Diurnal and seasonal activities of tabanids and muscids (Diptera) associated with dairy farms in relation to milk yield in Khartoum State, the Sudan

Abstract

Diel and seasonal flight activities of Tabanidae and Muscidae were investigated from November 2007 to October 2008 at Hillat Kuku, Khartoum North, the Sudan. Flies were caught each hour from dawn to dusk in Nzi traps and simultaneously intercepted in flight at 1.5X1.5m electric nets. Diel changes in activity were related to simultaneous changes in relative humidity, temperature, light intensity and wind speed in the open and shade. Data were also obtained on resting sites, fat content of trapped and electrocuted tabanids and, for the first time, diurnal activity of male tabanids. The statistical package, Basica- Epistat, was used to analyze data throughout. Four species of tabanids were identified in the study area including: Tabanus sufis Jennike, T. taeniola Palisot, T. gratus Loew and Atylotus agrestis Weidmann along with many unidentified Muscinae and a few Stomoxys species. Nzi traps caught female tabanids exclusively while electric nets caught both males and females. The species catch composition of tabanids differed significantly between seasons, irrespective of the catching device excepting trap comparisons between the cool dry season (November to March) and the rainy season (July to October). Unlike their electrocuted counterparts, the species composition of trapped muscids (proportions of Muscinae and Stomoxys species in the catch) differed significantly between 3 seasons. No significant heterogeneity was detected in species composition of male tabanids with season. Excepting the cool dry season, trapped or electrocuted female and male tabanids as well as muscids had roughly biphasic diurnal activity peaks, one in the morning and another in the afternoon, irrespective of species or season. No nocturnal activity was denoted for muscids or any species of tabanids, although a few flies of both groups were entering the trap or impinging on the electric net at twilight, and then, during the hot dry season only (April to June). Linear correlation analyses indicated that the diurnal activity of pooled and each species of tabanids was significantly correlated to temperature, light intensity and wind speed, regardless of the catching device or where these meteorological records had been taken (shade/open). The corresponding correlations with relative humidity records were indirect and inconsistent as well. Although hourly changes in catches of muscids at the electric net or trap were directly significantly correlated to corresponding changes in relative humidity there were no similar significant direct correlations with temperature, light intensity or wind speed. Mean monthly trap catches of each species and pooled tabanids were significantly correlated to those of the electric net over the study period. Both data suggested that tabanids had bimodal flight seasons: one in March/April at the end of the cool dry season and another 4 one from September during the rains to November which heralds the cool dry season. Electric net mean catches of muscids were not significantly associated with those of Nzi trap thus suggesting differences in flight seasons contingent upon the catching device. While mean monthly trap catches implied an extended flight season of muscids in the cool dry period from December to March, the corresponding electric net catches did not show any clear flight season for these flies. All resting tabanids had fed recently on blood but attempts to identify the sources of such blood meals using polymerase chain reaction (PCR) at the University of Khartoum were unsuccessful, and therefore not reported. Estimates of fat content for any one species of tabanids did not differ appreciably between catching devices (trap versus electric net). The mean quantity (kg) of milk produced per cow per day in each month was negatively correlated with mean trap catches of muscids, though not with the mean catches of tabanids in the trap or mean catches of both groups of flies at the electric net. These results are discussed in relation to inherent in efficiencies of trap and electric net in sampling of flies and effects of blood-sucking flies associated with dairy cattle on milk production as well as recommendations for further research leading to durable control of these flies