none Different levels of attack by hymenopterous parasitoids occurred in stem-boring larval Diptera in swards which had been re-seeded after conventional cultivation or by direct drilling or which had been improved by the addition of nitrogen fertilizer. Numbers of the major parasitoid of established grass, Chasmodon apterus Nees (a wingless braconid), were greatly reduced by ploughing and re-seeding, but were reduced to a lesser extent by chemical desiccation with the herbicide glyphosate and direct drilling. Other (alate) parasitoids showed greater prevalence in seeding grass than established grass. Changes in host population numbers were correlated with the initial level of parasitoid attack.