Mosquito Control

Pesticides used for outdoor mosquito control can be broadly divided into larvicides and adulticides, depending on the stages of the mosquitoes to be treated. Larvicides are intended for killing immature mosquitoes in their aquatic habitat while adulticides are intended for killing the flying adult mosquitoes.

Mosquito larvicides

When it is not possible to eliminate stagnant water that may become breeding grounds of mosquitoes, and there are no alternative methods of control, larvicides may be applied to the water bodies. The success of larval control depends upon many factors such as the chemical ingredients and formulation of the selected larvicides; the timing, rates and methods of application; and the physical and environmental conditions.

Bacillus thuringiensis subsp. israelensis (Bti)
Bacillus thuringiensis subsp. israelensis (Bti) is a microbial insecticide made from naturally occurring soil bacteria. The insecticide, which is proven to be effective against mosquito larvae, has a low level of toxicity to humans. With its highly specific mode of action agThere are basically three kinds of methods for the application of adulticides, namely thermal fogging, cold fogging (ULV treatment) and residual spraying.

  1. Thermal fogging is a space treatment against adult mosquitoes. The fog is produced by a device (thermal fogger) that uses heat to break up the chemical into very small droplets (usually in 5-30 micron diameter range) which will disperse in the air. When the chemical (usually diluted with oil-based carrier) is heated, it is vaporized in a combustion chamber and then expelled via an outlet tube to form a dense fog cloud when it condenses on contact with cool ambient air. It can even reach air spaces in areas obstructed by dense vegetation or other objects. When the mosquitoes in flight come into contact with the droplets, they will be knocked down and killed.
  2. The cold fogger may dispense formulations in a very concentrated form and generate the droplets (usually in the 5-30 micron diameter range) in a precise manner. When the mosquitoes in flight come into contact with the chemical droplets floating in the air, they will be killed. However, its ability to penetrate dense foliage or obstacles is not as good as that of the thermal fogging. Cold fogging is sometimes called Ultra Low Volume (ULV) treatment as it allows the utilization of only a very small amount of chemical for coverage of a large area.
  3. Residual spraying is usually carried out in the form of a fine spray made to the surface where mosquitoes may come into contact. As the droplets of the spray are much larger, they settle very quickly on the surface of treated areas. Residual effects usually last from a few days to about a week to several weeks depending on the product used. Mosquitoes rest or land in treated areas may be killed.

ainst a narrow host spectrum, it does not pose risks to other non-target organisms or the environment when used properly. However, it is not very effective in highly polluted water. Bti is most effective when used in the early larval stage when the larvae are actively feeding. When ingested by the larvae, it will disrupt their gut cells and cause death. If it is used for preventive control, repeated application at a 7-day interval is recommended in most situations. Some Bti products with a sustained release formulation may offer an extended larvicidal activity.

Bacillus sphaericus (Bs)
Bacillus sphaericus (Bs) is also a microbial insecticide with a nature and mode of action very similar to those of Bti. Bs, which is also highly specific to the larvae of mosquitoes, should be applied in the early larval stage when larvae are actively feeding. Comparing with Bti, Bs is particularly effective against Culex spp. and it can also be used in water with high organic contents, such as lagoons or stagnant catch basins.

S-Methopren
S-Methoprene is an insect growth regulator (IGR) that retards the development of larvae and prevents mosquitoes from developing into adults. Larvae treated with the chemical fail to molt successfully and simply die during the pupal stage. The target-specific S-Methoprene will not affect fish, waterfowl, mammals or the environment when applied at the recommended use rate. There are many different formulated S-Methropene products in the market; some only have little residual activity and are intended to target at and control a single brood of mosquitoes while others are slow-release formulations offering an extended period of control which lasts from 30 up to 150 days. It is important to note that the chemical must be present in the water when the larvae are in the late developmental stage for it to be effective.

Temephos
Temephos is an organophosphate with a relative low toxicity of its kind. It is highly effective against larvae and may be used on any of the larval growth stages.
With its neurotoxic mode of action different from other larvicides, it can be used in rotation with other larvicides to prevent the development of resistance. There are slow-release formulations that can provide control for 30 days or more. Since some aquatic invertebrates may be highly susceptible to the chemical, it must be used carefully so that non-target fauna may not be adversely affected.

Larvicidal oil
Larvicidal oil is a highly refined petroleum distillate designed especially for mosquito control. It can be applied to mosquito habitat where the water cannot be drained. As it spreads into a thin film over the water surface, only a small amount is needed. Larvicidal oil can provide effective control on both larvae and pupae as it basically kills by suffocating them. As the oil can be degraded rapidly by light and microorganisms into harmless components, it should not pose much environmental risk. When it is used in highly sensitive areas, it is necessary to exercise more care to avoid causing adverse impact to other non-target organisms.

Monomolecular film
The active ingredient of monomolecular film (MMF) is Iso-alcohol ethoxylate. It is a degradable surfactant with a mode of action similar to that of larvicidal oil. It forms a very thin monomolecular film on the water surface to which it is applied and causes suffocation of larvae and pupae. MMF, an environmentally friendly pesticide, has very little adverse impact on fish and plants.

Mosquito adulticides

There are basically three kinds of methods for the application of adulticides, namely thermal fogging, cold fogging (ULV treatment) and residual spraying.

  1. Thermal fogging is a space treatment against adult mosquitoes. The fog is produced by a device (thermal fogger) that uses heat to break up the chemical into very small droplets (usually in 5-30 micron diameter range) which will disperse in the air. When the chemical (usually diluted with oil-based carrier) is heated, it is vaporized in a combustion chamber and then expelled via an outlet tube to form a dense fog cloud when it condenses on contact with cool ambient air. It can even reach air spaces in areas obstructed by dense vegetation or other objects. When the mosquitoes in flight come into contact with the droplets, they will be knocked down and killed.
  2. The cold fogger may dispense formulations in a very concentrated form and generate the droplets (usually in the 5-30 micron diameter range) in a precise manner. When the mosquitoes in flight come into contact with the chemical droplets floating in the air, they will be killed. However, its ability to penetrate dense foliage or obstacles is not as good as that of the thermal fogging. Cold fogging is sometimes called Ultra Low Volume (ULV) treatment as it allows the utilization of only a very small amount of chemical for coverage of a large area.
  3. Residual spraying is usually carried out in the form of a fine spray made to the surface where mosquitoes may come into contact. As the droplets of the spray are much larger, they settle very quickly on the surface of treated areas. Residual effects usually last from a few days to about a week to several weeks depending on the product used. Mosquitoes rest or land in treated areas may be killed.