Understanding Sandfly Sampling Methods: The Sticky Traps are attraction-based and not interceptive sampling tools of Phlebotomus orientalis

Background : It is generally assumed that sticky traps are interceptive sandfly sampling methods, although no previous experimental evidence has supported this assumption. In this study, we test this assumption experimentally for Phlebotomus orientalis , the principal vector of visceral leishmaniasis in East Africa, and provide an explanation for why their collection is highly male-biased. Methods: Three field experiments were carried out in March 2016-June 2019, in Gedarif state, eastern Sudan. In a first experiment, we compared numbers of P. orientalis caught with sticky traps made of black, red, transparent, white, yellow, green and blue A4 size papers that were set simultaneously at different lunar light conditions. In a second and a third experiments, we compared numbers of P. orientalis captured on sticky traps placed side by side horizontally or vertically on the ground or horizontally on a stool. We also made observations on mating behaviour of sandflies following their landing on un-sticky papers placed on the ground. Results: Phlebotomus orientalis showed significant attraction to white, yellow and transparent traps; with negligible numbers caught on the black and the red traps. Similarly, significantly higher numbers of P. orientalis were attracted to the horizontal traps, resulting in 8-fold increase in their yield as compared to the vertical traps. Placing the traps on the stools resulted in significant reduction of this attraction. In contrast to the sticky traps that captured only very few females, our observations indicated that when male sandflies land on un-sticky white paper they successfully lure females and copulate with them. Conclusion: The study demonstrated that for P. orientalis, sticky traps are more attractant-based than interception-based sampling tools and supports the notion that males of this sandfly species probably utilize the bright surface of the papers of the traps to perform mating rituals to attract the females for copulation. However, their pre-mature death in the sticky oil hampers the completion of these rituals and thus the result in failure of the attraction of the females. The findings of the study have important implications for optimization of the ST design for vector surveillance purpose and understanding the behaviour of P. orientalis .