Insect Note #8 (Revised)

Dated 6/94
Placed on the Web 4/95 by the Department of Entomology, NCSU
Revised: 3/05

Insecticide Formulations for Home Gardeners

Proper mixing or formulation of insecticides and accessory components such as dust diluents, solvents and wetting, emulsifying, spread, penetrating, sticking and stabilizing agents has to take into account several factors: 1) Whether the preparation will be applied to plants or animals; 2) whether it will come in contact with food or feed; 3) the type of insect to be controlled; 4) the cost of the treatment; 5) the ease of application; 6) the effect of accessory materials on toxicity and, 7) the physical and chemical compatibility of both the formulated material and dilute sprays, dusts and granules.

Formulations:

1. Dusts: The concentration of a toxicant is low, usually ranging from 0.12 to 20%. Therefore, the properties of the carrier or diluent largely determine physical properties and quality of the finished product. Diluents are usually talcs, clays or diatomaceous earth. Sometimes wheat flour or powdered nut hulls are used as diluents for special purposes.

   The quality of formulated dust products can be determined by spreading a small amount of the material on a light-colored, smooth surface with a knife blade. Spots or streaks show that the material was poorly mixed. Improperly blended or mixed dusts will not spread the toxicant evenly on the treated surface. If the dust is unusually gritty, the material was not ground sufficiently. This also could cause uneven distribution of the formulated material to plants or other treated surfaces. This type of formulation would be very abrasive in a duster. Lumpy dusts are due to faulty formulation or improper storage. In either case, the material would not flow through a duster properly.

2. Granules: Granular formulations are primarily soil treatments and may be applied either directly to the soil or over plants. When applied over plants, the granules fall through the foliage to the soil. Some granules are applied to water either directly or over foliage. Since the granules do not adhere to the foliage of most plants, phytotoxicity and residue problems are reduced. Usually granules are prepared in advance and the toxicant either impregnated or bound to them with a binding or adhesive material. In treating granules with an insecticide, a rotating drum-type blender is used.

   Granules are described as being of a certain mesh size. For example, a 15/30 mesh range is common. This means that at least 98% by weight of the particle will pass through a screen with 15 openings per linear inch, but not more than 5% will pass through a screen with 30 openings per linear inch. Particles smaller than 60 mesh are considered dusts.

3. Wettable Powders: These are dry or powdered preparations containing surface active or wetting agents and usually other conditioning materials. When a wettable powder is mixed with water or other liquids, a suspension spray is formed. Similar formulations are called soluble granules and water dispersable granules.

   Wettable powders may be examined in the same manner as dusts. Streaks of light or dark materials mean poor blending or preparation. Coarseness or grittiness will be abrasive and cause damage to pumps and nozzles. In fact, wettable powders should not be used in low pressure gear pumps or nylon roller pumps because of the abrasive nature of the formulation.

   Check formulations for lumpiness, suspendability, etc., as follows: Mix two-thirds of an ounce of the wettable powder with one quart of water in a glass container. The material should mix well with water during mild shaking. Very little should settle out in 30 minutes. If there are distinct layers in the sediment at the bottom of the container, the particle size may not be uniform.

4. Solutions: Sprays prepared as solutions usually contain only the toxicant and a solvent. In many cases the solvent is a petroleum product. The type of solvent used to prepared the solutions will depend upon the use of the formulation. For example, household sprays require that a water-white kerosene type solvent or diluent be used. This would avoid staining of household items. Solutions properly prepared for special used do not leave unsightly residues. Most of these preparations will damage plants.

5. Emulsifiable Concentrates: These formulations are prepared by dissolving the toxicant and an emulsifying agent in an organic solvent. The solvent is usually not soluble in water; therefore, an emulsifier must be used so that the three or more chemicals will produce an emulsion spray when mixed with water. This is a solution-type spray and has no abrasive components; therefore, it is relatively harmless to any equipment in which it is used. Be sure to follow label directions when using emulsifiable concentrates, since some of the solvent or emulsifying components of the resulting spray may cause damage to certain types of plants.

   To check the quality of the emulsifiable concentrate, remove the glass container from the shelf and examine for a layering or settling out, crystallization, or other unusual appearance of the materials. If the container is metal, shake it thoroughly and pour out a sample in a clean glass container. Any sediment or unusual appearance may mean the emulsifiable concentrate will be ineffective against the pest to be controlled or even extremely toxic to both plants and animals.

   To further check the formulation, add two tablespoonfuls of the emulsifiable concentrate to one pint of water. Cover or seal the container and shake gently. The whole solution should become uniformly milky. the formulation of an opaque material at the top of the solution is called "creaming". After the solution has set for 30 minutes, there should be only moderate creaming and no free oil or water. At the end of two hours there should be no clear liquid and little, if any, creaming. A clear liquid at the top of the solution is usually the solvent. Solvent separated from the solution is likely to damage plants. It may mean that the emulsifiable concentrate is not properly emulsified.

   After 24 hours there will be separation of the components of the emulsion spray, but it should take on a uniformly milky appearance again with only mild agitation.

6. Aerosols: Aerosols are particles dispersed in gas. The liquified-gas aerosols consist of a toxicant dissolved in a liquid and held under pressure in a container. When the valve is opened, a fine spray emerges. The liquid solvent quickly vaporized leaving the small toxic particles suspended in the air. In general, there are space aerosols for control of flying insects and contact aerosols for control of ants and roaches. The main difference is the amount and type of toxicant dispersed. The particles in the contact aerosol are large and heavy, therefore would not remain suspended in the air.

   Some aerosols are prepared and may be used both in the house and on plant foliage. There are other types of aerosols such as those that depend upon smoke to carry the poison to an area, as in mosquito control operations.

7. Fumigants: Fumigants may be volatile liquids, solids or gases or mixtures of them which produce gas, vapor, fumes or smoke intended to destroy pests such as insects, rodents, nematodes, weed seed, plant pathogens, etc.

Glossary

Carrier or diluent:

an inert material used in the formulation of sprays and dusts. It is usually combined with a toxicant to dilute or otherwise make the mixed material more suitable for field application.

Compatibility:

refers to the combining or mixing of one or more materials without counteraction or the changing of their usual or predicted effects. There are two types of compatibility when dealing with pesticides; namely, physical and chemical. Physical compatibility is when there is an effect caused by the combination of materials that makes the material objectionable in the sprayer or duster. Chemical compatibility is when the materials are combined they become less effective than the originals.

Diluent:

see carrier.

Emulsifier or emulsifying agent:

is a surface active material which reduces the separation of droplets of one liquid in another.

Phytotoxic:

a material which causes damage to plants.

Solution:

this is when the molecules of the toxicant or solute are uniformly mixed with the molecules of the solvent. For example, when sugar is dissolved in water.

Spreader:

is a substance which increases the firmness of attachment of materials to surfaces.

Suspension:

is made up of particles of a solid dispersed in a liquid but not dissolved.

Toxic or toxicity:

refers to the quality, state or degree of being poisonous.

Toxicant:

is the substance or substances in a formulation that are intended to kill the intended pest.

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