Chemical control of mosquitoes

It is important to regularly calibrate chemical application equipment to prevent under or over-applying the product. Under-application can result in poor treatment efficacy and potentially increase the risk insect resistance. Over-application can be costly, and whilst there is a reasonable safety margin for most chemicals, applying a chemical at a rate higher than that specified on the label may have a negative impact on the environment. 

Application of a chemical according to the registered, label instructions is also a legal requirement by the APVMA. A factsheet on equipment calibration (PDF 188KB) is available to assist you in this process. Alternatively, you may wish to contact Medical Entomology for guidance.

Larvicides kill or disrupt the development of mosquito larvae, resulting in death before the adult can emerge and pose a potential health or nuisance risk. If used at specified label rates, larvicides are target-specific. Therefore, the environmental impact can be limited through appropriate product use. For these reasons, larvicides should be considered the preferred option over adulticides, if chemical control is deemed to be the most appropriate management strategy.

It is important to determine if the larvae collected are a species that is of concern as a nuisance or public health risk and therefore require chemical control. A photographic key for identification of mosquito larvae in the South West can be found here.

Pre-treatment surveys will need to be undertaken to determine where and when application is required, ensuring the product is applied at the correct stage of larval development. Post-treatment surveys are also recommended to evaluate treatment efficacy.

The advantages of larvicide application include: 

• target-specific when applied at the label rate, resulting in minimal impact on the environment
• require a potentially smaller area to treat as larvae are confined to breeding sites (compared to adulticide application)
• a range of product formulations are available allowing for flexibility in treatment
• no specialist equipment required when treating small areas.

• breeding sites need to be mapped, which can be labour intensive 
• different products are effective at different stages of mosquito lifecycle, leaving a small window of opportunity for larvicide application to be successful
• treating large areas can be impractical and expensive.

The active constituent in larvicide products used for mosquito control generally includes a bacterial toxin (Bti or Bs), an insect growth regulator (S-methoprene) or a combination of both. Their modes of action are very different, requiring application at specific stages of the mosquito life cycle in order to be effective (see table below for summary). A range of product formulations are available, including liquids, granules, pellets and briquets.  

Adulticides target adult mosquitoes and can be used in fogging activities or as a residual barrier treatment. It is important to note that adulticides are not target-specific and will kill other insects, such as bees and dragon flies, and can also be lethal to fish.

Fogging

Fogging involves the application of an adulticide, generally a synthetic pyrethroid, via thermal or ultra-low volume (ULV) space spraying equipment. The mode of action of synthetic pyrethroids involves disruption of the nervous system, resulting in paralysis and eventual death of the adult mosquito. As adulticides are not target-specific, fogging is only recommended when there is an imminent public health risk associated with mosquito-borne disease transmission. There is no residual effect from these products. Fogging activities should be planned appropriately to ensure wind conditions are optimal, there is no rain and the product will not drift over wetlands or water bodies where fish may be present. 

A common misconception is that fogging does not require any monitoring of the target population, or at the very least, only requires monitoring of adult numbers to evaluate treatment efficacy. It is important to note that larval monitoring helps to anticipate the timing of adult emergence and therefore, the likely timeframe in which fogging will need to take place.

Thermal fogging requires the adulticide to be first diluted in a carrier liquid (often oil-based). The fogging equipment uses hot gas to heat and vaporise the liquid. On application, the vapour hits cold air and condenses to form a visible white fog (as seen below). Ultra low volume (ULV) or cold fogging equipment uses large volumes of air at low pressure to break up the liquid into droplets. The spray droplets are generated without the need for heat. Water-based diluents can be used, which are more environmentally favourable. The white fog associated with thermal application is not observed with ULV application.

Residual barrier treatments

Residual barrier treatments involve application of a synthetic pyrethroid to any surface where adult mosquitoes may land. This may include internal/external building walls, eaves, fences, vegetation or foliage. If applied appropriately, the product binds well to surfaces and can provide control for 6-8 weeks. Residents can have these applications applied by a licenced pest technician. It is important to note that these products are not target-specific either, and will knock down all other insects that come in to contact with the surface. They are also toxic to fish and other aquatic fauna so should not be applied near waterways.

A thin film of oil, kerosene or other substance can be used to break the surface tension of a water body, making it difficult for mosquito larvae to attach to the surface. As a result, the larvae are unable to breathe and will drown. The application of surface films/oils can be useful in relatively small water-holding containers such as rainwater or septic tanks, where it is not possible to adequately apply insect screening to tank openings. 

Whilst the application of a surface film/oil results in the eventual death of mosquito larvae, these products are not typically considered larvicides, given the substance does not directly alter the organism’s physiology.