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Domestic Mosquitoes: New Challenges for Mosquito Control Districts
Christopher Lesser & Mark Latham
Manatee County Mosquito Control District (FL)
Traditional mosquito control relies upon the successful application of adulticides and larvicides. In order to be successful, adulticides are applied at times of peak mosquito activity (typically crepuscular hours when meteorological conditions are also favorable) and larvicides are applied to accessible mosquito breeding habitats where larvae are concentrated and susceptible (this typically taking place in rural salt marshes and freshwater swamps).
However, “domestic mosquitoes” such as Ae. albopictus and aegypti do not lend themselves to such control strategies as they are daytime-active with small, scattered and largely inaccessible breeding habitats (man-made containers located on private property). Widespread adulticiding of urban areas during peak mosquito activity is neither effective (due to unstable meteorological conditions) nor advisable (due to impacts on human outdoor activity). Source reduction through premise sanitation, generally recognized as the most appropriate control method, is typically not feasible in the U.S. due to locked properties, private property rights and inattentive/indifferent owners.
Until recently, control of domestic mosquitoes was unnecessary in all but the most southerly States where Ae. aegypti was a problem. Even in these States, salt marsh mosquitoes were still the primary problem species. However, the introduction of Ae. albopictus to the U.S. in the mid-1980’s and subsequent expansion up the eastern seaboard has brought the problem of domestic mosquitoes to the fore in areas previously free of the issue. Additionally, the re-emergence of dengue has significantly increased the importance of domestic mosquitoes and the need to evaluate and improve our control methods. In some areas, domestic mosquitoes are responsible for greater than 50% of all citizen mosquito complaints.
Currently, the majority of mosquito control districts simply “schedule a fog truck” to address domestic-mosquito public service requests although simultaneously recognizing the limitations of such activity. While the public is placated, the effectiveness of such activity is questionable. New research taking place in several areas of the country looks to improve upon domestic mosquito control strategies. Most research is investigating existing chemistries but applied via altered methodologies. The Florida Keys and Manatee County MCDs are individually investigating the efficacy of aerial larvicide sprays over urban areas. Preliminary results are promising but special attention must be given towards managing droplet sizes and evaporation rates for success. Also, ground ULV larvicide applications have been evaluated in Virginia, Delaware, Florida and New Jersey and all researchers found satisfactory control in sentinel larvae but others have noted that sentinel-larvae mortality does not translate to “population-level” reductions in natural populations using this technique. This observation is likely related to the inability of ground-released larvicides to successfully “find” and deposit within the multitude of small and cryptic breeding habitats often found in urban environments. Additionally, other research has investigated the effectiveness of combined ground ULV larvicide and adulticide applications made on weekly or bi-weekly intervals. This technique has proven to be an effective population-level reduction technique when evaluated on 100-200 acre communities (see Wing Beats, Fall 2011, Lesser and Latham; similar results have been documented by Rutgers University/Scott Crans in NJ). Industry has also taken notice of the emerging problem and is developing new chemistries/ strategies (release of genetically-sterile males, new larvicides formulations) that may prove to be effective but these products are in the very early stages of development and it remains to be seen which of these developments will pass legal registration approval processes and be acceptable in the field of public health entomology.
Ground ULV-larviciding when sprayed in conjunction with ground ULV-adulticiding has been shown reduce domestic mosquito populations by 70-80% when applied weekly or bi-weekly.
Aerial larvicide applications have shown promise in effectively delivering larvicides to small artificial-containers over large geographic urban areas and having significant impacts upon domestic mosquito populations.