Diet and nest resource exploitation in solitary wasps (Hymenoptera: Crabronidae): morphological, behavioural and environmental effects on species and individual specialization".

Apoid wasps of both families Crabronidae and Sphecidae (Hymenoptera) include insects that are predatory, free-living as adults and parasitoids, defenceless and immobile as larvae, and represent one of the best groups of animals to analyze for ecological topics such as diet and nest resource exploitation and their morphological, behavioural and environmental effects on species and individual specialization. In this study 7 different species are analyzed. Two of them are members of Cerceris genus: C. rubida (Jurine, 1807) and C. arenaria (Linnaeus, 1758); the other five represent all the species of Sceliphron present in Italy: S. spirifex (Linnaeus 1758), S. caementarium (Drury 1770), S. curvatum (F. Smith 1870), S. madraspatanum tubifex (Fabricius 1781), and S. destillatorium (Illiger 1807). Two aspects of the resources exploitation will be presented. The first (Section 1 and 2) is related to factors influencing foraging; the second (Section 3), instead, concerns wasp nesting habits. In particular, Section 1 provides information on the evolution of prey specialization through an in-depth analysis of diet preference in the ‘generalist’ beetle-hunting wasp C. rubida. Section 2, describes and quantifies a few factors, as wasp and prey size and weather condition, influencing foraging patterns of the digger wasp C. arenaria. Finally, Section 3 concerns the description of mud-balls building in 5 species of Sceliphron to highlight the differences, if any, which may exist in globule formation ethology. On the whole, the presented analysis investigate some main aspect of wasp environmental relations, for which, up to date, scanty information is available: prey selection, factors influencing wasp foraging pattern behaviour and, at last, ethological differences among species of the same genus in relation to a very stereotyped resource exploitation habit. Throughout this thesis, particular attention was paid to statistical aspects. Distributions of data as well as inferential models have been carefully studied and chosen. Hereafter, are briefly summarized aims and main results of the three sections listed above: Section 1: Opportunism and specialization appear to be widespread in apoid wasps, although the factors affecting the diet preference (and thus explaining the degree of specialization) are still largely unknown. Four hypotheses that stressed the importance of the size, sex, habitat, and taxonomic identity of prey of the beetle-hunting digger wasp C. Rubida were formulated and tested. The wasp population hunted for phytophagous beetles belonging to abundant families around the wasp nesting site. In practice, the prey appeared to be hunted only in two cultivated fields, thus habitat accounted for a majority of the observed diet. The size of wasps was correlated with that of their prey, accounting for the frequencies of hunted prey and the strong individual specialization for both taxa and size. However, some species were significantly over-hunted than expected and others significantly avoided by the wasps, causing an unexpected major role of prey taxon over the other explanatory variables (body size, body shape, sex, availability). This contrasts to that found in other wasp species for which prey selection is essentially based on their ecology and size or their relative abundance (opportunism). The result shows that even an apparent ‘generalist’ predator may turn out to be taxonomically specialized. One suggests that the effect of size constraints and developmental plan of prey (holometaboulous versus hemimetabolous) may have promoted either taxonomic opportunism or specialization in different lineages of apoid wasps. Section 2: Foraging patterns of digger wasps are strongly influenced by both size and weather conditions. For example, previous studies have shown that, in a North-European population of the solitary, weevil-hunting wasp C. arenaria, larger females benefit of a greater resistance to cooler temperature and make more provisioning trips per day. However, in a South-European population, smaller females have the highest provisioning rate per day, suggesting that some key factors interfere differently with the size-temperature-foraging relationship in the two areas. One has investigated the role of prey size, and its interaction with wasp size and weather conditions, on foraging patterns in this southern population. No relation has emerged between wasp head width and temperature for any of the foraging variables examined. Instead, prey size and weather conditions affect foraging. Firstly, wasps decrease the number of prey with the increase of their size, i.e. smaller prey were brought more rapidly (number/day) to the nest. Secondly, the first nest departures in a day were more frequently registered at lower temperatures. Thirdly, shorter foraging flights occurred more often at lower temperatures and longer flights at higher temperatures. These results suggest that the role of weather conditions could affect prey size-related behaviour (and then availability) more than wasp size-related behaviour, and in turn the foraging patterns of wasps. The much higher size variance (and species diversity) of prey in the Italian site compared to the northern population, and the generally hotter conditions in the southern site (in particular in the early morning) support this hypothesis. Section 3: Mud-dauber wasps of the genus Sceliphron build their aerial nests using mud collected at humid-soil sources, which carry to the nest in form of spherical mud balls. The analysis of a series of video-recordings of the 5 species of Sceliphron present in Italy was performed in order to compare their mud-ball building behaviour. Four phases were recognized common to all species: I) patrolling the ground in search of a suitable place for mud-ball building, II) removing leafs, twigs or small stones in the chosen restricted area, III) forming the mud-balls, and IV) flying off with these last ones. The duration of all these phases were remarkably similar among species, with the only exception for flying off which is shorter in S. curvatum. No particular attraction or repulsion was observed among wasps at the moment to chose the mud-ball formation area. Instead, major differences emerged in the way the ball was built. In particular, S. curvatum works the mud with the mandibles and the first pair of legs bringing the axis of the body parallel to the ground, while antennae tap quickly on the ball. On the contrary, in the other species the piece of moist soil is lifted below the chin and retained their laterally from the first pair of legs; the ball is shaped by the combined action of legs and mandibles; the body axis is maintained vertical (eventually reaching an angle of >90° in S. spirifex), and antennae are not involved in mud shaping. One suggests that mud-ball formation behaviour can potentially represent a useful character for phylogenetic studies of mud-dauber wasps.