Introduction

Many North American insects, including several that are important pests, undertake long-distance migrations (Table 2.1). Knowledge of the role of migration in the population processes of these species, and of the relation of migration to climate and atmospheric phenomena, has developed in four broad phases. At first it was simply recognised that serious pest problems in the northern latitudes of the USA were sometimes initiated by populations that appeared suddenly in association with southerly winds. Then in the 1920s and 1930s, airplane sampling of the upper air became possible (Felt, 1928, 1937; Glick, 1939) and it was found that insects, spiders, and mites were commonly to be found at heights up to 1.5 km and occasionally (<1% of specimens obtained) at greater altitudes, even up to ~4.6 km. Over 700 species were caught in these airplane samples, but certainly not all were long-distance migrants. Seasonality, light intensity, temperature, wind speed and direction, and some other meteorological variables were found to be important factors affecting the distribution of insects in the upper air. In the third phase of migration research, during the 1940s, 1950s and 1960s, the association between the sudden appearance of a species in an area and the passage of synoptic-scale (200–2000 km) weather systems was documented in a series of studies.