BIONOMICS OF CULEX FATIGANS WIED. (DIPTERA: CULICIDAE) IN SINGAPORE

Abstract

Adult Culex fatigans resting indoors during the day were collected once a week from twelve houses for slightly over a year. The qualitative and quantitative characteristics of the indoor resting catches such as age composition, endophily, filaria infection rate, source of blood meals, peak biting time, insemination rate and fecundity were discussed. Data from two short term catches namely outdoor day resting and outdoor night “Malayan baited and unbaited traps” were also analysed and used as supporting evidence to the conclusions and suggestions drawn from the indoor day catches. The overall picture gained from the study of the field population suggests that C. fabigans especially in an area where relatively large number of ovarian hosts are present. The mean indoor mosquito density per man-hour is 26.6 and agrees with that obtained by most authors. Relatively large numbers of indoor resting adults were blood-fed while the unfed comprised a small percentage of the total number caught. Examination of the abdominal condition and ovarian stage of development suggests that this species tends to be more endophilic in the earlier part of the genetrophic cycle than the latter part. Almost every egg hatch laid has been fertilized thus not wasting any reproductive potential. Retention of eggs was observed in all physiological age groups but this is not frequent. Data on the physiological age composition of the three types of catches suggest that newly emerged females prefer to rest outdoors before taking a blood meal. Its peak biting time was deduced to be around midnight and this is in accordance with most authors. The main effort was aimed at establishing and explaining the seasonal population fluctuation trend. This was analysed both from peeled and from corrected real time data. The latter has the advantage of being able to detect short-term or sudden changes in density and activity whilst the former reflects its overall population trend. Both these population curves were correlated with climatological factors as well as larvicidal operations imposed by man. Analysis showed that the number of mosquitoes taken at each collection represents changes in real density or activity or both. Age grading has partially helped to separate their influences. The final picture that has emerged from inspection of the available population as well as climatological data is that there exist two population as well as climatological data is that there exist two population density peaks in the area under investigation. The first peak (April/May) can be attributed to the final build up by a breeding population which has invaded the area from the north east. Presumably this is dispersed as the wind direction changes from north east to south west. Shortly after, a new breeding population builds up to the south west, giving rise to the second peak in July/August. The other partially related topics, (1) a life budget and (2) blood hosts and fecundity were studied in the laboratory. Parameters such as life cycle duration, fecundity, larval and pupal mortality percentage survival and sex ratio were derived. Such studies may provide a basis for understanding the processes that underlie increase, maintenance or decline overall density, varying efficiency of disease transmission and the effectiveness or otherwise of human effort to manipulate those phenomenon. The field studies, have to some extent succeeded in separating real density effects from those due to activity. Due to limited time, the integration of the field and laboratory studies has not been attempted, but it is believed that this is possible and should be developed by future work.