A four-year collaboration between the Pontchartrain Conservancy (PC) and St. Tammany Mosquito Abatement (STPMAD) on an EPA funded project has yielded important
findings for our understanding of sewage-associated mosquitoes. The project area, Ponchitolawa Creek Watershed, drains approximately 9,441 acres in St. Tammany Parish and is a tributary of the Lower Tchefuncte River, which flows into Lake Pontchartrain. The project watershed is within St. Tammany Parish, which has a watershed management plan developed by the Louisiana Coastal Protection and Restoration Authority that identified:
• 20 subdivisions not connected to community or centralized wastewater treatment systems;
• More than 1,686 residences that have septic tanks or Aerated Treatment Units (ATUs),
Water quality measurements taken by STPMAD staff at 311 sites in 2019 and 2021 demonstrated that failing OWTS are associated with more acidic aquatic habitats in
ditches receiving effluent (P = 0.008). In addition, mosquito larvae are 3.8 times more likely to be found in failing OWTS effluent than in ditches receiving partially-treated effluent from properly functioning systems. The probability of finding mosquito larvae was 3.5 times lower when fish were present, potentially due to predation or oviposition deterrence. Mosquitofish absence was correlated with worsening water quality indicators such as higher total dissolved solids (TDS) (P = 0.003), and lower oxidation reduction potential (ORP) (P = 0.027). Despite these relationships, we did not find a statistical relationship between OWTS functioning status and fish presence (P = 0.488).
Unfortunately, we were not able to determine whether repaired or remediated OWTS reduced mosquito populations in a direct pre-to-post intervention study.
However, correlative data suggesting that mosquito larvae are less likely to be found at OWTS that pass inspection indicate that remediating OWTS may limit mosquito
This study also determined that water depth is an important predictor of mosquitofish presence and mosquito absence as well. Mosquitoes were significantly more likely to be found in water less than three inches of depth. By contrast mosquitofish are 6.8 times more likely to be found in water greater than six inches than in
water between 0-3 in. Water depth may play several roles — as a refuge during drought for water-bound fish, accessibility for fish to find mosquito prey, and as additional
assimilative capacity for sewage pollutants.