Seasonal aspects of sciarid emergence in arable land (Diptera:Sciaridae)

In April-October 1990-95 soil sciarids emerging in two Danish barley fields wererecorded by means of emergence traps. In 1990-92 the fields were conventionally tilled, in the autumn of 1992 one of the fields was abandoned as annual set-aside (1993) and in 1994-95 most of the field was recropped. In the cropped and uncropped fields 480-527 ind. m(-2) and 369-133 ind. m(-2) emerged, respectively. Ten sciarid species were recorded, eight of which are well-known farmland inhabitants. Apparently, abandonment affected Scatopsciara atomaria negatively, whereas Corynoptera perpusilla was encouraged. The predominant species were probably bivoltine or trivoltine. In Lycoriella castanescens, Scatopsciara atomaria, Corynoptera perpusilla and Bradysia trivittata the female proportion increased significantly during the season. It is assumed that the primary sex ratio of the species is spanandrous. The seasonal change in sex ratio might arise from the unusual modes of reproduction demonstrated in Sciaridae. In Lycoriella castanescens, Bradysia trivittata and Corynoptera perpusilla a significant seasonal increase in wing length was recorded, whereas the wing length of Scatopsciara atomaria and Bradysia rufescens decreased significantly during the season. This suggests a seasonality in the production of large, potentially migrant females. In arable fields sciarid life history traits such as sex ratio, adult biometry and fecundity might be facultative. (C) 2004 Elsevier GmbH. All rights reserved.