The complementary effects of plant resistance and reduced pesticide dosage in field experiments to control the turnip root fly, Delia floralis, in swedes

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SummaryThe integration of different doses of chlorfenvinphos with plant resistance to different levels of Delia floralis attack in swedes was investigated at Saerheim Research Station in field experiments during 1987, 1988 and 1989. The indices of damage and grade 1 yields indicated different levels of resistance ranging from the partially resistant cvs Melfort, Angus and Vige down to the susceptible cvs Gry and Ruta. The relative resistance of the cvs changed with increasing levels of damage. Cvs Melfort and Angus were consistently the most resistant, while the resistance of Vige seemed to break down at high levels of attack. At these levels and in the absence of chemical treatment Gry was as resistant as Melfort and Angus. At the lowest level of attack, the resistance alone gave satisfactory control with Melfort, Angus and Vige. A half dose of chlorfenvinphos was sufficient to protect Melfort and Angus at higher levels of attack. Damage levels recorded in October showed that the different cultivars expressed widely different levels of resistance to D. radicum than to D. floralis.