Host location cues of Culicoides spp. (Dipera: Ceratopogonidae)
This study has investigated the nature of host derived cues used by parous female C. impunctatus in the location of blood meal hosts. Olfaction has been shown to be a vital part of the process. The responses of antennal receptors of females to a variety of host-derived volatile compounds were assessed using the EAG technique. High antennal sensitivity to olfactory cues was indicated by low thresholds for response and, taking into account the volatility of the compounds, a clear hierarchy of response was obtained in which octenol was the most stimulatory, followed by phenolics, lactic acid and finally ketones. Of particular note with regard to phenolic compounds was the finding that 3-derivatives (3-n-propylphenol & 3-methylphenol) were more stimulatory than their 4-derivatives (eg 4-methylphenol). Behavioural assays conducted within a Y-tube olfactometer showed that attraction to olfactory cues was dose dependent. Supra optimal doses which caused receptor saturation (as demonstrated in the EAG assay), also induced marked changes in behaviour with responses being either no different to random expectation, or the compound appearing to be repellent. Wind tunnel assays corroborated these findings for octenol and acetone when used in conjunction with 0.01% CO2. Upwind flight was found to be maximal, and significantly different from responses to the CO2 standard, at concentrations within the tunnel of 1.33x10-8 g 1-1 and 1.5x10-6 g 1-1, respectively. These concentrations are approximately those released by a bovid host for octenol and 1/1000x for acetone. Females exhibited a linear dose dependent response to increases in the concentration of CO2 in the wind tunnel up to 0.09% whether gas was released as a plume or as a lightly turbulent cloud. However, at cloud concentrations above this, CNS mediated habituation is suggested to have arrested upwind flight.