Efficiency

Trap Efficiency - How many flies get caught?

A well-designed trap should:

	Attract host-seeking flies from a distance
	Entice flies to circle, land on and investigate the trap
	Direct flies into a compartment where escape is difficult Experiments with both electrocuting nets and other approaches with sticky materials in Kenya suggest that the efficiency of an odour-baited Nzi trap is about 20-30% for two tsetse (G. pallidipes, G. longipennis), and for two stable flies (Stomoxys niger niger, S. n. bilineatus). (Mihok et al., 2007). Efficiency for the common tabanid Atylotus agrestis is high, about 50-70% (unpublished). For nonbiting Muscidae, efficiency is very low (e.g. no more than a few %). More limited data for Glossina swynnertoni suggest an efficiency of 12% for this species (Ndegwa & Mihok, 1999). Similar, properly-controlled and replicated experiments have not been done in temperate areas. Reliable estimates are very difficult to obtain without considerable effort. Preliminary observations with sticky materials during sunny, hot weather around a Nzi trap in Alberta suggests that the efficiency of an unbaited trap may be similar (32%) for Stomoxys calcitrans. In 2002, I gathered more information for both stable flies and tabanids at my home in Russell, Ontario.

Observations on Trap Efficiency in Canada

Methods for Trap Efficiency Estimates

A "good" trap is either very attractive, very efficient, or both. Finding out why a trap works well is not a trivial process. Catches often vary 10-fold from day to day, and from place to place. Hence, considerable effort is required to produce statistically useful results. Even with well-designed experiments, accurate estimates depend on satisfying numerous assumptions about how flies behave. These assumptions have been tested and partially verified for only a few species in Africa (mostly tsetse which fly at "high" speed), and for only a few basic variations on technique.

Net Ring 5k

A standard method is to place a trap in an incomplete ring of electrocuting nets (Vale and Hargrove, 1979). The nets intercept flies approaching and leaving the vicinity of the trap; they are assumed to be invisible to flies and to not affect fly behaviour (only partially true, Dransfield & Brightwell, 2001). By comparing numbers caught to the numbers electrocuted on the inside surfaces of the nets (Hargrove, 1980), one can obtain estimates of trap efficiency that appear to be "reasonable", but with some important caveats (Dransfield & Brightwell, 2001). I have found the same problems of interpretation in less intensive studies with both tsetse and biting flies in a similar setting (Mihok et al., 2007)

Practical details on the construction and operation of electric nets, including circuit diagrams, are available in Volume 4 of the FAO Training Manuals for Tsetse Personnel at the PAAT Information System. Constructing spark boxes and nets from local materials and from first principles is not straightforward.

An example of an experiment done in a woodland clearing at Nguruman, Kenya during the rainy season in May.

An odour-baited, Nzi trap was surrounded by six equally-spaced electric nets (1 x 1 m) at a radius of six metres. Two of the nets are visible in the background.

Tabanus taeniola male - Electrocuting nets often reveal interesting details of behaviour and biology that are not evident with simple counts of flies caught in traps. For example, male horse flies are rarely captured in traps, yet electrocuting nets will occasionally intercept males in good numbers (males only feed on nectar and hence rarely investigate traps that presumably mimic hosts).

Alternative Methods

In Africa, Nzi traps often catch about as many Tabanus taeniola as Atylotus agrestis, suggesting that both of these common tabanids are equally abundant. In reality, T. taeniola may be about 10x as abundant as A. agrestis. Equivalent catches appear to be the result of low trap efficiency for T. taeniola. This species flies higher than A. agresits and often lands on the upper portions of the trap, rather than entering through the low front entrance. This behaviour is shown at left for a trap set at Arba Minch in Ethiopia, covered in sticky plastic film. Many of the flies stuck to the upper blue shelf are T. taeniola.

Experiments with sticky materials placed on or near traps are particularly useful for understanding how flies react to specific trap elements. Sticky materials also provide an eminently practical alternative to electrocuting nets for obtaining qualitatively different estimates of trap efficiency. Unpublished sticky panel experiments in Kenya have produced estimates of trap efficiency of about 30-40% for G. pallidipes and G. longipennis - values similar to those obtained with the standard technique using an incomplete ring of electric nets.

For stable flies in Africa, it has been more difficult to design experiments to cross-validate estimates obtained with electric nets. Sticky films attached to trap surfaces have sometimes increased catches relative to controls (traps with only plain plastic film attached). Hence, sticky materials appear to be enhancing the attractiveness of the trap itself, confounding the experimental design. If one takes this effect into account, results suggest efficiencies of about 20-30% - again, values similar to those obtained with an incomplete ring of electric nets.

Transparency of Sticky Substances

Researchers use a variety of sticky substances to study the landing behaviour of tsetse and other biting flies (Tanglefoot is a popular retail product). In 1997, I measured the transmittance spectra of some key materials to document features that might affect fly behaviour. By doing so, I discovered that the dry chemical formulation used in Rentokil Fly Control Adhesive film (Stock No. FE22/G in the 1990's) strongly absorbs ultraviolet light (< 400 nm). Hence the film is not truly "transparent" to biting flies as they are highly sensitive to ultraviolet light. In contrast, other typical sticky materials were transparent across the entire spectrum.

This is illustrated in the graph below which shows the difference between the transmittance spectrum of the plastic base (likely polyester) from the Rentokil film and the same plastic base with the sticky chemical applied. This special film is still manufactured in a nearly identical format as in the 1990's for use in the FICS Unit (Flying Insect Control System). It is available from Rentokil Initial PLC under Stock No. FE26. Ultraviolet absorbing properties are the same as in the material from the 1990's. Cost of one roll about 10 m in length (30 cm sticky width) purchased from Rentokil in Canada was $75 in January 2007.

The other two sticky substances are typically used with sticky panel traps, e.g. for tsetse such as Glossina austeni. Tem-o-cid from Kollant in Italy is best for this purpose. Polybutene-30 from Chemmoditys in South Africa is also often used as a coating to retain flies on trays underlying electrocuting nets (a simple alternative is to use water with a surfactant, e.g. a few drops of liquid detergent).

Some Introductory References

Brach, E.J. & Trimble, R.M. (1985) Effect of adhesive on the spectral reflectance of insect traps. Canadian Entomologist 117, 1565-1568.

Cilek, J. (2002) Attractiveness of beach ball decoys to adult Stomoxys calcitrans (Diptera: Muscidae). Journal of Medical Entomology 39, 127-129.

Hall, M.J.R., Farkas, R. & Chainey, J.E. (1998) Use of odour-baited sticky boards to trap tabanid flies and investigate repellents. Medical and Veterinary Entomology 12, 241-245.

Mizell, R.F.,III, Mizell, R.F.,IV & Mizell, R.A. (2002) Trolling: a novel trapping method for Chrysops spp. (Diptera:Tabanidae). Florida Entomologist 85, 356-366.

Ryan, L. & Molyneux, D.H. (1981) Non-setting adhesives for insect traps. Insect Science and its Application 1, 349-355.

Snoddy, E.L. (1970) Trapping deer flies with colored weather balloons (Diptera: Tabanidae). Journal of the Georgia Entomological Society 5, 207-209.

Vreysen, M.J.B. (2000) Responses of Glossina austeni to sticky panels and odours. Medical and Veterinary Entomology 14, 283-289.

Sticky Spectra 6k

Technical Literature

Considerable literature exists on the use of electric nets and video techniques to study the behaviour of tsetse flies in Africa. Some key papers are given below as an introduction to this topic.

Baylis, M. (1997) The daily feeding rate of tsetse (Diptera: Glossinidae) on cattle at Galana Ranch, Kenya and comparison with trypanosomiasis incidence. Acta Tropica 67, 81-96.

Brady, J. & Griffiths, N. (1993) Upwind flight responses of tsetse flies (Glossina spp.) (Diptera: Glossinidae) to acetone, octenol and phenols in nature: a video study. Bulletin of Entomological Research 83, 329-333.

Brady, J., Griffiths, N. & Paynter, Q. (1995) Wind speed effects on odour source location by tsetse flies (Glossina). Physiological Entomology 20, 293-302.

Dransfield, R..D. & Brightwell, R. (2001) Trap efficiency for Glossina pallidipes (Diptera Glossinidae) at Nguruman, south-west Kenya. Bulletin of Entomological Research 91, 429-444.

Griffiths, N.B.J. (1995) Wind structure in relation to odour plumes in tsetse fly habitats. Physiological Entomology 20, 286-292.

Griffiths, N. & Brady, J. (1994) Analysis of the components of 'electric nets' that affect their sampling efficiency for tsetse flies (Diptera: Glossinidae). Bulletin of Entomological Research 84, 325-330.

Griffiths, N., Paynter, Q. & Brady, J. (1995) Rates of progress up odour plumes by tsetse flies: a mark-release video study of the timing of odour source location by Glossina pallidipes. Physiological Entomology 20, 100-108.

Groenendijk, C.A. (1996) The response of tsetse flies to artificial baits in relation to age, nutritional and reproductive state. Entomologia Experimentalis et Applicata 78, 335-340.

Hargrove, J.W. (1980) Improved estimates of the efficiency of traps for Glossina morsitans morsitans Westwood and G. pallidipes Austen (Diptera: Glossinidae), with a note on the effect of the concentration of accompanying host odour on efficiency. Bulletin of Entomological Research 70, 579-587.

Hargrove, J.W. (1980) The effect of model size and ox odour on the alighting response of Glossina morsitans Westwood and G. pallidipes Austen (Diptera: Glossinidae). Bulletin of Entomological Research 70, 229-234.

Odulaja, A. & Mohamed-Ahmed, M.M. (1997) Estimation of the efficiency of the biconical trap for Glossina fuscipes fuscipes along the lake Victoria shore, Kenya. Entomologia Experimentalis et Applicata 82, 19-24.

Packer, M.J. & Brady, J. (1990) Efficiency of electric nets as sampling devices for tsetse flies (Diptera: Glossinidae). Bulletin of Entomological Research 80, 43-47.

Packer, M.J. & Warnes, M.L. (1991) Responses of tsetse to ox sebum: a video study in the field. Medical and Veterinary Entomology 5, 23-27.

Paynter, Q. & Brady, J. (1992) Flight behaviour of tsetse flies in thick bush (Glossina pallidipes (Diptera: Glossinidae). Bulletin of Entomological Research 82, 513-516.

Paynter, Q. & Brady, J. (1996) The effect of wind speed on the flight responses of tsetse flies to CO₂: a wind -tunnel study. Physiological Entomology 21, 309-312.

Torr, S.J. (1994) Responses of tsetse flies (Diptera: Glossinidae) to warthog (Phacochoerus aethiopicus Pallas). Bulletin of Entomological Research 84, 411-419.

Vale, G. (1977b) The flight of tsetse flies (Diptera: Glossinidae) to and from a stationary ox. Bulletin of Entomological Research 67, 297-303.

Vale, G.A. (1974) New field methods for studying the responses of tsetse flies (Diptera: Glossinidae) to hosts. Bulletin of Entomological Research 64, 199-208.

Vale, G.A. (1977a) Feeding responses of tsetse flies (Diptera: Glossinidae) to stationary hosts. Bulletin of Entomological Research 67, 635-649.

Vale, G.A. & Hargrove, J.W. (1979) A method of studying the efficiency of traps for tsetse flies (Diptera: Glossinidae) and other insects. Bulletin of Entomological Research 69, 183-193.

Warnes, M.L. (1995) Field studies on the effect of cattle skin secretion on the behaviour of tsetse. Medical and Veterinary Entomology 9, 284-288.