Egression of flies from sewage filter-beds

Fly species that inhabit sewage filter-beds can be a public nuisance and have to be controlled. Chemical and bacteriological agents used currently for such control are washed rapidly from the beds, so, if these agents are to be used efficiently, they should be applied only when an emergence of adults abundant enough to cause a nuisance is expected. The present study examined the phenology of some fly species emanating from filter-beds in ten sewage works across a broad latitudinal range (51.13°N–57.17°N) in Britain, to establish whether environmental temperature alone could be used to predict fly egression patterns. Four species were widespread and abundant: the psychodids Psychoda albipennis (=P.severini) and P. alternata, and the chironomids Limnophyes minimus and Metriocnemus eurynotus (=M.hygropetricus). Average air temperature was a major environmental variable determining the phenology of these four species. When the observed polyvoltine patterns of egression were compared with predicted patterns based on generation time/temperature relationships, they were similar. A poorer match was obtained when the temperature of the settled sewage was used. Although the air temperature regime could be used to predict when the adults of a generation were likely to emerge from the beds, its relationship with the relative abundance of the various generations was less precise; other environmental variables were clearly also significant. It is concluded that larval populations have to be monitored if egression abundance is to be forecast accurately