L14: Chemical Control Mosquitoes

Physical control

Alteration of the breeding site to minimise or prevent mosquito breeding

Biological control

Use of biological means, such as predators, to reduce mosquito populations

Cultural control

Raising awareness in human populations to reduce exposure to mosquitoes through behavioural change

Chemical control

Use of chemicals to reduce larval and adult mosquito populations

Where all insecticides used for mosquito control must be registered in Aus

APVMA The Australian Pesticide and Veterinary Medicine Authority

Herbicides in mosquito control

Chemicals used for controlling vegetation that may harbour larval or adult mosquitoes

4 main types of chemical control

Insecticides (larvicides, adulticides)

Surface oils/films

Repellants (personal protection)

Herbicides

Which of the 4 main methods of chemical control are direct and which are indirect

Direct: Insecticides, Surface oils/films

Indirect: Repellants, Herbicides

Important considerations when selecting / using insecticides

• Pre-treatment monitoring (larval & adult)

• Target species

• Appropriate use

• Chemical resistance

• Cost $$$

• Public safety / Environmental safety

Chemical control of pupae

No known chemical products affect mosquito pupae, though surface oils or films can control them in small areas

Chemical control of eggs

Methylbromide and chlorine can be used against mosquito eggs in limited situations like water-holding containers

(restricted to biosecurity use only!)

Larvicide advantages (4)

Controls mosquitoes before they disperse (target smaller area)

Targets specific species

Can have slow-release for longer control (up to 3 months)

Doesn't need expensive equipment for small areas

Larvicide disadvantages (4)

Application and monitoring can be hard work

Difficult to treat large sites/access sites

Timing of application is critical (install stage)

Over-application can impact other animals, esp aquatic

Bacillus thuringiensis var israelensis (Bti):

What is it and How does it work

What life stage does it work in

Bio-larvicide toxin from a bacterium

destroys the lining of the larval gut when ingested by actively feeding larvae

must be applied while actively feeding (1st to 4th installs)

Bti advantages (3)

It kills within 24 hours

The most environmentally friendly larvicide available

Very low impact on non-target organisms

Bti disadvantages (3)

It has no residual activity after 24 to 36 hours

Must be applied while larvae are still feeding

Limited shelf life

Bacillus sphaericus (Bs)

A bio-larvicide endotoxin

Ingested by larvae and destroys gut lining

Replicates in dead larvae to produce more spores = residual treatment that can last up to 3 weeks

Bs usage

Effective in waters with high nutrient loads and controls Culex and Anopheles mosquitoes (NOT AEDES)

• low impact on non-target organisms

What forms does Bti and Bs come in

Bti: Liquid, granule, water dispersible granule

Bs: wettable granule

(S)-methoprene

A larvicide insect growth regulator that mimics juvenile growth hormone, preventing progression to pupae or emergence of adults

• absorbed through outer cuticle

(S)-methoprene forms it comes in

Liquid

Granule

Pellet

Briquet (suitable for water tanks)

S-methoprene safety

Minimal toxicity to non-targets

Typical margins of safety >100x

(S)-methoprene physical effects on larvae

Colour

• often paler than untreated larvae and pupae

Malformation

• larvae appear bloated, pupae often don’t tuck under their abdomens

Mobility

• both larvae and pupae slower moving and do not react as readily to shadows, pupae often swim on their sides

Development time

• often see very slow development through instars, this often can be weeks

Adulticiding

Using chemicals to knock down and kill adult mosquitoes, traditionally through fogging or residual barrier treatments

Two ways to do fogging

Thermal fogging

ULV (ultra low volume)

When is adulticiding often the most practical approach

In small towns surrounded by significant mosquito breeding habitats

Important once mosquito-borne viruses are detected + human disease evident

Adulticiding (thermal/ULV) advantages (3)

The only effective method once mosquitoes become adults

Important during disease outbreaks

More practical + lower cost compared to larviciding in regions with extensive breeding habitat

Adulticiding (thermal/ULV) disadvantages (8)

• Temporary (ineffective once fog has dissipated)

• Long term programs therefore expensive - repeated applications

• Not target-specific (not environmentally friendly)

• Special equipment – droplet size and $$

• Limited by weather (wind, temp & humidity)

• Timing important – fog when mosquitoes are active

• Some (e.g. synthetic pyrethroids) very poisonous to fish - don’t fog across wetlands!!

• Can polarise the community

Adulticides chemical product examples

Synthetic pyrethroids:

• phenothrin (Twilight ULV)

• cypermethrin

• pyrethrin

• deltamethrin – uses water as carrier: drift?

Adulticide application

Requires well-maintained and calibrated equipment to apply a correct droplet size of 10 to 20um

Timing VITAL: when mosquitoes are active

DONT fog wetlands

Residual/surface barrier adulticides target

Target resting adult mosquitos

Residual/surface barrier adulticides application

Large droplet sprays to ‘point of run-off’ applied to surfaces where mosquitoes rest or seek shelter, providing a residual effect for 6 to 8 weeks or more

-suitable for residential areas

Residual/surface barrier adulticides advantages/disadvantages

Advantages:

Long residual effect (6-8 weeks or more) - reduced application costs

Suitable for residential areas where larviciding/repeated fogging difficult

Apply at any time (eg. pre-emptively during favourable env conditions)

Disadvantages:

not target specific

Cannot use in wetlands- Lethal to fish/aquatic organisms

Residual/surface barrier adulticides example

Bifenthrin

(Biflex Aqua / Brigade T&O / Bifenthrin Aqua)

A synthetic pyrethroid residual formulation

Surface oils and films

Chemicals that reduce the surface tension of water to stop larvae and pupae from attaching to the surface to breathe, causing them to drown

Surface oils and films usage

Useful in small man-made situations like rainwater tanks but not to be used in natural environments

Examples of surface oils and films

Kerosene

Aquatin AMF

MosquitoDrops

Chemical resistance

A genetically inherited characteristic with an underlying physiological basis that allows insects to survive chemical exposure

Chemical resistance mechanisms (4)

Reduced penetration through the cuticle

Metabolic resistance

Site insensitivity

Changes in insect behaviour

Chemical resistance mechanisms

1) Reduced penetration through the cuticle

slows the entry of the insecticide or block completely

Chemical resistance mechanisms

2) Metabolic resistance

structural change in enzyme molecule (to bind or detoxify insecticide) and/or increase the amount of the enzyme produced

Chemical resistance mechanisms

3) Site insensitivity

alteration of the insecticide’s target site

Chemical resistance mechanisms

4) Behaviour

change in insect behaviour to reduce exposure to insecticide

Resistance management

Use chemicals only when necessary

Apply at registered rate

Avoid using larvicides and adulticides from the same chemical groups

Rotate chemicals during the season

When should chemical control be used (to help resistance management)

For disease or severe nuisance control when other options )biological, physical) are not possible

What can applying chemical control at too low/high rates result in

Too high – may kill other organisms

Too low – sub-lethal doses allow survival of ‘tough’ individuals – resistance