Indian River Mosquito Control District

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What is “ULV” Spraying?

From the 1940s through the late 1960s, IRMCD applied adulticides (chemicals applied to kill adult mosquitoes) which were commonly diesel fuel mixed with an insecticide. The formulation was heated until a thick white fog was generated which was applied from a truck. Not only was this fog a hazard to traffic but also had an unpleasant lingering odor. Today, our spray trucks apply a very fine mist of concentrate using a device most closely resembling a large spray-painting machine. This is known as an Ultra-Low-Volume (ULV) technique.

IRMCD pioneered the use of a sealed system for loading and transporting the adulticides used from our trucks. Each spray truck carries a single aluminum cylinder equipped with anti-spill connectors which holds about 55 pounds of liquid concentrate. The sealed system provides both our employees and the public maximum prevention of chemical spills.

How Much Spray is Used?

The application of the adulticide is computer-controlled so that the amount delivered per mile is always the same, regardless of truck speed. If the truck drives faster, the spray is pumped more quickly, if the truck drives slowly, the spray is pumped slowly, if the truck stops, the chemical flow also stops. This ensures that the correct amount of adulticide is applied every time. The computer control system is vastly superior to the old constant-flow method where the truck had to maintain a single exact speed regardless of road conditions or traffic situations.

In 2007 IRMCD updated the system to make use of GPS maps and spray control. This actually guides the driver through the route turn by turn, reducing missed streets and over/under treatments plus providing a detailed map for supervisors the next day.

IRMCD typically applies the adulticide “permethrin” from trucks at the rate of 8.0 ounces per mile driven (=0.002 gallons per acre). This small amount is quite effective in killing most mosquitoes within a few hundred feet downwind of the truck and also means that a truck can spray for hours without having to carry a large amount of liquid adulticide.

When Do We Spray?

The sprays used in our trucks are only for controlling adult, flying mosquitoes.

Mosquitoes prefer warm, dark, still and humid environmental conditions. High winds, heavy rain, low humidity, sunlight or cold temperatures can force mosquitoes to take cover deep within the vegetation, out of reach of our sprays. Under such circumstances, spraying becomes uneconomical and pointless. Only those mosquitoes which are up and flying when the spray truck comes by are likely to be killed. For this reason the IRMCD attempts to time its truck treatments so that the maximum number of mosquitoes will reached by the spray mist.

IRMCDs spray trucks normally begin at dusk and continue to spray for the two or three hours when mosquito activity is at its peak. On occasion our trucks may be seen spraying near dawn, but morning spraying is usually limited to special cases such as when there will be a school or public event that evening.

Why Not Spray During the Daytime?

We are often asked why we do not spray during the daytime hours. The reasons are fourfold. Firstly, a great many beneficial insects such as bees, ladybugs and dragonflies are active during the day and our spray can adversely affect them. Secondly, mosquitoes are most likely on the wing in the first few hours after dusk and this is the time when we can do the best job controlling their numbers. Thirdly, as a matter of good practice, the IRMCD attempts to minimize human exposure to our mosquitocides and this is impossible during daytime hours. For this same reason, we do not spray outdoor sporting events or any other crowded venue even during the evening hours. Finally, the fourth reason we do not spray during daylight hours is that sunlight produces thermal air currents which tend to make the spray mist rise up into the sky instead of staying down low where the mosquitoes are.

Does The Effect Last?

The mosquitocides we use are not persistent… that is they only kill the mosquitoes present and flying at the time of the treatment. There is no residual effect, so any new mosquitoes which infiltrate the area shortly after a ULV treatment are not killed. Non-persistence is important since it allows us to minimize effects on non-target organisms while maximizing effectiveness against mosquitoes. A long-lasting spray which coated every tree, bush and home might kill mosquitoes that evening, but it would also kill honeybees and other beneficial insects the next day and unnecessarily expose humans, pets and livestock.

Why Not Spray Individual Homes?

ULV truck spraying is normally used to treat broad areas, not single homes or neighborhoods. The reason for this is akin to trying to dig a hole in the ocean - take out one scoop of water and the rest immediately fills in the space from the edges. To achieve long-lasting mosquito control, it is important to kill all of the pests over a large area. Otherwise, if a small area is sprayed, the untreated mosquitoes from the surrounding areas will rapidly diffuse into the treatment zone leaving things just as bad as before.

About a week after a hard rain, or after citrus groves are flood-irrigated, mosquito levels can increase dramatically. In some areas of the county, suffering up to 100 mosquito bites per minute is possible once a new brood takes to the wing. Under these conditions, the IRMCD will usually send a number of spray trucks to blanket a large area around the infestation. Treatment may be repeated after a few days.

Why Didn’t You Respond Immediately?

Unfortunately, when the entire county is flooded by rain, we may not have enough spray trucks and drivers to get to all areas of the county as quickly as people would like. Our county has been divided into 62 spray zones and each zone usually takes 1-2 hours to spray. Under realistic conditions, it may take us seven to ten days to spray every zone in the county. We must prioritize our spraying after a countywide rain, covering the most heavily infested areas first.

For economic and ecological reasons, the IRMCD only uses truck spraying in areas known to have noteworthy mosquito activity. Information about local mosquito infestations can come from individuals living in the areas or from reports by our field inspectors. Indian River county is rarely completely free of mosquito activity except in the dead of winter. Encountering a mosquito every few minutes is normal and we will usually not send a spray truck to deal with such low levels of mosquito activity.

Mosquito “control” does not mean “total elimination”. Because of our climate and ecology, we can never totally eliminate mosquitoes, only keep them down to comfortable levels.

Why Doesn’t It Work Every Time?

ULV truck spraying can do a good job of controlling pest and disease-carrying mosquitoes. The primary limitation of using trucks is that the spray mist can only drift just so far. In the suburban and rural areas of the county, as the roads become fewer and further between, gaps develop in the treatment pattern. This leaves areas of woods and groves untreated and the mosquitoes there will soon infiltrate previously mosquito-free areas, causing new pest problems. Another treatment problem exists along the beach front were the prevailing easterly winds may prevent our spray from reaching those homes closest to the ocean. The only solution to the re-infestation problem is periodic retreatment.

Do You ULV Spray With an Airplane?

ULV spraying by air is an option which the IRMCD generally does not employ except under special circumstances. While aerial spraying achieves the best possible coverage, it is expensive and to be optimally effective, must be done on a very large scale. In addition, it has to be timed to coincide with favorable wind conditions and be late enough to avoid spraying beneficial insects like honeybees.

At this time, aerial ULV spraying has been reserved for dealing with Saint Louis Encephalitis (SLE)/ West-Nile epidemics and massive county-wide infestations.

Over the past 18 years, IRMCD has aerially adulticided only 12 times.  The District uses the adulticide “naled” (Dibrom) typically applied at the rate of 0.4 ounces per acre.

Why Did You Miss Our Street?

There are several reasons you may have heard a ULV truck in your area yet did not see it come down your street:

• You may live near the border of one of our spray zones and the truck was actually treating an adjacent zone.
• The driver simply missed your street. Although we train our drivers, even ones with great experience can miss a street while driving at night.
• If you live in a gated community, the gate codes may have been changed recently and IRMCD was not informed. If you live in a gated community and the code has been changed, please make certainthat our office is aware of the new code. If you have an individual gate and want us to come in, you must arrange this with our office.We will inform you when we are coming and you must leave the gate open. We will not open gates.
• Our spray trucks now are much quieter than in the past. If you commonly keep your windows closed, you may not even notice when the truck comes by.
• Did we ever spray your street ? In neighborhoods with closely-spaced roads, we may only treat the north-south or east-west streets because the spray drift is sufficient to cover the entire block.
• Do you live on a new street ? New housing developments are being added continuously. The street simply may not have yet been added to our route.
• Do you live on an unmaintained dirt road ? Especially after a hard rain, some dirt roads in the county can become so muddy, rutted or flooded that our drivers elect to skip them out of fear of becoming stuck or causing truck damage. In some areas, flooding may make it impossible to tell where a road ends and a deep canal begins while driving at night. We will resume spraying when conditions improve.
• Have you recently moved in? It is possible that the previous resident had asked us to not spray their end of the street because they kept beehives or believed they were allergic to the chemicals.
• Do you let your dog(s) run free in the evenings ? For liability reasons, the District may refuse to enter a property where dogs are known to chase our trucks. We try to keep track of “bad dogs” and will attempt to contact you if this is a problem for us. In any event, if you feel that the spray truck missed your house, call our office and we will check into the situation.

As a policy aside … for safety and liability reasons, the District discourages its ULV truck drivers from stopping to talk to people on the roads at night. Please do not try and chase-down one of our trucks if they missed your street or do something else which annoys you. The driver has no idea *what* you want. Instead, call our office and tell us about the problem.