Reproductive success balanced between abiotic and biotic factors: what is the role of aggregation for solitary wasps? Manuela Giovanetti, Josep D. Asís & José Tormos Área de Zoología, Facultad de Biología, Universidad de Salamanca, 37071 Salamanca (SPAIN) email: manuelag@usal.es This study mainly focused on those elements that may be working on the evolutionary stable strategy of nesting females of a well- established population. Are abiotic factors dominant? Is biotic pressure important on the level of local density? Can they justify the observed aggregation? Our research uses a native wasp as study species: Stizus continuus (Hymenoptera: Sphecidae) is nesting at least since 1985 in a protected area near Valencia (15 km south, Parque de la Albufera), forming the same aggregation in the same place every year. S. continuus is a solitary wasp: each female excavates its nest in the sandy soil near the coast, a fragmented habitat made of bare soil and small bushes. Nests are found on the bare soil as well as under vegetation. Females excavate subterranean tunnels and cells, not very deep, where larvae develop. Paralyzed othopteran prey are provided by the hunting females to the larvae (Fig. 1). Introduction Females success may be influenced by biotic factors as interactions during nesting activity, parasite pressure or other elements affecting the developing larvae inside the nest. We observed 8 females (4-6 hours/day for 6 days) during the excavation of a nest and its provisioning. At the end of the season we checked the nests content. General activities (for example the number of prey, see Table 2) did not present significant differences between females nesting under vegetation or on bare soil. Interactions with other females, males or parasites during excavation or when wasps return with prey did not seem to affect a wasp activity in terms of time lost for interruptions. Also if some differences exist between females nesting under vegetation and the ones nesting on bare soil in the number of cells and their content, these are not significant. Biotic Factors Figure 1: A marked female of Stizus continuus is entering its nest with a prey. Figure 2. Part of the total area observed, representing nests distribustion. Each square measures 3m X 3m. Plants and nests distribution are represented by colours, as explained in the legend. Colours Legend green: plants brown: bare soil red points: wasp nests Conclusions Our interest is understanding females preference and exploitation of two microhabitats offering different conditions. From the nest census it resulted that females prefer to nest under vegetation. In this situation the soil hardness is lower, which may facilitate the first attempt to dig a nest. This will represent an andvantage, as the time required for excavation will be reduced. Actually, the nests established under vegetation show a higher (although not significant) mean number of cells than the ones dug out on bare soil. We have to say that all these nests presented the entrance and a small part of the main tunnel under vegetation, but the cells were located under bare soil (Fig. 3). This is probably due to the thermal conditions required by the larvae development and may justify that in the end no differences exist as nest contents between females nesting in the two situations. Parasites seem to equally affect the nests, indipendently from the entrance location. They will likely use chemical cues to find their host nests; this may be confirmed by the fact that females close very carefully nests on bare soil, leaving no visible sign of the entrance. Another element that may influence females choice is the conspecifics nest density. Conspecifics presence may strongly influence a nesting female, due either to the information transmitted (a good place may result attractive to many females) or to the aggregation habit of the species (many hypothesis have been postulate, as to improve nest defence or steal prey). We think that the analysis of nest densisty may result the most important part of this project, but should still work on this topic and will process data in the future. Figure 3. Excavation of a wasp nest. The entrance was under the vegetation, while the main tunnel resulted under the bare soil. One of the cells, found at about 30 cm from the entrance, is visible. Abiotic Factors Table 2. Differences exist, from a biotic point of view, between the two type of nests, although they are not significant. Table 1. Abiotic factors as temperature, humidity and soil hardness compared for the two situations (under vegetation and on bare soil) The difference between the two resulted significant for each factor. ABIOTIC FACTORS STATISTIC TEST Air temperature Wilcoxon: z = 7,29 P< 0,001 Air relative temperature Wilcoxon: z = 7,12 P< 0,001 Soil temperature (at 13 cm of depth) Wilcoxon: z = 7,58 P< 0,001 Soil hardness Mann Whitney: U =22,0 z = 6,35 P< 0,001 BIOTIC FACTORS NESTS UNDER VEGETATION NESTS ON SPARE SOIL X 2 n. of cells 13,8 7,33 1,98 (n.s.) n. of Stizus pupae 6,8 4,33 0,548 (n.s.) n. of parasite pupae 3,4 1,66 0,598 (n.s.) n.of cells with rests (mortality not due to parasites) 3,6 1,33 1,045 (n.s.) mean n. of captured prey (6 days of observation) 7,8 10,33 0,353 (n.s.) Soil texture is mainly represented by sand, which is covered during the summer by a thin (a few mm) crust of salt. Bushes of Salicornia ramosissima (J. Wooods) and Sarcocornia fruticosa ((L.) A.J. Scott) form restricted areas of intricate vegetation, as mapped in Fig. 2. Under the vegetation cover, temperature and relative humidity of the air as well as temperature and hardness of the soil are lower than on bare soil (see Table 1). At the study site, the 40% of the area was covered by vegetation: 101 m 2 against the 151 m 2 of bare soil. We analyzed the number of nests on bare soil and under vegetation considering the sizes of the two microclimatic situations. We found that a higher number of wasps nested under vegetation, the difference being significant (  2 = 13,44, P<0,01).