RESPONSES OF GLOSSINA FUSCIPES FUSCIPES (DIPTERA: GLOSSINIDAE) TO TRAPS IN JUBA, SOUTHERN SUDAN

REACTIONS DE GLOSSINA FUSCPES FUSCIPES (DIPTERA: GLOSSINIDAE) AUX PIEGES A JUBA AU SUD-SOUDAN

 Yassir O. Mohammed¹, Khitma H. El Malik², Mohammed Ahmed M. Musa ³& Intisar E Elrayah⁴.

¹ Department of Tsetse and Trypanosomosis Control, Central Veterinary Research Laboratories (CVRL), Federal Ministry of Science and technology, Sudan.
² Department of Parasitology, Veterinary Medicine and Animal Production College, Sudan University of Science and technology, Sudan.
³Department Of Preventive Medicine and Public Health, Faculty of Veterinary Medicine,. University of Khartoum, Sudan.
⁴Trypanosomosis Unit, Tropical Medicine Research Institute (TMRI), National Center of Research , Federal Ministry of Science and Technology, Sudan.

Résumé

            Les réactions de Glossina fuscipes fuscipes à plusieurs types de piège ont été étudiées à Juba au Sud-Soudan. Le piège biconique conventionnel de couleur bleue attrapait beaucoup plus de mouches que le piège triangulaire Epsilon. Le piège biconique était aussi plus efficace que le piège Vavoua et le piège  monoécran. Le fait de changer  la partie supérieure de la toile blanche du cône avec une toile noire et la partie plus basse du cône bleu avec un coutil en coton n’avait pas un impact significatif sur les captures du piège biconique. Les trois versions du piège biconique étaient nettement meilleures que le piège Nzi pour la capture de Glossina f. fuscipes. Ces résultats sont discutés par rapport à la lutte contre la maladie du sommeil au Sud-Soudan à l’aide du piégeage des mouches tsétsé.

Summary

            The Responses of Glossina fuscipes fuscipes to several trap designs were studied in Juba area, Southern Sudan. The blue conventional biconical trap caught significantly (P<0.05) more flies than the triangular Epsilon trap. Also the biconical trap was more effective than the Vavoua trap and the monoscreen trap. Changing the upper white netting cone with a black netting and lower blue cone with a heavy drill cotton did not influence significantly (P>0.05) the biconical trap catches. The three versions of the biconical trap were significantly (P<0.05) better than the Nzi trap against Glossina f. fuscipes. These results are discussed in relation to control of sleeping sickness in southern Sudan by trapping tsetse.

Introduction:

            Southern Sudan has suffered series of epidemics of Human African Trypanosomosis (HAT), sleeping sickness since the last century. Efforts aimed at controlling sleeping sickness in general depended mainly on case-detection and treatment to sterilize the human reservoir of the disease, but this approach has not been without problems. The major ones are the emergence of drug-resistant trypanosome strains, (Pepin et al., 1995), inaccessibility of people in remote areas, and the uncontrolled movement of refugees and internally displaced people due to civil unrest. Hence there is a need to identify additional approaches to control the disease. Considerable attention has been given to exploitation of devices that suppress the tsetse population, to a level that reduces the disease challenge and consequently decreases the frequency of treatment.

            Efforts to control tsetse via the conventional tsetse control techniques such as insecticides, discriminative bush clearing and eliminating of the potential wild animal hosts constituting significant part of tsetse diet have been discarded for concerns about environment, cost and durability  (Robertson, 1983). The sterile insect technique (SIT), although environmentally friendly its high cost and the prior need to suppress tsetse by other means render it an unlikely sustainable control technique. Accordingly the advantage of control methods, which avoid employing of insecticides, is prerequisite from the environmental point of view. Ingeniously designed devices that attract and capture tsetse flies (Langridge, 1977; Vale, 1981) make it possible to hit the point. Attention has therefore been turned to utilize (Langley and Weidhaus, 1986) bait methods of controlling tsetse flies. Once we consider control alone and not tsetse elimination, suppression trapping becomes the most viable option (Dransfield et al., 1990).

            In the last decade, several discoveries have enhanced the prospects of improving tsetse trap designs (Flint, 1985; Mhindurwa, 1994; Ndegwa and Mihok, 1999; Mihok, 2002) sufficiently for economical control of various Glossina species (Vale et al., 1986; Brightwell et al., 1987; Laveissiére et al., 1987a; Dransfield et al., 1990; Brightwell et al., 1997). The traps low cost, ease of construction (Ryan and Molyneux, 1980) and simple application make them most suitable for community-based tsetse control (Dransfield et al., 1990) operations.

            The present work was conducted to compare the relative efficiency of different trap designs against G. f. fuscipes infesting Juba area for the purpose of selecting the simplest and the most effective design for sampling, controlling and monitoring this tsetse species. The long-term objective is to control Human African Trypanosomosis (HAT) in Southern Sudan using community-based tsetse suppression by traps integrated with case detection and treatment to ensure efficacy and sustainability.

Materials and Methods:

            Five trap designs were used including Epsilon (Hargrove and Langley, 1990), Nzi (Mihok, 2002), Vavoua (Laveissiére and Grébaut, 1990), monoscreen (Okoth, 1991), and white biconical (Challier, et al., 1977). All traps were locally made from Sudanese black cotton. The white Terylene mosquito netting and blue royal (phthalogen) cotton were imported from Kenya as recommended by scientists in the International Center of Insect Physiology and Ecology (ICIPE) and Kenya Trypanosomosis Research Institute (KETRI).  Some black netting and a heavier drill of blue cotton were borrowed from the Entomology Unit, Agriculture and Biotechnology Laboratory, NAAL, Seibersdorf Laboratories, FAO-IAEA, Austria.

Comparison between the biconical and Epsilon traps:

            A preliminary trial was conducted to compare the relative efficiency of the biconical and Epsilon trap. The traps were tested in the three main riverine vegetations including (1) gallery forest, (2) riverine thicket and (3) riverine farm hedge. Randomized 2X2 Latin square design was executed and the experiment was replicated two times. In each vegetation type traps were erected 200m apart and alternated between sites daily in each replicate of two days. Catches were collected every 24 hours. Traps were appropriately greased to prevent ants and lizards from raiding the catch.

Catch compositions in the biconical trap and Ep

            Catches of biconical and Epsilon traps sited in riverine farm hedges during the rainy season were compared for tenerality and sex ratio

Comparison between the biconical, Vavoua and monoscreen traps:

            Since the preliminary experiment has shown that only G. f. fuscipes exists in the study area, the biconical series of traps, namely, the white and blue biconical trap, Vavoua, and the monoscreen were compared for their efficiency in capture of G. f. fuscipes. Nine traps consisting of three biconical traps; three Vavoua traps and three monoscreen traps were used. Randomized 3X3 Latin square designs incorporating the effects of day, site and treatment were used. The experiment was replicated thrice in each of the gallery, hedge and thicket riverine vegetations.  In each vegetation type, traps were erected 200m apart and catches were treated as described in the preliminary experiment above.

Comparison between three modifications of the biconical trap and the Nzi trap:

            Since the blue-white biconical trap (Challier et al., 1977) proved to be highly effective against G. f. fuscipes, an experiment was conducted to further improve the efficiency of this design using two simple modifications of the trap.  Modification 1 (M1), the usual white and blue biconical trap, M2, the upper cone consisted of black netting, M3, the lower cone consisted of a heavier drill of blue cotton.  The Nzi trap was included because it is simpler than the biconical and Mihok (2002) claimed that the trap had been dually effective against both biting flies andtsetse flies including G. f. fuscipes.As in the two experiments above, the experimental design was 4x4 randomized Latin square replicated twice in each of the riverine gallery, thicket and hedge vegetations. Trap siting and precautions and collection were similar to those in the above-mentioned experiments.

Data analysis:

            The pooled catches of males and females (n) were compared by analysis of variance (ANOVA) after transformation of catches as above to log10  (n + 1). Significant differences between trap catches were detected by the Student-Newman-Keuls (SNK) (P = 0.05). Catch indices for each trap were calculated as back-transformed mean catch for the test trap divided by the back-transformed mean catch of the white and blue biconical trap (Challier et al., 1977). Back-transformed means are presented.

Results:

Comparisons between the biconical and Epsilon traps:

            Only one species of tsetse, namely, G. f. fuscipes, was caught during the present and subsequent experiments despite established reports of the coexistence of G. m. submorsitans and G. f. fuscipes in this fly belt (Ford and Katondo, 1977). In the preliminary experiment regardless of type of vegetation, the biconical trap caught up to 6 times more G. f. fuscipes than the Epsilon trap, which was alternated with it for two days in each of the three dominant riverine vegetation types i.e., farm hedges, riverine thickets and riverine gallery forests. These results confirmed the belief that the biconical trap and its modified versions were significantly more effective against species of the palpalis group tsetse, in this case, G. f. fuscipes, than the Epsilon trap (P<0.0008; Table 1).

Catch compositions in the biconical trap and Epsilon trap:    

            Overall the biconical trap caught 92 flies while the Epsilon trap caught only 18 flies with four males only. For this reasons tenerality and sex ratio were compared separately between tenerals and nonteneral in the total catch of the biconical and again between the total catch of the traps by adding one male to Epsilon trap’s males in order to use Chi2- test for comparison  (Table 2). There were no significant differences between the proportions of either tenerals (Chi2 = 2.5; P > 0.114) in males and in females or sex ratio (Chi2 = 2.5; P > 0.114) in tenerals and nontenerals of flies caught by the biconical trap. There was also no significant difference in the sex ratio between flies caught in the biconical trap and those caught in the Epsilon trap (Chi2 = 1.1; P > 0.291).

Table 1: The daily catches (log10 n + 1) of G. f. fuscipes using biconical (B) and Epsilon (E) traps in different riverine vegetation types.

Table 2: Catch composition of G. f. fuscipes in blue biconical trap and Epsilon trap sited during the rainy season in the riverine farm hedge.

Comparisons between the biconical, monoscreen and Vavoua traps:

            The monoscreen and Vavoua traps are simpler and therefore cheaper modifications of the biconical trap than the original design. If any one of these simpler modifications is statistically as efficient as the original biconical trap then the former must be adopted for sampling and controlling of G. f. fuscipes. Comparisons in Juba area showed that the original biconical trap caught significantly more flies (males + females) than the Vavoua and monoscreen modifications. The Vavoua also caught significantly more flies than monoscreen. This result shows clearly that the simpler monoscreen modification was not as efficient as the more complex designs (Table 3)

Table 3: Backtransformed mean catches and catch indices of G. f. fuscipes  in Juba area using three different trap designs.

** P < 0.002

Comparisons between three modifications of the biconical trap and the Nzi trap:

            In this experiment variations in the colours and materials of the upper and lower cones of the original biconical design were made and then compared them with the Nzi Trap.  Table 4 shows that all three modifications of the biconical trap were statistically significantly better than the Nzi trap.  The biconical trap with the black netting, with the heavy drill lower blue cone and the conventional one caught 22.5, 11.6 and 8.1 times more flies than the Nzi trap, respectively. It must be mentioned that the black netting biconical also caught 2.5 and 2 .0 times more tsetse than the conventional biconical or the biconical with the heavy blue drill, respectively.  As with its progenitor (Epsilon) the Nzi trap had performed poorly against G. f. fuscipes.

Table 4: The backtransformed mean catches and catch indices of G. f fuscipes  in the riverine gallery forest using Nzi and three different modifications of the biconical trap.

* Borrowed from Vienna ** index calculated by assuming the Nzi trap catch = 1; ***, significant at P < 0.001

 Discussion:

            It is now accepted that traps are important for both controlling and monitoring tsetse flies (Green, 1994). Novel methods have subsequently been developed to estimate their capture efficiency and range of attraction for several species (Vale and Hargrove, 1979; Dransfield, 1984; Kyorku et al., 1990; Odulaja and Mohamed-Ahmed, 1997). Trap performance is affected by several factors. These include vegetation type (Hargrove and Vale, 1980), location relative to vegetation (Dransfield et al., 1982), host abundance (Van Etten, 1981), time of day (Brady, 1972), changes in weather with time of day (Turner, 1987) and the fly’s physiological state (Rogers, 1978).

            The old distribution maps of tsetse species (Ford, 1963: Ford and Katondo, 1977) reveal that G. f. fuscipes and G. morsitans submorsitans are co-existing with overlapping distribution in the study area.  The biconical trap is highly effective for sampling the palpalis group tsetse including G. f. fuscipes (Challier et al., 1977, Odulaja and Mohamed-Ahmed, 1997) and the Epsilon trap is highly effective against savanna tsetse including G. morsitans subspecies (Hargrove and Langley, 1990), both traps were tested to assess their relative efficiency. Regardless of established reports of the existence of G. m. submorsitans in the study area, only G. f. fuscipes was encountered during this study.  Yet, both of the biconical and Epsilon traps caught G. f. fuscipes but the biconical trap was about 6 times more effective than the Epsilon trap (P<0.0029). This result agrees with Takken (1984) and Snow (1977) who respectively observed that the biconical trap was not effective against G. m. morsitans and G. m. submorsitans. In contrast the latter result disagrees with Hargrove (1977), Vale (1980) and Mohamed-Ahmed et al. (1993) who found that the odour-baited biconical trap had been highly effective against both subspecies. The failure to catch G. m. submorsitans might be attributed to the absence of odour near both trap designs in the present study.  We did not use odour in this work because an odour bait similar to those discovered for savanna tsetse has yet to be found for the riverine tsetse, G. f. fuscipes included. And the latter species is accepted as the main vector of gambiense sleeping sickness in southern Sudan (Snow et al., 1991).

            The catches in the biconical and Epsilon traps were compared for catch composition pertaining to tenerality and sex ratio. There were no significant differences between traps in the proportions of either tenerals or non-tenerals or the proportion of males in tenerals and non-teneral flies (Chi2 = 1.1; P > 0.291).  This result is in agreement with Flint (1985) in the Zambezi valley of Zimbabwe where no significant differences were seen in the catch compositions of G. m. morsitans and G. pallidipes in the biconical, F1 and F2 traps.  Similarly Filledier and Politzar (1985) could not find any significant change in the composition of catches of G. m. submorsitans and G. palpalis gambiensis with the trap type including the biconical in Burkina Faso. Ogwal and Kangwagye (1990) also reported no significant variations in the sex ratio of tenerals and non-tenerals in catches of G.f. fuscipes using pyramidal traps in Buvuma Island, Lake Victoria, Uganda.

            Capture by a trap is the end point of a series of behavioral responses by the fly, including visual appearance and orientation (Vale, 1982). Green (1986) investigated the importance of color in the attraction of Glossina to trapping devices. The use of the royal blue cloth in trap designs is in turn based on studies comparing several different cloth colours (Challier et al., 1977; Ryan and Molyneux, 1980; Gouteux et al., 1981; Green and Flint, 1986; Green, 1988). In the present study the royal blue biconical trap and its modified versions namely the monoscreen and Vavoua traps were compared for their effectiveness in capture of G. f. fuscipes with the objective of selection of the most cost-effective design for control and sampling. The results (Table 3) showed that the conventional design trapped significantly more G. f. fuscipes than the monoscreen and Vavoua. The high efficacy of the biconical trap against G. f. fuscipes had already been reported  (Madubunyi, 1988; Owaga et al., 1988; Odulaja and Mohamed-Ahmed, 1997; Mohamed-Ahmed and Mihok, 1999). It was unfortunate that the conventional blue and white biconical trap was much more effective than its simpler modifications, the Vavoua and the monoscreen traps.

            Since the conventional biconical trap is the most efficient for G. f. fuscipes (loc. cit.) we made small modifications such as a black netting upper cone and/or heavy cotton drill blue lower cone. The conventional trap and its two modifications were subsequently compared with the new Nzi trap (Mihok, 2002) against G. f. fuscipes. The biconical trap or its modifications were highly significantly better than the Nzi design (F = 32.01; P< 0.001). The NZI trap is the latest development in the triangular traps series including the Epsilon trap (Hargrove and Langley, 1990) and NGU2B and NGU2G (Brightwell et al., 1987), which proved ineffective against G. f. fuscipes in the present work and in Kenya (Mwangelwa et al. 1990).

            Accordingly we strongly recommend the use of biconical trap to suppress the population of G. f. fuscipes and consequently reduce the disease challenge. Since there is a need to control sleeping sickness via tsetse control due to the problems usually associated with case-detection and treatment to sterilize the human reservoir.

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