68 Pest ic ide Outlook – Apri l 2000

This journal is © The Royal Society of Chemistry 2000

IntroductionIt has traditionally been herbicides rather than insecticideswhose intrinsic biological activity or selectivity has beenoptimised through formulation. The use of adjuvants toincrease the spread of foliar deposits or cuticular uptake aretwo typical examples that have met with commercialsuccess. Most insecticides are produced as emulsifiableconcentrates, suspension concentrates, wettable granules orwettable powders, depending on the properties of the activeingredient (a.i.) and the target market, and provide cost-effective pest control along with convenience of handlingand compatibility with spray equipment. Past attempts toimprove insecticide formulations have usually centredaround greater rainfastness or photostability, rather thanreduced environmental risk or the complex issues related touptake of dried deposits of a contact-acting compound froma plant surface, systemic movement of a stomach-actingcompound or differential exposure to pests and naturalenemies.

In the 1990s, there has been an increasing demand forinsecticide products which are not only more active gram forgram, but safer to both operators and the environment,especially aquatic habitats. Registration schemes favoursafer and more selective crop protection products, which canbe achieved either through the intrinsic properties of thechemical itself, or through ways in which the effect isdelivered. The common use of aromatic solvents is underscrutiny for several reasons, including the so-called estrogenmimic effect, and the agrochemical industry has responded

by researching and developing a range of formulationswhich reduce both operator hazards and risks to naturalhabitats. Table 1 lists the range of insecticide formulationscurrently available.

Encapsulation technologyMany novel ideas are generated at the formulater’slaboratory bench (Woods, 1999), some of which are veryexciting biologically, but only those which meet the strictcriteria of official registration agencies and promisecommercial viability appear in the market place. Of recentdevelopments, capsule suspensions (CS) have particularlyappealing prospects for the future (Scher et al., 1998).

What are microcapsules?A microcapsule is a 10–3 m to 10–9 m diameter particle,composed of a core material and an outer wall (Tsuji, 1993).Several methods of producing these are available, but theuse of a process known as interfacial polymerisation basedon the condensation of isocyanate or aminoplast monomersor pre-polymers allows the highest pesticide loading and themost cost-effective manufacture. The outer wall isolates thecore material and protects it from environmentaldegradation and interaction with other materials; it shouldhave the following features:

● not react with pesticides● not be harmful to the environment

IMPROVING INSECTICIDES THROUGH ENCAPSULATION

Bob Perrin from Zeneca Agrochemicals at Jealotts Hill International Research Station discusses the controlleddelivery of insecticides achieved using capsule suspension formulations

FORMULATION

Table 1. Features of the major types of insecticide formulations

TYPE FEATURES

WETTABLE POWDER (WP) bulky, dusty, inconvenient, more hazardous to manufacture than liquids.

WETTABLE GRANULE (WG) low dust, low solvent, suitable for soluble packs and unit dosing, some dispersion problems. Lesseasy to measure than liquids.

EMULSIFIABLE CONCENTRATE (EC) simple, robust, versatile, proven, flammable, high solvent content seen as a pollutant.SUSPENSION CONCENTRATE (SC) simple, robust, generally water-based, not suited to many a.i.’s, some sedimentation problems.EMULSION IN WATER(EW) water-based, low solvent, some colloidal instability problems, less toxic than EC’s.MICROEMULSION expensive, chemical stability problems.CAPSULE SUSPENSION (CS) water-based, low solvent, robust, cost-effective, less toxic than EC’s.DRY MICROCAPSULE advantages of a CS, less bulky to store than CS or EC.TABLET convenient, unit dosing, easy to package, good image, some dispersion problems.GEL good image, suitable for soluble packs and unit dosing, intermediate properties of liquids and

solids.

● be stable in storage● be manufactured and processed easily● be economical● have suitable physicochemical properties to provide the

required release behaviour of the core materials

The core materials, e.g. an insecticide, are thus designed tobe released in a controlled fashion. Depending on theirparticular design, microcapsule formulations can providecombinations of the following characteristics :

● improved residual activity● longer application intervals● reduction in application dosage● stabilisation of core ingredients against environmental

degradation (light, air, humidity, microorganisms etc.)● masking of odour● reduction in spray drift● less impact on non-target organisms● better rainfastness● improved uptake and systemic movement in plants● less phytotoxicity● constant or delayed biological effect● reduced absorption on porous surfaces● improved mammalian toxicological profile● reduced environmental pollution● reduced volatilisation and leaching● improved compatibility with packaging materials● safer storage due to reduced flammability● seed coating with liquid insecticide without using an

absorbent carrier

Microcapsules are usually formulated as a slurry in whichmicrocapsules are suspended in water (CS formulations).Various surfactants and suspending agents are added tomaintain the stability of the formulation. Microcapsules canalso be formulated as dry powders, which can be used for

bait, dust or wettable powderfinished products. The rest of thisarticle refers to the water-based CSformulations.

Microcapsule releaseRelease rates are governed by thecapsule particle size, the thickness ofthe wall and the permeability of thewall. Small particles with thin wallsand low cross-linking density allowthe fastest possible release, whilelarge particles with thick walls andhigh cross-linking have the slowestrelease. The most practical way tochange release rates over orders ofmagnitude is to vary wall perme-ability through its cross-linking

density and chemical composition.One product concept is to have variations on theseparameters in the same container, for example, a proportionthat release very rapidly for quick knockdown of pests, anda proportion that release slowly to provide residual activity.An active ingredient and a synergist can also be madecompatible through different release rates.

Use of encapsulation technologyWith their controlled release of active ingredient to a greateror lesser degree, microcapsules have potential use in publichealth and animal health applications, such as control ofdisease vectors through treatment of building surfaces. Theextended residual activity achieved through encapsulation,however, can also find application in agricultural products.Modern capsules essentially bring the added potential to“dial up” the properties of the product which are requiredfor any particular outlet, all the way from very rapid releaseonce spray droplets reach their target, through release whichis triggered by a specific environmental cue, such as pH,temperature or light, to delays of weeks or months governedby slow biological degradation of the capsule material. Notonly release rate can be engineered to fit the purpose athand, but the polymer wall can be used to incorporate otherfeatures into the product, such as ultra-violet protection,substrate affinity, compatibility of several a.i.’s, colourcoding etc.

Microcapsules in public healthDEMAND CS capsules (Figure 1) have a volume mediandiameter (VMD) of approximately 12 microns and consistof a polyurea wall formed by an in situ interfacial condensa-tion process (Scher et al., 1998). The capsule wall is stronglycross-linked with a high ratio of polymethylene-polyphenol-isocyanate (PMMPI): toluene di-isocyanate (TDI), whichconfers a relatively low permeability to the capsule contents,namely the active ingredient, lambda-cyhalothrin, dissolved

Pest ic ide Outlook – Apri l 2000 69

FORMULATION

Figure 1. DEMAND CS capsules oncement 10 weeks after application.Magnification x 1400.

70 Pest ic ide Outlook – Apri l 2000

FORMULATION

in a small amount of aromatic oil. Release of a.i. is verylimited after spray deposits have dried on a substrate such ascement or wood, diffusion through the capsule wall beinginitiated by pick up of capsules on to the lipophilic legs orbodies of insects. Intact capsules have been observed bymicroscopy on the bodies of cockroaches, flies and ants thatmake contact with deposits during walking or resting(Figure 2) – the so-called ‘trampling effect’. Adult maleBlattella germanica cockroaches exposed for one minute toan unglazed tile treated with DEMAND CS at a rate of 30mg a.i. m–2 picked up a mean of 700 capsules, many timesthe lethal dose, which may contribute to the observedimprovement in efficacy against resistant strains ofcockroaches using microencapsulated formulations ratherthan emulsifiable concentrates (Wege et al., 1999).

In public health markets, there are other advantages toinhibiting the initial availability of the active ingredient. Forexample, the early symptoms of toxicity of pyrethroids canresult in flushing, that is the expulsion of cockroaches fromtheir harborages before a lethal dose is acquired. CS formu-lations generally reduce flushing compared to EC and WPformulations. Similarly, a CS formulation of lambda-cyhalothrin allows fire ants to be killed before any repellenteffect occurs.

Microcapsules in agricultureIn contrast to the requirements for months of persistence oninert surfaces when controlling flies, mosquitoes andcockroaches, agricultural formulations must provide a rapidknockdown effect, especially if the pest is a transmitter ofplant viruses, as well as residual activity for several days orweeks depending on crop growth rates and the pest inquestion. Fast-release capsules of lambda-cyhalothrin,marketed under the trade name “Zeon Technology”, aremade by an in-situ interfacial polymerisation where the

monomers reside only in the oil phaseand produce a very efficientasymmetric polyurea wall membrane(Perrin et al., 1998). Capsules have amean diameter (VMD) of 2.5 micronsand an ultra-violet absorber is incor-porated within the core. Activeingredient remains within the capsulesin the container, the spray tank andthe spray droplets during atomisation,offering greater protection to users.Once capsules lose their water barrieron a plant surface or insect cuticle,diffusion begins immediately and isthought to be complete within a fewhours (Figure 3). Insects pick up thereleased contents from the sprayedsurface rather than from intactcapsules. This results in a very similarbiological effect, whether on target or

non-target organisms, to an emulsifiableconcentrate, whilst significantly improving the toxicologicalprofile with respect to eye and skin irritation.

Zeon Technology has been widely evaluated for pestcontrol in major crops around the world, and provides thesame, and in some cases better, control than seen previouslywith emulsifiable concentrates. It is registered for sale inseveral countries, including the UK, and is the firstpyrethroid product to be endorsed by the British BeekeepersAssociation. There is potential to produce a wide range ofactive ingredient strengths and mean capsule sizes, mixturesof different active ingredients, and to co-formulate withsynergists or other agents to improve knockdown or residualactivity. Water-based microcapsules also lend themselves toincorporation in solid carriers for ease of handling orpackaging.

The future for encapsulationOne promising encapsulation technology is triggered (orstimuli-sensitive) release of capsule contents, where thetrigger to rupture the wall can be something that achieveshigh specificity of insecticidal effect, such as the alkaline gutof lepidopteran larvae like cotton bollworm. This has thebeauty of preventing exposure of chemical to predators andparasites, important components of IPM programmes, butlimiting the application to pests which eat treated planttissue and thus take capsules into their gut. Risks to aquatichabitats could also be minimised through thick-walledtriggered capsules. The ultimate trigger to release a toxin isconceivably the presence of the insect or its damage to thecrop. Insect saliva or plant defence chemicals, such assalicylic and jasmonic acids, released in response to feedingby pests would be a highly specific trigger that confinedexposure to those times when, and those locations wherecrop protection is needed. Another situation that would

Figure 2. Demand CS capsule on acockroach leg. Magnification x 1500.

Pest ic ide Outlook – Apri l 2000 71

benefit from trigger technology is the use of baits for antsand cockroaches. These insects are usually repelled by traceamounts of chemicals like pyrethroids, so a system whichlocks up the insecticide tightly until bait is ingested, wouldimprove the administration of lethal doses. Stimuli-sensitivepolymers to achieve on-demand delivery of activeingredients have already found considerable use in non-agri-cultural industries, and should play a valuable part in cropprotection in the future (Beestman, 1996).

Insecticidal action is usually required only at certainvulnerable periods, when a crop is at risk of yield loss orwhen disease vectors are active, and beyond this period thechemical should degrade rapidly before constituting a foodresidue or soil leaching hazard. Enzymes or micro-organismscould conceivably be incorporated in a separatecompartment alongside the active ingredient, and timed torelease when required to eliminate any remaining, unwanteda.i. (Beestman, 1996; Perrin, 1997).

Delivering some of these more sophisticated effects in acost-effective and reliable manner is a considerablechallenge, but the controlled release characteristics of micro-capsule formulations, coupled with their improved toxico-logical, ecotoxicological and environmental profiles, shouldensure for them a bright future.

ReferencesBeestman, G. W. (1996) Emerging Technology: The Bases For New

Generations of Pesticide Formulation. In: Pesticide Formulation

and Adjuvant Technology, edited by C. L. Foy and D. W.Pritchard, pp 43–68, CRC Press, London.

Perrin, R. M. (1997) Crop Protection: taking stock for the newmillennium. Crop Protection 16, 449–456.

Perrin, R. M.; Wege, P. J.; Foster, D. G.; Bartley, M. R.; Browde, J.;Rehmke, A.; Scher, H. (1998). Fast release capsules : a newformulation of lambda-cyhalothrin. Proceedings British CropProtection Conference – Pests and Diseases 1998 1, 43–48.

Scher, H.; Rodson, M.; Lee, K. S. (1998) Microencapsulation ofpesticides by interfacial polymerisation utilising isocyanate oraminoplast chemistry. Pesticide Science 54, 394–400.

Tsuji, K. (1993) Microcapsules of insecticides for household use.Pesticide Outlook 4(3), 36.

Wege, P. J.; Hoppe, M. A.; Bywater, A. F.; Weeks, S. D.; Gallo, T.S. (1999) A microencapsulated formulation of lambda-cyhalothrin. Proceedings of the 3rd International Conferenceon Urban Pests 1999, 301–310.

Woods, T. S. (1999) The formulator’s toolbox - product forms formodern agriculture. In : Pesticide Chemistry and Bioscience :The Food–Environment Challenge, edited by G. T. Brooks andT. R. Roberts, pp 120–133, Royal Society of Chemistry,Cambridge.

Bob Perrin has been a senior entomologist with Zeneca Agrochemi-cals for 22 years and is responsible for the laboratory and fieldevaluation of all novel insecticide formulations. In recent years, hehas co-ordinated extensive testing of Zeon Technology and iscurrently researching the application of triggered release capsules oflambda-cyhalothrin.

Figure 3. Dried deposit of ZeonTechnology capsules on an inertsurface, showing broken and partiallyemptied capsules. Magnification x 4500.