Efficacy of crop cover netting against cabbage pests and their natural enemies and relevance of oilseed rape

Crop cover nets are used in vegetables to reduce damage by pests. Recently, nets with 0.8 mm mesh size were recommended to protect Brassica vegetables from whiteflies, but farmers doubted their efficacy. We compared the impact of different mesh sizes on the abundance of insect pests and their antagonists on Brussels sprout in a field study and considered also important resource habitats, i.e. oilseed rape, in the surroundings. Cabbage whitefly was the most abundant pest followed by peach-potato aphid, cabbage aphid and diamondback moth. Aphid parasitism reached 48 % while syrphid larvae and spiders were the most abundant predatory taxa. The cabbage whitefly was less abundant under 0.8 mm nets in June and August compared to 7 mm nets, while no effect was observed in October. In general cabbage aphid, diamondback moth, syrphids and aphid parasitism were not affected by nets, but peach-potato aphid density increased under 7 mm nets late in the season. In contrast, spiders were less abundant under 0.8 mm than under 7 mm nets. Moreover, only the cabbage whitefly was positively affected by oilseed rape growing. In summary, 0.8 mm nets may be beneficial to reduce colonisation by cabbage whiteflies, especially in early summer and in landscapes with high colonisation pressure. Nets with wider mesh size may be appropriate in landscapes with low colonisation pressure by cabbage whiteflies since 0.8 mm mesh size had the disadvantages to support the peach-potato aphid and hinder colonisation by spiders.