Types of Pesticides
Pesticides are natural or synthetic compounds that are poisonous and can kill pests including insects and rodents. Before the Second World War, pesticides were mainly made up of minerals or plants. They were highly toxic to mammals and had long-term residual effects, easily causing environmental pollution. Since the end of the Second World War, more effective pesticides including organochlorines, organophosphates, carbamates and synthetic-pyrethroids have been gradually developed.
Classification of Pesticides
There are various ways by which pesticides are classified. Pesticides can be classified according to four distinctive functions, namely, stomach poisoning, contact poisoning, fumigation or systemic action.
Stomach poisoning - thepesticide enters the body of pests via their mouthpart and digestive system and causes death by poisoning. Pesticides that function in this way are known as stomach poison.
Contact poisoning - the pesticide enters the body of pests via their epidermis upon contact and causes death by poisoning. Pesticides that function in this way are known as contact poison.
Fumigation - the pesticide in gas form enters the body of pests via their respiration system and causes death by poisoning.
Systemic action - pesticides consumed by a host organism will stay in its body fluids. Pests feeding on the body fluids of the host organism will then be killed by poisoning.
Pesticides can be classified by their chemical nature:
1. Organochlorine Pesticides They are the synthetic organic pesticides that are earliest discovered and used. Their characteristics are broad-spectrum, long residual effect and relatively low toxicity. However, due to their stable chemical nature, they are hard to break down in the natural environment. Prolonged use in large quantities will easily lead to environmental pollution and accumulation in mammals, resulting in cumulative poisoning or damage. Organochlorine pesticides are therefore banned under general circumstances and gradually replaced by other pesticides.
2. Organophosphate Pesticides Organophosphate pesticides are characterised by their multiple functions and the capacity of controlling a broad spectrum of pests. They are nerve poisons that can be used not only as stomach poison but also as contact poison and fumigant. These pesticides are also biodegradable, cause minimum environmental pollution and slow pest resistance. Temephos and F enitrothion are examples of organophosphate pesticides.
3. Carbamates Carbamate pesticides work on the same principle as organophosphate pesticides by affecting the transmission of nerve signals resulting in the death of the pest by poisoning. They can be used as stomach and contact poisons as well as fumigant. Moreover, as their molecular structures are largely similar to that of natural organic substances, they can be degraded easily in a natural manner with minimum environmental pollution. Propoxur is an example of carbamate pesticides.
4. Synthetic-pyrethroid Pesticides Synthetic-pyrethroid pesticides are a pesticide synthesized by imitating the structure of natural pyrethrins. They are comparatively more stable with longer residual effects than natural pyrethrins. Synthetic-pyrethroid pesticides are highly toxic to insects but of only slight toxicity to mammals. A llethrin and Permethrin are examples of synthetic-pyrethroid pesticides.
5. Microbial Insecticides Microbial insecticides control pests by means of pathogenic micro-organisms including bacteria, fungus and viruses. Bacillus thuringiensis israelensis (B.t.i.) is an example of microbial insecticides.
6. Insect Growth Regulators Insect growth regulators are compounds developed by copying insect juvenile hormone. The main functions are to interfere with the growth and hatching of larvae into adults, and to prevent the formation of exoskeleton so as to prohibit the growth of the insect. As its ability to live as a living organism is curtailed, the insect may die eventually as well as the whole insect population. Methoprene is an example of insect growth regulators.
Application of Pesticides
When using pesticides, one should choose the suitable type and formulation according to the target pest and the purpose. The chosen pesticide should then be applied by suitable methods. Application methods can be generally classified as follows:
There are broadly two kinds of spraying: space spraying and residual spraying. Liquid pesticides are released from various types of sprayers, forming small droplets that float in the air or settle on object surfaces with which the passing insects will come into contact. Space spraying is to apply knockdown pesticide over the space where there are pest activities. It is mainly used to combat flying insects like mosquitoes, midges, flies and wasps. Residual spraying is to apply residual pesticide over the object surfaces on which there are pest activities. It is mainly used to combat crawling insects or those staying on an object surface for a long period of time, e.g. cockroaches, fleas, ticks and ants.
Sprayers are the most commonly used pest control equipment. They are light, convenient and come in various types, suitable for indoor and outdoor small scale / local control operations to eradicate pests of public health importance, e.g. mosquitoes, flies and cockroaches. Sprayers available on the market include manual sprayers and compression sprayers.
(I) Hand Pump
A handpump operates on a simple piston principle whereby the pesticide is drawn out from the solution cylinder. The holding capacity of the solution cylinder is usually about 0.5 to 1.0 litre. When the piston is pulled backward, the pesticide will be drawn out from the cylinder. When the piston is pushed forward, the pesticide will be released from the nozzle. Manual sprayers are usually used in small scale knockdown operations.
(II) Compression Sprayers
A compressionsprayer is composed primarily of three parts, including the solution tank , the pressurised device (piston) and the nozzle system (control / tube / resonator / nozzle). Air inside the solution tanks is compressed and pressurized by using the piston. When the trigger on / off (valve) is released, the liquid pesticide is pushed into the tube by high pressure and transferred to the nozzle system where it will be broken up into droplets, each with a volume diameter of about 100 to 400 microns.
The length of the application wand of the compression sprayer can be adjusted according to need. Its nozzle can also be replaced or adjusted to suit various spraying purposes -
Fan nozzle : offering the widest spraying angle, s uitable for residual spraying.
Cone nozzle : suitable for general sp ace spraying, such as the elimination of flies.
Jet : the cone nozzle can be adjusted to spray jet, allowing the liquid pesticide to reach its target (such as a wasp nest high up) at a far distance.
(III) Thermal Foggers
A thermal fogger is composed primarily of two parts including the pulse-jet engine and the formulation system. The pulse-jet engine generates a high temperature air stream, evaporating the formulation which will then be released through the resonator. When come into contact with cold air, the vapour will immediately condensed into small droplets. With a diameter of less than 50 microns, these droplets are highly effective in penetration, diffusion and adhesion, suitable for outdoor pest control work covering a large area. The fog produced by a thermal fogger is highly visible, facilitating the monitoring of aerosol diffusion and penetration.
(IV) Cold Foggers
A cold fogger is composed primarily of three parts including the driver (engine and air compressor), the solution tank and the nozzle system. Activated by the two-stroke petrol engine, the centrifugal fan of the fogger generates a high speed air stream channelling the fogging solution into the nozzle system via a delivery tube. The pesticide solution will then be broken up into droplets with a diameter of about 50 to 100 microns by the centrifugal force of a high-spin atomiser. The cold fogger is a high performance spraying machine with wide coverage. With a light breeze, the insecticide mist can cover 70 to 100 metres. The cold fogger is suitable for outdoor pest control work involving a large area, and can be fitted with different nozzle orifices to adjust the speed of fogging and droplet size.
(V) Ultra Low Volume (ULV) Sprayers
The structure and operating principle of ULV sprayers are the same as those of cold foggers. The size of droplets generated by an ULV sprayer may vary from several cubic microns (the tiniest) to less than a hundred cubic microns (the largest). Droplets of this size have strong penetration force and are most effective in controlling pests such as mosquitoes. In comparison with other cold foggers, ULV sprayers are more effective as they can deliver the correct-sized droplets with high uniformity.
Environmental conditions such as temperature, wind direction and wind speed should be noted when spraying is conducted, in particular during fogging applications. The size of droplets also has an important bearing on the effectiveness of spraying operations. Droplets with a big diameter will not be able to remain suspended in the air, and their contact with insects in flight will be reduced. Droplets with a small diameter will be susceptible to the turbulence created by insects in flight or other air currents. This will also reduce their contact with insects. In outdoor applications, most droplets will be carried away by the rising air current and dissipate in the atmosphere. The droplet size also affects the ability to penetrate into vegetation and other objects.
A duster is used to spray or scatter powder pesticides in areas needed to be treated. The body of pests will be stained with the powder as they pass through it. Powder pesticides are especially suitable for treating those areas / objects that cannot be wetted, such as carpet surface and books. By using the right nozzle, powder pesticides can be sprayed into the cracks and crevices or holes. Attention should be paid to the weather and situation on the ground during the application. Dusting should not be conducted outdoors under windy conditions. Dustable powder should not be applied on rainy days nor can it be applied in moist or damp areas. Dusting suitable for controlling fleas, cockroaches, centipedes and the larvae of certain insects .
Gas pesticides are used in fumigation. Its advantage lies in the small molecules which have a strong penetration force than any other pesticides. It is especially suitable for treating areas with many cracks and crevices where pests like to shelter. Another advantage of fumigation is the easy dispersion of gas, which will not leave any residue on the surface of treated objects. However, due to its strong penetration force, preparation must be thorough before fumigation is conducted, and the area to be treated must be completely enclosed to avoid leakage of pesticide. There is also a need to ensure that the pesticide is dispersed completely after application before the treated area is unsealed. Also, certain fumigant gas pesticides are highly toxic and are not suitable for use in places where there are human and animal activities.
In general, no special equipment is required for baiting. Only a simple device, such as a rodent bait box or cockroach bait box, is needed to put in the baits. It is most important to choose a suitable location for placing the baits. 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;
3. far away from food, utensils and food processing area.
You may also visit Agriculture, Fisheries and Conservation Department website
for more details about pesticides.