Artificial warthog burrows used to sample adult and immature tsetse (Glossina spp) in the Zambezi Valley of Zimbabwe

Background The biology of adult tsetse (Glossina spp), vectors of trypanosomiasis in Africa, has been extensively studied – but little is known about larviposition in the field. Methodology/Principal Findings In September-November 1998, in the hot-dry season in Zimbabwe’s Zambezi Valley, we used artificial warthog burrows to capture adult females as they deposited larvae. Females were subjected to ovarian dissection and were defined as perinatal flies, assumed to have entered burrows to larviposit, if oocyte sizes indicated >95% pregnancy completion. Perinatal flies were defined as full-term pregnant if there was a late third instar larva in utero, or postpartum if the uterus was empty. All other females were defined as pre-full-term pregnant (pre-FT). Of 845 G. m. morsitans captured, 91% (765) were female and 295/724 (41%) of females dissected were perinatal flies. By contrast, of 2805 G. pallidipes captured only 71% (2003) were female and only 33% (596/1825) of females were perinatal. Among all perinatal females 67% (596/891) were G. pallidipes. Conversely, in burrows not fitted with traps – such that flies were free to come and go – 1834 (59%) of pupae deposited were G. m. morsitans and only 1297 (41%) were G. pallidipes. Thus, while more full-term pregnant G. pallidipes enter burrows, greater proportions of G. m. morsitans larviposit in them, reflecting a greater discrimination among G. pallidipes in choosing larviposition sites. Catches of males and pre-FT females increased strongly with temperatures above 32°C, indicating that these flies used burrows as refuges from high ambient temperatures. Conversely, catches of perinatal females changed little with maximum temperature but declined from late September through November: females may anticipate that burrows will be inundated during the forthcoming wet season. Ovarian age distributions of perinatal and pre-FT females were similar, consistent with all ages of females larvipositing in burrows with similar probability. Conclusions/Significance Artificial warthog burrows provide a novel method for collecting tsetse pupae, studying tsetse behaviour at larviposition, assessing the physiological status of female tsetse and their larvae, and of improving understanding of the physiological dynamics of terminal pregnancy, and population dynamics generally, with a view to improving methods of trypanosomiasis control.
Author Summary Adult tsetse, vectors of trypanosomiasis, have been extensively studied for more than 100 years, but little is known about larviposition behaviour in the field. Pupae are generally collected in the field via arduous searches of putative larviposition sites. Females have never been sampled in the field as they deposit a larva, leading to confusion about the physiological dynamics at the end of pregnancy. We overcome these problems through the use of artificial warthog burrows, where tsetse deposit pupae during the hot dry season in the Zambezi Valley of Zimbabwe. When burrows were fitted with a retaining trap it was also possible to sample perinatal (full-term pregnant and postpartum) female tsetse. Comparisons of the numbers of pupae deposited in burrows without the trap, with the numbers of perinatal flies trapped in burrows, showed that many full-term pregnant female tsetse enter burrows but then leave without depositing a larva. G. pallidipes are more discriminating in this regard than G. m. morsitans. Capture of perinatal females will make it possible, for the first time, to compare the physiological status of female tsetse and the pupa they have just deposited, with important implications for the understanding of tsetse population dynamics.