Does the assessment of eggs in Ceutorhynchus pallidactylus and C. napi ovaries in spring make possible a date for one common spray treatment effective against the weevils and even pollen beetles?
Abstract: This study aims to improve timing of foliar insecticides against cabbage stem (Ceutorhynchus pallidactylus) and rape stem (C. napi) weevils at localities where C. pallidactylus usually highly predominates in relation to C. napi during spring. We also aim to address the question whether the stem mining weevils and pollen beetles could be controlled with one spring application when the spray timing is based on proper monitoring of the pests in oilseed rape crops. In the case when only one spring application was used, pyrethroids showed better results in decreasing the levels of stem damage induced by the larvae when they were applied earlier (first females prepared for oviposition). Conversely the combination of organophosphate + pyrethroid is better applied later (when more than 50% females are prepared for oviposition). The use of a combination of organophosphate and pyrethroid makes the delay in the date of first spring spraying more well-founded and it is more suitable when a farmer plans to combine control of stem weevils with control of pollen beetles. So, not only the detailed monitoring of flight activity and the subsequent dissecting of stem weevil females but also the choice of insecticide with substantially longer residual activity is important for the possibility to delay the first spring spray to later dates (and later growth stages).