Effect of soil hardness on aggression in the solitary wasp Mellinus arvensis

1. Two alternative nesting strategies are exhibited by soil-nesting Mellinu s arvensis females, digging a new nest (diggers) and searching for an old u noccupied burrow (searchers). Wasps appear unable to distinguish between oc cupied and unoccupied nests, and aggressive interactions between searchers and nest owners at nest entrances are frequent. 2. In aggressive encounters, there is an advantage in size and residency st atus. 3. The costs associated with the two nesting strategies vary across geograp hically separated populations: nest digging incurs costs in terms of time, and these vary according to the hardness of the soil substrate; nest search ing is variably costly in terms of risk of injury in aggressive encounters with nest-owning females. 4. Individual female wasps can switch between nesting strategies, and thus soil hardness, by affecting the cost of nest construction, affects the rela tive frequencies of the two nesting strategies within a population, favouri ng an increase in the searching strategy. This, in turn, affects the freque ncy and intensity of aggression between females at a nesting aggregation. 5. Female body size is correlated with soil hardness. As aggressive encount ers are more frequent in sites with hard soil substrates, there is increase d selective advantage in having large body size at these sites. 6. Body size is determined primarily by the availability of food resources during larval development, which is, to a degree, a function of the size of the adult female. There is a trade-off between provisioning a few cells wi th many provisions in each, leading to the development of few but large adu lts, as opposed to many cells with few provisions, leading to many small of fspring. The relative advantage of these two provisioning strategies is, at least in part, a function of the hardness of the soil substrate.