Abstract The effect of selenium on physiological and biochemical characteristics of broccoli (Brassica oleracea var italica) transplants under controlled conditions and on the plants and some of the brassica pests development under field conditions were studied. For studying the physiological and biochemical response, the transplants were cultivated under controlled conditions and fertilized with various concentrations of selenite or selenate: 2, 5, 10, 20 and 30 mg Se(IV) L−1 or 50 mg Se(VI) L−1. For the field trial only the treatment with 50 mg Se(VI) L−1 was compared to the untreated control. The addition of high selenium concentrations lowered photochemical efficiency of transplants, while 11 days after the exposure, the photochemical efficiency was not affected. Selenium had a minor impact on the amount of chlorophylls, while it increased the amount of anthocyanins. The content of selenium in plants varied between 0.38 and 5.76 μg g−1 (DM) for exposure to Se(IV) and between 2.09 and 45.73 μg g−1 (DM) for exposure to Se(VI). Under field conditions the treatment of broccoli transplants with Se(VI) increased the attractiveness of plants for Delia radicum female adults and Phyllotreta spp. in the first weeks after the selenium treatment. With high pest pressures increased oviposition of D. radicum and increased leaf damage by Phyllotreta spp. occurred which retarded the growth of plants. However, under low pest pressure, the selenium treated plants exhibited better initial growth. Despite increased oviposition of D. radicum in selenium treated plants the number of pupae recovered at harvest was significantly less than in control plants.