Pesticides are “formulated” in manyways, each with its own characteris-tics, to meet a wide array of pest con-trol conditions/needs, applicationmethods, applicator/environmental

safety, handling/storage conditions and actualpesticidal characteristics.

Pesticide behavior is important because allpesticides are poisons deliberately introducedinto the environment. To maintain minimal non-target exposure, the chemical and physical behav-ior of pesticides must be taken into account. Thefollowing terms and illustrations are meant toprovoke thought and understanding of someaspects of pesticide behavior.

Understanding FormulationsPesticides contain two groups of chemicals:

active ingredient(s) (AI) and inert or inactiveingredients. The active ingredient(s) is that part ofthe formulation designed to control the targetpest. The function of inert ingredients is toenhance application and effectiveness of the activeingredient(s). The combination of active and inac-tive ingredients is known as the pesticide formula-tion. The various formulations allow the pesticideto be applied by different means (sprays, dusts,granules) to achieve specific results.

Emulsifiable concentrate (EC). These formu-lations have the active ingredient dissolved in anorganic solvent because they have very low solu-bility in water. Emulsifying chemicals are addedto enable mixing with water (resulting in a“milky” solution). Wetting agents, UV lightblockers and various other chemicals may beadded to perform specific functions.

Microencapsulation is an EC technique where-by the AI is encapsulated as microscopic dropletsin a nylon-like material. Microencapsulationreduces dermal toxicity and extends residualtime because the amount of active ingredient islimited at any given time to that found on thesurface of each droplet.

Wettable powders. Wettable powders are for-mulations in which the AI is not dissolved inpetroleum solvents; rather, it is very finelyground and diluted with a powder such as tal-cum or other inert powders.

Aerosols. Aerosol formulations are usually inpressurized containers. By pushing the valve noz-zle, a very fine-droplet mist or aerosol is shot intothe surrounding air or onto a target surface.

Fumigants. Most fumigants are highlyvolatile. Some are kept as liquids under pres-

sure. When released they go directly into thetrue gaseous state. Others are formulated asdry tablets or pellets that upon exposure tomoisture in the air chemically react to producegases. Fumigants are not fine droplets like anaerosol but true gases.

Granular formulations. Granular formula-tions are made by placing the active ingredientson a core material such as clay, sand or groundcorncob. The core material imparts various quali-ties (e.g., flowability, dust, release rates) to theformulation. These granular formulations areapplied as purchased, i.e., dry. In contrast, water-dispersible granules have a granular form but aremixed with water, similar to wettable powders.Granular formulations generally have less dustthan dusts and wettable-powder formulations.

Understanding Persistence Persistence is the inherent stability of the pesti-

cide. Another term for persistence is “residualtime” — how long it effectively lasts. As repre-sented in Figure 1, below, two pesticides areapplied at the same time. In this example, chemi-cal A was more persistent, with six weeks of con-trol, than chemical B, with three weeks. This illus-trates the persistence (or residual time) of thesetwo chemicals under identical conditions of tem-perature, moisture and light.

Now look at Figure 1, below, differently.Imagine that both lines represent the same chem-ical applied under different light, moisture andtemperature conditions. Under existing levels oflight, moisture or temperature, curve A representsa 6-week residual time. Now consider that �

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Factors Affecting

PesticideBehavior

Figure 1. Most pesticides have different persistence durationsbecause they have different active and inactive ingredients.

Pesticide Persistence

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provided three weeks of residual control.However, though declining, it is still detectable forat least eight weeks (curve A lasts even longer).These “post-control residues” may affect subse-quent crops and are referred to as “carry over.”

Understanding Degradation Degradation is the AI breakdown process. It is

strictly a chemical breakdown, as illustrated inFigure 2, left. In chemical degradation, for pur-poses of illustration, the methyl group (CH

3) at

the top of the diazinon molecule has beenreplaced in the degraded state by a hydrogen (H)atom (as seen in Figure 2, right).

This form of degradation is a chemical reactionbetween the pesticide AI and the various chemicalsin the soil, on the leaf surface or wherever the pesti-cide happens to be. The rate of chemical degrada-tion is governed largely by temperature, that is, forevery 50° F rise in temperature, the chemical reac-tion rate will double.

The importance of chemical degradation is thefaster the pesticide compound degrades, the lesstime it is available for pest control. In some casesthe breakdown product(s) can be more toxic thanthe original AI. Therefore, when the U.S.Environmental Protection Agency registers anypesticide, it must also be concerned about thedegradation products.

Understanding Deactivation Some deactivation occurs when some of the

pesticide adheres so tightly to a soil particle,organic matter in the soil, leaf surface or someother compound in the environment that it is nolonger available. For example, a root hair grow-ing in soil where glyphosate is bound to the claymay not be affected by the glyphosate molecule.

If a soil sample containing deactivated chemicalsis taken into the laboratory for analysis, the amountof pesticide present may be chemically detected inthe soil sample even though it does not control thetarget pest from a biological standpoint.

Understanding Photodecomposition Ultraviolet (UV) light is a very high source of

energy that promotes the breakdown of manychemicals. Most of the pesticides we use today aresomewhat subject to photodecomposition. Somepesticide formulations contain UV light blockersthat lessen the amount of photodecomposition.

Ultraviolet light is an extremely destructivesource of energy and plays a very important rolein terms of the persistence of pesticides that areexposed to it. Some pesticides are packaged inbrown-colored glass. This cuts out the light andthereby stops the photodecomposition of the pes-ticide ingredient while it is on the shelf.

Understanding Hydrolysis Hydrolysis is the combination of a water

molecule with another molecule that results inthe splitting of the larger molecule. Hydrolysisin combination with photodecomposition isoften involved in the breakdown of various pes-ticide active ingredients.

Hydrolysis can take place in the containerbefore the pesticide is ever applied. Once thecontainer seal is broken, moisture in the airmoves in and out of the container with eachatmospheric pressure change, and the smallamount of moisture (water vapor) in the aircombines with the active ingredients in the con-tainer. When this occurs, the shelf life of thatparticular product is reduced.

Hydrolysis is an important mechanism in �

curve B represents a second application of thechemical later in the season when the factors oflight, moisture or temperature are even morefavorable for breakdown. The result is only threeweeks of residual time. Thus persistence is vari-able under field-use conditions.

There is a detectable amount of active ingredi-ent that is less than the lowest level required foreffective control. In the previous example, curve B

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pesticide breakdown.The faster the hydroly-sis reaction takes place,the less time the pesti-cide is available in theenvironment for pestici-dal activity, leaching orother movement.

UnderstandingTemperature

Temperature is an-other very significantfactor in the break-down of pesticides. AsFigure 3, right, shows,temperature can bedivided into two cate-gories: above the soilsurface and below thesoil surface. Above thesoil surface the air temperature is highly variable, fluctuating radically, butin general, it increases with height above the soil surface. For every 50° Frise in temperature, the chemical reaction rate doubles. Thus, if a pesticideis applied close to the soil surface versus the top of a crop canopy, the rateof chemical breakdown may increase up through the canopy due to highertemperatures at the top.

Below the soil surface, the soil temperature drops at a rather steady rate.For every 50° F drop in temperature, the chemical reaction rate is cut in half.As the pesticide moves further below the soil surface, the temperature iscooler, slowing chemical breakdown reaction to the extent it is dependent ontemperature. Therefore, if a chemical gets below the root zone, the soil tem-perature (along with oxygen level and microbial activity) will often be suchthat breakdown will be relatively slow.

Understanding pHA measure of the acidity of any substance, including where the pesti-

cide may be applied and the pesticide mixture itself, is known as pH, andit affects pesticides in many ways. If a slightly acidic pesticide is applied toa basic (alkaline) surface, the pesticide may break down more rapidly.Similarly, slightly basic pesticides applied to acidic surfaces will breakdown fairly quickly.

Pesticide reactions in the soil or on the leaf surface are not violentbecause the acids and bases are so weak, but the principle of breakdownor neutralization is the point. In some cases, buffering chemicals areincluded in pesticide formulations to help stabilize the pH and, in turn,reduce breakdown of the pesticide.

Soil is a mixture of many organic and inorganic bases and acids, all ofwhich react with each other to form a soil pH. If the pH of the soil is suchthat the pesticide is neutralized (broken down) relatively rapidly, the pesti-cide is unavailable for pesticidal activity or adverse environmental effects.

SummaryHalf-life is referred to in terms of how long a pesticide will last. As shown

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Effects Of Temperature On Pesticides

Figure 3. Temperatures above and below the soil surface canaffect a pesticide’s reaction rate. Warmer temperatures speedreaction rate; cooler temperatures slow reaction rate.

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in Figure 4, left, pesti-cide A is decomposingat the theoretical rateunder controlled condi-tions. The other lineindicates that pest-icideB, under field conditions,decomposes rather rap-idly then levels out. Itthen breaks down rapid-ly again only to slowdown and level outagain. In fact, these twobreakdown curves canindeed be the samechemical but under a dif-ferent complex of factors.

Temperature, light,moisture, bacteria, pH,etc., all affect pesticidesin different ways andcause them to breakdown at varying rates.

The point is, pesticide breakdown, thus half-life, is dependent on many andvarying factors when applied under normal use conditions. Some pesticidesare more stable than others under the same conditions. For this reason, half-life is not a single number (five days or 20 days, etc.). Half-life should only beused as a guide to pesticide residual time.

The behavior of pesticides is dependent on many factors, all affectingthe pesticide at the same time. The net result is that any given pesticideunder the various field situations can last for a short, intermediate oreven long period of time.

When applying any pesticide, it is important to recognize that all the fac-tors (light, temperature, moisture, pH, bacteria, etc.) will impact the activeingredient. The breakdown rate affects the time the pesticide is available forpest control, off-target movement and environmental contamination.

Donald Cress is with the Kansas State University Agricultural ExperimentStation and Cooperative Extension Service. He can be reached atdcress@ksu.edu.

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Theoretical v. ActualDecomposition

Figure 4. Line A represents the theoretical decomposition of apesticide under laboratory conditions. Line B represents theactual decomposition of the same pesticide under actual fieldconditions. The difference in decomposition is due to thevariability of decomposition factors in the field.

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