1966 Volume 7 Issue 2 Pages 44-49
In a previous report of the series a successful result of control against Culex pipiens pallens by an eradication of larvae in the breeding places was presented. It was clearly demonstrated that the age-composition of adult population showed a remarkable trend of getting older as the size of population was reduced. In the present experiment it was intended to investigate the effect of control against adults on the age of adult population. When the control of a population of adult mosquitos could be successfuly achieved one may expect to observe a younger population on reappearance of mosquitos. Three villages in Setouchi-cho, Amami Oshima were selected. These are involved in an area of bancroftian filariasis. The vector is known to be Culex pipiens fatigans. Each house was visited from 20 to 22 hrs at night nearly once a week during the period from June to September 1964. Mosquitos resting indoor were collected by an aspirator. The degree of abundance of mosquitos was expressed by the number collected per house per collector and per 30 minutes of collection. Seventeen species of six genera were collected and among them Culex pipiens fatigans, Culex tritaeniorhynchus, Culex vishnui and Anopheles sinensis were indicated to be main species. Age determination was carried out according to the observation of relics in the ovariolar pedicle as well as of changes in the ovarian tracheoles. Residual spraying of organophosphoric insecticides, namely, Baytex was applied to Katetsu village and Sumithion was adopted for Isu- and Konasa village. 0.5 % emulsion of concentrate of each chemical was sprayed once during the observation period on the walls inside of houses at a dose of 50 ml per square meter. All houses including animal sheds were treated at the same time in respective villages. C. fatigans disappeared in houses in each village immediately after the spraying. The effect remained for more than three weeks then a few number were occasionally collected. However the original level of indoor density had never been regained during two months observation after the treatment. Regarding other main species there were no evidence observed to indicate a significant reduction in the size of indoor population which is due to the spraying. Neither the difference of parous rate or age structure of the four main species of mosquitos were observed between before and after the residual spraying. It seems to be of particular interest that C. fatigans was remarkably suppressed in number by the spraying but the age composition was apparently not affected on reappearence inside of houses. A possible interpretation may be that the residual spraying of insecticides could not affect on the whole adult population of this species but merely repelled its invasion into houses although the possibility of specific and lasting repellency to fatigans should be checked. Based on the present observation the residual spraying of Baytex or Sumithion seems useful to some extent for filariasis control. should the majority of infections be acquired indoor.