Impact of oilseed rape on initial colonisation and pre-harvest infestation of Brussels sprouts by cabbage aphid, cabbage whitefly and whitefly parasitoids

Abstract: Most crops are annually recolonised by insect pests from source and overwintering habitats. Detailed knowledge on these source habitats is of vital interest for plant protection. This knowledge can be used for risk assessment of crop fields and for development of pest prevention strategies. Here we evaluate the relevance of oil seed rape farming for colonisation and infestation of cabbage by phloem sucking pests and their natural enemies. We were in particular interested in initial colonisation in July and infestation shortly before harvest in October. Nineteen farms growing organic Brussels sprouts in landscapes with different amounts of oil seed rape were selected. The colonisation process and infestation by pests and natural enemies on Brussels sprouts was monitored from June to November. Results showed that cabbage whitefly (Aleyrodes proletella) and cabbage aphid (Brevicoryne brassicae) were the main pest species in the region. The peak of colonisation was in June for both species, while highest whitefly infestations were reached in October. The abundance of cabbage whiteflies on monitoring plants increased with oilseed rape growing, while no effect on cabbage aphids and whitefly parasitoids could be detected. Generally, an increase in oilseed rape growing offers additional overwintering and reproduction sites for cabbage pests and therefore is supposed to benefit the colonisation on cabbage. We found evidence that oilseed rape growing increased colonisation and peak infestation by cabbage whiteflies. So far no significant effects on cabbage aphid and whitefly parasitoids could be detected. For aphids this may be explained by higher susceptibility of aphids compared to whiteflies to insecticide spraying in oilseed rape. The investigations are being continued taking into account larger landscape areas, temperature and wind data.