[Technical Report] Pesticides

An in-depth look at how pesticides are developed and how the various formulations work.

In modern professional pest control there are many options, including a variety of non-chemical methods, such as sanitation to remove breeding sites and harborages, the use of traps to remove individual pests from the infested site, mating disruption to gradually reduce infestations, and chemical application to provide immediate and long-term control. In this article the chemical applications most commonly used in the U.S. will be reviewed.

DEVELOPMENt. Developing a new pesticide for professional pest control is a lengthy process; it can take 10 years from discovery to introduction of a commercial product. The process usually starts in the laboratory with the screening of potential molecules. Once human toxicity and environmental contamination tests are complete, then the chemical progresses through small-scale evaluations on a variety of pests. Extensive field testing is used to determine effective concentrations and application rates, and the most cost-effective application method. After several years of laboratory and field testing, the data is submitted to the U.S. Environmental Protection Agency for approval. EPA reviews the toxicity data and the potential environmental impact of the chemical before granting the manufacturer a label and permission to sell the product. The application rates and delivery methods of all pesticides used in the U.S. are approved by the EPA and are on the label of the product. Professional applicators are required by (federal and state) law to read and strictly follow the mixing instructions, application rates, and delivery methods contained in the label. Failure to do so can result in regulatory action that may includes fines and the loss of applicator license.

PEST CONTROL PesticideS. Most pesticide products consist of an active ingredient (AI) and one or more inert ingredients. This mixture is considered the formulation, and it may be in the form of a solid, liquid, or gas. Some formulations are ready-to-use, while others must be diluted with a solvent, water, or air before application. The formulation influences how an pesticide is applied, its persistence, and how it is transferred from treated surface to target insect.

GASES. These are highly volatile compounds that exist as a gas at temperatures above 40°F. Their small molecular size enables them to penetrate organic material, and they can easily pass through the skin or hard covering of an insect. The gases used as pesticides are usually delivered with specialized equipment and only by manufacturer-trained service technicians. They are effective in treating large structures for extensive infestations, but the gas has to be contained for long periods to achieve complete control. Common pest control fumigants methyl bromide and sulfuryl fluoride act on the nervous system. They accumulate in tissues and nerve membranes. The mode of action is suppression of the nervous system to induce inactivity and eventual death.

SOLIDS. Granules, dusts, and gels contain an AI and usually an inert carrier (e.g., clay or gelatin). The primary role of the carrier is to make the application easier. These formulations are usually designed for a specific pest, such as gel baits for cockroaches and ants, or for a specific location, such as the use of dusts in enclosed voids.

Granules. Granular pesticides deliver the pesticide to the surface area immediately around the particle. The use of granular pesticides in professional pest control is primarily for outdoor and building perimeter pests. They can be applied to bare soil, turfgrass, and mulch as a spot treatment, or entire perimeter treatment. The AI is incorporated into clay or organic material that acts as a temporary carrier; the pesticide migrates from the carrier to the substrate. Some formulations are small to enhance penetration and coverage of ground cover, such as mulch. Granular baits may be designed to be transported to the nest of social insects such as ants.

Dust. Dusts are ready-to-use formulations of a low concentration of pesticide mixed with a carrier, such as clay or talc. These formulations can be effective in controlling indoor pests that utilize narrow harborages because particles can penetrate crevices to create a residue on all surfaces. Dust particles are easily picked up by crawling insects and may remain as an active residue for long periods. The advantages of dust formulations include low or no odor, nonstaining, long residual activity, and coverage of horizontal and vertical surfaces. Some modern formulations resist wetting and can be used in locations prone to moisture or high humidity.

Gels. These are semi-sold formulations that are a mixture of a low concentration of pesticide with water and inert ingredients, such as a gelatin to provide a gel or paste. Gel baits contain a high (50-75%) percentage of water, and a feeding stimulant that ensures that once the insect eats the bait it will ingest a lethal dose. The concept of combining a pesticide with a palatable food to make bait is one of the oldest methods of insect control.

WATER-BASED FORMULATIONS. The majority of pesticides used to control structural pests are usually available as liquid formulations. They are typically in the form of dilutions in water. Solutions in water are perhaps the simplest to mix and apply, unlike spray liquids prepared from emulsion concentrates and wettable powders. However, there are few organic pesticides soluble in water and the degree to which they are may be very limited.

Wettable and soluble powders, pellets and granules. Wettable powders do not form solutions since the powdered pesticide is only suspended and not dissolved in water. These formulations consist of finely-ground pesticide mixed with a mineral to keep the particles separated during storage. The efficacy of wettable powder sprays is linked to the presence of the active ingredient on the surface of the dried residue. The particles are picked up on the legs and other body parts of insects walking on the surface. The pesticide in soluble powders is finely ground with a small amount of wetting agent; these formulations can be a true solution. They may be dose packaged in water-dissolvable bags that can be dropped into the water in the mixing tank. Water dispersible granules, grains, pellets, and tablets also can be placed in water-soluble packets for accurate mixing.

Emulsifiable concentrates. These are solutions that contain small amounts of emulsifying agents, which permit the suspension of minute (1-10 microns diameter) droplets of oil in water. Emulsifiable formulations (EC) can be added directly to water in the spray tank. The suspension of fine droplets in the water is apparent by a white or opaque appearance of the concentrate and usually the finished dilution. Once applied to the surface the oil and water gradually disappear by absorption and evaporation to leave the active ingredient exposed to the target pest.

Suspension concentrates. Suspension concentrates (SC) contain finely ground solid pesticide in a liquid. The particles in these formulations are typically small, ranging from 0.5 to 17 microns. These particles adhere to insects walking on treated surfaces. Enhanced manufacturing processes can produce formulations with a specific range of active ingredient crystals: small crystals may penetrate quick knockdown and kill, while large crystals provide residual activity.

Foam. Water-based foam is an effective means of delivering professional and wood protection pesticides. Foam provides increased lateral spread around objects and into crevices that conventional liquid sprays would not reach. It is created by adding a small amount (1-3 ounces) of foaming agent to the finished dilution of pesticide, then combining air with the liquid before or as it is sprayed. Combining air and liquid produces the foam. Dry foam is characterized as having a high ratio of foam to liquid (>25:1), and consisting primarily of air and little liquid. The surface residue resulting from dry foam is limited because of the small amount of liquid. Wet foam has more liquid than dry foam, and it collapses quickly to create an effective residue.

OIL-BASED FORMULATIONS. Oil-based formulations consist of an active ingredient dissolved and applied in light oil or other organic solvent. Since the pesticide is soluble in the oil these are true solutions. Most of these are packaged as a ready-to-use product. A commonly used solvent spray for wood protection involves a borate and ethylene glycol. The ethylene glycol maintains a high (20%) concentration and dilution of disodium octaborate tetrahydrate; the formulation is further mixed with water for delivery.

AEROSOL FORMULATIONS. An aerosol is a gaseous suspension of solid or liquid particles in the air for a short period of time (hang time). Aerosol spray is an efficient way to deliver small amounts of liquid pesticide to a substrate or void space. Inside the typical aerosol can is the main ingredient (pesticide), and a propellant. Early propellants were liquefied gases, such as hydrocarbons, carbon dioxide, and nitrous oxide. The combined ingredients, pesticide product and propellant, are pressure-sealed then released by a hand-pressed valve.

LIQUID Pesticide APPLICATION. Liquid pesticides first became available for professional pest control in the 1940s, and remain today as the most common formulation. The early chemicals included chlordane and aldrin for termite control, and diazinon and DDT for crawling insects. Along with these chemicals the first professional application tool was developed: the B&G compressed-air sprayer. B&G termiticide application tools followed soon after the 1-gallon tank sprayer. Over the years, pesticides and formulations have changed dramatically and so has the equipment and techniques used to apply these new formulations. The professional service technician has to have a thorough knowledge of modern pesticides and the equipment required to deliver them. The following review of liquid technology and application tools will help technicians achieve the most effectiv control of the targeted pest.

LIQUID CHARACTERISTICS. Every liquid pesticide has a unique set of characteristics, based on features of the active ingredient and other components of the formulation.

Viscosity. This is basically the "thickness" of the liquid. An extremely viscous liquid has the flow characteristics of syrup, while a non-viscous liquid flows like water. Viscosity does not play an important role in most liquid formulations, since most professional pesticides have a viscosity similar to water. Some borates are prepared in very viscous formulations, and require careful mixing to prepare a final dilution that can be sprayed. The gel of some bait formulations responds to high temperatures and becomes liquid during application. The higher the viscosity of a liquid, the more difficult it becomes to move it through a nozzle to produce small droplets. Generally, spraying a viscous liquid results in an uneven spray pattern or, in some instances, no spray pattern at all. In some cases the effect of viscosity can be overcome with increased tank pressure.

Surface tension. This may be understood in terms of how ‘slippery’ a liquid is. An example of a liquid that has low surface tension might be a liquid soap. Surface tension can affect the capacity of a liquid to be dispersed through a nozzle: the less surface tension the better the flow through a small orifice. The surface tension of most pest control pesticides is similar to that of water, and usually there are no difficulties in spraying these liquids.

Specific gravity. This may be understood in terms of the ‘weight’ (compared to water) of the liquid. Oil is an example of a liquid that has a specific gravity less than water, and it will remain on the surface (float). The specific gravity of most pest control pesticides is similar to that of water, and usually there are no difficulties in mixing these liquids.

Temperature. Except in the case of viscous liquids, temperature has little effect on the actual distribution or application of a liquid pesticide. Temperature has the most effect on application equipment: materials tend to expand and contract as the temperature change. An example of the influence of temperature can be seen in a compressed-air tank sprayer. It might be initially pressurized to 30 psi, but when left in the hot sun or exposed to heat, the pressure in the tank might increase to 60 psi due to the expansion of (hot) air in the tank.

Chemical composition. This is an important feature when choosing the application equipment for a pesticide. Most modern pesticides are water based, and these can be easily applied though conventional application equipment. However, there are some formulations that are solvent based and may not easily flow through typical nozzles. Typically in today’s chemicals, the composition is usually made of a high volume of "base" material, which typically is water, although there is still some solvent based material being used. In addition to the base material, you have the active ingredient and in some cases there are some other ingredients. The selection of high quality equipment is critical to assuring that you will not have chemical damage to any component of your equipment, causing it to fail. With the increasing usage of ‘green’ and ‘natural’ pest control products, this can be extremely critical. Many of these chemicals are citric acid based which tends to be very corrosive on most materials. Before using any chemical, if you have any questions, please check with both the chemical and equipment manufacturer to make sure there are no chemical compatibility issues.

Effective LIQUID APPLICATIONs. For effective pest control, liquid must be dispersed so that insects may come in contact with the chemical. The most effective way to do this is to spray small droplets on the surface. A nozzle or spray tip along with a pressure device is used to create this type of spray. A spray tip serves the purpose of distributing and atomizing or breaking up the liquid into small droplets. There are two different ways used in pest control to atomize a liquid, one is using liquid (hydraulic) pressure alone, and the other by combining liquid pressure with external pressure, such as compressed air or a propellant (as in aerosols). Each method has specific advantages.

Hydraulic pressure. When using liquid pressure alone, liquid is under pressure until it reaches the nozzle orifice, as the liquid exits the orifice "shears" it into droplets. Other factors such orifice configuration and spray nozzle type will create the spray pattern and distribute the liquid.

External pressure. With an external atomizing force such as compressed air, the external atomizing force will assist in shearing the liquid into droplets and help in distribution. Typically these spray droplets are much smaller as compared to a spray that was created with liquid pressure only. Each different type spray creates different spray characteristics. This is important when applying pesticides to ensure that delivery is efficient and effective.