Range of attractiveness of carbon dioxide to Hybomitra spp. (Diptera: Tabanidae).

Carbon dioxide (COz) is a known attractant to female tabanids and is frequently added to various traps to increase numbers collected (Roberts 197 I, t97 5, 1976). The attractiveness of different amounts of COz for tabanids has been investigated (Roberts 1975), as well as its possible selective attractiveness to flies of a certain age composition and a specific physiological state (Leprince and Lewis 1983, Leprince and Jolicoeur 1986). Roberts (1971) reported that each tabanid species has a definite individual response to CO2, and Leprince and Lewis (1983) found that reactions of parous and nulliparous females to COz may also differ among species. This note reports observations on the range over which COz is attractive to female Hybomitro spp. These observations were made during an indepth study of mosquitoes in southwestern Ontario during the summer of 1986. Experiments were conductedbetween }-4'ay 27 and August 5, 1986 near Aberfoyle, Ontario (43"C 35'N,80'C 15'W) in an open field where the vegetation consists primarily of grass (Poa sp.) and goldenrod (Solidago sp.). Twenty ramp traps, similar in design and size to those of Gillies (1969), but constructed from aluminum window framing and black wire screen, were arranged radially with 4 traps located at 3,7 , l!, 15 and 19 m from a central CO2 source. The openings of the traps faced away from the COz source. On 7 ofthe 14 days when the study was conducted, COz was released at a rate of 1,000 ml/min from a 20 lb pressurized tank via a regulator. This rate was comparable to the release of CO2 by Hereford beef heifers (1.2 to 1.8 liters/min, Roberts 1972). No COz was released on the other 7 days. Carbon dioxide and nonCOz trials were carried out alternately, and trials were only conducted on days without cloud cover and a slight (0-10 km/h) breeze. The temperature range was 20-31"C in the course of the experiments. Trials commenced at 0930-1000 EDST and terminated at 1500-1530 of the same day. Tabanids in the traps were removed using a battery-powered aspirator and transported to the laboratory for identification. Three species of tabanids, I{yb omitra epistates (Osten Sacken),I/. nudo (McDunnough) and.E[. Iasiaphthalma (Macquart), were caught in sufficient numbers for inferences to be made regarding the range of attraction of COz (Table 1). Table 1. Total number of female Hybomitra in relation to distance from carbon dioxide source (4 traps/range; ? days with and 7 without COz).