July 2002

Adulticiding

            Adulticiding, the killing of adult mosquitoes, is the most obvious aspect of mosquito control programs. It’s accomplished by either ground ULV (ultra-low volume) equipment mounted on a pickup truck or aerially by plane or helicopter. Spray equipment used for adulticiding aerosolizes chemical pesticides to a specific droplet size, designed to kill actively flying mosquitoes. “Active flying” is the main ingredient needed for successful results from adulticiding efforts. Proper droplet size, dose rate, and favorable environmental conditions are essential but an actively flying mosquito population is paramount and, for that matter, required, before adulticiding with chemical pesticides is allowed. Droplet size for adulticiding by ground ULV should be between 5-15 microns, about the size of water vapor particles in a light, naturally occurring fog event. This micron size allows the cloud created by our ULV equipment to stay suspended in the air without settling out on the ground or vegetation. The chemical dosage is determined by the rate dictated on the label of the product being used. This rate will kill mosquito size insects and smaller. The ideal environmental conditions are high humidity, wind speeds of 1 to 3 mph, and temperatures between 65° and 85° F. These conditions favor mosquito activity. The high humidity allows the mosquitoes to be very active without drying out, the low wind speeds allow the mosquito to home in on a blood meal, and the warm temperatures are the temperatures at which the cold-blooded mosquito is most active. Luckily, these conditions also favor the application of aerosolized pesticides. The high humidity provides a dense air column which helps the aerosolized pesticide to stay suspended longer, making it more available for contact with flying mosquitoes. A low wind speed moves the pesticide cloud slowly through an area but not so fast as to blow it out of the target area. Warm temperatures reduce the viscosity of the pesticide, thereby allowing proper atomization of the product. These conditions occur during the summer, at night, which also happens to be prime time for mosquito activity. During the summer these conditions are usually the norm, except when it rains. When it rains, we don’t spray, basically because mosquitoes don’t fly when it rains, they seek shelter, out of harm’s way.

            When conditions are favorable, mosquitoes will seek a host by flying into the prevailing wind to detect and follow a carbon dioxide trail and other scents. This movement, actively flying into the wind while seeking a blood meal, is the inertia required to cause impingement of the pesticide droplets onto the mosquito. When mosquitoes are at rest the pesticide droplets will flow by and not readily contact the insect because there is no force to cause impingement. With ideal conditions, proper timing of application, proper dosage and droplet size, adulticiding efforts can reduce our light trap counts by 90% or more against most mosquito species. I say most species because there are a few container breeding mosquitoes that adulticiding will not control, but that’s another story.

            In conclusion, timing of adulticide efforts is critical for effective results. It must be done when environmental conditions are favorable and mosquito populations are significant and active. Our surveillance activities provide us with the information needed to make the decision of whether or not to spray. It must be remembered that the object of adultciding is to kill mosquitoes and keep their population below a nuisance level, but also to conduct adulticiding activities so that it has a minimal impact on non-target organisms. Chemical pesticides are indiscriminate. Enough exposure at high enough dosages can kill just about anything. For mosquitoes, the dose rates needed are lower than what is required for most other insects, because of their size. The form of the pesticide, an aerosol, is designed to contact just flying insects. The timing of application, at night, avoids exposing day active insects, while the surveillance data, required by the State of Florida, ensures that the adulticiding activities that we perform are justified.