These characteristics are the principal disadvantages of suspensions. The major
advantage of the suspension is the low-cost and easily obtainable diluent (water).
Suspensions are also relatively safe to apply since they are generally not easily
absorbed through the skin and will not cause burning of the plant foliage. Suspensions
usually have less odor than solutions or emulsions because of the water diluent, as
opposed to an organic solvent. They are especially valuable for treating outbuildings,
adobe, concrete, and thatch structures because the active material is deposited on the
surface. Solutions and emulsions, on the other hand, tend to penetrate such materials
and lose much of their residual effectiveness.
A solution is a liquid formulation consisting of a solution concentrate dissolved in
a diluent or solvent. The ideal solution would be one using water as the solvent for
reasons of economy as well as convenience. However, most of the synthetic pesticides
are relatively insoluble in water. Therefore, the solvents most commonly used are No. 2
fuel oil (used in domestic heating), diesel oil, or kerosene. Other solvents sometimes
used are xylene, benzene, and other organic liquids. The solvent selected must be one
in which the concentrate is soluble. An advantage of the solution is that constant
agitation is not necessary, which facilitates the use of spraying equipment. It is effective
as a contact insecticide as the oil base easily wets and penetrates the insect cuticle
(outer covering). In residual spraying, the solvent evaporates from treated surfaces,
leaving a deposit of the insecticide in relatively pure form. Disadvantages of solutions
are that they are readily absorbed through the skin; the solvents are flammable and
relatively expensive; they are staining; and they are, in general, toxic to vegetation.
2-10. EMULSIONS (EC)
Emulsions are prepared by diluting emulsifiable concentrates with water and
thoroughly mixing. The emulsifiable concentrate is a concentrated solution of the toxic
agent in a solvent to which an emulsifying agent (wetting agent), such as soap or a
detergent, has been added. The emulsifying agent enables the small droplets of
solvent carrying the toxicant to remain dispersed through the water, much like the fat
globules in homogenized milk. The emulsion, like the solution, does not need to be
constantly agitated. This advantage, combined with that of a cheap, readily available
diluent (water), makes the emulsion a commonly used pesticide formulation in the Army.
Emulsions are similar to solutions in their effects on insects, and they can be used for
most residual sprayings. They do not usually stain surfaces. Their principal
disadvantages are that they are readily absorbed through the skin and that they may be
toxic to plants.