Overview

Research into olfactory responses of bloodsucking insects to host odours is contributing to a better understanding of vector behaviour as well as to the identification of more effective trapping methods for monitoring or control purposes. For further reading see e.g. Gibson and Torr (1999).

In the field of host seeking behaviour in sand flies, measurements of host preference or anthrophily have mainly been performed by the identification of bloodmeal origin from wild caught female sand flies or the use of human and animal baits (e.g. Christensen and Herrer, 1980; Quinnell et al., 1992; Montoya-Lerma and Lane, 1996; Campbell-Lendrum et al., 1999). But bloodmeal data has according to Pinto et al. (2001) to be interpreted with caution as they reflect not only innate vector preferences, but also the relative abundance and accessibility of host species in the local environment. For a more obvious judgement of the preferences reflected by different bloodmeals the forage ratio and the host selectivity index have been designed.
Generally adult phlebotomines reportedly exhibit specific activity periods for events such as host-seeking, sugar- and blood-feeding, mating, and oviposition. Flight activity is triggered by daily changes in light intensity (Chaniotis et al., 1971) and further modified by other abiotic factors such as temperature, relative humidity, cloud cover, rain, and wind velocity (Hoogstraal et al., 1962; Quate, 1964). Most phlebotomine sand flies are crepuscular and nocturnal biters (Chaniotis et al., 1971). Feeding, however, is discontinuous and sporadic within the course of a single evening (Rosabal and Trejos, 1965; Chaniotis et al., 1971). For further information see 'Circadian and seasonal activity'.

Direct evidence for odour-mediated host preferences results from studies on different attraction to a range of animal baits in the field (Christensen and Herrer, 1973; Quinnell et al., 1992; Montoya-Lerma and Lane, 1996; Campbell-Lendrum et al., 1999), or to extracted volatiles of different hosts in the laboratory (Hamilton and Ramsoondar, 1994; Rebollar-Tellez et al., 1999).

Feeding Habits

Diverse feeding habits are distinguished in haematophageous insects:

• anthropophagic / anthropophilic: sand flies biting (preferably) humans
• zoophagic / zoophilic: sand flies biting lifestock and other animals
• endophagic: sand flies biting mainly inside houses
• endophilic: sand flies resting indoors after blood-feeding while the meal is digested and the eggs mature
• exophagic: sand flies biting mainly out of doors
• exophilic: sand flies resting outdoors after blood-feeding while the meal is digested and the eggs mature

CO2

Sand flies being attracted to a range of hosts must be responsive to ubiquitous host odour cues such as CO₂ (Pinto et al., 2001). This explains why the rate of attraction seems to increase with host size in many sandfly species (Quinnell et al., 1992; Campbell-Lendrum et al., 1999), while this is still debatable (e.g. Christensen and Herrer, 1980).

Generally, haematophagous insects are attracted by CO₂ concentration gradients, and the attractiveness of other host odours is synergized by CO₂ (Nicolas and Sillans, 1989; Mboera and Takken, 1997; Gibson and Torr, 1999). In the field CO₂ from dry ice has proven to be an attractant for sand flies (first indicated by Knudsen et al. (1979)), but not always shown effective in laboratory assays (Nigam and Ward, 1991). Using so-called odour-baited entry traps (OBETs) (Costatini et al., 1993), working along the principle to isolate the tested odour from other cues as vision, temperature and humidity, and to generate wind flow carrying odours to exploit anemotaxis behaviour (of flying insects), Pinto et al. (2001) could prove that anthropophilic sand flies (Lu. intermedia and Lu. whitmani) were attracted by human kairomones in addition to CO₂. The role of the single odours respectively CO₂ varied with sandfly gender (Pinto et al., 2001).

Other Host Odour Compounds

Information on host odours apart from CO₂ in sand flies is minimal. Octenol, a well known cattle-derived attractant for tsetse flies has not proven to increase attractivity in Ph. argentipes in traps in field studies (Cameron et al., 1991).

Human skin odour deposited on a rubbed petri dish did show various attraction to Lu. longipalpis (Hamilton and Ramsoondar, 1994). Even transfer of volatiles responsible for attraction to new petri dishes after extraction has been successfully performed (Rebollar-Tellez et al., 1999). None of these studies identified single components or provided information on the chemical constituents of the kairomones. Whereas Dougherty et al. (1999) managed to create a synthetic mix of compounds mimicking fox odour and being comparable attractive. And even though the single compounds did not induce the same response as whole fox odour, benzaldehyde, 4-hydroxy-4-methyl-2-pentanone and methyl-2-pentanone attracted sand flies in wind-tunnel choice experiments.

Sandfly Pheromones

Sandfly pheromones have also been proven to be attractive. Ward (1986) showed a significant increase in host attractivity to female flies when in the presence of Lu. longipalpis males. Hamilton et al. (1994) demonstrated pheromone fractions which were responsible for female sand fly attraction.
Pinto et al. (2001) suggest the initial arrival of male sand flies on a host as response to attractive odour cues, but male aggregations on hosts as presumably mediated by sandfly pheromones (Dye et al., 1991).

In the genus Phlebotomus no chemical or biological evidence for pheromones has so far been given.