Nutritional states of male tsetse flies (Glossina spp.) (Diptera: Glossinidae) caught in odour-baited traps and artificial refuges: models for feeding and digestion

Abstract

Male Glossina morsitans morsitans Westwood and Glossina pallidipes Austen caught in artificial refuges in Zimbabwe had ca. six times as much haematin and up to 32% more fat than flies from odour-baited traps, but haematin-specific fat levels did not differ significantly between methods. G. pallidipes estimated to have fed < 9 h prior to sampling contained ca. 3.3 mg fat - only 10% less than peak levels. A differential equation model for blood-meal metabolism was developed which described well the changes in fat levels of laboratory G. m. morsitans and the relationship between fat and log haematin in field data. The model predicts a mean feeding interval (T) of 54 – 65 h and mean fat levels of ca. 2.8 mg for G. pallidipes at feeding. When haematin frequency data were analysed as suggested in the literature, estimates of feeding rates and intervals, and of the non-feeding phase, varied with sampling method. Published estimates of activity levels related to feeding suggest a model where feeding rates increase approximately linearly during the trophic cycle. For T = 58 h the model gives good predictions of mean fat levels and variances in both species, with starvation rates < 1% / day. Models with long non-feeding phases predict fat levels up to 40% lower than observed and with smaller variances. For constant feeding rates, fat levels were well predicted for T = 54 h, but predicted death rates (> 5% / day due to starvation alone) were impossibly high. It is suggested that a proportion of tsetse flies with high fat levels feed on mobile hosts early in the trophic cycle and that this effect is more marked for G. m. morsitans than for G. pallidipes. Over-estimates of T result from the failure to consider these tsetse flies and not to errors in the assumed time scale, nor failure to catch high-fat tsetse flies which visit stationary traps.