Tsetse flies CONTROL. Ecological control. Arthropod vectors Tsetse flies

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1 Arthropod vectors Tsetse flies Tsetse flies Author: Dr Reginald De Deken Licensed under a Creative Commons Attribution license. CONTROL Tsetse fly control is only one of the trypanosomosis control methods and not always the most adequate. Unsubsidised, effective and sustainable vector control managed by local communities is often unrealistic. Situations that justify the use of vector control are: a trypanosomosis risk which remains high (e.g. prevalence >10%) notwithstanding regular chemotherapy or chemoprophylaxis, the protection of valuable trypanosomosis-free areas when the aim is to eradicate the disease. Notwithstanding the poor reproductive capacity of the tsetse fly and its unusual high susceptibility to insecticides, results of past tsetse control campaigns were often unsustainable. Major reasons for these failures were: conflicting objectives between governments, local farmers and donors, campaigns aiming control instead of eradication, whereby barriers to stop reinvasion of tsetse flies from neighboring uncontrolled regions were abandoned because too costly to maintain, vector control campaigns not foreseeing participation from the local communities resulting in a complete stop of the activities once the program ended, and insufficient concerted effort among affected countries to control the trans boundary problem of trypanosomosis. In those countries, where eradication is aimed for, complete isolation of the area by natural or artificial barriers as well as political and economic stability is essential in order to reach the objective. The size of the eradication area has to be carefully evaluated through assessment of habitat suitability, risk analysis and entomologic surveys (Bouyer, 2010). When isolation of the eradication zone is impossible, one can try to eradicate the fly in areas where the environment is the most hostile to the tsetse fly (at the edge of the fly s distribution area). When, instead of eradication, control of tsetse and/or trypanosomosis is the main objective it is important to prioritise the areas that are most suitable for an intervention on the basis of agricultural potential and carrying capacity of the soil, human and livestock densities, socio-economic impact of the disease as well as environmental considerations. Tsetse control may be ecological, chemical, biological or genetic. Ecological control 1 P a g e

2 Arthropod vectors Tsetse flies It consists of the modification of the biotope in order to render the biotope less suitable for the tsetse fly. Total vegetation clearance (see Bush-clearing tsetse.jpg), partial discriminative vegetation clearing (mainly used against riverine species), game elimination and game fences, or combinations of these strategies have all proven to be effective in eliminating tsetse. Complete bush-clearing to prevent tsetse to enter Ruanda-Urundi from the Akagera National Park Since these methods are costly and nowadays unacceptable from an environmental point of view, they are no longer in use. However, the gradual expansion of the human population has similar effects since arable land also does constitute an environment unfavourable for tsetse, especially for those of the fusca and morsitans group. The influence of humans on the palpalis group of flies is not always adverse, and these flies can often exist in close contact with people and their domestic stock. Traditional ecological methods such as using smoke and a continuous wooden or netting wall, 1.5 m high, to protect cattle reduce significantly the numbers of tsetse flies feeding on the animals (Torr, 2011). Chemical control When chemical agents are used to combat tsetse flies, it must be taken into account that tsetse flies spend about 50% of their lifespan under the ground as pupae. Therefore, either insecticides must be used, which remain active for at least the maximal pupal period, or repeated treatments with a shortacting product must be foreseen. Another issue is the possible reinvasion by flies coming from untreated areas. This requires the creation of barriers (natural or chemical) between the infested and control zones which constitute an unbridgeable gap for the tsetse fly. In case of artificial barriers, the larger the control zone surface, the smaller the proportion of barrier costs in the total costs. Different methods of chemical tsetse control have been applied: 2 P a g e

3 Arthropod vectors Tsetse flies 1. Ground spraying: Method: Preferably during the dry season, a residual insecticide is applied to the resting and refuge sites of the flies. Spraying is discriminating (only certain types of vegetation are treated, generally 7 to 15% of the total surface) and selective (only the parts of trees and bushes are treated where the tsetse flies rest during the day). Persistent insecticides (DDT 3% and dieldrin 2% at 15 to 60 kg a.i./km 2 ) were used for this purpose. Deltamethrin (w.p.) 0.1% (1 to 3 kg/km²) and alpha-cypermethrin are also effective and less harmful to the environment, but these products are more expensive. Ground spraying may be used to create chemical barriers. Because of environmental concerns and organizational difficulties this method is nowadays seldom used. 2. Sequential aerial spraying: This method includes the application of a non-residual insecticide by airplane (see Aerial spraying plane.jpg) as an ultra-low volume aerosol (droplets of 20 to 50µ) generated by a rotary atomiser. Aerial spraying plane Aerial spraying atomizer for ULV One droplet has to contain sufficient insecticide to kill the least susceptible tsetse fly (some Glossina spp. and especially pregnant females can tolerate higher insecticidal doses). However doses used during aerial spraying must be low enough to have no residual action and negligible toxicity for non-target organisms. Four to seven treatments are carried out. Timing of the spray sequence depends on the duration of the first interlarval period and of the puparial period (and thus environmental temperature). Care must be taken that newly emerged flies are sprayed before they have the opportunity to deposit their first larva. Conventionally spray cycles are scheduled two days short of the first interlarval period and stopped only once two sprays subsequent to the emergence of the last pre spray pupae (one puparial duration) have been carried out. The airplane flies at a height of approximately 20 to 30 m above the tree canopies (therefore a flat terrain is required) and at a distance of 200 to 400 m from the preceding flight line. To reduce aerosol drift, applications are made under conditions of temperature inversion early in the morning or late in the evening (1 h before sunset or up to 2 h after sunrise). This method was shown to be very effective against the morsitans group of tsetse flies in wooded savannah on flat terrain. Major problems with this method are that, unless the sprayed area is isolated or other techniques are used in combination, there is nothing to stop flies invading from 3 P a g e

4 Arthropod vectors Tsetse flies beyond the sprayed area. Equally, if there are any survivors left after the completion of the final treatment, there is no hindrance to the growth of the population. Insecticides that are used are Endosulfan (e.g. 20% E.C.) at 10 to 20 g a.i./ha and Deltamethrin 0.2 to 0.5 g a.i./ha. The technique is fast (a surface of 50 km² is treated in 1 h time), environmentally acceptable and effective but demands a lot of expertise and accurate navigation systems (see: Aerial spraying instrumentarium.jpg) (Kgori 2009). Aerial spraying instrumentarium 3. Aerial spraying by helicopter: An insecticide is applied on the nocturnal resting sites of the tsetse flies. Using this method the most inaccessible sites can be sprayed but the helicopter is the most expensive way to apply insecticides. Therefore generally persistent insecticides are used, in order to obtain a residual effect and thus to reduce costs. Harmful effects on terrestrial and water fauna are to be expected. Insecticides that are used are Endosulfan (100 to 1000 g/ha), Dieldrin (800 to 1000 g/ha), and Deltamethrin (12.5 to 30 g/ha) 4. Insecticide target screens or impregnated traps: This method of reduction of the population of adult tsetse flies make use of systems to capture flies (traps or screens, which act as traps because they are treated with "Temo-o-cid" glue) or to kill flies with persistent insecticides applied to screens or traps. 4 P a g e

5 Arthropod vectors Tsetse flies Target screen spraying Target screens consist of a blue, black or blue and black cloth screen that has been sprayed with insecticide. In some versions the cloth is flanked by bands of insecticide treated black mosquito netting, which is not readily visible to tsetse flies. Various designs of traps have been developed for use against particular target species in particular environments. Biconical and Monoconical trap 5 P a g e

6 Arthropod vectors Tsetse flies Trap Epsilon and NZI Table: Comparison between traps and target screens to control tsetse flies Traps Target screens Manufacturing and price more complicated and costly easier to produce and cheaper than traps Insecticide application not essential essential Size Attraction Efficacy large higher than target screen variable according to species, usually ± 30% if trap is untreated, and > 50% if trap is treated with insecticide Vulnerability high low even very small screens may have a relatively good efficacy (Lindh 2009) less than half the number of flies attracted by a trap variable according to species, usually ±50% of attracted flies alight on screen and are killed Efficacy visualised yes (visible capture) no (flies die not immediately) Impact on non-target organisms low to very low if untreated low Knowledge of tsetse species behaviour essential to ensure good results The major advantages of screens and traps are that: an asset the techniques are rather simple and adapted to the requirements of community participation, it is relatively cheap, easy to produce and with almost no pollution of the environment, it allows the spreading of the operation over time and is easily combined with other methods, it allows some errors without causing the failure of the entire operation but eradication is difficult to attain, 6 P a g e

7 Arthropod vectors Tsetse flies the efficacy of the technique especially against flies of the group morsitans can substantially be improved using attractive odours, it can be used to produce barriers to prevent reinvasion of tsetse flies. Target screen barrier The major disadvantages of screens and traps are: its effectiveness, which is variable according to the species (e.g. G.pallidipes is 4 times more susceptible to a control by targetscreens that G.m.morsitans which generally requires 4 screens/km 2 ). Far more traps and screens are necessary against riverine tsetse than against savannah species as riverine species are less attracted to odours, the larger the control area the better the results but the more difficult the management of traps and screens, sometimes many access roads have to be created (pernicious in game reserves), maintenance of the traps and screens is essential if the control has to be continued. However, community participation fades once control has diminished disease impact, with the current knowledge these methods do not always lead to eradication, the type of landscape which does not always allow the use of the screens or traps, problems of theft (necessity to inform the community) and of degradation of the materials (e.g. fires, wildlife, swelling of rivers). Insecticides: pyrethroids (deltamethrin, alpha-cypermethrin, lambda-cyhalotrin) are usually used to spray screens or traps. Immersion of the screens in an insecticidal solution for 15 minutes and afterwards drying them on a horizontal surface is preferred to simple spraying. Glossinex (deltamethrin 19% + UV-absorber) is used at concentrations up to 1g deltamethrin/m² in order to reduce the number of sprayings (once a year) and avoid maintenance. The frequency, with which the insecticidal spray on the screen must be renewed, depends on the insecticide (type, 7 P a g e

8 Arthropod vectors Tsetse flies formulation and dose) but also on the quality of the fabric used to manufacture the trap or screen and on the season. The chitin synthesis inhibitor triflumuron (Langley, 1995) and the macrocyclic lactone "spinosad" (De Deken et al., 2004) are alternative insecticides should it become undesirable to use pyrethroids. Attractants: Kairomones that are used in the field to attract tsetse flies to traps or targets are acetone, methyl ethyl ketone (butanone), 1-octen-3-ol (octenol), 4-methylphenol, 3-npropylphenol and 3-methylphenol. Not all of these substances are effective against all species and not all need to be included in a bait for it to be effective (Vale et al; 1988; Vale & Torr, 2004; Rayaisse et al, 2010). The way in which tsetse flies move through the habitat has an important bearing on where traps and targets should be positioned so as to be most effective (Vale, 1998; Kuzoe & Schofield, 2004; Bouyer et al, 2010). Density of baits: The use of olfactory products makes it possible to reduce considerably the density of the baits for the control of tsetse flies of the morsitans group. A density of 2 to3 screens/km² for G.pallidipes and 4 to 5 screens/km² for G.morsitans is sufficient to control these flies. In Burkina Faso 33 biconical traps or 25 blue-colored screens/km² were used against G.m.submorsitans, but by adding olfactory products the density could be reduced to 5 traps or screens/km². For riverine tsetse flies (e.g. G.p.gambiensis and G.tachinoides) the screens (or traps) are posted along the river banks and in isolated forest galleries at a rate of 10 to 12 screens/km or 3 to 10 traps per km. It is often necessary to withdraw the traps or screens during the rainy season in order to prevent that they are carried away by the flood. However, if the biotope of flies belonging to the palpalis or fusca groups is not strictly riparian, screens and traps have to be used at much higher densities, since these flies are poorly attracted by odours and some species in this group (e.g. G.fuscipes) are not very mobile. In the human trypanosomosis focus of Vavoua 180 blue screens/km² were used to control G.palpalis in its biotope (primarily cocoa and coffee plantations). The screen density could be decreased to 32 screens/km² by using screens with blue and black bands side by side. In Uganda to control G.f.fuscipes 8 to 10 pyramidal traps/km² were set up at the transition between forest and other biotopes (Lancien, 1993). Creation of barriers using target screens: The barrier surrounding the control area to prevent invasion of tsetse flies must be created before the start of the control campaign (for example control by aerial spraying). Broad barriers (4 to 8 km), having a width of up to 8 times the distance covered by the tsetse species in 1 day and made up of a number of screens or traps/km² equal to or twice the usual density, are to be preferred on small barriers with higher screen densities (Hargrove, 1993). 8 P a g e

9 Arthropod vectors Tsetse flies 5. Insecticide treated hosts: In livestock breeding areas tsetse flies can be controlled by dipping the cattle regularly (e.g. every two weeks) in a solution of 0, 0038% to 0,005% of deltamethrin (Decatix 5% SC), applying up to 20 times/year a pour-on insecticide (Cylence 1%, SpotOn 1%, Renegade 1, 5%, Bayticol 1%), or spraying the animals every two weeks with 0,005% of deltamethrin (Decatix) using a portable compressed air sprayer. Such treatment frequencies are too expensive for most African cattle owners. Treatment frequency may be decreased when the area is not subjected to invasion. However, since 75-90% of tsetse flies feed on the legs or belly of cattle, spraying may be restricted to these body parts of all animals in the herd or only of the larger animals in the herd (Torr, et al, 2007). This strategy reduces insecticide costs by 40 to 80%, with only a 20% to 30% reduction in efficacy and makes restricted application affordable for poor livestock owners. These treatments will in most situations have no or little impact on endemic stability against local tickborne diseases. Treatment of only a part of the herd is cost beneficial as the percentage of tsetse fly feeds from an animal is correlated to its live weight. However, depending on the region and its local customs the benefits of this particular strategy may vary heavily as Masaai graze their adult cattle often separately from the calves while small stock of the Shona in Zimbabwe graze all their livestock together. If the application of insecticides on cattle is used as the sole method to control the vector, the zone, in which the cattle have to be treated simultaneously, must be significant. The effectiveness of the method can be reduced in case of alternative hosts, such as an abundant wild fauna and village herds not treated simultaneously, or when the treated cattle do not penetrate all the infested tsetse biotopes (Van den Bossche et al, 2004). In absence of behavioral resistance the applied insecticide has no influence on the number of tsetse flies attracted towards the host or on the percentage of tsetse flies taking a meal on the treated animal. Thus insecticide treated animals are not completely protected from trypanosomosis. Through the years chemical control of tsetse changed gradually from area-wide application of insecticides towards a more targeted control using insecticide treated hosts, traps or target screens. These devices attract and kill tsetse with limited environmental impact and allow participation of local communities in the control. However, it is still not sure that these devices may eradicate all tsetse flies. Especially in case of riverine species the Sterile Insect Technique may be required to obtain complete eradication after reduction of the tsetse population by insecticides. Biological control Predators: Many animals (e.g. lizards, snakes, birds, bats, mongoose, rodents, insects, spiders, ants) predate on tsetse flies but it is very difficult to evaluate their impact. Parasitoids: some deposit their eggs in tsetse fly pupae, but usually the percentage of parasitized pupae fluctuates strongly according to the time of the year. Parasitoids belonging to the Hymenoptera 9 P a g e

10 Arthropod vectors Tsetse flies - Nesolynx spp. (Syntomosphyrum spp.) do not attack the pupae of tsetse flies exclusively. Establishment of breeding colonies of this parasite is possible, but significant releases in Eastern Africa were not successful while this insect is not able to dig the ground while searching for the pupae of tsetse flies. - Chrestomutilla spp. (Mutilla spp.) are able to reproduce in a parthenogenetic way, the notfertilized eggs giving rise to males. The female lays 2 to 3 eggs in the same pupa, but only one adult parasitoid will emerge. The development of the immature stages takes approximately a month and half. Breeding of this insect in the laboratory has not been successful. Parasitoids belonging to the Diptera - Twelve different species of Exhyalanthrax (Thyridanthrax) were found to parasitize tsetse flies as well as other flies; 10 in Eastern Africa and 2 in Western Africa. The cycle from egg to adult generally lasts 20 to 35 days, but sometimes there is a kind of hibernation of the larvae so that the adults hatch only after several months (up to 14 month). The breeding of these diptera could not been achieved. Pathogens: a microbe (Bacterium mathisi) several fungi and a Baculovirus (salivary gland hypertrophy virus) are pathogenic for tsetse flies but none of these pathogens can be used as a biological control agent yet. Genetic control This method of control aims to alter the reproductive potential of the vector or its vectorial competence. The sterile insect technique via irradiation: This method considers primarily the eradication of isolated populations of tsetse flies. It involves the breeding of thousands of tsetse flies of each subspecies present in the area and the male tsetse are then sterilised using gamma (or X-) radiation and released at regular intervals, thus swamping the population with males that are unable to fertilise females successfully. Glossina austeni has been successfully eradicated from Zanzibar using this method (Vreysen et al., 2000). During this campaign 5.5 million sterile males were released in total. Before flooding the area with the sterile males the local wild tsetse population must be largely reduced by other control methods. It is estimated that a proportion of 10 to 30 sterile males per fertile male is needed in order to assure that most couplings with the few remaining wild females will occur with sterile males and, since most female tsetse fly does copulate only once with a male, the population will be eradicated. Sterilisation by irradiation is possible for adult male or female tsetse as well as for pupae. Before being released the sterile males are nourished with blood from animals treated with trypanocides in order to decrease as such as possible the chances of transmission by the sterile flies. For riverine tsetse fly species it seems easier for the sterilised, released male to find wild flies of the opposite sex than for savannah species. The sterile insect technique via chemical sterilisation: Mass breeding followed by gamma ray sterilization of males could possibly be replaced by the direct sterilization of the males or/and females in the field thanks to systems of traps or screens allowing a sufficient contact between the insect and 10 P a g e

11 Arthropod vectors Tsetse flies a sterilizing chemical product (e.g. bisazir, tepa, hempa). As these products are relatively dangerous and toxic to mammals, they may be substituted by analogues of the juvenile hormone (e.g. pyriproxyfen: (Langley et al., 1994). An amount of 0.02 µg of pyriproxyfen applied on a tsetse fly female inhibits any hatching of pupae by this female. Pyriproxyfen being relatively stable under tropical conditions, screens treated with a mixture of oil (Cerechlor S45) and of pyriproxyfen 2 mg/cm² were efficacious for a period of 9 months. This juvenile hormone mimics the chitin synthesis inhibitor, triflumuron, and may provide a safe way to effectively autosterilize female tsetse flies. These methods may eliminate the need for the costly artificially maintained tsetse fly colonies. Genetic control via transgenic tsetse fly symbionts: Current research on genetic control of tsetse flies involves the development of transgenic symbionts of the tsetse fly (Aksoy, 2005). The 3 symbionts of the tsetse fly (Sodalis glossinidius, Wigglesworthia glossinidium and a strain of Wolbachia) are all maternally transmitted to progeny. The research aims to alter the genome of the endosymbiont, Sodalis glossinidius, so that the symbiont expresses trypanolytic substances into the fly. Wolbachia will be used to introduce cytoplasmic incompatibility in the natural tsetse population. Cytoplasmic incompatibility is achieved when wild-type females are mated with males infected by a Wolbachia strain that is non-existent in the female. The intracellular Wolbachia will then cause embryonic mortality. When both methods are combined it must, in theory, be possible to replace the wild-type, trypanosome-susceptible tsetse population with the engineered, trypanosome-refractory line. PATTEC Recently the Organisation of African Unity decided to launch the Pan-African Tsetse and Trypanosomosis Eradication Campaign (PATTEC). PATTEC promotes an implementation policy designed to select feasible intervention areas, suppress tsetse populations using any appropriate combination of trap and target deployment, insecticide-treated cattle, and/or Sequential Aerial Technique, to be followed by the Sterile Insect Technique if necessary for definitive elimination of the target population. The first phase of this project attempts to free isolated zones or zones close to the limit of the current fly distribution. Parts of Burkina Faso, Ghana and Mali in West Africa and parts of Kenya, Ethiopia and Uganda in East Africa will be concerned. Whether or not it will be possible to eradicate tsetse from Africa is the subject of much debate. Some are sceptical about the chances of success of such a campaign due to the extent, complexity and trans boundary nature of the problem. Furthermore, many environmentalists are concerned about the indirect impact tsetse fly control may have on the environment. They fear that eradicating tsetse may open the path to unsustainable pastoral encroachment of national parks and game reserves and are overtly opposed to any eradication attempt. It is correct that many tsetse control campaigns are carried out because of scarcity of arable land or pasture and in these situations it may be difficult to prevent movement of people into tsetse-cleared area, if the future use of the tsetse cleared land is not well planned in advance and implemented afterwards. Otherwise, the benefit of control campaigns can easily be lost by the damage inflicted to the ecological system. Therefore, simultaneously with the planning of the control campaigns, future land-use must be laid down in consultation with potential users, senior civil servants and experts in environmental conservation. Now that vector control in the framework of PATTEC has started, it is vital that not only the entomological efficacy of the different control techniques is examined, but also their relative cost. 11 P a g e

12 Arthropod vectors Tsetse flies Such cost estimations are not simple to be determined as was demonstrated by Shaw et al., 2007 & 2009 and most African countries haven t the resources to eliminate flies within their borders. Available multimedia Available multimedia Research on tsetse fly control (Wellcome Foundation): P a g e