Substrate Firmness Controlling Nesting Behavior of Bembix Oculata (Hymenoptera, Bembicinae)

The influence of substrate firmness as shear strength on the nesting behavior of the wasp Bembix oculata is analyzed in “natural” conditions. The experimental area (a landing pit at the athletics stadium in Granada, southern Spain), is characterized by the existence of a non-random distribution of nesting holes made by Bembix oculata. The analysis of sediment features shows homogeneity in some substrate properties such as grain size and mineralogy. However, the quantitative analysis of substrate firmness leads to recognition of a close relationship between shear-strength values and burrow distribution. Experiments performed by Torvane testing in 13 sampling sites reveal changes in shear strength related to variations in water content of the substrate. These changes in shear strength are closely associated with distribution of nests. Thus, a gradual increase in substrate shear strength, over 1000-1500 Pa, is recorded toward the area where nesting holes appear, whereas burrows are most abundant where shear strength is higher (above 2500 Pa). Substrate firmness is revealed to be of prime importance in the nesting behavior of Bembix oculata. The relationship between the obtained shear-strength values and the soil water content could be interpreted as the influence of water content in substrate firmness. However, we do not discard the influence of water content as an independent factor (soil moisture).