Most people know little about pesticides, yet they play a very important role in food production and personal wellbeing. Even though they are important products, we must not forget that they are poisons and may present serious dangers if not stored and used properly. The best place to learn about a pesticide that you intend to buy and use is the label printed on the container. This is a legal document that contains information developed by scientists, government regulators and suppliers over a period of several years. By following timing recommendations listed in this bulletin and on the pesticide label, you can achieve results equally good as those of the scientists that rigorously tested the product.
All insecticides and miticides are poisons. However, some are much more toxic than others. The pesticide container issues a precaution statement that indicates just how toxic the product is. For example, a skull and crossbones indicates that the most highly toxic materials. In most cases, these products should not be purchased, stored or used by those untrained in pesticide usage. Most of these products also will state "RESTRICTED USE PRODUCT," which means that only certified applicators may purchase or apply these products. County OSU Extension offices can supply pesticide certification information.
The toxicity of all pesticides is measured by a term called LD50. LD stands for Lethal Dose, or the amount of material that causes mortality of, in this case 50% of the pest population tested. Most LD50 studies are conducted with mice, rats or rabbits under laboratory conditions. Since these animals and humans are warm blooded mammals that share biochemical processes, scientists extrapolate from these tests to predict just how toxic products might be to humans. In any case, LD50 is a relative measure of toxicity. We must remember that many individuals in a population will be sensitive to the product at a level well below the LD50.
Insecticides and miticides may be purchased in forms such as dusts, wettable powders, liquid concentrates, flowables, granules, oil emulsions, aerosol sprays, baits and fumigants. Here are some good and bad points of these formulations.
Dusts (abbreviated "D") are dry mixtures of insecticides with inert powders such as organic flours, minerals, talc, or clay. The dusts are composed of fine particles, about 250 to 350 mesh. They are usually sold in strengths of 0.5-10% and are applied in the form purchased. Dusts can be used on almost any surface without harming it, but visible dust will usually create an unsightly appearance. Due to their small particle size, dusts will float in air and are easily blown away during application. They generally leave an effective residue as long as they remain dry, but when they get moist, they may cake and become ineffective. However, a slight amount of moisture on a plant may actually aid in the distribution and adherence of the dust to the treated surface. Dusts are of little use in treating large trees.
Liquid concentrates (usually denoted with "E" or "EC") are high concentrations of the pure pesticide dissolved in a solvent. Other materials are added to the concentrate to make the pesticide mix with water. Liquid concentrates are sold in strengths of about 18 to 75%. Because they are concentrates, it takes only a small amount mixed in water to make an effective spray. A disadvantage of liquid concentrates is that they may cause plant injury under certain weather conditions because of the solvents they contain.
Wettable powders (denoted by "W" or "WP") are usually made by impregnating an inert powder with an insecticide or by grinding a dry pesticide into a powder and then adding a wetting agent so that the powder particles can be suspended in water. Sprays made with wettable powders must be constantly agitated to prevent the large particles from settling to the bottom of the sprayer. Because the water used as a carrier of the powder during application evaporates, most of the insecticide is left as a residue. However, the powder itself is quite visible and this residue may be undesirable. Wettable powders are sold in strengths from 15 to 80%. As they are not formulated with solvents, they are preferred over liquid concentrates for use on plants which may be injured by solvents. However, during mixing, wettable powders have a tendency to drift or blow about. This problem is often addressed by bagging in water soluble packets ("WSP") or granulating into dry flowables ("DF").
Flowables (denoted with "F") are finely ground pesticide particles suspended in a liquid, usually water based, carrier. They have the same characteristics as wettable powders but are easier to handle during mixing.
Granules (denoted with "G") commonly contain from 1 to 20% of the insecticide impregnated onto highly absorptive materials like clays, limestone, corn cob or nut hull pieces or even fertilizer particles ranging in size from 30 to 60 mesh. Granules are heavy. This minimizes drift and prevents undue loss of insecticide and undesirable contamination of areas bordering those being treated. Granules are used mainly for ground treatment and not on foliage.
Oil emulsions contain an insecticide mixed in a highly refined oil and are used primarily for the control of household pests like cockroaches or wood borers. These formulations are sold in strengths from 4 to 5% in low-pressure, atomizer-type applicators. These oil solutions should never be used on plants because the oil will kill living plant tissue.
Aerosol sprays usually contain a mixture of several insecticides in a pressurized can. Most of them contain only a small percentage of insecticide and are designed for small jobs. Until recently, aerosol sprays were used mainly for killing flying pests in the house. Today, many of the aerosols can be applied to rose bushes and other outdoor plants. They are not practical for large scale use on ornamental plants because they are rather expensive for the amount of insecticide contained.
Baits contain a food substance attractive to the insects, along with an effective stomach poison. Formulations are available for use both inside and outside the house. In general, they need to be applied at several intervals to be most effective.
Fumigants may be purchased in solid or aerosol forms and are generally used in closed areas where a lethal concentration of the poison can be built up in the air. Fumigants are of limited value for use on ornamental plants, unless they are grown in enclosed greenhouses or polyhouses.
A systemic insecticide/miticide is one that is absorbed by plant tissue and translocated by the movement of sap from the area treated to additional parts of the plant. Systemics are effective against many different kinds of sucking and chewing insects as well as mites. When absorbed by the plant, they actually become a temporary part of it, and as the plant grows the systemic is distributed to foliage and other growing areas. This built-in poison may continue to be a toxic meal to various pests for several weeks.
One of the advantages of systemics is that when a plant is growing rapidly, even the new growth is being protected by the insecticide. Depending on application method, another good aspect of systemics is that they have little or no direct affect on beneficial insects which prey on the destructive pests feeding on the plant tissue. Also, systemics in a plant are not subject to breakdown by environmental factors such as rain, wind, temperature, and sunlight, at least not as readily as externally applied materials. Systemics cannot be used on all kinds of plants because they may burn foliage.
Systemics are available in granular and liquid concentrate formulations. Granules may be broadcast, used as a side-dressing, or incorporated in the soil at planting time. Liquids may be injected into the soil, watered onto the surface of the ground, painted or sprayed on the bark, sprayed on the foliage, or injected into tree trunks.
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Keith L. Smith, Associate Vice President for Ag. Adm. and Director, OSU Extension.
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