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Observations On the Flight Activity of the Red Locust, Nomadacris Septemfasciata (Serville)

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[From 1953 to 1956 observations were made on the flight activity of non-swarming populations of the Red Locust, Nomadacris septemfasciata (Serville) in the Rukwa Valley, Tanganyika, a recognised outbreak area of this species. An attempt was made to obtain quantitative as well as qualitative data on flying locusts. Observations were also made of air temperatures and locust body temperatures, relative humidity, wind speed and light intensity. No flight was seen at air temperatures below 18°C. and it is considered that the minimum permissive thoracic temperature for forced flight was about 20°C. Extensive spontaneous flying was common only between air temperatures of 29°C. and 35°C., being reduced at higher temperatures. Sustained flight usually occurred only in bright sunshine. Flight was seen at all relative humidities from 12% to 80% and a seasonal variation in the humidities at which flight occurred appeared to result from a close correlation of humidity with temperature. Marked displacement of whole populations was observed only when the saturation deficit was low. Extensive flying occurred only at wind speeds not exceeding 3 m.p.h. whereas, in general, locusts that had fed recently flew less. The state of maturity also affected flight, which was least in fledglings and mature females. High-density populations flew in conditions considered unsuitable for flight in those of lower densities. Fanning was seen to occur mainly at temperatures considered too low for flight. It was regarded as a response to stimulation in conditions that raised the threshold for flight. Take-off occurred in lulls in the wind and was probably initiated by a sharp rise in body temperature following the dropping of the wind. At higher wind speeds the speed itself may be important. Other factors causing sharp temperature changes may also promote take-off. The movement of locusts from the roosts was associated with the first marked increase of body temperature relative to air temperature. It was regarded as a special case of take-off promoted by a sharp increase in temperature, all the locusts being similarly conditioned to temperature at this time of day. After moving from the roosts, the locusts were less active, the reduction in activity being associated with a period of feeding and also with temperatures relatively high for locusts conditioned overnight to low temperatures. Extensive flight started later in the day. In March-June, population displacement resulted, but later in the year the activity did not lead to marked displacements. The beginning of extensive flight was related to the end of feeding and possibly also to conditioning to higher temperatures. Late in the afternoon the migratory flights became less extensive and led to roosting. This was always associated with falling body temperatures. Flight stopped at temperatures much higher than those at which it began. This was probably due to conditioning, as was the seasonal variation in the temperature at which the last flight occurred. Active feeding at this time may also have reduced flight. Flight direction was studied, using the mirror method to provide quantitative data. Wind speed and direction were found to be important. At low wind speeds, the locusts were orientated predominantly upwind, at higher wind speeds predominantly downwind. The evidence suggested that the sun was not important in the determination of orientation, but that it might assist, through a sun-compass reaction, in maintaining a particular orientation despite short term fluctuations in wind direction. Humidity probably played no part as a directing influence during these observations, but it could not be excluded as a possible factor under some circumstances. Landscape features were thought to be unimportant in orientation except in so far as they affect wind speed and direction. The finer structure of the grasslands, however, probably elicited an optomotor reaction which made orientation to wind possible. Movements to and from the roosts were visually directed. Gregarious alignment helped to maintain a given orientation, while mutual stimulation promoted flight in high winds and so might be virtually responsible for a downwind orientation at high densities. Displacements of the locust population were studied by marking and recovery methods. The results supported the conclusions drawn from the study of factors affecting flight direction. Locusts in low density moved into the wind. Having reached particular areas, characterised by large stands of Echinochloa, they became relatively static and concentration resulted. The high-density populations were displaced downwind since they often flew in winds too high for upwind orientation on the optomotor theory. The buildup of swarm-density populations by "real" concentration of adults appeared to be a characteristic of Nomadacris in the Rukwa Valley., From 1953 to 1956 observations were made on the flight activity of non-swarming populations of the Red Locust, Nomadacris septemfasciata (Serville) in the Rukwa Valley, Tanganyika, a recognised outbreak area of this species. An attempt was made to obtain quantitative as well as qualitative data on flying locusts. Observations were also made of air temperatures and locust body temperatures, relative humidity, wind speed and light intensity. No flight was seen at air temperatures below 18°C. and it is considered that the minimum permissive thoracic temperature for forced flight was about 20°C. Extensive spontaneous flying was common only between air temperatures of 29°C. and 35°C., being reduced at higher temperatures. Sustained flight usually occurred only in bright sunshine. Flight was seen at all relative humidities from 12% to 80% and a seasonal variation in the humidities at which flight occurred appeared to result from a close correlation of humidity with temperature. Marked displacement of whole populations was observed only when the saturation deficit was low. Extensive flying occurred only at wind speeds not exceeding 3 m.p.h. whereas, in general, locusts that had fed recently flew less. The state of maturity also affected flight, which was least in fledglings and mature females. High-density populations flew in conditions considered unsuitable for flight in those of lower densities. Fanning was seen to occur mainly at temperatures considered too low for flight. It was regarded as a response to stimulation in conditions that raised the threshold for flight. Take-off occurred in lulls in the wind and was probably initiated by a sharp rise in body temperature following the dropping of the wind. At higher wind speeds the speed itself may be important. Other factors causing sharp temperature changes may also promote take-off. The movement of locusts from the roosts was associated with the first marked increase of body temperature relative to air temperature. It was regarded as a special case of take-off promoted by a sharp increase in temperature, all the locusts being similarly conditioned to temperature at this time of day. After moving from the roosts, the locusts were less active, the reduction in activity being associated with a period of feeding and also with temperatures relatively high for locusts conditioned overnight to low temperatures. Extensive flight started later in the day. In March-June, population displacement resulted, but later in the year the activity did not lead to marked displacements. The beginning of extensive flight was related to the end of feeding and possibly also to conditioning to higher temperatures. Late in the afternoon the migratory flights became less extensive and led to roosting. This was always associated with falling body temperatures. Flight stopped at temperatures much higher than those at which it began. This was probably due to conditioning, as was the seasonal variation in the temperature at which the last flight occurred. Active feeding at this time may also have reduced flight. Flight direction was studied, using the mirror method to provide quantitative data. Wind speed and direction were found to be important. At low wind speeds, the locusts were orientated predominantly upwind, at higher wind speeds predominantly downwind. The evidence suggested that the sun was not important in the determination of orientation, but that it might assist, through a sun-compass reaction, in maintaining a particular orientation despite short term fluctuations in wind direction. Humidity probably played no part as a directing influence during these observations, but it could not be excluded as a possible factor under some circumstances. Landscape features were thought to be unimportant in orientation except in so far as they affect wind speed and direction. The finer structure of the grasslands, however, probably elicited an optomotor reaction which made orientation to wind possible. Movements to and from the roosts were visually directed. Gregarious alignment helped to maintain a given orientation, while mutual stimulation promoted flight in high winds and so might be virtually responsible for a downwind orientation at high densities. Displacements of the locust population were studied by marking and recovery methods. The results supported the conclusions drawn from the study of factors affecting flight direction. Locusts in low density moved into the wind. Having reached particular areas, characterised by large stands of Echinochloa, they became relatively static and concentration resulted. The high-density populations were displaced downwind since they often flew in winds too high for upwind orientation on the optomotor theory. The buildup of swarm-density populations by "real" concentration of adults appeared to be a characteristic of Nomadacris in the Rukwa Valley.]

Affiliations: 1: International Red Locust Control Service, Abercorn, Northern Rhodesia

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