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Each pesticide contains a chemical or 'active ingredient' (a.i.) which is responsible for its pesticidal effect, but pesticides are rarely supplied as preparations of neat, concentrated or technical grade chemical. The active ingredient must be FORMULATED with other non-pesticidal compounds before it is ready to use. Formulation generally improves the properties of a chemical for: storage, handling, application, effectiveness and safety. The FORMULATION of a pesticide is the form in which it is sold, not necessarily that in which it is used.
By far the most popular and frequently used formulation is a spray, used for around 75% of pesticides(Ware 91). The formulation of pesticides as sprays or dusts allows small amounts of pesticide to be applied directly onto the pest or its immediate environment, but both of these formulations tend to have a problem with spray drift and distribution of pesticide onto non-target areas. Where the pesticide doesn't have to be applied directly onto the pest but only delivered to a particular location where it will be available to the pest granules can be very successful, especially as they do not have the associated problem of spray drift.

Sprays
While sprays may be a very popular means of applying pesticides, only a few pesticides are very soluble in water and can be supplied as aqueous solutions or water-soluble powders. Most pesticide compounds are effectively insoluble in water and need an organic solvent or very specialised formulation to enable them to be mixed with water for spraying.

Emulsifiable concentrates
Concentrated solutions of a.i. in oil with an emulsifier to allow the concentrate to mix readily with water. The emulsifier is a detergent-like material that allows microscopically-small oil droplets to be suspended in water to form an emulsion. The concentrate disperses uniformly in water and will usually remain evenly suspended for at least a day.

Water miscible liquids/powders
Liquids or finely ground solids that mix readily with water. The a.i. is miscible with water or alcohol and so produces true, clear solutions that don't precipitate out.

Wettable powders or water-dispersible powders
The a.i. is mixed with a fine dust (usually clay or talc) and a wetting agent (usually dry soap or detergent), this allows the pesticide to be dispersed in water before spraying. The a.i. is insoluble in water and so without the wetting agent the powder would simply float on the water and be impossible to mix. These formulations tend to 'settle' quite quickly and need to be used quickly or agitated regularly if the concentration of the pesticide in the spray liquid is not to vary during spraying.

Flowable/sprayable suspensions or suspension concentrates
Pesticides that are not soluble in water can also be formulated as suspension concentrates, that is as a suspension of very finely ground dust dilutent and a.i. in a non-solvent liquid (usually water). This suspension will then mix well with water and can be sprayed in the same way as wettable powders. These formulations have a tendency to sediment out if stored for long periods of time but this can be reduced by 'thickening' the liquid concentrate with polymers that tend to fix the solid in suspension.
A similar formulation is the 'flowable microencapsulated' in which the pesticide is held in small, permeable spheres of polymer or plastic 15-12 microns in diameter.

Oil solutions
The a.i. is dispersed in oil and applied as an oil-based spray. Many 'ready-to-use' pesticides come in this form.

Dusts
The a.i. is mixed with a solid, particulate dilutent (usually with a size range of 50-100 microns) that is then mixed with the air with the aid of a dusting machine. Historically, these have been the simplest to make and easiest to apply. However, application rates are high, pesticide concentration rates low and hence while pesticide deposition rates on the target tend to be low they are also prone to spray drift onto non-target areas. Dusts may be formulated as:

• undiluted toxic agent - e.g. sulfur dust in agriculture
• toxic agent + active dilutent - e.g. insecticide + sulfur dust as dilutent
• toxic agent + inert dilutent - e.g. insecticide + clay as dilutent.

Granules
Granules are small pellets (usually 0.3mm - 1.3mm) of inert carrier (often clay) mixed with the pesticide a.i. to give the desired concentration. Granules can be formulated to allow either rapid release of the pesticide or slow, controlled release over time. Granules can also be particularly useful when dealing with very toxic pesticides which can be 'shut away' in the granule, allowing them to be handled more safely.

Aerosols
To be used in an aerosol the pesticide must be soluble in a volatile, petroleum solvent when pressurised. When the aerosol is sprayed and atomised the solvent evaporates rapidly leaving micro-droplets of toxicant suspended in the air. Usually used for indoor application of insecticides, repellents and disinfectants.

Fumigants
Fumigants must be vaporous liquids or gases with a relatively high vapour pressure so that they can exist as a gas in high enough concentrations to kill pests in soil or enclosed spaces. The toxicity of the fumigant is proportional to its concentration and the exposure time. Fumigants are widely used for soil treatment to control insects, nematodes, soil-borne diseases and weed seeds and in stored-product pest control. The number of chemicals suitable for use in fumigation is limited, examples include hydrogen phosphide, methyl bromide, ethylene dibromide and dichlorvos (Dent 1991).

Baits
The pesticide is incorporated with a 'bait' which attracts the pest to the pesticide. The bait may either be food which is then eaten by the target animal (as in vertebrates and molluscs) or an attractant which is used to lure the target to the bait where the pesticide is transferred by contact (as in many insects). One of the main advantages of baits is that, used properly, there is practically no release of pesticide into the environment.

Slow Release Formulations
There are many potential advantages to being able to control the rate at which a pesticide is released into the environment. Rather than applying one, single, big dose of pesticide the same amount released over a period of time will have a much greater pesticidal effect. In theory, it should be possible to deliver a constant dose of pesticide to the pest/pest environment over time. Slow release formulations include:

1. microencapsulation & laminated strips

In both of these case the pesticide is surrounded by an insoluble polymeric membrane. Provided that the concentration of the pesticide inside doesnot change the pesticide diffuses through the membrane at a constant rate, giving a constant dose outside over time. This means that the pesticide is effective for 2-4 times longer than if applied as an emulsifiable concentrate.

2. polyvinylchloride strips & rubber pellets

these are impregnated with volatile pesticides (e.g. dichlorvos insecticide). The rate of loss and therefore the dose generally decreases over time, although this can be controlled to some extent.

References

Barlow, F (1985) Chemistry and formulation. In: Pesticide Application: Principles and Practice. Ed: P T Haskell. Oxford Science Publications: Oxford. pp 1-34.
Dent, D R (1995) Integrated Pest Management. Chapman & Hall: London, Glasgow, Weinheim, New York, Todyo, Melbourne, Madras.
Rombke, J & J M Moltmann (1995) Applied Ecotoxicology. Lewis Publishers: Boca Raton, New York, London, Tokyo.
Ware, G W (1991) Fundamentals of Pesticides. A self-instruction guide. Thomsom Publications: Fresno USA.