Insecticides fall into two types; inorganic and organic. Organic molecules always contain carbon and inorganic don't. For example Methyl Bromide (CH3Br) is organic and Ammonia NH3 is inorganic. Clicking on the name of the chemical will take you to a page with a bit of information on that chemical.

The inorganic insecticides in main use are:

  • Silica (SiO2); this acts as a dessicant and strips off the waxy coating off the cuticle of the insect thus causing suffocation. This is sometimes referred to as diatomaceous earth or kieselguhr and is made up of the frustules of diatoms (Bacillariophyceae), this material also has a tremendous surface area which explains why it is a good absorbent. These are unicellular algae characterised by the silicified cell made by two halves. (Silicified meaning infiltration or replacement of organic tissues or of other minerals such as calcite by silica).
  • Boric Acid (H3BO3); also known as Boracic Acid and is used for incorporating into baits for ant control.

The organic insecticides are split up into the following main groups:

  • Organophosphorous compounds (OP); these are compounds made up of an organic molecule to which has been added Phosphorous. There are many compounds on the market which have this basis, clicking on them will give a brief description:
  • Organochlorine compounds (OC); these are compounds made up of an organic molecule with the addition of chlorine. The downside to these types of insecticide is that they are very persistent. Some studies have shown that when Lindane has been used it is still active after a number of years. As a consequence these compounds are largely banned as they threaten the environment. Having said that, they really were good insecticides..! A few examples are listed below:
  • Carbamates(C); These are effective against a wide range of pests. Moderately residual and effective at higher temperatures, but broken down if alkalinity is high. There are loads of carbamates, I just cover the one we mostly use..
          1. Bendiocarbamate
  • Pyrethrum is a natural insecticide obtained from the flower heads of tropical chrysanthemum and has excellent knockdown properties at low concentrations. The downside to using natural pyrethrum is that it is very expensive. Another natural insecticide is Rotenone which is obtained from the Derris tree, useful as a contact insecticide.
  • Pyrethrins/Synthetic Pyrethroids(SP); These fall into two categories; those that are photostable and those that are not photostable and chemically stable. These products are sometimes mixed with another compound such as piperonyl butoxide to give a synergic effect enabling high residuality and good knockdown. I am covering one of the best known, which is Permethrin.
  • Insect Growth Regulators(IGR); These are hormones which interfere with the insects growth cycle inhibiting full development.
          1. Methoprene
          2. Hydramethylnon
          3. Pyriproxyfen
          4. Flufenoxuron
  • Fumigants; These are volatile gases and only to be used by qualified personnel.
          1. Methyl Bromide (CH3Br)
          2. Aluminium Phosphide
          3. Magnesium Phosphide
          4. Calcium Cyanide
          5. Hydrogen Cyanide

All these chemicals have to be prepared so enabling them to be applied and this is called formulation and is the different forms in which the active ingredient can be obtained as follows:

Suspo-Emulsions (SE): a suspo-emulsion is a combination formulation, consisting of a suspension concentrate coupled with an oil based emulsion. It is considered that the ideal insecticide would be one that is active immediately upon application but with good residuality. A suspo-emulsion is designed to take advantage of the high residuality of the particulate SC with the speed of the oil-in-water (emulsion)(EW). Whilst one part of the formulation (the EW) is fast acting, it is also broken down quickly in the environment but this is balanced by the particulates in the suspension that start to become effective after the water carrier has evaporated and remain active for longer.

Wettable Powders (WP): This formulation also includes water dispersible powders (WDP). WPs consist of an inert powder impregnated with the active ingredient and incorporating a wetting agent to aid dispersion in water. WPs can be used on all surfaces effectively but are particularly useful on absorbent surfaces such as unpainted wood and concrete where the insecticide particles remain on the surface of the substrate thus being available to insects walking on it. The suspension of particles in water means that the sprayer should be well agitated to prevent the particles settling out. Filters should also be checked regularly.

Suspension Concentrates/flowables (SC): The active ingredient is ground into a fine form in a liquid base and when diluted in water forms a fine suspension of particles. This formulation combines the ease of liquids with the efficacy of powder based formulations.

Emulsion Concentrates (EC): These are oily liquids in a solvent. When diluted with water a milky emulsion is formed in which the insecticidal oily droplets are finely dispersed. They should be used immediately after dilution. They should not be used on absorbent surfaces and care should be taken with some plastics and rubbers due to the solvent; terrazzo tiles are an example where one should be very careful to avoid staining, using the wrong preparation can turn them yellow. ECs are suitable for surface application but are not suitable for mist application.

Dusts: These contain a low concentration of active ingredient mixed with an inert powder. In domestic and food premises they should be applied primarily to inaccessible places (It should also be remembered that when applying dusts to pipe runs or cable runs, which are deemed inaccessible to the client, they are not inaccessible to plumbers and electricians, who could end up covered in the aware). They can be very effective in small quantities for the control wasps and ants nests, in addition to effective barrier treatments. The can be applied by small puffer packs or with a range of pressure dusters and Motor-Blo etc.

Micro-encapsulated formulations (ME): The active ingredient is encapsulated in a plastic polymer coat of polyurea. This type of formulation can improve photo-stability. The active ingredient diffuses slowly through the coating giving good residual control.

Lacquers (LQ): The active ingredient is formulated into a resin which can be brushed or sprayed onto a surface where it dries to a hard lacquer finish (obviously not suitable for carpets, though I have known some people use them in this situation). Lacquers are particularly residual and can be washed or wiped frequently with a new layer of insecticide being exposed. They are unsuitable for surfaces where discolouring is undesirable as well as carpets..!!

Smokes: The active ingredient is formulated with a pyrotechnic compound which when ignited burn to produce smoke which carries the insecticide and eventually settles out. It is usual practice to inform the fire brigade when using these in roof spaces for the simple reason that anyone passing the premises where smoke is coming out of the roof, will phone the fire brigade who will turn up to find you....later, a bill drops through your letter box for the cost of an abortive call-out.

Gels and Baits: The active ingredient is formulated in an edible bait. This is taken in by the target pest, usually having a longer term effect. This method is particularly useful for the control of cockroaches and Pharaohs ants, where the bait is also taken back to the nest.

Ultra Low Volume (ULV): What is ULV, weeellll...liquids are passed through a specially designed unit which generates an airborne mist of droplets. These droplets diffuse through the treated area and settle out on surfaces, meaning that the material used will be active both as a space spray and a surface spray. The key to successful use of ULV is the production of an optimum size of spray droplet. It must be small enough to remain airborne without being too small to hit the target. - research has shown the optimum droplet size to be around 15 microns...fogging droplets are about 5 microns and conventional spray is 80 microns. "ULV spraying utilises the minimum volume of insecticide formulation required to produce the desired biological effect with maximum economy" (quote from World Health Organisation).

Fumigants: This involves the use of volatile gases (not sprays). Fumigation is a specialised treatment and therefore requires specialised training and qualification. This process should not be carried out unless the operators have successfully completed the necessary training.

Controlled Atmosphere: Again a form of fumigation but without the use of toxic gas, instead, nitrogen or carbon dioxide and in some cases heat, are used. The downside to this type of treatment is the length of time that the material or object would need to be under the gas. This can run to weeks in some cases, depending upon the pest which is being treated.

LD50/LC50: A common measure of the acute toxicity is the lethal dose (LD50) or lethal concentration (LC50) that causes death (resulting from a single or limited exposure) in 50% of the treated animals, known as the population. LD50 is generally expressed as the dose, in milligrams (mg) of chemical per kilogram (kg) of body weight. LC50 is often expressed as mg of chemical per volume (e.g., litre (L) of medium (i.e., air or water) the organism is exposed to. Chemicals are considered highly toxic when the LD50/LC50 is small and practically non-toxic when the figure is large, (some people have difficulty getting their heads round this, as they think that if the number is large, then so is the toxicity, not so...!). However, the LD50/LC50 does not reflect any effects from long term exposure (i.e., cancer, birth defects or reproductive toxicity) that may occur at levels below those which cause death, these are covered by things such as OEL (Occupational Exposure Limit) which we are not about here.


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© Stuart M Bennett 2012