How Insecticides Are Mixed

H. L. Haller.

Insecticidal chemicals have to be mixed properly before they can be used as insecticides. To use them in pure or undiluted form would often be too costly, and their physical properties usually they are coarse or sticky solids or viscous liquids make them unsuited for direct application. Dust diluents, solvents, and wetting, emulsifying, spreading, penetrating, sticking, and stabilizing agents are added to them.

Proper mixing of the accessory components has to take into account several factors: Whether the preparation will be applied to plants, animals, or humans; whether it will come into contact with foods or feeds; the insect to be controlled; the cost of the treatment; the ease of application; and the effect of the accessory materials on the toxicity.

For example: An oil solution would be chosen to protect a wooden post against termites because oil helps the insecticide to penetrate the post and give more than surface protection. But oils harm plants and on them water emulsions or dusts are used.

Five types of insecticide formulations are employed to control insect pests: Dusts, wettable powders, emulsions, solutions, and aerosols. Dusts are applied in hand dusters or power-driven devices; wettable powders, emulsions, and solutions are applied as sprays. Aerosols are of the liquefied gas type or smokes or mechanically generated oil clouds.

Before proceeding to consider them, let me define some terms that apply here and to other chapters in this book.

A pesticide is a substance or mixture of substances that may be used to destroy or otherwise control any unwanted form of plant or animal life. (The ending -tide means killer.)

Among the many types of pesticides are:

An insecticide is used against insects and their near relatives. Insecticides of more specific use are often designated by such terms as larvicide, aphicide, or miticide, which kill larvae, aphids, and mites, respectively.

A fungicide is used against fungi, particularly those causing diseases of plants. Some fungicides also act as insecticides.

A herbicide is used against plants growing as weeds. It often is called a weed killer.

A rodenticide is used against rodents, especially rats and mice.

Some of the terms pertaining to insecticides are:

Wettable powders are insecticidal materials manufactured into powders that can be readily mixed with water. They often contain wetting and conditioning agents.

Suspension sprays are mixtures in which the finely divided particles of powdered insecticide are dispersed in a liquid.

Emulsion concentrates are insecticidal materials manufactured into liquid concentrates so formulated that they will form an emulsion when mixed with water or another liquid.

Emulsion sprays are mixtures made with emulsion concentrates and a liquid, usually water.

Conventional or dilute sprays contain a relatively small amount of insecticide in a relatively large amount of water, such as 4 pounds per 100 gallons.

Concentrated sprays contain large amounts of insecticides in small amounts of liquid, such as 1 pound in 1 to 5 gallons.

Terms like stomach insecticide and contact insecticide indicate the way the insecticide enters the body of the insect. A stomach insecticide is eaten and swallowed. A contact insecticide enters through the skin. The terms have no significance as to how or where the materials exert their effect. Some substances can enter in only one way. Others reach the vital organs in both ways.

INSECTICIDAL DUSTS usually are made with talcs, clays, and diatomaceous earth. Sometimes finely ground plant material, such as walnut-shell flour, is used.

Diluents of dusts are classed according to whether they have low or high bulk density. By this is meant the weight of the dust occupying a definite volume. The low bulk density, or light, type is illustrated by silica gel, hydrated alumina, calcium silicate, and diatomaceous earth. Examples of the high bulk density, or heavy, type are pyrophyllite, talc, calcite, and clays. Mixtures of both types often are used to prepare products that have practical bulk-density values and will also resist caking on storage at high temperatures. The use of the heavy diluents alone may yield products that become packed or lumpy on storage.

Dusts also may be prepared by mixing a solution of the insecticidal chemical in a volatile organic solvent, such as acetone or benzene, with the dust diluent; the solvent is then allowed to evaporate, and the mixture is ground. Or a solution of the insecticide may be sprayed into the dust diluent during the mixing and grinding process. Sometimes the chemical is dissolved in a nonvolatile solvent and mixed with the diluent. When this is done, care must be taken so that the amount of the solvent used is not so great as to impair the dusting qualities of the finished dust. The concentration of the active ingredient of dusts ranges from 1 to 20 percent, depending on the insecticide and its use.

Wettable powders, which can be dispersed or suspended in water for use as sprays or dips, are made by adding wetting agents to dusts. With some kaolin types of clay as the diluent the addition of a wetting agent is unnecessary. The wetting agent may be adversely affected by the type of diluent or kind of water, such as extremely hard or highly alkaline. The amount of wetting agent must be carefully adjusted to avoid excessive run-off when the spray is applied to plants.

EMULSIONS are obtained by adding water to an emulsifiable or emulsion concentrate. Such concentrates are made by dissolving the insecticidal chemical and an emulsifying agent in an organic solvent. Usually the solvent is substantially insoluble in water as water-miscible solvents have not in general proved satisfactory.

Two general types of solvents have been used : (1) Solvents, such as toluene or xylene, which evaporate after spraying or dipping to leave a deposit of the toxicant; and (2) nonvolatile solvents, such as alkylated naphthalenes or a petroleum oil, which leave the treated surface coated with a solution of the toxicant in oil after the water has evaporated. Solvents such as toluene and xylene under certain conditions may constitute a fire hazard. The use of high-boiling aromatic solvents, such as the alkylated naphthalenes, may be dangerous when the emulsion is applied to animals.

Three classes of emulsifiers are generally recognized anion-active, cation-active, and nonionic. Soap is typical of an anion-active agent. Lauryl pyridinium chloride is an example of a cation-active emulsifier. Nonionic emulsifiers, as the name implies, do not ionize. An example of a nonionic emulsifier is the reaction product obtained from 10 to 12 moles of ethylene oxide to 1 mole of dodecyl alcohol. Several hundred emulsifiers are commercially available under various trade names. No one class of emulsifiers may be said to be superior to another. The type best suited will depend on the insecticide and can only be determined by experimentation. When extremely small particle size or permanence of the emulsion is not essential, or if agitation can be maintained after the concentrate has been diluted with water, the proportion of emulsifying agent in the concentrate may be reduced considerably.

Oil solutions of insecticidal chemicals are usually made with crude or refined kerosene and other petroleum oils. The selection of a solvent depends on its ability to hold the chemical in solution at ordinary temperatures, whether it is toxic to plants, and whether it constitutes a fire hazard. Sometimes more than one solvent is used, particularly when a preferred solvent does not dissolve sufficient of the insecticidal chemical to provide a solution of the desired concentration. An example is DDT in refined kerosene. This solvent does not dissolve enough DDT to permit the preparation of a 5-percent solution. One has to add an auxiliary solvent. Auxiliary solvents dissolve larger quantities of the chemical. Cost, toxicity, and fire hazard keep them from being used as the only solvent. When a solution is made up of two solvents, one of which is a poor one and the other a good one for a particular chemical, the solubility in the mixed solvent may not equal the sum of the solubilities in the individual components. The quantity of a chemical that a solvent will dissolve varies widely with the temperature.

All in all, so many factors are involved in the formulation of insecticides that it usually behooves a person to buy the ready-to-use insecticides rather than to try to mix the chemicals himself.

H. L. HALLER is assistant chief of the Bureau of Entomology and Plant Quarantine. His duties cover the various chemical aspects of the Bureau's problems and involve the development and use of products for the control of insect n pests. He has been engaged since 1929 in studies on insecticides in the Department, which he joined in 1919 following service in the First World War. From 1923 to 1929 he was on the staff of the Rockefeller Institute for Medical Research as an associate in chemistry.