Effects of within-field and landscape factors on Dasineura brassicae in winter oilseed rape cultivations in Skåne, southern Sweden : a field survey with a special approach to pesticide-free control zones

Abstract: Winter oilseed rape (Brassica napus) is the most common oilseed crop grown in the province of Skåne in southern Sweden, where this study was performed. B. napus is often associated with various insect pests. This study regards two of these; the cabbage seed weevil (Ceutorhynchus obstrictus) and the brassica pod midge (Dasineura brassicae). While direct infestations by C. obstrictus usually do not pose a major threat to B. napus cultivations, the attacks will often facilitate infestation by D. brassicae, which can cause considerable damage and subsequent yield loss through its destruction of pods. This study focuses on within-field and landscape factors that may affect abundance and damage by D. brassicae in 18 rape fields during spring and summer of 2018. Parameters investigated include chemical treatment, abundance of C. obstrictus, landscape complexity (proportion of e.g. forested areas), the proportional area of oilseed rape grown in the previous year, and distance to the nearest previous years’ oilseed rape field. Effects of chemical treatments were assessed by establishing a pesticide-free control zone in each of the study fields. Samples were collected for nearly 10 weeks from April to July. Two field surveys of pod damages were performed during this period; the first during early pod set and the second during late pod set. The results showed that chemical treatment had an effect on damages in early pod set. There was a progression in damage at the field edges to the field interior in early- and late pod set and in damages early in the crop season to later in the season. No relation between damages and the abundance of C. obstrictus and D. brassicae was found, nor between abundances of the study organisms. The gender ratio of D. brassicae displayed more males than females. An exceptionally warm and dry climate during the spring and summer of 2018 did have an impact on this study. The weather conditions affected the growth of the rapeseed plants and complicated the surveys of damages. The climate may also have affected the abundances of weevils and pod midges; indeed, a considerably lower number of weevils during this season compared to the number of weevils in a study carried out during the season of 2017 was shown. Analyses of damage between regions within the province showed no significant differences between them, and landscape complexity was not correlated to abundances of the study organisms or damages by D. brassicae. Positive correlations between the proportion of rapeseed fields from the previous year and damage by D. brassicae could be reported, however, these relationships were not consistent between damages in early- and late season or between damages at the field edge and in the inner parts of the fields. The distance to the nearest previous year's rapeseed field could not explain the abundances or damages, but a probable cause for this is the dense cultivation of rapeseed fields in Skåne.