Phenology and spatial distributions of Dasineura brassicae and its parasitoids in a crop of winter oilseed rape: implications for integrated pest management

The phenology and distributions of Dasineura brassicae and its parasitoids were studied in relation to the value of spatial and temporal information for the conservation of parasitoids in integrated pest management. Insects were sampled at 40 spatially-referenced points within a crop of winter oilseed rape and the following crop of winter wheat. In 1999, two generations of mature D. brassicae larvae were collected into water trays placed below the crop canopy, as they fell from rape pods to the soil for pupation. Insects emerging from D. brassicae larval cocoons in the soil were collected in emergence traps pre-diapause (1999) and post-diapause (2000). Spatial distributions were analysed and compared using Spatial Analysis by Distance IndicEs (SADIE). Only 7% of first generation larvae gave rise to emerging insects pre-diapause and 0.2% of first and second generation larvae to emerging insects post-diapause. Parasitoids comprised 42% and 49% of insects emerging from D. brassicae cocoons pre-diapause and post-diapause, respectively. Omphale clypealis was more abundant than Platygaster subuliformis in 1999. Pre-diapause, the start of emergence of both parasitoid species was coincident with the emergence of adult D. brassicae but the emergence of parasitoids was more prolonged. Emergence of O. clypealis post-diapause peaked a month later than either D. brassicae or P. subuliformis. All insects were markedly edge-distributed and spatially associated pre-diapause but only O. clypealis remained edge-distributed post-diapause. The implications for conservation management of parasitoids are discussed.