Release kinetics of a synthetic tsetse Allomone based on waterbuck odour from a Tygon silicon dispenser

Abstract

This study was conducted in order to develop an appropriate dispenser for the waterbuck-derived repellent for tsetse flies. The waterbuck-derived repellent consisted of pentanoic, hexanoic and heptanoic acids, guaiacol, geranylacetone, 2-undecanone and 8- octalactone. The reservoir tube of the dispenser was made from aluminium (or polypropylene) of diameter 10 rom and length 10 em. The diffusion area was made from tygon silicon tubing of internal diameter 6.4 rom, outer diameter 9.6 mm, thickness 3.2 mm and length 2 em (diffusion area 6.028 cnr') or 4 em (diffusion area 12.056 crrr'). Preliminary trials were conducted under semifield conditions with a synthetic repellent (2-methoxy-4-methyl phenol) to determine the effect of surface area on the release rates. The rates were found to be directly related to the surface area of the tygon tubing. Increasing the surface area increased the weight loss of 2-methoxy-4-methyl phenoL These trials enabled the selection of appropriate lengths of the tubing to be used in both the laboratory and semifield trials with the waterbuck-derived repellent blend. Laboratory tests were conducted in a two choice wind tunnel in which the windspeed was maintained constant at 20cm/sec while the room was maintained at 24 ± 1°C and 65 ± 5 % relative humidity. The compounds were dispensed either singly or as a blend from the dispensers, with 6.028 cm2 diffusion area The weight loss of the individual compounds was assessed gravimetrically and the release of the individual compounds in the blend quantitatively determined by gas chromatography (GC). Zero-, first- and second-order rate models were tested to determine the release kinetics of the individual compounds and the blend. Comparison of the models using correlation coefficients (~) indicated that the release of the individual compounds followed fIrstorder kinetics while the release of the blend followed zero-order kinetics. In the semifield trials, dispensers were placed either under direct sunlight or under the shade. Weight loss was assessed gravimetrically and the release of the blend compounds quantitatively determined by GC. The individual compounds dispensed singly followed first-order release kinetics while the blend of the compounds followed zero-order release kinetics. It's however interesting to note that the release of the individual components of the blend-mixture follows zero-order kinetics under semi-field conditions contrary to the behaviour exhibited by the individual components dispensed singly. The repellents placed in dispensers exposed to direct sunlight exhibited higher rate constants than those in the dispensers placed in the shade. The rate of release was found to be slightly higher during the first 24 hours and then became steady, obeying Fick's law of diffusion. The release rates were observed to depend on the surface area of the tygon tubing and generally increased with temperature. Semifield data was more variable than laboratory data due to the changing temperature conditions in the field. Rate constants established under laboratory conditions were slightly lower than those obtained under semifield conditions. The results indicate that temperature could be the major environmental determinant of release rates with other variables like relative humidity having little or no effect. However, the magnitude of the effect of temperature on the release rates was not easily demonstrated with the field data. It is thus evident that the release of the compounds was not a simple function of temperature; with the release rates at higher temperatures being lower than would be expected. The zero-order rate equation best described the release of the blend, which was found to be diffusioncontrolled. Controlled release of the blend was therefore achieved using the dispenser.