Dynamics and forecasting of frit fly populations in Sweden

The frit fly, Oscinella frit, is a migrating insect pest which, in Sweden, causes severe damage on oats. This thesis examines the influence of various environmental factors on (1) the number of flies migrating from grasslands to oat fields in spring, (2) the time when flies arrive in oats and (3) their reproductive success after colonising fields. Such data are vital to forecast crop damage. Spring population size is correlated with weather conditions of the previous year. An analysis of samples of frit flies caught by suction traps reveals that spring migrants are more numerous in years following sunny summers. Furthermore, spring migration is completed earlier if the previous autumn was warm. Spring catches can, however, not be predicted from earlier catches. Variation in mortality of the overwintering population seems to be a factor of minor importance for spring population size. During five years, mean parasitism rates of oscinellid flies in late autumn differed little (range 10 to 20%). Likewise, the decline in frit fly densities during winter was relatively constant. Mean winter mortality ranged from 35 to 65%, except for one winter when it was close to zero. Densities of overwintering frit flies are higher in permanent pastures than in leys. This indicate that pastures form the most important overwintering habitats in Sweden. The time of frit fly arrival in oats is determined by spring temperatures. Based on data from water trap catches in oat fields, we constructed a degree-day model for regional prediction of first frit fly occurrence. Validation of the model showed that the prediction errors most often were less than five days. Experimental evidence shows that oats are susceptible to infestation during a limited period. Seedlings are primarly susceptible during DC 11 and 12 (one and two leaves unfolded), but damage may occasionally occur in DC 13. Temperature also affects damage; seedlings are not infested on days with maximum temperatures lower than 14 deg C. In warmer weather, infestation rates increase with temperature. In conclusion, these results imply that weather factors, in combination with crop sowing time and landscape features, are the most important determinants of frit fly damage on oats.