DISTRIBUTION AND POPULATION-DENSITIES OF STEM-BORING FLY LARVAE (DIPTERA) AND THEIR PARASITOIDS IN DANISH FARMLAND

Grass samples from various field types in Denmark were dissected for s temboring fly larvae in order to estimate population densities. Oscine lla species were predominant. The results support the assumption that 0. vastator and 0. pusilla are stationary species building up high pop ulation densities in perennial established fields, whereas 0. frit is highly expansive primarily invading annual crops. From an O. frit aber rant larval type four species emerged: O. hortensis, 0. nigerrima, 0. pusilla and an O. frit resembling fly. O. hortensis was primarily foun d in stems of Festuca rubra and F. ovina, but also of Deschampsia flex uosa. Of the stemboring larvae 15.5 % were parasitized by hymenopterou s parasitoids, the wingless braconid Chasmodon apterus was responsible for 67,6 % of the parasitism.