Simplified and improved monitoring traps for sampling sand flies

Authors


Almost every attempt to study natural behavior in the field or the outcome of applied control of sand flies involves intensive population sampling (Alexander and Maroli 2003). The three standard surveillance techniques for adult phlebotomine sand flies are human landing collections, sticky papers, and CDC light trap collections (Killick-Kendrick 1987). Human landing collections often attract the largest number of sand flies (Hanafi et al. 2007) but catches depend on the skill and attractiveness of the individual collector and often expose collectors to an increased risk of Leishmania infections. Accordingly, CDC light traps (either with or without CO2) and sticky papers (with or without lights) have become the standard surveillance techniques (Hoel et al. 2007, Orshan et al. 2010). Carbon dioxide is the single most powerful attractant for blood questing sand flies, but for practical reasons it is often not used (Killick-Kendrick 1987).

Numerous methods and trap designs for sand fly collection have been reviewed by Alexander (2000). Non-attractive traps, like simple sticky papers and unlighted, un-baited CDC traps, only catch flies from their immediate area and accordingly, tend to yield relatively low numbers of sand flies (Burkett et al. 2007). In contrast, CDC traps equipped with incandescent or UV light catch significantly more sand flies than unlighted traps and from distances up to several meters (Killick-Kendrick et al. 1985). Light, especially of a short wavelength like UV, though generally regarded as an attractant, is upsetting the orientation of night active flying insects rather than attracting them in a classical sense (Nowinszky 2004). Female and male sand flies with compromised orientation are drawn into the direction of the light source and are disabled to avoid capture mechanisms of traps (Junnila et al. 2011).

The purpose of this study was to improve the design of sticky traps and to develop a simple and inexpensive alternative for efficient but often expensive CDC-UV traps especially for use in remote areas.

The study was conducted in Israel in early-autumn, 2010, near Jericho, about 10 km north of the Dead Sea, at an altitude of about 300 m below sea level. This region is an extreme desert and belongs to the Saharo-Arabian phyto-geographical zone (Ashbel 1951, Danin 1988).

The traps were tested along an unpaved road in a natural oasis with a date plantation containing a large population of Phlebotomus papatasi Scopoli. The sand fly P. papatasi is the only local species at this site and is associated with colonies of fat sand rats Psammomys obesus (Cretzschmar) (Müller and Schlein 2011).

For this study, two commercial CDC trap models, one equipped with an incandescent light (model 512, John Hock, Gainesville FL, U.S.A.) and another with a UV tube (model 1212), and a conventional sticky paper (constructed from Din A4 white, plastic coated paper painted with castor oil as the adhesive) were compared to one novel sticky trap, one novel incandescent trap design, and two novel UV trap designs. The modified sticky trap was made from rigid plastic netting (size equal to Din A4, white color) with 0.1 cm wide netting separated by 0.3 cm square holes (as used in previous studies for sand fly emergence traps by Müller et al. 2011) and painted with tangle foot, (Rimi, Petah Tiqua, Israel) as an adhesive (Figure 1–1). Both types of sticky traps were mounted on 0.6 m wooden sticks that were driven 0.1 m deep into the ground. The experimental UV and incandescent traps were built from small portable money checkers (Tragbarer Geldschein-Prüfer mit Leuchte, model 751778–62, Conrad Munich, Germany) equipped with a 4 W, 6 V UV tube (as used in the CDC Model 1212) and a small incandescent light bulb for an additional flashlight option (similar to the one used in CDC Model 512). For one trap design, this unit was placed horizontally on a stainless steel net (1 mm wide netting separated by 1.0 cm square holes) which was covering an aluminum tray (26×20×6 cm). Disposable aluminum trays for cooking were half-filled with 70% ethanol. For the incandescent tray trap design, the unit was operated in flashlight mode, while for the UV trap design it was operated in the UV mode (Figure 1-2).

Figure 1.

(1) Sticky trap made from rigid plastic netting; (2) the unit can be operated in incandescent or UV mode; (3) the UV bottle design, dismantled. (4) UV bottle view from the front; (5) UV bottle view from the side.

Figure 2.

Average catch of P. papatasi per trap and day from five types of light and two types of sticky traps.

For the second UV trap design, the unit was mounted vertically inside a clear 2-liter plastic bottle with three windows cut in the sides that were covered by the same steel net as used for the previous design to keep larger non-target insects out of the trap. The UV unit was mounted inside a fourth adjusted window with the tube protruding into the bottle while part of the body of the unit was external (Figure 1–3, 1–4, 1–5). The bottle traps were suspended inverted (with the closed openings facing down) from tripods. The lower parts of the bottles were filled with 70% alcohol that could be retrieved with the catch by opening the bottle in its inverted position and pouring the liquid into a tray.

For seven consecutive nights three units of each of the seven trap models (21 repetitions per trap) were operated from late afternoon to the early morning of the following day (18:00 to 08:00) along an unpaved road crossing the plantation, with a distance of 20 m between each trap. The CDC traps and the bottle traps were hung on bamboo tripods with the opening 50 cm above the ground, glue traps were mounted on wooden poles, and the UV and candescent tray traps were placed on the ground. To eliminate positional bias, traps were rotated clockwise each day. Insects drawn to the traps were collected promptly at 08:00 to prevent degradation. Traps were powered by 6 V motorcycle batteries which were recharged daily.

Statistical analysis was carried out using GraphPad Prism 5.0 statistical package (La Jolla, CA, U.S.A.). Trap types and sex differences were compared using the student's t-test and significance was taken at P < 0.05.

In this trial, 5,917 female and 4,825 male P. papatasi (female/male ratio 1.23) were caught in 147 trapping nights. The best performing traps, the UV tray and the CDC UV, both caught significantly more sand flies than the other traps. The UV bottle caught significantly fewer sand flies than the first traps but significantly more than the others. The incandescent CDC caught significantly more sand flies than the incandescent tray and the sticky net caught significantly more sand flies than the simple sticky paper (Figure 2). There was no significant difference between the catches of the incandescent tray and the sticky net.

Apart from sand flies, the CDC UV, the UV tray, and the UV bottle caught considerable numbers of Anopheles sergentii mosquitoes and to a smaller extent some Culex species. Metal grids of 1 cm, 0.5 cm, and 0.3 cm did not influence the sand fly catches, but the two last reduced the caught mosquitoes and non-target insects collected (data not shown). Compared to the UV tray, the UV bottles were more bulky, took more time for construction, and ultimately caught fewer sand flies. Likewise, the incandescent tray offered little advantage over the UV tray; though the incandescent light attracted fewer non-target insects, the catches of sand flies were >90% smaller.

UV tray traps are not only inexpensive (about US$5 per unit), they are also lightweight and need little space for transport. In peridomestic environments, the CDC and the tray traps can also be operated with small inexpensive 6 V transformers from the main power. Cables distant from the transformer can be up to 20 m long. In remote areas, power supplies can be a crucial problem. The UV traps can be operated with four D cell, 1.5 V batteries for about 5 h (with eight batteries a whole night); these types of batteries are available in remote areas and are fairly inexpensive. In addition, 12 V batteries can be used to replace the traditional 6 V (6–10 Amp.) motorcycle batteries since in some regions they are no longer available. In such situations, two CDC traps as well as two tray traps can be connected in series to form a single 12 V unit and they can be separated with a bridge cable for up to 20 m. However, UV traps often catch large catches of non-target organisms even if protective grids are used to keep larger insects out. In our experience, to sort the catch of a UV tray trap in a large Petri dish with sufficient alcohol takes almost twice as much time as the dry catch of a CDC UV trap. On the other hand, complete unsorted catches in alcohol can be transferred to 50 ml plastic tubes and can be stored there almost indefinitely while dry catches with lots of non-target insects can easily degrade.

A major advantage of glue traps is that they are independent of a power supply and, depending on the consistency of the adhesive, only a few small non-targets are trapped. Single sticky panels are usually directional, but if combined in a right angle to a wind vane or if mounted at right angles to each other, insects from all directions can be caught (Service 1993). A disadvantage of solid sticky panels is that eddies form around their edges and consequently not all insects blown towards them are caught (Fröhlich 1956), but in the case of sticky nets the air flows more smoothly past them. This may explain why the sticky nets designed for this study caught more than three times as many sand flies as the solid panels. The kind of rigid plastic mesh we used to construct the panels was cheap (a single trap was less than US$0.10) and they can be reused almost indefinitely.

In the present trial, the UV tray and the sticky net proved to be inexpensive, easy, and convenient alternatives for existing monitoring trap designs. Further evaluations are needed to determine if this is also the case with sand fly species other than P. papatasi.

Acknowledgments

The authors thank Mr. Nadim Rishmawi, The Hashemite University Zarka, Jordan, for his assistance in the field. This study was funded in part by the United States Department of the Army, Space and Missile Defense Command, Deployed War Fighters Protection Project (DWFP) grant #W81R6000325007.

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