Managing Salt Marshes for Mosquito Control: Impacts of Runnelling, Open Marsh Water Management and Grid-ditching in Sub-tropical Australia

Abstract

Three salt marsh sites in south-east Queensland, Australia, have been modified for mosquito control. The problem species is Ochlerotatus vigilax, a vector of Ross River virus that is an epidemic polyarthritic disease. All sites have similar vegetation and tidal influences. Each site has a different form of modification to manage the mosquitoes: runnelling, Open Marsh Water Management (OMWM) and grid-ditching. Results are compared of a treatment and control experimental design, similar for each site, but analysed separately for each site, and over the same 3-year time period. Environmental variables monitored included: for the water table, the depth, salinity and pH; for the substrate, its moisture, salinity and pH; for the vegetation, the size and density of the dominant grass (Sporobolus virginicus) and the number of mangrove (Avicennia marina) pneumatophores. Crab activity was indicated by crab hole numbers. ANOVA analyses, comparing treatment and control at each site, indicated that runnelling had the least significant impact on the environmental variables, whereas most impacts were at the grid-ditched site. Of the 10 variables included here, 5 were affected by runnelling, 7 by OMWM and 9 by grid-ditching. Statistically significant results are summarised, compared to the relevant controls, as follows: Water table: the level was significantly higher near ditches in the grid-ditched site; pH was higher at the treatment in the OMWM site but was lower in the grid-ditched site; salinity was significantly lower in the runnelled and in part of the grid-ditched marsh. Substrate: the moisture was higher at both the OMWM and grid-ditched sites; pH was higher in the OMWM treatment but lower in the grid-ditched marsh; salinity was significantly lower at the runnelled and grid-ditched sites, but was higher near the ditch at the OMWM site. Vegetation and crabs: The grass Sporobolus virginicus was less dense in the treatments in both the OMWM and grid-ditched sites, but was taller in the runnelled site and in part of the grid-ditched site. There were fewer mangrove pneumatophores in the runnelled area, but more in the OMWM one. There was more crab activity in both the runnelled and OMWM sites. It was concluded that runnelling has least impact and that grid-ditching has the most, but that none of these appears to have destroyed the marsh environment.