Studies on oviposition characteristics of Mansonia (Mansonioides) mosquitoes in southern Thailand.

Abstract

Oviposition characteristics of Mansonia (Mansonioides) mosquitoes were studied to assess future possibilities for their control. Special emphasis was given to the study of the factors, especially the host plant species, which guide the gravid females towards the oviposition site. Initially, egg dispersal throughout the breeding place was assessed by comparing egg densities between so-called areas of 100-200 m2, each characterized by reasonable homogeneity in its flora. Some of these areas were preferred by gravid females, indicating an aggregative type of distribution. There was no evidence that the attractiveness of areas was correlated with the availability of preferred host plants. Egg dispersal within areas was then studied using randomly chosen plots of 1 X 1 m or 15 X 15 cm as sampling units. This again revealed aggregative egg distributions which could be fitted to a negative binomial expectation. The amount of visible water surface, rather than the availability of host plants, appeared to determine the attractiveness of the plots. The dispersal within 1 m2 experimental plots containing the host plants Pistia, Eichhornia and Salvinia was then investigated and this showed clearly that specific levels of attractiveness of plant species influenced egg dispersal. These levels were modified by (i) the presence of grasses in the immediate surroundings, (ii) features of grass growth and (iii) the type of host plant arrangement. It was concluded that the factors responsible for egg aggregations vary with the size of the samples compared, and that the host plant species plays a significant role only for the distribution within quite small units of the breeding place.