33 Seasonal progression and not facultative policing explains pattern of worker reproduction in the Saxon wasp Chapter 4 53 Hydrocarbon signatures of egg maternity, caste membership and reproductive status in the common wasp Chapter 5 69 Chemical mimicry by the beetle Metoecus paradoxus in Vespula vulgaris wasp colonies General conclusions and perspectives 77 References 81nrpages: 118status: publishe