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Essential Pesticide Readings for Beginners (二)

 Pesticide Formulations

1. Introduction to major formulations

Currently there are tens of pesticide formulation types. But only a few of them are the widely used formulations. Hereinafter we just introduce a few major formulations. All the other formulations and their codes and definitions are given in appendix 5.

(1) Emulsifiable Concentrate

Emulsifiable Concentrate formulations are a blend of active ingredient, organic solvent, and surfactants. When the solution is diluted into water, a spontaneous milky emulsion forms with dispersed phase droplets in the size range of 1 to 25 μm.  When sprayed, this dilute emulsion gives a uniform and accurate application of active ingredient on the crop, which is essential for effective pest control.  The formulation of the active ingredient into an Emulsifiable Concentrate can ensure uniform spreading and wetting under normal spray and weather conditions.

Table 1  Advantages and Disadvantages of EC Formulation

Advantages

Disadvantages

(1) Relatively easy to handle, transport, and store. 

(2) Little agitation required—will not settle out or separate when equipment is running.

(3) Not abrasive.

(4) Will not plug screens or nozzles.

(5) Little visible residue on treated surfaces.

(1) High a.i. concentration makes it easy to overdose or underdose through mixing or calibration errors.

(2) May cause damage to desirable plants (phytotoxicity).

(3) Easily absorbed through skin of humans or animals.

(4) Solvents may cause rubber or plastic hoses, gaskets, and pump parts and surfaces to deteriorate.

(5) May cause pitting or discoloration of painted finishes.

(6) Flammable—should be used and stored away from heat or open flame.

(7) May be corrosive.

(2) Microemulsions

Microemulsions consist of a dispersed phase of small droplets with diameters of 10 – 100 nm suspended in a continuous phase.  The dispersed and continuous phases are usually an oil and water or vice-versa.  It is commonly believed that microemulsions can provide superior efficacy relative to macroemulsion formulas having the same level of activities.  It is believed that the small size of the emulsion droplets may allow for better transport of the pesticide active through cell membranes (plant and insect) thereby resulting in enhanced efficacy.  Microemulsions are considered to be infinitely stable, thereby providing improved stability over traditional macroemulsion systems.  The only real disadvantage of microemulsions is the relatively higher level of emulsifier required.

(3) Wettable Powders

Wettable Powder formulations are applied as suspensions after dispersion in water. They consist of one or more active ingredients which are blended and mixed with inert diluents and surfactants. The active ingredient can be either a liquid or solid.  Wetting agents are used to facilitate the suspension of the particles in water.  A dispersing agent is added to prevent any flocculation of the suspension before it is applied.

Wettable powders are dry, finely ground formulations that look like dusts. They usually must be mixed with water for application as a spray. A few products, however, may be applied either as a dust or as a wettable powder—the choice is left to the applicator. Wettable powders contain 5 to 95 percent active ingredient by weight; usually 50 percent or more. The particles do not dissolve in water. They settle out quickly unless constantly agitated to keep them suspended. They can be used for most pest problems and in most types of spray equipment where agitation is possible. Wettable powders have excellent residual activity. Because of their physical properties, most of the pesticide remains on the surface of treated porous materials such as concrete, plaster, and untreated wood. In such cases, only the water penetrates the material.

Table 2  Advantages and Disadvantages of WP Formulation

Advantages:

Disadvantages:

(1) Easy to store, transport, and handle.

(2) Less likely than ECs and other petroleum-based pesticides to cause unwanted harm to treated plants, animals, and surfaces.

(3) Easily measured and mixed.

(4) Less skin and eye absorption than ECs and other liquid formulations.

 

(1) Inhalation hazard to applicator while measuring and mixing the concentrated powder.

(2) Requires good and constant agitation (usually mechanical) in the spray tank and quickly settles out if the agitator is turned off.

(3) Abrasive to many types of pumps and nozzles, causing them to wear out quickly.

(4) Difficult to mix in very hard, alkaline water.

(5) Often clog nozzles and screens.

(6) Residues may be visible on treated surfaces.

(4) Suspensions Concentrates

A Suspension concentrate or flowable combines many of the characteristics of emulsifiable concentrates and wettable powders. The active ingredient in these formulations is usually a solid that does not dissolve in either water or oil. Although occasionally liquid active ingredients have been used but these must first be absorbed onto a solid carrier. The active ingredient, impregnated onto or combined with a substance such as clay, is ground to a very fine powder. The powder is then suspended in a small amount of liquid (usually water).

Other ingredients used in the formulation include; wetting agents, dispersants, viscosity modifiers (thickeners), preservatives and antifreeze.

The formulation in blended together with a hi-shear mixer and then liquid milled through a media, using glass beads, zirconium oxide beads or sand as the grining media. The mean particle size generated by this type of milling can be extremely small, often below 1 micron.

It is important that the active ingredient and any solid carriers used in this type of formulation have a very low solubility in the suspending solvent (usually less than 0.2%) otherwise Ostwalt ripening (crystal growth) will occur in the product.

(5) Water Dispersible Granules

Water-dispersible granules, also known as dry flowables, are like wettable powders except instead of being dust like, they are formulated as small, easily measured granules. Water-dispersible granules must be mixed with water to be applied. Once in water, the granules break apart into fine particles similar to wettable powders. The formulation requires constant agitation to keep them suspended in water. The percentage of active ingredient can be high, sometimes as much as 90 percent by weight. Water-dispersible granules share many of the same advantages and disadvantages of wettable powders except:

(1) They are more easily measured and mixed;

(2) Because of low dust, they cause less inhalation hazard to the applicator during handling Water dispersible granules can be made by several techniques. The most common ones include: Extrusion – to produce cylindrical pellets; Pan granulation (tumbling in a pan) to produce spherical balls – this is done by spray a liquid, usually water, onto the tumbling powder to cause agglomeration by the “snow-ball effect”; Compaction – in this process a powder with an included binder is compressed into a small sphere, disc or obloid. Tablets may be considered to be a form of disc shaped compacted granule.

(6) Soluble Powders

Soluble powders are similar to wettable powders with one major difference: all of the ingredients in the formulation are totally water soluble at the use rate.

They are applied as solutions in water. They consist of one or more active ingredients which are blended and mixed with water soluble inert diluents and surfactants. The active ingredient can be either a liquid or solid. Wetting agents are used to facilitate rapid dissolution in water and spreading on plant foliage. A dispersing agent is not normally required because of the total water solubility of the product. A free-flow aid can be included to inhibit clumping and facilitate easy handling of the product. These formulations are often packaged in water soluble bags to minimize user exposure to the pesticide.

Table 3  Advantages and Disadvantages of SP Formulation

Advantages

Disadvantages

(1) Mix readily with water

(2)No agitation required for water soluble herbicides when mixed with water

(3) Non-volatile

(4) Not abrasive

(5) Do not plug screens or nozzles

(6) Equipment cleans easily

(7) Soluble powders are easy to store, transport and handle

(1) Inhalation hazard when pouring soluble powders

(2) Some products may react with unlined steel spray tanks

(3) Eye irritation with some salts

 

(7) Baits

A bait formulation is an active ingredient mixed with food or another attractive substance. The bait either attracts the pests or is placed where the pests will find it. Pests are killed by eating the bait that contains the pesticide. The amount of active ingredient in most bait formulations is quite low, usually less than 5 percent.

Baits are used inside buildings to control ants, roaches, flies, other insects, and rodents. Outdoors they sometimes are used to control snails, slugs, and insects such as ants and termites.

Table 5  Advantages and Disadvantages of Bait Formulation

Advantages:

Disadvantages:

(1) Ready to use.

(2) Entire area need not be covered, because pest goes to bait.

(3) Controls pests that move in and out of an area.

 

(1) Can be attractive to children and pets.

(2) May kill domestic animals and non-target wildlife outdoors.

(3) Pest may prefer the crop or other food to the bait.

(4) Dead vertebrate pests may cause odor problems.

(5) Other animals may be poisoned as a result of feeding on the poisoned pests.

(6) If baits are not removed when the pesticide becomes ineffective, they may serve as a food supply for the target pest or other pests.

Pastes and gels are mainly used in the pest control industry for ants and cockroaches. Insecticides formulated as pastes and gels are now the primary formulations used in cockroach control. They are designed to be injected or placed as either a bead or dot inside small cracks and crevices of building elements where insects tend to hide or travel. Two basic types of tools are used to apply pastes and gels—syringes and bait guns. The applicator forces the bait out of the tip of the device by applying pressure to a plunger or trigger.

(8) Ultra-low Volume Sprays

These concentrates may approach 100 percent active ingredient. They are designed to be used “as is” or to be diluted with only small quantities of a specified carrier. They are used at rates of no more than 1/2 gallon per acre. These special purpose formulations are used mostly in outdoor applications, such as in agricultural, forestry, ornamental, and mosquito control programs. 

Table 6 Advantages and Disadvantages of ULV Formulation

Advantages

Disadvantages

(1) Relatively easy to transport and store

(2) Remain in solution; little agitation required

(3) Not abrasive to equipment

(4) Will not plug screens and nozzles

(5) Leave little visible residue on treated surfaces

(1) Difficult to keep pesticide on target- -high drift hazard

(2) Specialized equipment required Easily absorbed through skin of humans or animals

(3) Solvents may cause rubber or plastic hoses, gaskets, and pump parts and surfaces to deteriorate

(4) Calibration and application must be done very carefully because of the high concentration of active ingredient

(9) Microencapsulations

Microencapsulation is the entrapment of a core material (sometimes referred to as the payload or internal phase) inside an inert coating (usually referred to as shell, capsule wall or membrane). Three general classes of microencapsulation exist: (1) Nanoencapsulation 30-200nm;(2) Microencapsulation 0.1-1000nm;(3) Macroencapsulation >1 mm.

Microencapsulated pesticides are mixed with water and sprayed in the same manner as other spray able formulations. After spraying, the capsule wall breaks down and slowly releases the active ingredient. Microencapsulated materials have several advantages:

(1) Highly toxic materials are safer for applicators to mix and apply

(2) Delayed or slow release of the active ingredient prolongs its effectiveness, allowing for fewer and less precisely timed applications

(3) The pesticide volatilizes more slowly; less is lost from the application site.

In residential, industrial, and institutional applications, microencapsulated formulations offer several advantages. These include reduced odor, the release of small quantities of pesticide over a long time, and greater safety. Microencapsulated materials offer fewer hazards to the skin than ordinary formulations. Microencapsulated materials, however, pose a special hazard to bees. Foraging bees may carry microencapsulated materials back to their hives because they are about the same size as pollen grains. As the capsules break down, they release the pesticide, poisoning the adults and brood.

Breakdown of the microencapsulated materials to release the pesticide sometimes depends on weather conditions. Under certain conditions, the microencapsulated materials may break down more slowly than expected. This could leave higher residues of pesticide active ingredient in treated areas beyond normal restricted-entry or harvest intervals with the potential to injure fieldworkers. For this reason, regulations require long restricted-entry intervals for some microencapsulated formulations.