Observations on the diurnal rhythms of biting and resting behaviors of Culex pipiens fatigans in Bangkok

Abstract

Observations were made on the biting and resting behaviors of Culex pipiens fatigans in Bangkok. Human bait collections were repeated at the intervals of about a month by four technicians asigned to stations inside and outside of a building, and the biting rhythm as well as population density could successfully be observed. The biting activity was found to become highest at about midnight and to take chronological periodicity similar to that of the microfilarial density in the blood stream. The results also reflected the seasonal fluctuations of the mosquito density which considerably dropped in the dry season. All the mosquitoes collected by the human bait method were females with the follicular stage I-b (ovarial follicles undeveloped and midgut unengorged), same as those collected by the dryice traps. The resting behavior was observed by the box-traps set inside the building and by the water-jar trap set outdoors. Both equipments were the favorite daytime resting places for Culex pipiens fatigans, and several hundreds of the mosquito could be collected everyday. The males were always higher in the numbers than the females, and females of all stages of ovarial developments and blood engorgements were found to use the traps as the daytime shelters. All these mosquitoes were found to escape from the traps during a few hours' period from about the sunset, and come back into the traps in a short period in the morning just before or soon after the sunrise. Comparative studies were made on the ovarial developmental stages and parous rates of the females collected from various resting places, such as the above two types of the traps, animal houses, human dwellings and outdoor shelters. Examinations of physiological ages of the females as estimated from the numbers of ovarial follicular relics indicated that the species were relatively short-lived in nature, with the parous rate of about 20% in the average.