Retention and efficacy of ultra-low volume pesticide applications on Culex quinquefasciatus (Diptera: Culicidae)

To combat mosquitoes and the public health hazards they present, spraying chemical adulticides is an efficient and timely control method for immediate reduction of adult populations. With the growing consciousness of environmental and public health concerns, effective mosquito control means not only maximizing the effectiveness (in terms of mosquito mortality rates) of the pesticide application, but also minimizing the unintended effects (health hazard and environmental pollution). A series of experiments was carried out to assess the efficacy and deposition of ultra-low volume (ULV) sprays on adult mosquitoes which included the influence of chemical type, spray volume, spray concentration, droplet size, and deposit location (where the droplets land on the mosquito). A modified Potter Tower was used to apply an extremely fine spray (volume median diameter ∼20 μm) on caged adult mosquitoes (Culex quinquefasciatus). Reslin® (50 g/L bioresmethrin) was diluted in either water or D-C-Tron® plus spray oil (782 g/L paraffinic petroleum oil), Twilight® (89 g/L phenothrin) was diluted in D-C-Tron®, and the mosquito mortality was assessed 24 h after spraying. A fluorescent tracer was added to the spray mixture to determine the amount of spray on mosquitoes. A fluorescent microscope was also used to view the deposit of droplets on mosquitoes. It was found that droplet retention and mortality were reduced with the larger droplet sizes. Large water-based droplets tend to bounce off adult mosquitoes. There is a tendency for droplets approximately 20 μm in size to be retained on the fine hairs on the mosquito. The largest spray deposit was found on the adult mosquito wings and the lowest deposit on the head. Mortality was higher for formulations diluted with oil compared to those diluted with water. ULV applications with ultra-fine sprays (VMD 20 μm) and oil-based products resulted in maximum target efficacy under laboratory conditions, at minimum cost, and with the minimum amount of chemical adulticides.