Although chemicals are widely used to treat Ae. aegypti larval habitats, larviciding should be considered as complementary to environmental management and – except in emergencies – should be restricted to containers that cannot otherwise be eliminated or managed.
Larvicides may be impractical to apply in hard-to-reach natural sites such as leaf axils and tree holes, which are common habitats of Ae. albopictus, or in deep wells. The difficulty of accessing indoor larval habitats of Ae. aegypti (e.g. water-storage containers, plant vases, saucers) to apply larvicides is a major limitation in many urban contexts.
Larvicides in water-storage containers should have low toxicity to other species and should not significantly change the taste, odour or colour of the water. WHO’s Guidelines for drinking-water quality provide authoritative guidance on the use of pesticides in drinking-water. Understandably, placing chemicals in domestic water, particularly drinking-water, is often viewed with suspicion and may be unacceptable in some communities.
More information on safety, quality control, guidelines for testing, insecticide resistance and application of larvicides is available from WHOPES.
Methods of chemical control that target adult vectors are intended to impact on mosquito densities, longevity and other transmission parameters. Adulticides are applied either as residual surface treatments or as space treatments.
Perifocal treatment, as described above, has both adulticiding and larviciding effects. Suitable insecticides can be applied with hand-operated compression sprayers. Care must be taken not to treat containers used to store potable water.
Indoor residual spraying (IRS) is the application of long-acting chemical insecticides on the walls and roofs of all houses and domestic animal shelters in a given area, in order to kill the adult vector mosquitoes that land and rest on these surfaces.
More information on safety, quality control, guidelines for testing, insecticide resistance and application of residual treatments is available from WHOPES.
Space spraying is recommended for control only in emergency situations to suppress an ongoing epidemic or to prevent an incipient one. The objective of space spraying is the massive, rapid destruction of the adult vector population.
Any control method that reduces the number of infective adult mosquitoes, even for a short time, should reduce virus transmission during that time, but it remains unclear whether the transient impact of space treatments is epidemiologically significant in the long run.
If space spraying is used early in an epidemic and on a sufficiently large scale, the intensity of transmission may be reduced, which would give time for the application of other vector control measures that provide longer-term control, including larviciding and community-based source reduction.
Thus, if disease surveillance is sensitive enough to detect cases in the early stages of an epidemic, and if the resources are available, emergency space spraying can be initiated at the same time as source reduction measures and larviciding are intensified.
Space spraying efficiency is dependent on:
- Method of release (aircraft, vehicle, hand-held equipment);
- Fog types (cold or thermal);
- Droplet size, application rate, climatic conditions;
- Building structures, configuration and penetration of space sprays;
- Target area size;
- Terrain and accessibility;
- Peak flight times.
More information on safety, quality control, guidelines for testing, insecticide resistance and application of space spraying is available from WHOPES.
The treatment cycle of larvicide will depend on the species of mosquito, seasonality of transmission, patterns of rainfall, duration of efficacy of the larvicide and types of larval habitat. Two or three application rounds carried out annually in a timely manner with proper monitoring of efficacy may suffice, especially in areas where the main transmission season is short.
When a rapid reduction in vector density is essential, such as in emergencies, space treatment should ideally be carried out every 2–3 days for 10 days. Further applications should then be made once or twice a week to sustain suppression of the adult vector population. Continuous entomological and epidemiological surveillance should be conducted to determine the appropriate application schedule and the effectiveness of the control strategy.