Effects of dropleg application technique during flowering of oilseed rape on insect pests

Spraying of insecticides and fungicides during the flowering stage of oilseed rape (Brassica napus L.) could be necessary to control insect pests (such as Ceutorhynchus obstrictus (Marsham) and Dasineura brassicae (Winnertz)) and pathogens (Sclerotinia sclerotiorum (Lib.) de Bary). In contrast to the conventional application technique, dropleg nozzles are kept below the flowering canopy to restrict the exposure of pollinators to active ingredients of insecticides. The Julius Kühn-Institut (JKI) tested the efficacy of different insecticides using the dropleg technique compared to the conventional technique, on insect pests in field trials conducted from 2015 to 2017 in the area of Braunschweig. The crop was treated at full flowering (BBCH 65) for three consecutive years. The tested insecticides were Biscaya (72 g a. i. ha−¹ thiacloprid), Mavrik (48 g a. i. ha−¹ tau-fluvalinate), and Mospilan SG (40 g a. i. ha−¹ acetamiprid). The adults and larvae of oilseed rape pests that drop to the ground were collected using water trays. The emergence of new generation beetles (Brassicogethes aeneus (Fabricius), C. obstrictus) was recorded from BBCH 78 onwards, using photoeclectors. At least twice during each season, oilseed rape plants were assessed for D. brassicae injury. The new dropleg application technique showed similar efficacy on pest insects compared to the conventional spraying technique. Nevertheless, the abundance of pests was not always sufficient to get clear results. Results especially on D. brassicae indicate that the dropleg technique had a comparable efficacy under moderate pest pressure conditions observed in our study. Further research under conditions with a higher abundance of C. obstrictus, as well as research on the effects of dropleg technique on parasitoids of oilseed rape pests, is needed. The population of B. aeneus population was reduced slightly less using dropleg instead of conventional application technique. In all the years, the yields did not differ significantly with regard to the conventional and dropleg application techniques.