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
- 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..
- 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.
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
- Fumigants; These are volatile gases and only
to be used by qualified personnel.
- Methyl Bromide (CH3Br)
- Aluminium Phosphide
- Magnesium Phosphide
- Calcium Cyanide
- 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:
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.
(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.
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 stuff....be 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.
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.
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
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).
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.
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.
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
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© Stuart M Bennett 2012