MOSQUITO CONTROL

Any type of control should adopt an Integrated Mosquito Management (IMM) strategy that involves careful consideration of mosquito biology and the environment, as well as, based on scientific surveillance. A response to control nuisance mosquitoes may look very different from a response to control disease-vectoring mosquitoes. Education is crucial to the success of a program; public expectations and concerns must be addressed. In all cases, mosquito population “thresholds” should be established, to validate any treatment if at all necessary, and larval control should be considered as the first option for abatement. This involves location of larval habitats, followed by their modification or treatment in such a way that the integrity of the habitat is preserved but the mosquito larvae are reduced in numbers. By controlling larval mosquitoes, the adults may never become a problem. Larviciding has the greatest control impact on mosquito populations because the larvae are concentrated, immobile, and accessible.

The larval habitats of spring and summer floodwater mosquitoes can be permanently eliminated through environmental sanitation and civil engineering, and should be the first thing to consider for mosquito control. Because of the temporary nature and small size of mosquito floodwater habitats, they often can be altered to prevent mosquito production. However, there are laws and policies regulating alterations of wetlands, and the Michigan Department of Environmental Quality, Land and Water Management Division must be consulted before these activities take place. Indeed, professionals responsible for mosquito control are in the unique position of finding a balance between preservation of our wetlands and elimination of mosquito sources, but this balance can often be achieved with the careful planning and consultation with authorities. Landscape planners should consider carefully the kinds of mosquito habitats they may be creating when wetlands are integrated into landscape or neighborhood designs. However, it is entirely possible to reduce larval mosquito sources and at the same time preserve wetlands and other desirable habitats.

Source reduction of larval mosquitoes may involve: (1) installation of catchments; (2) installation of tile leading to a catchment or drain; (3) modification of grade to permit drainage; or (4) conversion of a mosquito-producing area to a non-mosquito-producing body of water such as an ornamental pond, water hazard, or permanent wetland. For tiling purposes, "sock" tile, which allows water entry but prevents roots and debris from clogging the tile, is very useful when dealing with woodland mosquito habitats. Another type of source reduction is the removal of artificial habitat, such as the filling of abandoned swimming pools or collection and shredding of abandoned tires.

Education is another important aspect of source reduction. Communicating with mosquito control recipients individually, group settings, social media, mass mailings, or through other means allows control programs to educate property owners in mosquito biology and their preferred habitat. Artificial containers found in the yard setting account for a large portion of the summer mosquito population in many urban areas; educating property owners about this habitat and the importance of its removal may be just as effective as trying to treat all that habitat. This communication also offers the opportunity to educate about mosquito avoidance, with respect to peak mosquito activity, and effective repellents like those containing DEET or Picaridin.

Often, larval mosquitoes must be controlled through the use of insecticides that are applied directly into the water where larvae occur. In such instances, presence of larvae in numbers at or above treatment thresholds should be confirmed with use of a mosquito dipper and visible inspection. There are many EPA registered larval mosquito insecticides. Larviciding application equipment varies depending on larvicide formulation. Application equipment for granular or pelletized larvicide formulations include hand-cranking equipment or motorized backpack sprayers.   Liquid formulations can easily be applied with compression sprayers.

Products used for Larviciding*

Biological formulations. Two bacterial formulations are available for larviciding activity: Bacillus thuringiensis israelensis (Bti) and Bacillus sphaericus. Both materials cause cellular breakdown in the alkaline midgut resulting in rapid death, usually within 8 to 12 hours. It is important to note that these formulations must be ingested by actively feeding mosquitoes so they have no effect on late 4th instar larvae or pupae. Bti is available in liquid, briquette, and granular formulation from commercial sources under trade names such as Vectobac®, Aquabac® and Teknar®. Bti works well in a variety of freshwater habitats. Granular formulations are particularly effective against spring mosquitoes when applied during April when the larvae are the second and third stage. B. sphaericus probably does not have the Bti’s broad spectrum of activity, but has a longer period of larvicidal activity. In addition, B. sphaericus, trade name Vectolex® works well in highly organic habitats. Because of its extended control and effectiveness in organic water, B. sphaericus is effective in treating catch basins. The active ingredient Spinosad, the biological derivative of another soil bacterium, represents the other biological formulation. Under the trade name Natular™, this active ingredient kills mosquito larvae through ingestion, as well as, contact. Spinosad products carry a Group 5 classification, which makes it valuable in insecticide resistance management programs. There are various spinosad formulations suitable for most mosquito habitats.

Insect growth regulators. Methoprene, trade name Altosid®, disrupts the larval mosquito’s normal growth pattern by artificially stunting the mosquito’s development making it impossible to reach the adult stage. It may be used to control 2nd, 3rd, and 4th instar larvae. Treated larvae will pupate but adults will not emerge from the pupal stage; it is not toxic to existing pupal or adult stages. This product provides effective control against spring and summer floodwater mosquitoes. Mode of delivery includes liquid, charcoal pellet, briquette, and sand granules.

Abate® (Temephos) is an organophosphate material delivered as a plaster pellet, liquid, or sand granule with a relatively low toxicity. The larvicide is effective against the floodwater mosquitoes, but is often used as a larvicide in polluted larval habitats. This insecticide has been used by the World Health Organization to treat stored drinking water. However, careless handling or ingestion of any organophosphate increases health risks. As with any insecticide special attention to the label is necessary relative to site use, mixing, and application of material.

Petroleum Hydrocarbons, with trade names Kontrol, BVA 2, and Golden Bear, are highly refined mineral oils used in larviciding/pupaciding practices. The mineral oil covers the surface of the water and then enters the breathing tubes of the larvae/pupae as they surface, making it impossible for them to breath. This larvicide offers effective larval control in all larval habitats.

Monomolecular films, trade name Agnique®, spread across the water to decrease its surface tension making it difficult for larvae, pupae, and emerging adults to attach to the water surface, causing them to drown. These films are effective in treating mosquito habitat without significant surface vegetation.

Adult mosquito control can also be accomplished through the application of EPA registered insecticides. Essentially, there are three ways to accomplish this. First, adult mosquitoes can be killed on the wing during their normal flight time (dusk and dawn) using ultra-low volume (ULV) equipment (a type of sprayer that is hand-held, mounted on a vehicle, or fixed to aircraft) and an insecticide. This method is sometimes called "cold fogging," although the droplet size of ULV application comprises a cloud that is technically not a fog. This is an excellent method for controlling mosquitoes, because it allows for use of a small amount of material (generally about 1-5 fl oz per acre) in tiny droplets (usually 12-18 microns) in a narrow band of time and space. In Michigan, malathion (an organophosphate), sumithrin, permethrin, and etofenprox (synthetic pyrethroids) work well as adulticides applied as ULV. These insecticides present minimal risk to humans when used at the labeled dosages.  In general, malathion in a ULV formulation is more effective on warmer evenings, while pyrethroids (synergized with piperonyl butoxide) are more effective in cooler evening temperatures.

A second approach to killing adult mosquitoes is using thermal fogs. In this technology, an insecticide is heated along with another combustible material such as kerosene or oil, thus creating a fog that moves through the air, around vegetation, among flying insects. For mosquito control, the best time to make a thermal fog application is in the evening when thermal inversion conditions exist. A thermal inversion occurs when the warm air (heated by the earth during the day) has not yet mixed with cooler air above it. The insecticidal fog remains most stable and near the ground under conditions of thermal inversion. Thermal foggers can be purchased commercially in sizes small enough for backyard use to sizes large enough for widescale application. Currently, formulations of malathion, resmethrin or permethrin are recommended, follow the label directions.

Another way to control mosquitoes is to use "harborage" or "barrier" techniques. This involves spraying the adulticide onto the vegetation surrounding the area to be protected. This area could be a backyard, a cemetery, a park, fairway, etc. The insecticide provides a residual of active ingredient on plant leaf surfaces, and when mosquitoes fly from the harborage areas (the woods) through this zone, they die or are repelled and do not move into the open to bite. Equipment for harborage application varies with the size of the area to be protected, but can range from a small hand pump sprayer to a motorized backpack sprayer to a large Buffalo turbine rig. Barrier treatments most often utilize synthetic pyrethroids, such as deltamethrin and tau-fluvalinate which are available under trade names such as Suspend® and Mavrik®, respectively.

Products used for ULV Adulticiding*

Pyrethrin insecticides are derivatives of a substances (pyrethrums) isolated from the flowers of the chrysanthemum. Pyrethrins are fat-soluble and act on insects to interrupt transmission of nerve impulses. Humans quickly breakdown pyrethrins in the body; however, insects cannot breakdown pyrethrins as easily. Hence, there is a selective toxicity to insects. These chemicals are combined with compounds (synergists) that increase their insecticidal activity up to 300-fold. Pyrethroids are manmade chemicals with a similar mode of action to pyrethrums, but have increased chemical stability and effectiveness. Toxicity of these substances is nonspecific in regards to insects. Thus proper dosage, application method and timing must be followed to minimize damage to non-targets. Pyrethrin insecticides vary in toxicity to humans, other mammals, birds, and fish so important attention should be paid to the label when utilizing this group of adulticides.

• Pyrethrins are botanical, derivatives of pyrethrums and may be registered under various trade names such as Pyrocide®. Are most environmentally friendly, but are often the most expensive.
• Resmethrin is a synthetic pyrethroid that may be registered under various trade names such as Scourge®. It is used by both ground and aerial equipment. It is a restricted use pesticide so handlers must be certified by the Michigan Department of Agriculture.
• Permethrin is a synthetic pyrethroid registered under various trade names such as Biomist® and Evoluer™.
• Sumithrin is a synthetic pyrethroid that may be registered under various trade names such as Anvil®. It is applied both by ground and aerial equipment.
• Etofenprox is a non-ester pyrethroid registered under the trade name Zenivex™, and is classified by the EPA as reduced risk. This formulation does not contain the synergist piperonyl butoxide (PBO).

Organophosphate insecticides act on the nervous tissue to prevent breakdown of a substance acetylcholine responsible for nerve conduction. This group is nonspecific regarding toxic effects on other insects, and varies in toxicity to humans, other mammals, and fish. To prevent damage to non target organisms it is important to read, understand, and follow the label (It Is the LAW!). Proper dosage, application method and timing will also minimize effects on non-targets.

• Malathion may be registered under various trade names such as Fyfanon®. Used in truck-mounted ULV machines.
• Chlorpyrifos may be registered under various trade names such as MosquitoMist®. Used in truck-mounted ULV machines.
• Naled may be registered under various trade names such as Trumpet®. This product is primarily applied by airplane.

Always read the insecticide label before buying, mixing, loading, applying and storing insecticides. A few adulticides available are restricted use pesticides and must be purchased and applied under the direct supervision of a certified pesticide applicator. All applicators should review and carry the insecticide labels with them and keep MSDSs on file or with them while conducting control activities.

Since most mosquito species are night fliers, adulticiding should generally be done after sunset. However, some species’ peak activity may occur at a time other than sunset so treatment should be adjusted to effectively control these species. Special attention should be paid to wind direction and speed. A change in wind direction may move the material off target, while a moderate to strong wind will cause the material to disperse too quickly, lessening its effectiveness. Spray trucks should be driven slowly at a speed between 10 to 20 mph depending on adulticide flow rate. This assures application of the insecticide at a uniform and labeled rate.

Machines must be kept in proper working order and calibrated every 40 hours of running time. Calibration of machines may also be needed after maintenance or after changing insecticides. Both flow rate and droplet size should be calibrated in ULV machines and flow rate in thermal foggers. Generally, equipment manufacturers and distributors are available to aid in calibration. When using corrosive insecticides, machines should be flushed after each use.

There should always be a basis for adulticiding; thresholds are met either through a mosquito disease threat, increased adult activity in mosquito traps, or substantiated complaint calls. Care should be taken not to waste insecticide, expose the public to pesticides unnecessarily, and promote buildup of mosquito populations resistant to the insecticides.

Biological Control

Many organisms have been or are being evaluated as potential biological control agents for mosquitoes. A few of these agents have been used in mosquito control for years. The mosquito fish (Gambusia affinis) has been used by the World Health Organization and others in many parts of the world since the 1940s. A nematode parasite (Romanomermis culicivorax) has been used with measured success, but is limited by commercial availability. The release of sterile male mosquitoes into native mosquito populations has also had some success. As mentioned above, the bacteria Bacillus thuringiensis israelensis and Bacillus sphaericus have been on the market for several years, and are some of the most historically successful biological control agents currently used.

Each biological control agent has its own unique merits and restrictions. To use a biological control agent successfully, mosquito control personnel must have knowledge of the biology of each agent used. Some biological control agents are limited by climatic and physical factors such as temperature, oxygen availability, and organic pollution. Some are more effective on certain types of mosquitoes than others. Biological agents also differ in the ways in which they can be formulated, transported, stored and applied. Cost is also a factor in any responsible control program. Parasitic nematodes and sterile male mosquitoes, for example, can be very costly due to the laboratory rearing associated with them. All of these factors must be considered when selecting the proper biological control agent for a specific habitat or to control a specific mosquito.

Flying predators are often cited in the popular press as means for controlling mosquitoes by predation. However, scientific studies do not support the contentions that bats, swallows, purple martins, dragonflies, or other flying predators are effective, even thou gh these methods might sound appealing and the animals themselves have aesthetic and intrinsic value. One has to bear in mind that predation is a natural process that is ongoing, yet we have mosquitoes anyway, often in large numbers. Actually, birds and bats do not include many mosquitoes in their diets, despite some claims to the contrary. The idea that they eat thousands of mosquitoes per night comes from statements in the natural history literature indicating that these predators would have to eat this many to maintain their existence. Outdoor, electronic bug zappers with ultraviolet lights do not control mosquitoes. So-called "mosquito plants" do not effectively repel mosquitoes, and are not recommended for this purpose despite advertisements to this effect. Other devices such as those advertised to repel mosquitoes by high frequency sound do not actually repel mosquitoes.

Related Resources

There are several sources of information on mosquitoes and their management that are appropriate for Michigan conditions. The Michigan Department of Agriculture’s Michigan Mosquito Manual contains material on mosquito control activities, mosquito biology, mosquito-borne diseases such as encephalitis and dog heartworm, and organizational tips. This and other information are available from our website on the “Publications” page. Mosquito control districts in Bay, Midland, Saginaw or Tuscola Counties have brochures and other information available.

The Michigan Department of Agriculture certifies persons who apply insecticides for mosquito control. The initial certification process requires taking two examinations, one a core exam and the other a specialty exam for mosquito control (Category 7F). Re-certification can be accomplished either by re-testing or attending MDA approved seminars. Interested parties can call their local Michigan Department of Agriculture office to schedule an examination or to get further information. Study booklets for both of these exams are available from the Michigan State University Bulletin Office and from local County Extension Services offices. These manuals are also available on the Saginaw County Mosquito Abatement Commission website (www.scmac.org) under “Employment”. The Michigan Department of Agriculture’s website (www.michigan.gov/mda) has the core manual available.

*The use of proprietary or trade names is for example only and does not constitute product endorsement.