Insecticide Application: the Dose Transfer Process

In the broadest sense, the application of pesticides involves the transfer of a toxicant from the manufacturer to the target organism. This is the dose transfer process, which can be broken down into several major events. However, difficulties at any step can result in a loss of biological activity, excessive environmental contamination or both. The dose transfer process is a roadmap for the events that take place in delivering a toxicant to a target organism. In field agriculture, a small quantity of toxicant must be spread over a large area (acres or hectares). On a smaller scale, the same process is used for home garden and turf application. However, many steps in the dose transfer process are reduced when traps are used (e.g., cockroach and ant control in houses), or when the toxicant is used as a barrier (e.g., treated lumber for insect and fungus control). Each step in the dose transfer process is unique for each toxicant, its formulation, its intended use and the environment where it will be used. A general outline of the steps in the dose transfer process and emphasize their interrelatedness is presented here. While most applicable to water-dispersed toxicants applied to field crops via atomization through a small orifice, the discussion could be applied to most pest control activities with some judicious modification (Fig. 1). The act of formulation is the addition of chemicals other than the toxicant or diluent to the overall sprayed product. These chemicals are collectively called „adjuvants.‟ The manufacturer of the toxicant, distributors of the toxicant and persons involved in applying the toxicant determine the formulation that is applied to the field. While the applicator is not usually thought of as a formulation chemist, he is altering the physico-chemical properties of the toxicant-diluent mixture when adjuvants are added. Usually the objectives of adding adjuvants are: 1) consumer usability; 2) dispersal in diluent; 3) retention by foliage; 4) reduced redistribution through the drift of small droplets in the air, or the washing off of the toxicant from rainfall; 5) improved biological activity, including improved pick-up of the toxicant by the insects and redistribution to the chemically active site in the insects; and 6) correction of existing problems like the presence of specific ions in the water used for the diluent. This description of the dose transfer process focuses on the application of toxicants with water as a diluent. The second most common diluent is oil. As oil is different from water, the following discussion may not always apply to applications with oil as the diluent.