A Comparison of the Migratory Strategies of Two Milkweed Bugs, Oncopeltus Fasciatus and Lygaeus Kalmii

Abstract

Views on the nature of migration and dispersal in insects have changed markedly in the past 25 years. Williams (1957) in the 1940s and 1950s was largely responsible for calling attention to the large number of insect species which migrate. However, he stressed that insect migrations were mass occurrences, often triggered by unfavorable conditions, in which flight was unidirectional and under the control of the individual. Dispersal was considered to be mainly a passive and accidental process in which weak-flying insects were scattered by the wind. In the late 1950s, Rainey (1963) demonstrated that movements of African migratory locusts were correlated with prevailing wind patterns and suggested that the direction of migration in these strong fliers was independent of their orientation and thus also passively determined. This led to the questioning of whether or not there was a valid distinction which could be drawn, on the basis of directionality, between the so-called active migrants and passive dispersants. The problem was further complicated when considering the vertical and horizontal components of orientation and the fact that there was then, as there still is, so little data on the orientation of flying insects (Kennedy 1961). Waloff (1972) has recently presented evidence that locusts do in fact orient their direction of flight down wind, thus pointing to the difficulties in attempting to distinguish among dispersal strategies using orientation as a criterion.