Seasonal variation in nutritional levels of male tsetse flies Glossina morsitans morsitans (Diptera : Glossinidae) caught using fly-rounds and electric screens

A total of 4420 male Glossina morsitans morsitans Westwood were captured on man fly-rounds in Katete District, Eastern Province, Zambia between Februa ry 1991 and December 1993. Of these flies, 1680 were captured before June 1 992, during which period 989 flies were also captured on odour-baited elect ric screens operated in the same area. Non-teneral flies were analysed for fat, haematin and residual dry weight and their wing-vein length was measur ed. There were well marked annual cycles in wing length, fat and residual d ry weight. Flies were biggest at the end of the rainy season, and smallest at the end of the hot dry season. Fat levels were lowest before the onset o f the rains and highest in the cool season. Residual dry weight was a funct ion of haematin content and the degree of wing-fray; these factors were use d to correct the residual dry weight to zero haematin. Corrected residual d ry weight and wing-vein lengths were most highly correlated with relative h umidity in the month prior to capture (r > 0.8 and 0.6 respectively). Corre lations with saturation deficit were weaker; temperature accounted for <20% of the variance. Fly-round flies had a consistently higher residual dry we ight than those from the electric screen, but their fat levels were lower. The distributions of log haematin levels differed little between the two sa mpling methods and were adequately described by a model where capture and f eeding rates increased exponentially after each meal. The increase in the f eeding rate after each meal differed little with season and was closely sim ilar to that estimated for female G. pallidipes Austen in Zimbabwe.