NUISANCE MOSQUITO SPRAYING

Most nuisance mosquitoes are the direct result of significant rainfall. The mosquitoes produced from such rains are primarily the common nuisance floodwater mosquito Aedes vexans. This mosquito has not been proven capable of transmitting West Nile Virus (WNV) to humans. We follow the following guidelines before initiating our night spraying program:

Night Spraying for the biting adult mosquitoes at the District is initiated when light traps located throughout the District average over 35 female mosquitoes per trap/night for a consecutive three-day period. We also follow weather guidelines necessary for an effective kill. These include after sunset temperatures 60 °F or warmer and consistent wind speeds not exceeding 10 mph. Mosquitoes are much less active at cooler temperatures and not likely to contact our insecticide if they are not actively flying (reduced kill ). Similarly, mosquitoes are weak fliers and less likely to be flying when wind speed increases. We also cannot spray during the daytime due to thermal updrafts limiting the ground coverage of our ultra low volume sprays.

Some residents have commented that “Although your light traps are below the threshold level (to initiate night spraying) and the temperature was below 60°F then why was I eaten alive by mosquitoes at my BBQ last night? Answer- mosquitoes are primarily sedentary insects preferring to rest on vegetation. They are much more sedentary at cooler temperatures (because they are cold-blooded) and only likely to fly if disturbed or in the direct vicinity of a blood meal. Your activity (exhaling carbon dioxide & heat) in the vicinity of resting mosquitoes prompts them to fly and seek a blood meal; otherwise, they would remain in vegetation until the opportunity arises. At warmer temperatures mosquitoes actively seek a blood meal. For night spray control purposes unless mosquitoes are actively flying at the time our insecticide is applied it will not contact enough of them to make a difference because they are resting & protected in the vegetation.

Night Spraying for adult mosquitoes is considered by some experts as a less effective method of controlling a mosquito problem simply because of its perceived temporary effectiveness. The reason for this is because our primary nuisance mosquito can move considerable distances in any given evening and night spraying by itself only kills a percentage of the actual population. When trying to spray the whole outdoors there is also a dilution effect as well as obstacles from barriers which the spray encounters (vegetation, buildings) which may reduce its effectiveness. The District uses synthetic pyrethyroid insecticides called sumithrin and resmethrin which have limited residual properties because of health considerations to the public. For this reason repetitive sprayings may be necessary to bring mosquito populations to an acceptable level. Night Spraying only reduces the mosquito population, it will not eliminate it. It is, however, the only practical control tool available to kill mosquitoes infected with viruses that may bite and infect humans and reservoir animals. Additionally, it may be the only useful mosquito control tool in situations of nuisance mosquito migrations from untreated areas.