P-3: The impact of insecticide applications on the mortality of pollen beetle larvae and parasitism in winter oilseed rape buds

Abstract: In 2021 we carried out a trial to understand the degree of parasitization of pollen
beetle larvae in oilseed rape in response to 17 insecticide treatments. Twenty inflorescences
oilseed were collected from each plot. Inflorescences were immediately frozen after sampling
from plots (25 m2 /one plot) and later assessed in the laboratory. Assessments included the
number of flowers per inflorescence, the number of larvae in each flower, the number of flowers damaged by larvae and the number of larvae in inflorescence (analyzed 16,000 specimens). The number of parasitoid larvae and the number of eggs in larval individuals were determined by dissection with usage a binocular microscope. The assessments in field plots were repeated 3 days, 5 days, 7 days after the application of insecticides. The results of this trial demonstrate the efficacy of insecticides on pollen beetle larvae and their impacts on parasitization. Mostly, the registered doses of pyrethroids were used (lambda-cyhalothrin, gamma-cyhalothrin, cypermethrin, alpha-cypermethrin, zeta-cypermethrin, deltamethrin, tau-fluvalinate, etofenprox, esfenvalerate, indoxacarb), two neonicotinoids (thiacloprid, acetamiprid) and other products consired to be ecologicall (spinetoram, spinosad, azadirachtin and orange oil). Monitoring with yellow water traps during the whole season showed the most frequent parasitoid of the pollen beetle wass Tersilochus heterocerus (Tersilochinae, Hymenoptera) around city Šumperk, Czechia (parasitation 25-98 %). Etheric pyrethroids did not reduce the number of larvae in the flowers at all. Neonicotinoids with systemic efficacy significantly reduced the number of larvae and also reduced the level of its parasitation. However, an ’ecological produkt’ – “Radiant” (spinetoram), had no negative effect on parasitation but reduced the number of larvae, indicating potential to decrease the abundance of pollen beetles in the next season.