Mosquito Management at Ni-les'tun Marsh

Mosquito Monitoring

Refuge staff and partners continue to monitor and control mosquito larvae at Bandon Marsh National Wildlife Refuge.


During the restoration of the Ni-les’tun Marsh from a dairy farm to tidal wetland in 2010 and 2011, the linear drainage ditches used to keep pastures dry were filled and the marsh disked to create a more natural surface to improve habitat for fish and wildlife. Although this effectively eliminated the ditches, it also created a series of small pools that would retain water once the tide receded.  This inadvertently produced breeding sites for the salt marsh mosquito (Aedes dorsalis). From 2013 to 2015, the salt marsh mosquito population flourished, with surveys finding close to 500 larvae per dip in multiple sampled pools across the tidal wetland. The situation became a potential health hazard and a nuisance to neighboring property owners.

Early Management

In 2014, the refuge began its mosquito abatement and management program, employing a variety of techniques to resolve the issue with the goal of maintaining a healthy wetland while posing a low risk to fish and wildlife and their habitats and to the human environment. These techniques included surveying and treating larval pools with the larvicide Bacillus thuringiensis israelensis (Bti) and dredging new channels to better drain the marsh and eliminate the pools.

Working with Coos County Commissioners, the refuge also hired a contractor specializing in mosquito control to help resolve the issue. Vector Disease Control International (VDCI) assisted the refuge with larvae dipping, adult trapping around the perimeter of the marsh, and the development of a plan for ongoing management. After assisting with monitoring and managing the mosquito issue on the marsh for two years, VDCI deemed the mosquito issue to be resolved. Since resolution of the issue in 2016, refuge staff have continued to monitor pooling on the Ni-les’tun Marsh monthly throughout the summer months to ensure that the larval numbers remain low.

Current Conditions

In 2020, the refuge hired VDCI to complete a post-restoration assessment that included a thorough marsh-wide larvae and adult mosquito survey with the intent of identifying any problem areas or specific breeding pools that may have gone undetected by refuge surveys, and to advise on management actions for maximum effectiveness. VDCI completed the survey in July of 2020 and concluded that pooling conditions were similar to that of 2016, the year the issue was deemed resolved, and that conditions for mosquito production remain very limited.

The salt marsh mosquito is still the most common species found on the refuge. Standard dip procedures were used to conduct larvae samples at 120 test sites, of which 81 had no presence of larvae while only 6 sites had more than 50 per dip. At historic sampling sites, land rate counts were conducted for feeding adults and CO2 traps were used to collect adults. Compared to previous years’ surveys, the average number of mosquitoes collected in 2020 (avg=35.8) was higher than in 2015 (avg=24.7) and in 2016 (avg=24.8). The fact that 2020 larval samples are consistent with sampling in 2016, but 2020 adult trapping numbers are higher, may indicate that higher levels of mosquito production is occurring off refuge and that a majority of the adult mosquitoes were not likely being produced on the refuge.

The refuge is committed to maintaining a healthy tidal marsh and will continue to monitor larval pools annually and provide treatment if numbers increased.

Fast Facts:

  • The Ni-les’tun Marsh restoration inadvertently caused an increase in mosquito production on the refuge in 2013.
  • Through ditching, applying larvicide, and increasing the number of sinuous channels to drain the marsh, the problem was resolved by 2016.
  • The Refuge developed a mosquito action plan in 2017 for ongoing management.
  • Refuge staff annually monitor mosquito populations through monthly larvae surveys during active mosquito breeding months.
  • VDCI, mosquito contractor, was hired to complete in-depth investigations into marsh health in 2020 and confirmed that mosquito larval numbers have not increased.
  • Saltmarsh mosquitoes can fly up to 20 miles from their breeding grounds.