Geostatistical evaluation of integrated marsh management impact on mosquito vectors using before-after-control-impact (BACI) design

Abstract

BackgroundIn many parts of the world, salt marshes play a key ecological role as the interface between the marine and the terrestrial environments. Salt marshes are also exceedingly important for public health as larval habitat for mosquitoes that are vectors of disease and significant biting pests. Although grid ditching and pesticides have been effective in salt marsh mosquito control, marsh degradation and other environmental considerations compel a different approach. Targeted habitat modification and biological control methods known as Open Marsh Water Management (OMWM) had been proposed as a viable alternative to marsh-wide physical alterations and chemical control. However, traditional larval sampling techniques may not adequately assess the impacts of marsh management on mosquito larvae. To assess the effectiveness of integrated OMWM and marsh restoration techniques for mosquito control, we analyzed the results of a 5-year OMWM/marsh restoration project to determine changes in mosquito larval production using GIS and geostatistical methods.MethodsThe following parameters were evaluated using "Before-After-Control-Impact" (BACI) design: frequency and geographic extent of larval production, intensity of larval production, changes in larval habitat, and number of larvicide applications. The analyses were performed using Moran's I, Getis-Ord, and Spatial Scan statistics on aggregated before and after data as well as data collected over time. This allowed comparison of control and treatment areas to identify changes attributable to the OMWM/marsh restoration modifications.ResultsThe frequency of finding mosquito larvae in the treatment areas was reduced by 70% resulting in a loss of spatial larval clusters compared to those found in the control areas. This effect was observed directly following OMWM treatment and remained significant throughout the study period. The greatly reduced frequency of finding larvae in the treatment areas led to a significant decrease (~44%) in the number of times when the larviciding threshold was reached. This reduction, in turn, resulted in a significant decrease (~74%) in the number of larvicide applications in the treatment areas post-project. The remaining larval habitat in the treatment areas had a different geographic distribution and was largely confined to the restored marsh surface (i.e. filled-in mosquito ditches); however only ~21% of the restored marsh surface supported mosquito production.ConclusionThe geostatistical analysis showed that OMWM demonstrated considerable potential for effective mosquito control and compatibility with other natural resource management goals such as restoration, wildlife habitat enhancement, and invasive species abatement. GPS and GIS tools are invaluable for large scale project design, data collection, and data analysis, with geostatistical methods serving as an alternative or a supplement to the conventional inference statistics in evaluating the project outcome.

Effects of Open Marsh Water Management on Numbers of Larval Salt Marsh Mosquitoes

“…On both marshes, larval densities declined in manipulated marshes relative to control marshes, resulting in a percentage of control of larval mosquito production in excess of 90% for at least 2 yr after the manipulations. Similar Þndings were reported for a recent study on Long Island, NY, where OMWM-type manipulations decreased the frequency of larval mosquito production by 70% on a treatment marsh compared with a control marsh (Rochlin et al 2009). The effects of the interventions on the proportion of sample stations that were wet and on the proportion of sample stations with larvae were present shed light on the mechanisms of these two approaches to marsh management.…”

“…Therefore, these manipulations decreased the need for larvicide applications at these marshes. Rochlin et al (2009) also observed a decreased need for larvicide applications after OMWM-type alterations on their treatment marsh. An additional note that is worth mentioning is that another treatment site at Parker River NWR (site A), which had undergone ditch plugging in 1994 to control mosquito production, also exceeded larvicide threshold criteria on isolated sampling dates during this study (James-Pirri et al 2008).…”

Impacts from ditching salt marshes in the mid-Atlantic and northeastern United States

“…Nixon (1982) judged that ditching was of "questionable value" for mosquito control. Recent studies of ditch modifications compared the treatments with control ditched marshes; the ditched marshes produced measurable numbers of mosquitoes (Rochlin et al 2009;James-Pirri et al 2009;Leisnham and Sandoval-Mohapatra 2011;Rochlin et al 2012), which of course suggests the ditches are not that effective at mosquito control.…”

Comparison of salt marsh creeks and ditches as habitat for nekton

“…Although not feasible at the federally designated Wilderness Area, another restoration scenario might be to fill the linear ditches, while also creating meandering tidal creeks and marsh pools, providing essential nekton habitat. This was recently done at a nearby salt marsh at Great South Bay, NY (Rochlin et al 2009). Often, historic tidal channels that had been lost in response to grid ditching (Adamowicz & Roman 2005) can be relocated and restored.…”

“…Some restoration efforts are underway or are being considered to remove ditches from the marsh landscape by filling, plugging at the mouth to encourage natural filling, or other techniques (e.g. Adamowicz et al 2004, Baustian & Turner 2006, Rochlin et al 2009). Also Open Marsh Water Management (OMWM), an alternative method to grid-ditching, is now widely implemented for mosquito control and has been offered as a technique to restore hydrology of grid-ditched marshes (Meredith et al 1985, Meredith & Lesser 2007.…”