Flights after sunset by the Australian plagye locust, Chortoicetes terminifera (Walk.) and their significance in dispersal and migration

Abstract

Adult C. terminifera take off shortly after sunset, climb steeply into the air, and fly out of sight. This steep take-off differs from that of locusts that take flight during the day. One important factor which induces take-off appears to be decreasing light intensity. Take-off has been recorded at air temperatures as low as 17*5"C, relative humidities that ranged from 9 to 70%, and surface wind speeds (at 2 m) which varied from 1 .7 to 21 -2 ft/sec. Initially the locusts take off into wind. At low surface wind speeds individuals subsequently fly out of sight, dispersing in various directions, but at winds in excess of 6 ftlsec the locusts orient and fly downwind. Hundreds of individuals were seen flying away from concentrations of sexually immature adults on successive evenings in 1969. It appears that flight occurs on practically every evening when temperatures are suitable. Significant numbers of C. terminifera appear at lights during the night only during disturbed weather caused by the approach and passage of depressions. In view of the random orientation of flying locusts with respect to wind at low wind speeds, it is suggested that under dry anticyclonic conditions with slight pressure gradients flights after sunset would lead to widespread dispersal, i.e. scattering of individuals over large areas. The downwind orientation at higher wind speeds, which would result in greater displacement, is associated with the approach and passage of depressions (low pressure systems) and the development of storms. In addition to this the increased turbulence of the winds accelerating ahead of the depression (mainly northerlies) and the convergent air flow from the opposite quarter behind the depression would lead to concentration of the locusts in flight. The hypothesis outlined above would explain the striking changes in distribution and densities that frequently occur among populations of adult C. terminifera. Dispersal under dry anticyclonic conditions, and concentration and mass displacement during periods of disturbed weather during which rain-storms may produce conditions favourable for breeding, play an important part in the survival of C. terminifera. In the major part of its distribution area the probability of effective rain in any particular locality is low.