The Effect of C02, Sweat, Chemical Vapours and Air on Simulium ornatum: Implications for Control

Studies on the response of the blackfly Simulium ornatum s.l (Diptera Simuliidae) to carbon dioxide (CO2), acetone, 1-octen-3-ol and air was conducted in the laboratory using a Y-tube olfactometer. The blackflies were found to exhibit a high degree of activity in the olfactometer and responded to the various odours. The results showed that CO2 and l-octen-3-ol were attractive to the flies at low concentrations (< 1% CO2 & < 2.5% 1-octen-3-ol) and repellent at high concentrations (> 2% CO2 & > 4% 1octen-3-ol). Humidified air was found to be an attractant, and dry air a repellent. Acetone at low concentrations (< 0.2%) did not appear to have any effect, while at higher concentrations (> 1%) it repelled. The results indicate that the use of attractive odours could be beneficial in reducing vector biting of hosts if used in areas where the host is present. Introduction Laboratory investigation of the orientation responses of insects, especially Diptera, to odours have generally involved olfacto-meters of various designs, such as Y or T tubes (Giannakakis & Fletcher, 1978). Orientation response is a stage in the host location behaviour of blood-feeding flies, and is influenced by several factors such as colour, shape, size and host odours. Using a Y-tube olfactometer, Read et al. (1970) concluded that Delia rapae locate their aphid host by a combination of odours emanating from the aphid host plant and visual searching for the aphid. Similar investigations of the host searching behaviour of syrphid parasitoids using a T-tube olfactometer showed that aphid odours are important cues in host location (Rotheray, 1979). It was reported that Aedes aegyptii (mosquito) females kept in a CO 2 -free atmosphere oriented to an olfactometer port through which human odour-laden air passed in preference to one through which clean air passed (Smith et al., 1970). With a choice chamber olfactometer, Laarman (1955) found that the vapour from citrated rabbit blood was over twice as attractive to Anopheles atropavus as that of physiological saline. Willis (1947) used an olfacto-meter to show that the vapour from the arm and hand of a human was nearly twice as attractive to Aedes aegyptii and almost three times to Anopheles quadrimaculatus as moist air. Investigations of the responses of tephritid fruitflies (Anastrepha suspense) to odour in an olfactometer showed that male A. suspense release pheromones that attract females (Nation, 1972). In a similar experiment in a T-tube olfactometer, male German cockroach were shown to prefer the side contaminated with their own faeces to a clean one (Kitamura et al., 1974). The adult blackfly, S. ornatum s.l, are collected using sweep nets, aspirators, etc. while S. damnosum s.l are collected using human bait. These methods of collection are not only a health hazard exposing the trapper to risk of disease, but costly and inefficient, and often required skilled labour which may not be available in the long term. The successful use of odours in collection of mosquitoes, tsetse flies and many other insects suggested that the use of odour could be a fruitful avenue to explore for the blackfly. The study was, therefore, initiated to determine the effects of odours in the hostseeking behaviour of S. ornatum using different techniques. The paper presents and discusses the responses of the fly to CO 2 , 1-octen-3-ol, air and acetone using an olfactometer. Materials and methods Study area