Pesticides are natural or synthetic compounds that used to kill pests including insects and rodents. A pesticide is composed of two parts, active ingredient(s) and inert ingredient(s). Active ingredient(s) is responsible for the pesticidal effect on target pests and inert ingredient(s) is added to facilitate the delivery or enhance the action of the pesticides. The combination of these two ingredients in a pesticide product is known as pesticide formulation which determines the chemical and physical characteristics of a pesticide.

Types of Pesticides

Pesticides are commonly classified according to the types of pests they control, e.g. insecticides for killing insects and rodenticides for killing rodents.  Pesticides can also be classified by the chemical or biological natures of the active ingredient(s).  The commonly encountered ones including:

1. Organochlorines
Synthetic organochlorines are one of the earliest groups of chemicals being used as pesticides.  Benefits from their broad spectrum and long residual effect, they are very effective in controlling various pests.  However, they are hard to breakdown in the natural environment due to their stable chemical nature.  Their prolonged usage in large quantities will easily result in environmental pollution and bioaccumulation in living organisms.  Organochlorine pesticides are therefore banned in many countries/areas, including Hong Kong, under general circumstances and gradually replaced by other types of pesticides.

2. Organophosphates
Organophosphate pesticides are characterised by their multiple functions and their capacity of controlling a broad spectrum of pests.  They can be used not only as stomach poisons but also as contact poisons and fumigants.  Members of these pesticides are biodegradable, cause low environmental effect and show low pest resistance.  Temephos and fenitrothion are samples of organophosphate pesticides.

3. Carbamates
Carbamate pesticides work on the same principle as organophosphate pesticides by poisoning the target pest nervous system.  As the molecular structures of carbamates are largely similar to that of natural organic substances, they can be degraded in a natural manner with low environmental impact.  Propoxur is an example of carbamate pesticides. 

4. Synthetic Pyrethroids
Synthetic pyrethroids are synthesised by imitating the structure of natural pyrethrins, but they are comparatively more stable and have longer residual effects.  These pesticides are highly toxic to insects, but have relatively low toxicity to mammals.  Allethrin and permethrin are examples of synthetic pyrethroid pesticides.

5. Microbial Pesticides
Microbial pesticides control pests by means of pathogenic microorganisms, such as bacteria, fungi and viruses.  These pesticides are target specific, i.e. have very low or no effects on non-target organisms.  Bacillus thuringiensis israelensis (B.t.i.) is an example of microbial insecticides. 

6. Insect Growth Regulators (IGRs)
IGRs are compounds developed by mimicking insect juvenile hormones.  Their main function is to interfere with the growth of the insects and prohibit them from developing into adults.  As the growth of insect pests is curtailed, they eventually die leading to the depression of the whole insect pest population.  Methoprene is an example of IGRs. 

Pesticides can act in four distinctive ways as poisoned baits, contact pesticides, fumigants and systemic pesticides to kill the target pests.  For instance, rodenticide mixed with grains to form poisoned baits used to kill rats via ingestion.  Temephos can be applied to water bodies and acts as contact poison to kill mosquito larvae upon contact.  Permethrin formulated in ‘Flea bomb’ functions as fumigant and kill the pests once inhaled into the body.  Systemic pesticides consumed by a host organism will stay in its body fluids; and pests feeding on the host’s body fluids will then be killed by poisoning.  Systemic pesticides are more commonly applied in agriculture for plant protection.

Application of Pesticides

When using pesticides, one should select the suitable type and formulation according to the target pest and the control purpose.  The selected pesticide should then be applied by appropriate methods.  Application methods can be generally classified as follows:

1. Spraying

Spraying is one of the most common forms of pesticide applications.  It converts liquid pesticides into droplets of various sizes for different control purposes by applying through sprayers.  It can be broadly divided into two kinds – space spraying and residual spraying.  Space spraying is to apply knockdown pesticides over the space where pest activities are detected.  These pesticides release as tiny droplets that suspend in the air and kill the passing insects that come into contact.  It is mainly used to combat flying insects, e.g. mosquitoes, flies and wasps.  Residual spraying is to apply residual pesticides over the surfaces where pest activities are detected to provide long term control.  It is mainly used to combat crawling insects, e.g. cockroaches, fleas, ticks and ants, or treat resting places of the pests.  In order to have the pesticide effectively applied, right kind of sprayers must be selected, properly maintained and used correctly in any operations.

Sprayers

Sprayers are the most commonly used pesticide application equipment.  They enable the production of pesticide vapours in different sizes for various indoor and outdoor pest control operations.  Sprayers available on the market include manual sprayers or power-operated sprayers ranging in different forms.  Sprayers that usually use in the control of public health importance pests include:

(I) Hand Pump

A hand pump operates on a simple piston principle whereby the pesticide is drawn out from the solution compartment.  The holding capacity of the solution compartment is usually about 0.5 to 1.0 litres.  When the piston is pulled backward, the pesticide will be released from the nozzle.  Hand pumps are usually used in small scale knockdown operations.

(II) Compression Sprayers

A compression sprayer is composed primarily of three parts – a pesticide tank, a pressurised device (e.g. plunger) and a discharging system (e.g. trigger valves, supply tube, hose and nozzle).  Air inside the pesticide tank is compressed by the plunger and the tank is pressurised.  When the trigger valve is released, the liquid pesticide is pushed into the supply tube and through the hose to the nozzle where it will be broken up and sprayed out as small droplets.  The nozzle can be replaced or adjusted to spray in different patterns for various control purposes.  There are basically three nozzle patterns.  They are:

1. Fan pattern: It offers the widest spray angles and is suitable for residual spraying.
2. Cone pattern: It offers better penetration power into and coverage of foliage or irregular target areas.  It is suitable for general space spraying, such as elimination of flies.
3. Solid stream pattern (jet): It offers a jet to spray distant and specific target, such as wasp nest on tree.

(III) Hand-carried Thermal Foggers

A thermal fogger is composed primarily of two parts including a pulse-jet engine and a formulation system.  The pulse-jet engine generates a high temperature air stream, evaporating the pesticide formulation which will then be released through a resonator.  When released pesticide vapours contact with the colder air, the vapour will immediately condense into tiny droplets with a diameter mostly less than 20 microns.  These droplets are highly effective in penetration, diffusion and adhesion, and therefore, suitable for outdoor pest control work covering a large area.  Besides, the fog produced by a thermal fogger is highly visible facilitating the monitoring of fog diffusion and penetration.

(IV) Knapsack Cold Foggers

A cold fogger is composed primarily of three parts including a driver (engine and air compressor), a pesticide tank and a nozzle system.  Activated by the two-stroke petrol engine, the centrifugal fan of the fogger generates a high speed air stream channelling the liquid pesticide in the pesticide tank into the nozzle system through a delivery tube.  The pesticide solution will then be broken up into droplets by the centrifugal force of a high-spin atomiser.  The cold fogger is a high performance spraying machine with wide coverage.  The cold fogger is suitable for outdoor pest control work involving a large area, and can be operated in different speed and fitted with different nozzle orifices to adjust the droplet size.

(V) Ultra-Low Volume (ULV) Foggers

ULV foggers operate in the same principle as those of cold foggers.  The size of droplets generated by ULV foggers can reach below 30 microns in diameter.  These droplets have high uniformity, high penetration power and ability to remain airborne for a substantial period of time.  Therefore, ULV foggers are comparatively more effective than other cold foggers.

As the size of pesticide droplets released from foggers are relative small, they can remain suspended in the air long enough to be contacted with insects in flight.  During outdoor applications, these droplets are very susceptible to air currents / turbulence, and may drift off from the treated area.  If the temperature is high, the droplets may be carried away by the rising air current and dissipated in the atmosphere.  Therefore, effect of environmental conditions such as temperature, wind direction and wind speed on the pesticide application should be considered before, during and after fogging.

2. Dusting

Dusting is a method that sprays or scatters pesticides in fine dust form (dustable powder) in areas needed to be treated by a duster.  The body of pests will be stained with the powder as they pass through the treated areas.  Pesticides in dustable powder formulation are especially suitable for treating areas / objects that cannot be wetted, such as carpet surface and books.  By using the right dusting nozzle, the pesticides can be sprayed into cracks, crevices or holes.  Dusting should not be conducted in outdoors under windy conditions or damp conditions, as the dustable powder will be blown away from the target area or clumped together becoming ineffective.  Dusting is suitable for controlling fleas, cockroaches and centipedes.

3. Fumigation

Fumigation kills the target pests within an enclosed area by filling the area with gaseous pesticides (fumigants).  Its advantage lays in the strong penetration power of the fumigants that allowing treatment of areas with many void spaces (cracks and crevices) to shelter pests.  Another advantage of fumigation is the ease of gas diffusion, which will leave no residue on the surface of treated objects.  However, due to its strong penetration power, the area to be treated must be completely enclosed to avoid leakage of fumigants.  Besides, it is necessary to ensure that the fumigants disperse completely after application before unsealing the treated area.  Otherwise, humans and animals may easily take up the fumigants by inhalation and get harms.

4. Baiting

Baiting is to place poisonous baits at a suitable location for the target pests to ingest.  In general, no special equipment is required for baiting.  A simple device, such as a bait box, may need to protect the baits to be taken by non-target organisms or tampered by external factors, e.g. weathers.  To have a successful baiting operation, the location to place the baits is crucial.  The baits should be placed:

1. near to active runways / activity areas of pests so that the baits can be easily discovered;
2. in areas not easily accessible to humans and non-target organisms; and
3. far away from food, utensils and food processing area.

When applying pesticides, applicators should wear suitable personal protection equipment and take appropriate measures to protect themselves, occupants and the surrounding environments before, during and after the pesticide applications.

You may also visit Agriculture, Fisheries and Conservation Department website
http://www.afcd.gov.hk/english/quarantine/qua_pesticide/qua_pesticide.html
for more details about pesticides.