Hymenopterous parasitoids of stem-boring Diptera (e.g. Oscinella frit (L.)) in perennial ryegrass (Lolium perenne) in Britain

Abstract

AbstractA study was made at Hurley in southern England of the hymenopterous parasitoids of the stem-boring larvae of Oscinella spp. and Geomyza tripunctata Fall, in perennial ryegrass plants {Lolium perenne) taken from swards managed in different ways. Six species of parasitoids were identified after their emergence from stem-borer pupae; four of these were also identified in the parasitoid larval form parasitising stem-borer larvae. Parasitism exceeded 45% during 1980 but was only about 20% in 1981. Of the species of Oscinella, larvae of O. frit (L.) were always the most highly parasitised and those of O. vastator (Curt.) the least, irrespective of their frequency of occurrence. G. tripunctata also showed less parasitism than O. frit. The braconid Chasmodon apterus (Nees) was responsible for 95 % of the parasitism in 1980 and 85% in 1981. The higher parasitism in 1980 probably resulted from greater synchrony between the populations of the adults of C. apterus and those of the stem-boring larvae. Samples from eight other sites in the UK gave results similar to those obtained from Hurley.