Chapter 3: Traps, targets and electric nets

Colour
The colour of the target or trap is very important in influencing the number of tsetse attracted or entering.

For all tsetse species studied so far, blue and white are the most attractive colours, with phthalogen blue, royal blue, considered to be the best. Black, and possibly red, are more likely to promote a settling or entry response. Yellow, green and brown are unattractive.

Tsetse can detect a wider range of the wavelength spectrum than humans, and are sensitive to wavelengths in the near ultraviolet (UV) part of the spectrum. In insectaries UV light from fluorescent tubes is attractive to tsetse, but for morsitans group flies, the UV reflectivity of the trap or target does not seem to affect the catch. For the palpalis group, a high UV reflectivity promotes settling, so various combinations of black and UV reflective blue are preferred for targets.

Traps usually have a netting cone above the body of the trap, and targets may be flanked by netting. For targets and electric nets, the aim is to use material which is invisible to tsetse. This will increase efficiency by ensuring that flies attracted to the device and which do not land on the visible section may collide with the invisible netting and be killed by electrocution or insecticide. A fine, open mesh black netting is thought to be least visible to an approaching tsetse.

For trap cones, the upward escape response is important, and not surprisingly the degree of light transmission is the most critical factor. However, netting colour may have some effect on catches since very new 'shiny' netting seems to reduce the catch compared to old or brown netting. White or grey netting is usually used for traps cones, or where the cone is recessed, black or grey is more common.

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Materials
Selection of the appropriate materials for trap and target construction is a compromise between attractiveness to tsetse, insecticide retention, durability, cost and availability. When selecting a material for use in an insecticide treated device it is important to ensure that the insecticide does not become "bound" and thus unavailable to be transferred to the fly on contact. This can be tested through a bioassay.

(a) Attractiveness
Given the information above, it would seem relatively simple to decide on the most attractive material. However the exact blue is very critical and few workers have access to the facilities required to assess spectral reflectances. When selecting materials by eye, it is useful to compare it with a sample of material that is known to be attractive, such as phthalogen blue cloth for morsitans group flies. Colours should always be compared in daylight, not artificial light. Shiny surfaces may reduce the settling response of morsitans flies, and this may reduce the effectiveness of synthetic fabrics and plastics.

(b) Insecticide retention
The main insecticides used are various synthetic pyrethroids, particularly deltamethrin and cypermethrin. Pure cotton cloth does not retain insecticide well, whereas a cotton/polyester mixture is better. However, insecticide applied to closely woven cloth is not readily transferred to the fly. Synthetic materials such as polyester, acrylic and nylon give the best results in this respect, but the dye may fade and insecticide retention may also be affected. It is also indicated that reflectivity, which varies with colour, may adversely affect insecticide persistence.

When using targets against palpalis species, a combination of royal blue cotton/polyester cloth with black nylon is probably the best. Coloured plastic sheeting has been used for pyramidal traps. Black cotton and terylene netting is most commonly used for morsitans species, since shiny materials are less effective.

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(c) Durability
Materials must be durable under field conditions. Fading and other colour changes can be a serious problem; for example white, although highly attractive to morsitans species, is now rarely used, as it yellows quickly. When using white for experimental purposes, it is advisable to repaint the traps before each experiment.

Black cloth is particularly prone to fading. Blue dyes vary greatly in their durability, but phthalogen blue on cotton cloth is remarkably colour fast. Blue dyes on pure synthetic materials may fade very fast to an unattractive grey colour. When selecting materials, the manufacturers should be asked which dyes they use, and how colour fast they are when exposed to sunlight and rain.

If using polyethylene (polythene) make sure it includes a UV stabiliser, or be prepared to replace it every few months. Some samples should be set out under field conditions; plastic sheeting often tears more readily than cloth, its lifespan being dependent on its thickness.

The choice of a suitable netting may be difficult. Only lightweight cotton net is usually made locally, and this is easily damaged and rots if exposed to rain; in addition many nets stretch asymmetrically which makes trap construction difficult. Nylon netting only lasts a few months if exposed to sunlight, and other more expensive synthetics are preferable.

d) Cost/availability
Maintenance of traps or targets constitutes a major component of total costs. Hence it may be cheaper to use more expensive materials that last a long time and need less maintenance.

In view of the difficulties of importing materials in many instances, local availability is likely to determine the choice of materials. Shortage of a relatively cheap item which has to be imported, or is intermittently available, can seriously disrupt control programmes. Some imports (e.g. acetone) are in such general use that this may not be a problem, but even locally made supplies may fluctuate in availability. Before existing trap materials are substituted by new ones, their effectiveness should be checked (see Chapter 4).

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Construction and support

(a) Size and shape
Within limits, the bigger the trap or target the more tsetse will be attracted and caught, but the relationship is by no means linear in terms of catch per square meter. Moreover, since the proportion of the overall fly population available to any trap at any given time is limited by their mobility, there is inevitably an upper limit to this effect, and experiments are needed to decide on the optimum size. If the catch could be doubled by increasing trap size, servicing requirements would be halved. It may not be necessary to increase all dimensions equally; there is some evidence that species not attracted to man are attracted to wider shapes, whilst those that are attracted to man, prefer a more vertical shape. Species that feed low for example G. austeni, G. tachinoides and G. pallidipes may be more readily caught by traps whose entrance is closer to the ground.

(b) Free standing or hung from a branch
Where very high trap/target densities are required, the costs per trap become critical. For some traps, biconical, monoconical and pyramidal, one means of reducing costs may be to hang them from trees rather than their being "free standing". Apart from reduced cost, this has the added advantage that they cannot be easily knocked over, although they may still be affected by strong wind.

However, there is not always a convenient tree branch available, and some traps due to their design (NGU, F3 and epsilon) must have supports, either internally using a frame or externally by means of poles or guy ropes. External supports are usually cheaper, and have the advantage that guy ropes can be adjusted when the cloth stretches.

(c) Imported or locally made
Imported traps or targets can be used, but being commercially manufactured they are usually more expensive. For community-based control operations, there are advantages to having them made in villages within the control area. This enhances local involvement and sustainability.

If traps or targets are to be made in a village or homesteads, there are certain essential rules to be followed. Designs should be as simple as possible; staplers should be considered rather than sewing machines; welding should be avoided as much as possible and locally available materials such as rush matting, bark cloth or sacking should also be considered. Waste materials may be used to good advantage. Old tins make good odour bait raincovers. Some workers have even tried using insecticide-treated old car tyres painted blue as targets, but any new design must be tested before putting it into general use.

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Cage design
The most widely used cage design is still the Geigy cage (Fig. 3A) measuring 10 x 7 x 20 cm. However, once it contains more than a few flies the light entering the top of the trap cone is reduced, so inhibiting entry of flies to the cage and reducing the catch. Tilting the cage (Fig. 3B) reduces this problem, but the cage is then more likely to blow off; bending the cage to give a trapezium (Fig. 3C) achieves the same result, whilst allowing the cage to clip properly onto the metal cone.

An alternative is to use a much larger cage measuring 50 x 25 x 25 cm (Fig. 4A), or the arrangement of plastic bottles shown in Fig. 4B, which concentrates live flies away from the cone apex and allows dead ones to fall into a bag for ease of collection. For control purposes where very large numbers of flies are being caught, the above designs can still become overloaded, and the cage shown in Fig. 4C may be more effective.

For moderate numbers of flies, an internal collecting jar containing preservative (Fig. 4E) can be used to store dead flies away from the cone apex and allows catches to be monitored. Where monitoring is not needed, a treated jar (Fig. 4F) can deal with any number of flies, uses little insecticide, is rain proof and does not block up.

The cage designs above are intended for killing the flies, but various sterilizing devices have also been developed. The only one used so far in a field trial is shown in Fig. 4D. It consists of a cut-off plastic bottle, containing a cone of 'mutton' cloth soaked in the sterilant.

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Introduction
All the main trap and target designs commonly in use at present are described and illustrated below, and their efficacies for the different species are compared. The diagrams of traps show both plan (from above) and three dimensional views. The scale on all traps and screens is the same, so that they can be directly compared. The key for the different colours and materials is given in Fig. 5.

Fig. 3 Geigy cages A. standard; B. set sloping to prevent flies accumulating above entry hole; C. cage modified to allow correct fitting on to metal cone support.

Fig. 4 Novel cage designs A. large cage; B. plastic bottle multi chambered cage; C. NGU-type high capacity cage; D. simple sterilising chamber; E. Lancien-type internal collector; F. self-clearing cage.

Fig. 5 Key to figures of traps (plan and three dimensional views); netting viewed from above is not shaded for clarity. Unless otherwise stated all dimensions are in cm.

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Savanna species

(a) The F3 trap
Developed for sampling G. pallidipes and G.m. morsitans, the F3 has found widespread use From outside, the trap is a blue box (Fig. 6C), the front lower half of which is folded in to give an entrance with a horizontal shelf above. Other than the rear, all inside surfaces of the upper half of the trap are black, including the shelf (Fig. 6A). All inside surfaces of the lower half are blue, except for the rear which is black,the target, (Fig. 6B). The F2 trap is identical in design to the F3, but is white where the F3 is blue.

The cone is recessed half way into the trap, and is an asymmetric pyramid with its apex to the fore of centre and level with the trap top (Fig. 6D). Earlier versions used a large wire gauze cage to prevent overcrowding, later replaced by an arrangement of chambers made from plastic bottles and a collecting bag (Fig. 4B). A blue tarpaulin groundsheet forms the floor of the trap, and this can be greased or sprayed with insecticide to deter ants; the groundsheet is, however, often omitted. The trap is supported internally by a tubular frame, which also provides an external cage support.

(b) The NGU traps
These were developed primarily to provide an effective, cheap and easily made trap for community-based control of G. pallidipes. Three of the series have subsequently been used for both survey and control.

From above the NG2B is an equilateral triangle (Fig. 7A). The rear two sides are blue, the shelf is black and slopes down into the trap from the top. The black target base is attached half way along the base of the two sides and its top is fixed to the upper rear corner. The pyramidal net cone is not recessed and a 12 mm hole in its apex admits flies to the cage. A large polythene cage in the form of a modified tetrahedron is used to avoid overcrowding. The trap and cage are supported externally by poles; the cone is supported internally by a centre pole with three nails in its end (Fig. 7D). The NG2G has one 1.0 m blue wing on one side of the entrance (Fig. 7B), and the NG2F has one 0.5 m blue wing added on each side of the entrance (Fig. 7C & F).

Both the NG2G and NG2F catch more G. pallidipes, and G. longipennis , than the original NG2B, but the NG2F version is preferred as it is symmetrical, and hence easier to make and more robust once erected in the field.

Fig. 6 F3 trap A. plan view of upper half; B. plan view of lower half; C. three dimensional view including cage; D. cross section through trap.

Fig. 7 NGU trap series A. plan view of NG2B; B. plan view of NG2G; C. plan view of NG2F including cage; D. cone support; E. three dimensional view of NG2F and cage.

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(c) The epsilon trap
Developed as an alternative to the F3, from above this trap is an equilateral triangle (Fig. 8A & B). Like the F3, it is blue outside, with the lower half of the front folded back into the trap to give a horizontal shelf. The target is a vertical 0.5 x 1 m piece of black cloth sewn into the rear of the trap, all other inside surfaces are blue. As in the F3 the cone is recessed, with its apex level with the top and forward of centre. It uses the same plastic cage design but lacks a groundsheet. It is supported internally by aluminium poles held upright by guy ropes.

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For riverine species

(a) The biconical trap
Since the biconical trap was developed in the early 1970's for G. palpalis many other designs have been developed either for sampling or for control of particular species of fly. The biconical trap (see vol. 1, section 7.2.2.) is however still one of the most widely used traps for sampling, especially of palpalis group species, and for this reason a detailed figure of the original design is given (Fig. 9A & B).

Fig. 8 Epsilon trap A. plan view; B. three dimensional view.

Fig. 9 Biconical trap A. plan view; B. three dimensional view; C. metal support cone and modified Geigy cage.

The trap consists of two cones each 80 cm wide, the upper cone 73 cm high and the lower cone 60 cm high, joined at their widest point. The trap body is kept open by a metal or plastic hoop sewn into the seam where the two cones join. The blue lower cone has four entrances, approximately 30 cm high and 20 cm wide. The upper netting cone has a 12 mm hole to allow flies to enter (but not exit) the cage. Vertically dividing the inside of the trap is a black cruciform, which acts as both a target and baffle. For sampling, the trap is supported by a central pole; for control it is frequently hung from a convenient branch. When free-standing, the weight of the trap is supported at the upper cone apex by a welded wire cone, which also supports the Geigy cage (see section 3.1.4 on cage designs).

The portability and ease of setting of this trap are particularly useful when sampling. Its principal drawback is the need for skilled tailoring and the amount of cloth required.

Despite its widespread use, tsetse workers have often not used the standard design, and dozens of versions of the biconical trap are used around Africa. They vary in all the dimensions, as well as in the colour and materials used. This is unfortunate because it not only makes it difficult to compare catches in different parts of a species' distribution but also the efficiency of the trap can be greatly decreased by random modifications. This is especially true if the wrong shade of blue is used, if the entry holes are made too small, or if the hole at the top of the upper cone is too large.

In general traps should be made to follow as closely as possible to the original specifications. If any modifications are made they should first be tested as described in Chapter 4.

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(b) The monoconical trap
First designed as a simplified biconical trap for control of G.f. quanzensis, it has a polyvinyl chloride cone used as a raincover for the impregnated material below (Fig. 10A & B). Instead of a lower cone, blue streamers hang vertically from the cone rim. As in the biconical trap there is a black cruciform target; this is the same width as the cone and extends from the trap top to below the cone rim. The cone is nearly half the size of a biconical, and is self-supporting. There have been many subsequent versions of monoconical traps. One of the better known has no blue streamers, and the cruciform target is black above the level of the cone rim and blue below.

Fig. 10 Monoconical trap A. plan view; B. three dimensional view.

(c) The pyramidal trap
This was designed primarily for control of G.p. palpalis and G.f. quanzensis, as a simpler, cheaper alternative to the biconical. Instead of a lower cone or streamers, one diagonal of the black cruciform target is replaced by blue (Fig. 11A & B). The upper net cone is pyramidal with the blue and black reaching only half way to its top. If free standing, the upper part of the baffles are netting; when used with insecticide and externally suspended for control, the baffles are modified to accommodate an internal net funnel and a collector filled with diesel fuel, gas oil, as a preservative. The cone is kept open by two horizontal pieces of wood inserted diagonally across its base.

(d) The blue/black target monoconicals and the Vavoua trap
Developed for control of G. palpalis, the blue/black monoconicals have an upper netting cone, with the same cruciform target below as the pyramidal trap. Type A monoconical has one screen black and the other blue, whilst type L monoconical has the central portion of each screen black and the outer portion blue. In the Vavoua trap (Fig. 12A & B) one arm of the cruciform of the type L monoconical is omitted, giving three half screens at 120o to each other. They only reach half way up into the cone and there is no netting baffle above. The net cone is held open by a hoop sewn into its rim.

Fig. 11 Pyramidal trap A. plan view; B. three dimensional view.

Fig. 12 Vavoua trap A. plan view; B. three dimensional view.

(e) The monoscreen trap
Developed for community-based control of G. fuscipes, this trap has a single half black, half blue screen reaching half way up into a small net cone (Fig. 13A & B). Flies collect in a Geigy-size cage, and insecticide is not used. The cone is held open by a hoop sewn into its base and the trap is free standing.

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Choice of trap design

(a) G. pallidipes
This species enters traps readily, and was the first species to be controlled by trapping (using the Harris trap in the 1920's). The biconical trap has been used widely in the past in East Africa to sample this species. It is adequate for ecological work when fly densities are fairly high, but more sensitive traps have now been developed. The most commonly used being the F3, those of the NGU series and the epsilon.

In south-western Kenya, the F3 and the winged NGU's (NG2F and NG2G) perform similarly, catching about twice as many males and 3-5times as many females as the biconical; the epsilon catches fewer flies than the NG2G, although it is still better than the biconical. On the Kenya coast the NG2G and epsilon catch about 1.4 times more males and twice as many females as the biconical, with the F3 catching similar numbers of males and about 1.5 times more females than the biconical. In Somalia the F3 is about 2-3 times better than the biconical, whilst in Ethiopia the NG2B is about 2-3 times better than the biconical.

In Zimbabwe, the F3 catches 10 times more G. pallidipes than the biconical. Unlike in East Africa, the F3 is about twice as effective as the NG2B; the epsilon catches similar numbers to the F3 trap.

For survey and monitoring of G. pallidipes , either the NG2F, F3 or epsilon traps are recommended.

Fig. 13 Monoscreen trap A. plan view; B. three dimensional view.

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(b) G. morsitans
This species is more reluctant to enter traps than G. pallidipes and for monitoring, especially of low density populations, supplementary sampling with mobile electric nets or odour-baited fly rounds is recommended. The biconical, the F2 or F3, the epsilon and the NG2B traps have all been used in recent years for sampling.

Again, the newer traps are more effective than the biconical for G.m. morsitans, although the difference is not as great as it is with G. pallidipes. In Zimbabwe, the F3 is about 4 times as effective as the biconical. For G.m. submorsitans in The Gambia, there is no significant difference between catches in F2 and biconical traps.

F3, epsilon or NG2F traps are recommended for survey and monitoring of G. morsitans, although more testing is needed for the various subspecies.

(c) Other morsitans species
G. austeni is very reluctant to enter traps, although high catches have sometimes been recorded in biconical traps. The pyramidal may also be effective for this species. Sticky two and three dimensional targets have proved more effective and are recommended for this species.
Little recent work has been done on trapping of G. longipalpis or G. swynnertoni, although the biconical will certainly catch them. The F3, epsilon and NGU traps should be tested for these species.

(d) G. palpalis and G. tachinoides
The biconical is used very extensively for sampling both species. It is very efficient for G. tachinoides but less so for G. palpalis. The early monoconical traps were primarily developed for control purposes but can also be used for sampling. Most trials have indicated that they catch fewer flies than the biconical.

The pyramidal trap, which was also developed for control, catches 2-5 times more G.p. palpalis in Congo than the biconical trap, but similar numbers to the biconical in CÌte d'Ivoire. The Vavoua trap catches similar numbers to both the biconical and pyramidal traps, and is considerably cheaper.

The biconical may be recommended for both control and monitoring of G. tachinoides. The biconical, pyramidal and Vavoua are recommended for G. palpalis.

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(e) G. fuscipes
The most frequently used traps for sampling G. fuscipes are the biconical and the pyramidal. For G.f. quanzensis, the pyramidal is 1.6-4.2 times more effective than the biconical. For G.f. fuscipes in Kenya and Uganda, results are conflicting, with the pyramidal sometimes more effective than the biconical and sometimes vice versa.

Early trials in Uganda suggest that the monoscreen trap may catch more G.f. fuscipes than the pyramidal, but it is less effective than either the biconical or pyramidal in Kenya. The F3 and the NG2B and NG2G are certainly less effective than either the pyramidal or biconical traps.

Either the pyramidal or the biconical (unbaited) are recommended for monitoring, with either of these or the cheaper monoscreen trap being effective for control.

(f) Other palpalis group flies
Little comparative work has been done on comparing trap types for G. caliginea and G. pallicera. Mark-release-recapture and trapping studies on G. pallicera in CÌte d'Ivoire have shown that the biconical is an effective sampling tool for this species, and is about as sensitive as it is for G. palpalis.

(g) G. longipennis
The biconical, the F3, those of the NGU series and the epsilon trap have all been used for G. longipennis. In south-western Kenya, the F3 is considerably more effective than the biconical, especially when used without the blue floor, about double for males and about 8 times more effective for females. The NG2B is about 1.2 times more effective than the biconical for males and 4.7 times for females. The winged NGU's are similar to the F3 , being about 1.7 times better than the biconical for males and 7.5 times better for females. On the Kenya coast, the NG2G, the epsilon and the F3 traps all catch about twice as many males, but only about 2-3 times as many females as the biconical. The F3, the epsilon, or the NG2F trap are all recommended for monitoring and possibly control of this species.

(h) Other fusca group flies
Various traps have been tested for G. brevipalpis, including the biconical, NG2B and NG2G. The biconicalis probably the best and is recommended for monitoring purposes.

The biconical will also catch a number of other species such as G. medicorum, G. tabaniformis, G. nashi and G. nigrofusca. Numbers are usually small, but normally it is not known if this reflects a low efficiency of the trap, as is the case with G. austeni, or low densities. The biconical is, however, known to be less sensitive for G. nigrofusca than it is for G. palpalis.

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For savanna species

(a) Black cloth/netting or S-type
This has been the most widely used target to date and consists of a piece of black cotton cloth measuring 100 x 70-100 cm. with black terylene netting panels, about 100 x 50-55 cm, attached at each end (see Fig. 14A). The reason for having the two netting sides was to intercept flies that might otherwise fly around the black cloth but not land on it. The target is attached to a frame, which is pivoted on a pole driven into the ground. The target is thus free to rotate. It has been used primarily for control of G. pallidipes and G. morsitans.

(b) All black cloth
This consists of a single piece of black cloth measuring approximately 100 x 180-200 cm (Fig. 14A insert). It appears to be as effective as the S-type, but is easier and cheaper to make and the black cloth is less liable to damage or fading than the netting. It has been used for the same species as the S target, and is now replacing it.

(c) Blue cloth/netting
This follows the same basic design as the S target, but with the black cloth replaced by blue (Fig. 14A insert). This has been recommended for use against G.m. submorsitans and the riverine species in West Africa.

(d) Blue cloth/black cloth/netting
This also follows the same basic design as the S target, but with the black cloth being replaced by one piece of black cloth (35 x 70 cm) and a similarly sized piece of blue cloth (Fig 14B). It has been used against G.m. submorsitans in West Africa.

Fig. 14 S-type targets A. standard type; inserts from left to right show standard (black cloth + netting); blue S-type (blue cloth + netting) and all black target (black cloth only); B. two-colour S-type target (blue and black cloth and netting.)

(e) Natural objects as targets
Research has recently been carried out on the attractiveness of black painted tree trunks and stumps, odour baited with acetone, octenol and phenols, for the tsetse G. pallidipes and G.m. morsitans. Although blackened trees were less effective than a free-standing 1 x 1 m screen of black cloth, the use of netting attached to a blackened stump as a target showed potential for control.

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For riverine species

(a) All blue screen
The all-blue target (Fig 15A) has been the most widely used target design against riverine species of tsetse, especially G. palpalis and G. tachinoides. The size varies from 100-150 x 90 cm. These targets are either suspended from the branch of a tree, or are supported on a wooden or metal pole. Holes can be cut in them to reduce the probability of theft, but this also may reduce their effectiveness.

(b) Black/blue/black screen
This consists of a piece of blue cotton/polyester cloth measuring 50 x 110 cm, flanked by two pieces of black nylon (polyamide) cloth each measuring 17.5 x 110 cm (Fig. 15B). The screen is hung on an iron support.

(c) Three-dimensional screen
This is made of two rectangular screens, one blue and one black, which cross at right angles and are supported by sticks (Fig. 16A). It is covered with a plastic roof, with flaps of white mosquito netting attached to the roof and projecting part-way down the sides (Fig. 16B).

Fig. 15 Screens A. all blue screen; B. black/blue/black screen.

Fig. 16 Screen-trap A. plan view; B. three dimensional view.

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Target design

(a) morsitans species
Although the S-type target has proved very effective for control of G. pallidipes and G. morsitans, it is more susceptible to damage than the all black target, and the latter is now generally recommended. Target designs have not been adequately tested on other morsitans species.

(b) palpalis species
All-blue targets have proved effective for G. palpalis and G. tachinoides, but the black/blue/black screen is about twice as effective as an all blue screen in CÌte d'Ivoire and is now recommended for control of this species. Target designs have not been adequately tested on other palpalis or fusca species.

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Sticky substances and water plus detergent have been used both to catch flies attracted to coloured screens or trays and to hold flies killed or stunned by other methods, e.g. electric screens.

There are various commercially available sticky substances including Tanglefoot(R) and Stickem(R). Before any particular type is used for sampling, it is essential to ensure that the flies cannot pull themselves free. This is best done by just simply watching a sticky trap in operation over a period of time.

The sticky substance is normally applied over a coloured metal, wooden or cloth screen up to 1 x 1 m in size. Sticky screens are used at present to sample G. austeni. Various designs have been developed, including a 3-dimensional white target and a 61 x 70 cm plywood target that slots into a metal frame which rotates freely in a rod sunk into the ground. (Fig. 17A). In the latter case a blue or white target is more effective than a grey one.

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General principles
An electric net consists of a frame with wires spaced close together. A high voltage is applied between the wires so that any tsetse colliding with the grid are electrocuted and drop down into a collecting device. There are two basic types of high tension (HT) units used to produce the high voltages:

Continuous voltage HT systems
A continuous voltage is supplied to the net by a circuit very similar to that used in electronic flash units for cameras (Fig. 18). A high voltage is obtained using a toroidal, or ring shaped, transformer fed with alternating current from an oscillator running at around 1 kHz; this voltage is increased by a capacitor/diode voltage multiplier network to give around 10 kV DC. This high voltage is stored in a large capacitor until an insect makes contact and bridges any two wires on the net, through which it then discharges thus electrocuting the insect.

Since current is only drawn from the capacitor when an insect is present, the power requirements are minimal resulting in a small, relatively cheap unit, which can be run from torch batteries (D cells). It generates little heat and hence is generally reliable.

Unfortunately if any large insect is caught between the wires the continuing current drain through it prevents the storage capacitor from being recharged; in addition the current through the insect is insufficient to burn it off the net, so it must be removed. In practice the only way to check that a screen is functional is to move two of the wires together using a non-conducting object (e.g. a dry grass stem or plastic pen) to produce a spark, thus necessitating frequent visits to the working screens.

Fig. 17 A. Sticky target for G. austeni; B. water trap for G.m. submorsitans.

Fig. 18 Typical circuit diagram of continous HT (high tension) unit for electric net (oscillator and voltage multiplier boxed for clarity).

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Pulsed voltage HT systems
A continuous stream of high voltage pulses are supplied to the net by the circuit shown in Fig. 19. In this case a voltage of only 300 V is obtained from an ordinary transformer supplied with alternating current by an oscillator running at around 100 Hz. This voltage is converted to direct current and stored in a large capacitor through a current limiting resistor connected to a car ignition coil.

A thyrister, a type of electrical switch with no moving parts, discharge circuit is connected across the capacitor and the ignition coil; between 25 and 100 times a second the thyrister is switched on by its triggering circuit. When in the 'on state' the thyrister presents a very low resistance allowing the large capacitor to discharge rapidly through the ignition coil, thus producing a very high voltage pulse. When the voltage in the storage capacitor drops to zero the thyrister turns itself off allowing the capacitor to recharge again, producing a much smaller pulse in the opposite direction to the previous one.

Because the storage capacitor is discharged at around 50 times each second, this type of unit requires much more power than the previous one. The result is a larger, more expensive unit, run from car batteries, which gets hot and is prone to electrical failure. Efficient and reliable units require lots of power, good cooling arrangements and very robust electronics.

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There are two possible power sources for high current systems. Vehicle batteries are most frequently used, and one of these can be connected directly to each HT unit. A battery charger, and access to a 240 V power supply (generator or mains), are required to charge up the batteries at regular intervals.

Fig. 19 Typical circuit diagram of pulsed HT unit for electric net (oscillator and thryister discharge circuits boxed for clarity).

Alternatively the screens can be run directly from a generator or from a vehicle. A 12 V power supply cannot, however, be used for this since the engine must be some distance, at least 200m, from the screens and too much voltage will be lost using a long cable on a 12 V system. Either the 240 V output from a generator can be used to feed the HT units from the battery charger situated close to the screens, or an "inverter" can be attached to the car battery to step up the voltage to 240 V, and used to feed the battery charger.

A major advantage of this type of unit is its higher catching efficiency. Insects caught between wires on the net normally only reduce the efficiency until the constant stream of sparks burn them off. If the net is not working, e.g. due to crossed wires, very large insects or water, it is immediately apparent. Working screens can thus be checked at a distance either audibly or by binoculars.

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Design of electric net
A diagram of the electric net is given in Fig. 20A & B. It normally consists of two grids of wires (about 95 x 95 cm) spaced 12 mm apart and separated by a sheet of fine black terylene mosquito netting. Each grid is made up of 0.2 mm diameter copper wires, running parallel and vertical, 8 mm apart. Alternate wires are electrically connected to the top or bottom of the frame via a spring and insulated from the other by a nylon loop. Sharp wire ends should be covered by plastic sleeving to prevent the screen mainly sparking at these points.

The top of the frame is insulated from the remainder by corners of plastic, hard rubber or wood. The sides and bottom of the screen are connected to the battery and, for safety, to the ground. The screen top is connected to the high voltage output of the HT unit using heavily insulated wire.

Fig. 20 Electric net A. correct way to connect battery, HT unit and electric net (insulators are ommitted for clarity); B. diagram of part of electric net cut away to show the two layers of electrified wires with netting between.

Collecting devices
Since many flies are only stunned by the nets some form of retaining system is essential.

(a) Funnel collector
This is suitable for back-pack or vehicle mounted electric nets.

(b) Sticky metal sheets/trays
A piece of corrugated iron or a plastic tray about 1.2 x 0.6 m is coated with polybutene or other sticky substance.

(c) Water trays
A shallow, 4-5 cm deep, tray of the same dimensions as above is filled with water and a little detergent added. The tray is usually painted light brown to match the soil colour, white water trays are themselves attractive to several tsetse species.

Sticky metal trays were widely used initially, but most workers now prefer water trays for stationary electric nets. This is because it is much easier to collect and handle the flies, and they are in better condition for subsequent studies such as counting, age dissection etc.

Efficiency of electric nets
Early work on the high current system indicated that a very high percentage (94%) of flies which collided with the net were electrocuted and fell into the collecting trays. Subsequent work using video to film the flies approaching the nets has, however, suggested that this is an overestimate, and that the efficiency of commercially available electric nets is only about 45-55%. Work is ongoing to improve the electrical system

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Refuge traps work on the principle that when the ambient temperature rises above 30C, tsetse naturally seek favourable microclimates. Artificially provided cool dark places attract resting flies during hot weather. A variety of refuges have been used ranging from the highly complex to the remarkably simple (e.g. Fig. 21). These have only been used for research purposes and sampling.

Fig. 21 Simple refuge trap.

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