The challenge of targeted and effective insecticide treatment of flowering oilseed rape (Brassica napus L.) avoiding non–target effects on pollinators and ichneumon parasitic wasps by using dropleg technique

Abstract

Oilseed rape (Brassica napus L.) is damaged by plenty of insect pests and is therefore regularly treated with insecticides. During the flowering period, the cabbage seedpod weevil (Ceutorhynchus obstrictus (Marsham)) and the brassica pod midge (Dasineura brassicae (Winnertz)) migrate into the crop. Their larvae can cause crop damages of up to 30%. In this context, the cabbage seedpod weevil serves as a precursor for a strong infestation by the brassica pod midge. Therefore, the threshold for an application of insecticides is based on the extent of the cabbage seedpod weevil infestation at full flowering stage. Yet, there is a lack of integrated pest management strategies, especially regarding the brassica pod midge. The application of insecticides during the flowering of oilseed rape potentially endangers pollinators and many parasitoid ichneumon flies, which are present in the crop during this time. Residues of pesticides can be ingested by honey bees (Apis mellifera L.) and may result in contamination of bee products such as honey or pollen. With the so-called dropleg technique, pesticides can be applied spatially targeted below the flowering canopy of the crop. Thereby, pesticide residues in nectar and pollen can be minimized and beekeepers’ associations promote this innovative application technique. However, there has been a lack of data on the effectiveness of insecticides applied using the dropleg technique on pod damaging insect pests. In addition, non-target effects on important parasitoids of the pollen beetle (Brassicogethes aeneus Fabricius) and the cabbage seedpod weevil need to be studied.

In five years of field trials (2016–2020), the dropleg technique was tested near Braunschweig, Germany, under standard practical conditions in large-scale plots and compared with conventional application from above into the flowering plant stand. The applications of insecticides from the group of neonicotinoids (Biscaya (a.i. thiacloprid, 72 g ha^-1) and Mospilan SG/SL (a.i. acetamiprid, 40/42 g ha^-1)) and a pyrethroid (Mavrik (a i. tau-fluvalinate 48 g ha^-1)) were carried out at the peak flight of the brassica pod midge (BBCH 65–67). The efficacy of the applied insecticides on pod damaging insect pests was investigated using water trays to collect larvae and adults dropping from the plants. Additionally, full plants were examined twice in each season for damaged pods. Effects on the new generations of insect pests were examined using photoeclectors. In order to derive possible side effects of the dropleg application on parasitoids, the vertical distribution of the oilseed rape insect pests and their parasitoids was investigated with the aid of a portable aspirator during the flowering period in 2018 and 2019. In parallel, the parasitisation rates of pollen beetle larvae in the dropleg field trials were assessed from 2016–2019. Additionally, in 2019 and 2020, the parasitisation rates of cabbage seedpod weevil larvae were determined.

The results showed, that the effectiveness of insecticides applied with dropleg technique in the field trials was slightly reduced compared to conventional application technique. This is in accordance with the investigations on the vertical distribution of pod damaging insect pests, which were present in the flowering canopy at 80% of all cases. The vertical distribution of parasitoid species was not independent from the crop layers and differed species-specific. With regard to the larval parasitoids of the pollen beetle, hints for a niche separation of key species Phradis interstitialis (Thomson) and Tersilochus heterocerus (Thomson) was found. The latter was abundant at 84% of all cases in the flowering canopy, whereas P. interstitialis was collected mostly below the horizon of pods and open flowers. In the field trials, parasitisation rates of pollen beetle larvae by T. heterocerus were significantly reduced after conventional application in three out of four years. The use of dropleg-technique resulted in reduced parasitisation rates compared to the untreated control only in the year 2019. The strength of the observed effects can be explained by the coincidence between the immigration of the parasitoids into the crop and the date of application. Regarding the cabbage seedpod weevil, average parasitisation rates of the larvae were about 60%. Only in 2019, treatment effects occurred after the application of Biscaya and a weak, positive effect of dropleg application on the species Mesopolobus morys (Walker), but not on Trichomalus perfectus (Walker), was found. The evaluation shows, that there is a trade-off between a slightly reduced efficacy on pod damaging insect pests on the one hand and benefits for certain parasitoid species on the other using dropleg technique for the application of insecticides in flowering oilseed rape. In general, insecticides applied during the flowering period of oilseed rape can be crucial in preventing yield losses. However, there is a need of improved monitoring methods for the brassica pod midge. This would help to develop damage thresholds and to prevent superfluous applications in advance.