Behaviour and activity patterns of the scuttle fly Megaselia oxybelorum Schmitz (Diptera : Phoridae) at nest aggregations of two host digger wasps (Hymenoptera : Crabronidae)

We report on a field study of the behavioural ecology of Megaselia oxybelorum Schmitz at nest aggregations of its hosts, the digger wasps Philanthus triangulum F. and Cerceris arenaria L. (Hymenoptera: Crabronidae). The flies flew across the P. triangulum nesting site both as single individuals and as females and males paired in copula, and the former case was recorded more than twice as often as the latter, while only single individuals were recorded at the C. arenaria site. Individuals both alone and in copula were seen at the P. triangulum site during the day roughly coinciding with the host provisioning activity, while at the C. arenaria site the fly's daily activity followed a bimodal trend in contrast to the normal distribution of the host provisioning. Visits of host nests were frequent at the P. triangulum site and null at the C. arenaria site. Single individuals spent less than 1 min inside a nest, while a female entering while still in copula spent generally 1-3 min inside, males exiting after 1-5 s, showing that only in this second case was an oviposition possible. The number of Megaselia oxybelorum increased with increasing host nest density and decreasing nearest neighbour distances of nests. Behavioural patterns of M. oxybelorum, when compared to other Megaselia spp. associated with fossorial Hymenoptera, showed differences possibly related to the biology of the hosts. In addition, some morphological variation within and between host sites are discussed.