Phenologies and diel periodicities of within-crop flight by pests and parasitoids in winter oilseed rape in the UK

Abstract: Parasitoids are important natural enemies of insect pests of oilseed rape (Brassica napus) in Europe yet they are vulnerable to effects of insecticides. Temporal targeting of insecticide against the cabbage seed weevil (Ceutorhynchus obstrictus, syn. C. assimilis) reduces harm to its parasitoids and so benefits conservation biological control. The objective of this study was to establish whether the same principle could be used to protect parasitoids of other pests when insecticides are applied during bud or flowering stages for control of pollen beetle (Meligethes aeneus) or cabbage seed weevil. Yellow water traps in a crop of winter oilseed rape were used to record the phenology of these pests and of three species of tersilochine parasitoids, Phradis interstitialis and Tersilochus heterocerus (both larval parasitoids of pollen beetle) and Tersilochus obscurator (a larval parasitoid of cabbage stem weevil, Ceutorhynchus pallidactylus). The diel within-crop flight periodicities of the pollen beetle and two tersilochine species were established using Malaise traps with automated time-sorting heads and were analysed in relation to meteorological data. None of the parasitoids were active in the crop at bud stage and they would therefore not be at risk from insecticides applied at this recommended time for pollen beetle control. However, all three species were active in the crop at mid-flowering and therefore potentially at risk from insecticides applied for seed weevil control. Peak flight activity of the pollen beetle, T. obscurator and P. interstitialis was around midday and the pollen beetle and T. obscurator showed marked diel periodicity. Flight activity was positively correlated with solar energy and average air temperature and weakly negatively correlated with wind speed. Few insects were caught before 10.00 h. The difficulty of defining days when insecticide applications do not risk significant injury to parasitoids is discussed. It is suggested that parasitoids present in the crop might be less at risk from selective insecticides if applied a times of day when the insects are not flying and that this is worthy of further investigation.