Constructed wetlands for sewage effluent treatment and mosquito larvae at two sites in subtropical Australia

This study of 2 wetlands in subtropical Australia, constructed to treat sewage effluent, examined the relationships between dips positive for mosquito larvae and water quality, operational status of the system, vegetation, and nontarget macroinvertebrates. One site is inland and the other is close to the coast. Larvae of disease vector mosquitoes were present at various times in the wetlands, especially in summer and autumn. The proportion of early instars (1st and 2nd) was greater than that of later ones (3rd and 4th). Dissolved oxygen was negatively, and temperature was positively, associated with the proportion of dips containing larvae. For the coastal site we noted that larvae were more common during draw-down of water for maintenance and also as the system started to come online. Vegetation associated with larvae included dense Typha orientalis and algae. Where there were several types of plants, such as at the coastal site, plant density and water depth were not significantly related to larval presence. Where there were several types of macroinvertebrates there were fewer dips positive for larvae. To provide water treatment capacity and minimal mosquito production we concluded that design should include a variety of plant types, discouraging low dissolved oxygen (for example, by aeration) and ongoing maintenance should be carried out in winter or spring, when mosquitoes are fewer than in summer.