Light-trap and suction-trap catches of insects in the northern Gezira, Sudan, in the season of southward movement of the Inter-Tropical Front

Abstract

Catches in light-traps adjoining cotton were obtained at the time of seasonal southward movement of the Inter-Tropical Front (ITF) in October, and during most of the following two months. Taxa studied were mostly Orthoptera and moths, many associated with sorghum, others long-distance migrants. Suction-trap catches at three heights up to 50 ft were obtained for short periods in October and November, and aircraft catches at 250 ft were also available on two days. Suction-trap catches of grass-feeding Homoptera suggest that displacement of these insects was associated with changes in wind direction marking movement of the ITF in October. The exact form of the displacement system in relation to the front cannot be reconstructed from catches at a single place, but it seems likely that proximity of the front at or soon after the time of a brief period of crepuscular activity stimulates insects to take flight and rise to 50 ft or more so that they are displaced. In many taxa, light-trap catches showed a regular pattern of increase, with only slight nightly fluctuations from a logarithmic trend, following full moon. Other increases were superimposed on this pattern at times when the ITF passed north of the trap site, and in some taxa particularly when it was far north. The pattern of change after full moon, shown most clearly in taxa with source populations close to the trap, was related to the moon's influence on the range of trap effectiveness. But various qualitative variations suggest that, in addition, aspects of behaviour or development may have adaptive relationships to the lunar cycle; variations include differences between taxa, particularly in timing of catch changes, and increasing proportion and decreasing maturity of females of certain taxa at the time of the regular increases in catch.