Density and mortality of overwintering Oscinella frit and other oscinellids in Swedish grasslands

Aiming to assess the mortality of the overwintering generation of Oscinella frit, populations of oscinellid flies and their parasitoids were sampled d uring five consecutive years, before and after overwintering. Samples were taken in permanent pastures, leys and ryegrass plots in Sweden. Densities o f O. frit were higher in pastures than in leys. The percentage of oscinelli ds parasitized showed little variation among years, but was higher in ryegr ass plots (37%) than in pastures (14%). The parasitoid complex was dominate d by one species, Rhoptromeris heptoma. The percentage decline in density o f unparasitized O. frit from autumn to spring ranged from 5 to 65% during t he five studied winters. This mortality was not related to O. frit density in autumn, nor were any weather effects found. My results suggest that the yearly variation in mortality of overwintering O. frit is moderate and that , consequently, during most years it should be possible to predict spring p opulation size based on population estimates made the previous autumn.