Edge distributions of Ceutorhynchus assimilis and its parasitoid Trichomalus perfectus in a crop of winter oilseed rape (Brassica napus)

The edge distributions of Ceutorhynchus assimilis and its parasitoid Tricho malus perfectus within a crop of winter oilseed rape were investigated betw een May and July using yellow water-traps arranged on a grid system. A rand omisation test, which compared the observed arrangement of trap counts with permuted arrangements, was used to quantify the observed tendency of C. as similis to congregate at the edges as it migrated into the crop; this tende ncy declined during the season. Trichomalus perfectus were distributed at t he edge of the crop only during the early phase of its immigration in May, which occurred later than immigration of its host. It is considered that th is simple means of quantifying insect spatial distribution could prove a us eful tool in precision targeting of insecticides.