Surveillance of above- and below-ground mosquito breeding habitats in a rural midwestern community: baseline data for larvicidal control measures against West Nile Virus vectors.

BACKGROUND: Mosquitoes in the genus Culex are thought to play a major role as vectors in the transmission cycle of West Nile virus (WNV) and other arboviruses in the United States. Effective control of mosquitoes through larviciding and adulticiding is expensive for communities and should be guided by reliable surveillance data on the distribution of mosquito breeding habitats. However, few small to medium sized cities in rural areas of the midwestern United States have this type of baseline information available.
OBJECTIVE: During the summer of 2004, we investigated the characteristics of Culex and other mosquito-breeding habitats in a rural central Wisconsin community with a population of approximately 19,000. Such baseline information will aid in the development of rational strategies to control mosquito populations and prevent human exposure to WNV and other mosquito-transmitted viruses.
METHODS: Mosquito larvae were collected and identified weekly from 14 below-ground storm water catch basins and 10 above-ground standing water sites distributed throughout the community. Collection began June 4, 2004 and continued through September 24, 2004. For each collection site the primary and adjacent land use patterns were determined.
RESULTS: Over the study period, 1,244 larvae were collected from catch basins; 94% were Culex species. Breeding activity was first detected in early July. Peak breeding was observed during a period of several weeks when average daily temperatures were at the maximum observed and rainfall had declined. Organically enriched catch basins in low intensity urban sites adjacent to forests and wetlands were found to be more productive breeding habitats compared to catch basins having little organic debris located in isolated high intensity urban sites. Above-ground standing water sites produced 1,504 larvae; 66% of which were Culex species. Flood control ditches and permanent wetlands with stagnant water were most productive, while ditches with moving water were least productive habitats. Larvae were produced earlier in the season by above-ground sites than were produced by catch basins. However, larvae production was more variable in above-ground sites since half the sites became dry at some point during the study period.
CONCLUSION: The observed differences in Culex larvae production based on the variables of habitat-type, temperature, and precipitation support the need for ongoing surveillance in communities to guide public health officials in planning for and prioritizing mosquito control efforts.