How Do Tsetse Recognise Their Hosts? The Role of Shape in the Responses of Tsetse (Glossina fuscipes and G. palpalis) to Artificial Hosts

Palpalis-group tsetse, particularly the subspecies of Glossina palpalis and G. fuscipes, are the most important transmitters of human African trypanomiasis (HAT), transmitting >95% of cases. Traps and insecticide-treated targets are used to control tsetse but more cost-effective baits might be developed through a better understanding of the fly's host-seeking behaviour. Electrocuting grids were used to assess the numbers of G. palpalis palpalis and G. fuscipes quanzensis attracted to and landing on square or oblong targets of black cloth varying in size from 0.01 m2 to 1.0 m2. For both species, increasing the size of a square target from 0.01 m2 (dimensions = 0.1×0.1 m) to 1.0 m2 (1.0×1.0 m) increased the catch ∼4x however the numbers of tsetse killed per unit area of target declined with target size suggesting that the most cost efficient targets are not the largest. For G. f. quanzensis, horizontal oblongs, (1 m wide×0.5 m high) caught ∼1.8x more tsetse than vertical ones (0.5 m wide×1.0 m high) but the opposite applied for G. p. palpalis. Shape preference was consistent over the range of target sizes. For G. p. palpalis square targets caught as many tsetse as the oblong; while the evidence is less strong the same appears to apply to G. f. quanzensis. The results suggest that targets used to control G. p. palpalis and G. f. quanzensis should be square, and that the most cost-effective designs, as judged by the numbers of tsetse caught per area of target, are likely to be in the region of 0.25×0.25 m2. The preference of G. p. palpalis for vertical oblongs is unique amongst tsetse species, and it is suggested that this response might be related to its anthropophagic behaviour and hence importance as a vector of HAT. Author Summary: While the numbers of cases of human African trypanosomiasis (HAT) is now less than 10,000 reported cases per year, progress against the tsetse species that spread the disease is poor, with ∼10 million square kilometres of sub-Saharan Africa still being infested. This widespread persistence of vectors and reservoir hosts threatens the long-term sustainability of recent gains against HAT. Better progress against the vector would be achieved by developing cheap, effective and practical methods of tsetse control. Toward this end, we are improving the design of insecticide-treated targets to attract and kill tsetse. Here we show that for two important vectors of HAT, Glossina palpalis palpalis in Côte d'Ivoire and Glossina fuscipes quanzensis in the Democratic Republic of Congo, small (between 0.25 m and 0.5 m square) targets of black cloth with equally sized panel of fine black netting are ∼10x more cost-effective than the larger (∼1 m square) targets or traps commonly in use.