lowest in cost, easy to apply, nonstaining, nontoxic to plant life, and generally not readily
absorbed through the skin. They may be dangerous if inhaled into respiratory
passages. Since they are dry, they are the preferred formulation for use around
electrical connections. Due to their small particle size, they can be used to penetrate
small cracks and crevices. Two disadvantages of dusts are that they do not adhere well
to vertical surfaces and they are easily removed by wind and rain.
Granules are pellets varying from 16 to 30 mesh in size. The granules are
usually impregnated with 5 to 50 percent of the toxicant. The carrier is generally
bentonite, although vermiculite has been used. The granular form of insecticide is
particularly desirable in mosquito breeding areas covered by heavy vegetation, which is
not easily penetrated by liquid sprays. In mosquito control work, the granules may be
applied to large breeding sites by aircraft using special dispersing equipment.
Backpack sprayers may be used for treating smaller breeding sites.
Suspensions are liquid formulations. They are prepared by diluting water
wettable powders with water and thoroughly mixing. A water wettable powder is a toxic
ingredient blended with an inert dust to which a wetting agent -- usually a soap or
detergent -- is added to facilitate mixing the powder with the water. This forms a
suspension in which the fine particles are suspended -- not dissolved -- in the water.
Suspensions require constant agitation during application to prevent solid particles from
settling to the bottom of the sprayer. They are also more prone to clog the nozzles of
sprayers. 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 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
relatively insoluble in water. Therefore, the solvents most commonly used are No. 2
fuel oil (used in domestic heating), diesel oil, or kerosene. Other organic solvents may
also be used. 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