Predicting Oscinella frit population densities from suction trap catches and weather data.
Abstract
With the aim of developing a model for the prediction of Oscinella frit populations, migrating adults were sampled using suction traps at two locations in Sweden for 10 and 14 years, respectively. Catches were separated into two periods: spring migration, representing the size of the overwintering generation; and summer migration, representing the total size of one or two progeny generations. The temperature threshold for sustained flight of O. frit was estimated as 14°C, measured as daily maximum temperature. Current weather conditions during the migration periods had weak effects on the seasonal progress of migration. There was, however, a lagged weather effect: spring migration ceased earlier in years following a warm autumn. Although the number of spring migrants could not be predicted from suction trap catches in the previous year, the number was correlated with weather factors in the previous summer. Sunshine conditions early in summer, i.e. from May to July, seemed to be most important. The long time lag between this period and migration in the following spring implied that weather affects O. frit population dynamics indirectly, possibly by influencing the amount of food resources available in the autumn. The number of summer migrants was correlated with the number of spring migrants, except for three observations with very small summer catches due to extremely cold and cloudy weather in early summer. The number of migrants of O. frit varied up to 100-fold among years, a fact that needs to be accounted for in damage forecasts. The number of spring migrants can be predicted fairly well from weather conditions in the previous and current year, whereas the number of summer migrants can, in most years, be predicted from spring catches in suction traps.