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An undersowing system with additional intercropping of flowering plants was assessed in field trials in Germany and Japan to estimate regulating effects on pests and possible negative effects on white cabbage (Brassica oleracea var. capitata). In particular, we tested cabbage undersown with wheat (Triticum aestivum L.) and cabbage undersown with wheat plus additional sweet alyssum (Lobularia maritima L. Desv.) intercropping. Counts of the aphid species Brevicoryne brassicae (L.) and Myzus persicae (Sulzer), as well as related predators on cabbage plants, were determined. Abundance of Phyllotreta spp. flea beetles and their feeding damage on cabbage plants were recorded and cabbage yield was compared. In both countries, trials showed that wheat undersowing reduced the abundance of M. persicae but not B. brassicae. The occurrence of natural enemies on cabbage plants was not significantly affected by any of the companion plants. Additional sweet alyssum intercropping increased the abundance of adult hoverflies at the German but not at the Japanese location. However, it also significantly increased flea beetle infestation on cabbage plants at both locations. Neither wheat undersowing nor additional sweet alyssum intercropping significantly reduced cabbage harvest weight. In conclusion, adding companion plants can be a promising method to improve pest control in vegetable crops. However, intercropping crucifer crops with sweet alyssum may not be recommended in regions where flea beetles are a relevant pest because of the observed enhancing effect on them. In contrast, to prove the positive effect of wheat undersowing on white cabbage, results from further years of investigation are needed. [ABSTRACT FROM AUTHOR]