Degradation of insecticides used for indoor spraying in malaria control and possible solutions

12 Pages·2011·0.99 MB·English

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http://www.malariajournal.com/content/10/1/307
RESEARCH Open Access
Degradation of insecticides used for indoor
spraying in malaria control and possible solutions
Mthokozisi M Sibanda1†, Walter W Focke1*†, Frederick JWJ Labuschagne1, Lumbidzani Moyo1, Nontete S Nhlapo1,
Arjun Maity2, Herminio Muiambo1,3, Pedro Massinga Jr1,3, Nico AS Crowther4, Maureen Coetzee5 and
Gordon WA Brindley1
Abstract
Background: The insecticide dichloro-diphenyl-trichloroethane (DDT) is widely used in indoor residual spraying
(IRS) for malaria control owing to its longer residual efficacy in the field compared to other World Health
Organization (WHO) alternatives. Suitable stabilization to render these alternative insecticides longer lasting could
provide a less controversial and more acceptable and effective alternative insecticide formulations than DDT.
Methods: This study sought to investigate the reasons behind the often reported longer lasting behaviour of DDT
by exposing all the WHO approved insecticides to high temperature, high humidity and ultra-violet light.
Interactions between the insecticides and some mineral powders in the presence of an aqueous medium were
also tested. Simple insecticidal paints were made using slurries of these mineral powders whilst some insecticides
were dispersed into a conventional acrylic paint binder. These formulations were then spray painted on neat and
manure coated mud plaques, representative of the material typically used in rural mud houses, at twice the upper
limit of the WHO recommended dosage range. DDT was applied directly onto mud plaques at four times the
WHO recommended concentration and on manure plaques at twice WHO recommended concentration. All
plaques were subjected to accelerated ageing conditions of 40°C and a relative humidity of 90%.
Results: The pyrethroids insecticides outperformed the carbamates and DDT in the accelerated ageing tests. Thus
UV exposure, high temperature oxidation and high humidity per se were ruled out as the main causes of failure of
the alternative insecticides. Gas chromatography (GC) spectrograms showed that phosphogypsum stabilised the
insecticides the most against alkaline degradation (i.e., hydrolysis). Bioassay testing showed that the period of
efficacy of some of these formulations was comparable to that of DDT when sprayed on mud surfaces or cattle
manure coated surfaces.
Conclusions: Bioassay experiments indicated that incorporating insecticides into a conventional paint binder or
adsorbing them onto phosphogypsum can provide for extended effective life spans that compare favourably with
DDT’s performance under accelerated ageing conditions. Best results were obtained with propoxur in standard
acrylic emulsion paint. Similarly, insecticides adsorbed on phosphogypsum and sprayed on cattle manure coated
surfaces provided superior lifespans compared with DDT sprayed directly on a similar surface.
Keywords: Indoor residual spray, DDT, pyrethroid, carbamate, stabilization
*Correspondence:[email protected]
†Contributedequally
1InstituteofAppliedMaterials,DepartmentsofChemistryandChemical
Engineering,UniversityofPretoria,LynwoodRoad,Pretoria0002,South
Africa
Fulllistofauthorinformationisavailableattheendofthearticle
©2011Sibandaetal;licenseeBioMedCentralLtd.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
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Background Itmayalsoundergooxidationonthe─P═Smoietygroup
The World Health Organization (WHO) Global Malaria to form the oxon and ester cleavage to form the corre-
Action Plan promotes indoor residual spraying (IRS) as sponding phenol [20]. Malathion is generally stable to
a primary operational vector control intervention to photolysis.Thismaybeduetoalackofchromophoresto
reduce and ultimately eliminate malaria transmission. In absorb radiation in the UV- range [21]. Pirimiphos-
some southern African countries DDT is regarded as methylphotodegradesrapidlyforming2-diethylamino-6-
the most effective insecticide for this purpose. Depend- methylpyrimidin-4-ol as the major degradation product
ing on the dosage and substrate nature, DDT retains its [22]. Carbamate photodegradation involves the cleavage
efficacy against malaria vectors for up to 12 months. In of the carbamic acid ester to form the corresponding
South Africa DDT was temporarily replaced with the alkylphenyletheroralkylphenolether[23].Carbamates
pyrethroid deltamethrin between 1996 and 1999. How- isomerize on irradiation through the photo-Fries re-
ever, DDT was reintroduced in 2000 when malaria arrangement. Photo decomposition of carbamates also
transmission reached epidemic proportions. The failure produces cholinesterase inhibitors [24]. DDT undergoes
of the pyrethroid was attributed to the return of the homolytic cleavage of the C─Cl bond on the trichloro-
major vector mosquito, Anopheles funestus that was methyl group to form DDD as the product [25]. DDE
shown to be resistant to pyrethroids but fully susceptible may beproduceddue toexothermic effectsof theinitial
to DDT [1,2]. Other WHO-approved pyrethroid, orga- photodechlorinationstage.Theoverallphotodechlorina-
nophosphate and carbamate insecticides are limited in tion is proposed to be dominated by the sequential
effective IRS residual life. Furthermore, repeated applica- dechlorinationpathwaywitheachsuccessivedehalogena-
tion of these alternatives is required in order to provide tion proceedingmore slowly. Apart frometofenprox, all
year-round protection and this significantly increases pyrethroids are structurally similar. They all feature the
the costs of IRS [3]. Formulations based on micro- characteristiccarboxylicestermoiety,thedihalogensub-
encapsulated insecticides have been tested with great stituted vinyl moiety and the phenoxy ether group. Eto-
success [4-7]. These results show that shielding the fenprox only contains the phenoxy ether group. The
insecticides from the outside environment stabilizes carboxylic ester moiety is susceptible to photodegrada-
them against premature degradation. However, the tion and hence decarboxylation of these insecticides is
higher costs associated with such formulations may the main route of photodegradation [26,27]. However
limit their widespread implementation as replacements photodecarboxylation of these pyrethroids is a minor
for DDT in IRS. transformation process accounting for not more than
The stability of WHO-approved insecticides for IRS is 15%ofthetransformationproducts[28].Theamountof
affected by the pH of the environment [8-14], tempera- decarboxylationdependson thehalogen substituentson
ture [15-19], exposure to ultraviolet (UV) light [20-30] thevinylmoiety,i.e.themoreelectronegativethehalogen
and the availability of degrading bacteria [31-36]. Pyre- themorethedecarboxylation.Themainprocessoftrans-
throids, organophosphates and carbamates degrade via formation of these pyrethroids is photo-isomerization
hydrophilic attack of the carboxylic and carbamic ester (cis/trans and E/Z) on the triplet diradicals formed on
linkages [11-13]. DDT undergoes alkaline dechlorination exposuretoultravioletlight(>300nm),e.g.deltamethrin
to yield DDE [14]. [29], cypermethrin and cyhalothrin [30]. Cyfluthrin is
Onthermalexposure,phenylcarbamatesrepresentative subject to photolysis rapidly forming 4-fluorophenoxy-
ofbendiocarbandpropoxurdegradetothecorresponding benzaldehydeviaesterhydrolysisandtherelease of cya-
phenolandmethylisocyanate[15]. Pyrethroidstransform nideionfromthecorrespondingcyanohydrin[21].
byisomerization,estercleavageandprimary oxidationof Extensive work has been done on the fundamentals of
the final products [16,17]. Organophosphates, e.g. insecticidebiodegradationtodevelopbioremediationtech-
malathion,initiallyisomerizetoS-alkylorganophosphates niques to detoxify contaminated environments [31].
beforetheyeventuallydecompose[18].Theprimarystep Microbiological degradation of insecticides relies on the
inthermal decomposition of p,p’-DDT istheelimination availabilityoforganismsthatcansecretespecificdegrading
of HCl, resulting in the formation of p,p’-DDE at 152°C enzymes. Bacteria and fungi have the ability to produce
[19]. DDE starts to volatilize at the onset of the process. these enzymes under both aerobic and anaerobic condi-
Thedecompositiontemperatureisdependentonthetype tions. Carbamates, pyrethroids and organophosphates
ofDDT,foro,p’-DDTthedecompositionstartsat higher haveacommonestermoietythatprovidesarouteforbac-
temperatures. terial-mediatedenzymaticbiotransformation.Theenzyme
All the insecticides classes, except for pyrethroids to groups that are able to hydrolyse these ester moieties
some extent, are degraded by exposure to ultraviolet include carboxyesterases for carbamic and carboxylic
light. Fenitrothion undergoes photo-oxidation on the esters, andphosphotriesterases andcarboxylesterasesfor
benzene methyl group to form a carboxylic acid group. triphosphate esters[32].DDTisdegraded byoxygenases
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anddehydrogenaseenzymes[33]. Aperiod ofadaptation Phosphogypsumisawasteproductgeneratedinthepro-
isrequiredbeforedegradingmicrobesmanagetoestablish duction of phosphoric acid. It was chosen because it is
themselvesandsignificantdegradationoftheinsecticideis readily available in immense quantities and because the
observed[34].Thisacclimationperiodentailsthebuilding phosphoricacidimpurityimpartsanacidicnaturetothis
ofviablemicrobialcoloniesthathaveacapacitytoinduce filler. Phosphogypsum powder was supplied by Gypsum
enhanceddegradation[35].Thelengthoftheacclimation Industries.Thed(0.5)particlesizewas7.6μm(Mastersi-
periodisalsoaffectedbytheavailabilityofsuitablemeta- zer Hydrosizer 2000). Attapulgite with a d (0.5) of 32.0
bolicsubstratesforgrowth.Thesecanbeacarbonsource, μm was supplied by G&W Base Minerals. The particles
amineralnutrientsource,orboth.Thepresenceofalter- of this mineral have an elongated shape with parallel
nativecarbonsubstratesandnutrientsgenerallyincreases channels through the lattice that impart a high adsorp-
the rate of biodegradation. This is supported by the tion capacity and a highsurface area [38]. Calciumben-
observedincreaseintherateofDDTdegradationinhighly tonite powder (d = 20.1 μm) was supplied by G&W
50
fertilizedsoils[36]. BaseMinerals.Dellite67Gorganoclay,amontmorillonite
Mineralpowderssuchasbentonite,gypsum,montmoril- intercalatedwithditallowdimethylammonium(d =12.2
50
lonite,kaolin,attapulgite,diatomiteetc.havebeenusedas μm)wassuppliedbyLaviosa.
insecticide carriers in granular insecticide formulations A sulphonated mimosa extract (Mimosa WS supplied
[37,38]. These minerals have the ability to slowly release by Mimosa Extract Company) was used as dispersant.
the adsorbed insecticide into the environment [39,40]. Makeean Polymers MCP 503 acrylic copolymer emul-
Lagaly[41]presentedanexcellentreviewonpesticide-clay sion was used as binder. The emulsion had a solids con-
interactions and formulations relevant to formulating tent of 56 wt.% and a pH of 7.5 - 8.5.
insecticideladenclayformulations. Ethanol (96% Univar rectified), methanol (Univar)
Impuritiescan mediatecatalyticdegradationofinsecti- and acetone (Univar) were supplied by Merck. Two
cidesadsorbedonsoilsurfaces.Stabilitycanbeimproved soil samples were used to prepare substrate surfaces.
bydeactivatingsurfaceactivesitesresponsible[42].Addi- They were collected at Thomo (Clay 10%, Silt 25.0%
tivessuchaspolyethyleneglycolarereportedlyveryeffec- and coarse sand 59.8%) and Makoya (clay 10%, silt
tive deactivators. A useful property of mineral powder 10% and coarse sand 95.1%) locations in Mozambique.
carriersisthattheycanstabilizephotolabileandthermola- The pH values of the soil samples were 7.1 and 6.5
bileinsecticides[43-45]. respectively.
Ourultimateaimistodeveloplong-lasting,“green”and Accelerated ageing mediated by temperature, ultra-violet
cost effective alternatives to DDT for IRS. This paper light or high humidity
reportsoninvestigationsdirectedtowardsfindingwaysto Polytetrafluoroethylene (PTFE) infrared sample cards
extendtheactivelifeofinsecticidesformulatedinenviron- suppliedby LifeandAnalytical Sciences(PTY)Ltdwere
mentallystable,pesticide-based“whitewash”orequivalent usedassampleholders.PTFEwaschosenassubstrateon
“paint”.Thegoalsofthisinvestigationweretoovercome account of its chemical inertness, UV and temperature
thetime-limitedeffectivenessofcurrentWHO-approved stability as well as its transparency in the infrared range
DDTalternativesbyevaluatingtheinteractionsbetween 1300 cm-1 to 3400 cm-1 wave numbers. Neat insecticide
selectedinsecticideswiththemicro-environmentonwhich filmsweredepositedonthesubstratebycoatingitwitha
theyareappliedandalsowithlow-costpaintingredients. 1wt.%solutionofthe activeinacetone andallowingthe
solventtoevaporate.
Methods The state of the insecticides deposited on the Teflon
Materials substrates was followed as a function of ageing time
The technical grade insecticides used in this study are using Fourier Transform Infrared (FTIR) spectroscopy.
listed in Table 1 together with their suppliers. A Perkin Elmer Spectrum 100 spectrometer was used in
Table 1Insecticides tested andWHO recommendeddosage rangeforIRS
Insecticideclass/Insecticide IRSdosageg/m2 Activity*months Supplier
Organochlorine
DDT 1-2 >6 Avima
Carbamates
Bendiocarb 0.1-0.4 2-6 Bayer
Propoxur 1-2 3-6 Avima
Pyrethroid
Alphacypermethrin 0.02-0.03 0.02-0.03 Bilag
*Durationofeffectiveaction
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transmission mode. The resolution was 1 cm-1 and 20 temperature was immediately ramped at a rate of 25°C.
scans were recorded per sample. min-1tothefinaltemperatureof275°Cwhereitwasheld
A Labcon economy type EFDO oven, set at 80°C, was for 10 minutes. The injector and detector temperatures
used for high temperature ageing. Samples were exposed werebothsetat250°C.
to this elevated temperature for consecutive 50 h peri- Spray surfaces and insecticide formulations for ageing and
ods. FTIR spectra were recorded after each exposure bio-assaying
period. A Labcon model FSIM forced circulation incu- Soilsamplesweremixedwithwarmwatertoformathick
bator was used for humidity ageing. The incubator con- paste. This paste was then packed in substrate holders
trolled the temperature to within ±0.2°C and humidity madefromhighdensitypolyethylene(HDPE).Thesewere
to within ± 2% ℜH. The samples were exposed to a round shallow dishes with diameter 11.6 cm and depth
relative humidity of 90% at 60°C for 168 h. Accelerated 2 cm or diameter 15.5 cm and depth 2.5 cm. The mud
artificial weathering was conducted in a QUV tester substrateswerethenovendriedat90°Cfor24h.Oneset
fitted with A340 UV lamps. A dry cycle was used with ofthedriedsoilplaqueswascoatedwithfreshcattleman-
the temperature set at 63°C and an irradiance of uredilutedwithsomewater.Theserepresentedthemicro-
0.67 W.m-2. biologically active surfaces typically found in rural mud
houses.
Hydrolytic stability of the insecticides
Since most insecticides are effectively insoluble in water, Preparation of spray “paints”
thefollowingprocedure was usedto explore theeffect of Paints were prepared using alphacypermethrin (pyre-
mineral powders on hydrolytic stability. Stock solutions throid),bendiocarbandpropoxur(carbamates)andDDT
containingca.1%insecticidewerepreparedbyfirstdissol- (organochlorine). Organophosphates were excluded
ving8gbendiocarboralphacypermethrinin800gacetone because they were considered too unstable. Insecticides
and then adding 32 g of distilled water. Mineral powder were dissolved injustenough acetone tocompletely dis-
(1g)wasaddedtotwosetsofsixSchottglassbottles.To solve the insecticides. Since the insecticidesare all water
thiswasadded21goftherespectiveinsecticidestocksolu- insoluble, the insecticides precipitated on the gypsum
tions. The top of the bottles were sealed with Parafilm whenthesolutionwasdispersedintothephosphogypsum
beforecappingandthenwrappedinaluminiumfoiltopro- slurry. Next,dispersantwas added at a loading of 2wt.%
tectagainstlightexposure.Thebottleswereagitatedona basedonthemassofgypsumused.Thislevelwaschosen
shakertableatfrequencyof180cyclesperminuteduring on the basis of its viscosity reducing effect on a 50 wt.%
theageingperiodatambientconditions(temperatureran- phosphogypsum slurry. These dispersion formulations
gingfromca.15°Cto25°C).Twosetsofsolutions(without weresprayedonmudplaques.Experimentsconductedat
mineralpowders)foreach insecticidewere usedasrefer- alaterstageestablishedthatmorestabledispersionswere
ence and control standards respectively. The Reference obtainedbyaslightmodificationoftheprocedure.Inthis
sample was stored in a fridge and the (negative) Control method the insecticide was dissolved in just sufficient
sample was kept under the same conditions as the test acetonetoallowcompletewettingofthedryphosphogyp-
samples. A small amount of sodium ethoxide, a strong sum powder. This mixture was then slurried into water
alkali, was added to another pair of Schott glass bottles usingvigorousstirringandthenaddingthesameamount
containing the bendiocarb and alphacypermethrin stock ofdispersant.This second set of dispersionformulations
solutions. These samples provided a positive control and wassprayedontothecattlemanure-coatedmudplaques.
were prepared inorder to confirm the instability ofboth The following three types of “paints” formulations
insecticidesinhighlyalkalineenvironments. were prepared and tested:
The degradation of the insecticide in the presence or
absence ofmineralpowders wasfollowed using a Varian (cid:129) Gypsum “white wash”. Insecticide-coated phos-
Starmodel3400CXgaschromatographfittedwithanAgi- phogypsum slurry (50 wt.% solids with 1 wt.% dis-
lent HP5GC column.Thecolumnlength was 30mand persant) was used as is. The final insecticide content
the internal diameter 0.25 mm with a film thickness of based on dry solids was 0.37 wt.% alphacypermethrin
0.25μm.Thecarriergaswasheliumandthegassplitratio and 9 wt.% bendiocarb. The dispersions were
was1:30.Aflameionizationdetectorwasused.Forbend- sprayed on soil surfaces and cattle manure coated
iocarb the initial oven temperature was120°C, a holding surfaces.
timeof1minute,aramprateof25°C.min-1,finaltempera- (cid:129) Gypsum “white wash” with minimal acrylic bin-
ture of 250°C and a final holding time of 10 min. The der. Phosphogypsum (45 wt.%), paint binder (4.5 wt.
injector temperature was 200°C while the detector tem- %),dispersant(0.9wt.%),water(45wt.%)plusbendio-
perature was set at 265°C. For alphacypermethrin the carb(4.5wt.%)wasused.Thefinalinsecticidecontent
initial oven temperature was set at 245°C and the based on dry solids was 8 wt.%. This “paint” was
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sprayedonthemudsurfaces.Theideabehindadding temperature, UV light exposure or high humidity.
the acrylic binder was to improve adhesion of the Selected members of this class survived more than
insecticidetothehousewallsurface. 250 h oven ageing at 80°C, up to 150 h 0.67 W.m-2
(cid:129) Acrylic paint. Carbamate insecticides (bendiocarb QUV exposure at 63°C and all were still present after
or propoxur), dissolved in a minimal amount of 168 h high humidity ageing at 40°C and 90% ℜH. In
acetone, were mixed into the neat acrylic emulsion. contrast, DDT and the other classes were either comple-
The final insecticide content was 15.2 wt.% based on tely degraded or lost after only 50 h of exposure to high
dry solids. This “paint” was sprayed on standard soil heat or UV light.
surfaces.
Thermal ageing
The FTIR spectra (Figure 1) showed that alphacyperme-
Surface spraying thrin was still present after 250 h exposure to a tem-
The insecticide“paints”weredepositedonthesubstrates perature of 80°C. However, a reduction and broadening
using an Aircraft Pneumatic Systems, Model SG AS in the characteristic absorption peaks is apparent after
1001Aspraygun.Thefeedtothespraygunwasbygrav- about 50 h suggesting isomerization of the parent com-
ity. The nozzle diameter was 1.4 mm and the working pound and, possibly, partial volatilization. No DDT,
pressure 2bar.The spray patternwasadjusted such that organophosphate or carbamate was detectable by FTIR
the widest possible spray width was achieved. A smaller on the sample substrates after 50 h at this temperature.
Aircraft Pneumatic Systems Model SG A138 spray gun
wasusedtospraytheDDTacetonesolutions.Thefeedto UV light exposure
the spray gun was via suction. This nozzle diameter was The temporal behaviour of the pyrethroid insecticides
0.8mmandaworkingpressure3.45bar.Thecompressed exposed to UV light is exemplified by the FTIR spectra
airusedforsprayingwastappedfroma6barcompressed recorded for alphacypermethrin (Figure 2A). The spec-
airlineviaacommonmanifold. trum recorded for the neat insecticide shows strong and
Each “paint” was sprayed onto soil plaques and cattle well-resolved absorption bands located at 1740 cm-1
manure coated surfaces. Triplicate samples of each type (ester carbonyl C=O stretch), 1605 cm-1 and 1485 cm-1
was produced and tested. Since the present purpose was (aromatic C-C in-ring stretch modes). With increase in
to studydegradationunderaccelerated laboratorycondi- exposure time these initially very sharp bands broaden
tions, the final insecticide loading was set at twice the considerably and diminish in strength. However, they
upper limit of the WHO recommended dosages [46] are still present following 250 h of UV light exposure.
(Table1).DDTwassprayeddirectlyontothestandardsoil The observed broadening is attributed to the formation
surface and cattle manure coated surface as a 10 wt.% of degradation products with slightly shifted absorption
solutioninacetone.Fourtimestherecommendedamount bands that are observed superimposed on those of car-
of DDT was employed on the soil substrates because it bonyl and aromatic functional groups of the parent
performedpoorlywhentestedattwicetheupperlimit. compound [48].
The temporal changes in the FTIR spectra for DDT
Bioassays are presented in Figure 2B. The strongest absorption
Thetreatedsurfacesweresubjectedtoacceleratedageing band for DDT is the aromatic C-C in-ring stretch mode
in a humidity cabinet set at 90% relative humidity and a located at ca. 1500 cm-1. It is still present after 50 h of
temperatureof40°C.WHObioassays methods[47]were exposure but has completely disappeared after 100 h
usedtoperiodicallytracktheresidualefficacyagainstmos- irradiation. The appearance of two new absorption
quitoes.Inthesetests,25non-blood-fedfemaleAnopheles bands 2355 and 2365 cm-1 after merely 50 h UV light
arabiensis mosquitoes (KGB colony housed at the exposure indicate the onset of UV mediated degrada-
NationalInstituteforCommunicableDiseases,Johannes- tion. This doubled dominated in the spectra recorded
burg),3-5daysold,wereexposedfor30minutestosam- after 100 h and 150 h UV light exposure. The increased
ples of the treated or untreated substrate surfaces. intensity of the absorption band at 2968 cm-1 suggests
Mortalityrateswererecorded24hoursafterinitialexpo- that the aromatic rings were oxidized to phenols or
sureofthetriplicatesforeachtreatedsurface. quinones.
The absorption bands characteristic of fenitrothion
Results disappeared within the first 50 h of ageing (Figure 2C).
The results obtained for thermal ageing, UV degradation The spectrum of the degraded residue is consistent with
and resistance to high humidity are summarized in the formation of phenols (ca. 2970 cm-1) and phosphates
Table 2. As a group, the pyrethroids showed the greatest (ca. 2343 cm-1). Bendiocarb also degraded rapidly when
resistance to degradation whether mediated by elevated exposed to UV light (Figure 2d). The strong absorption
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Table 2FTIRrecorded performance under accelerated ageing conditions
Insecticideclass Insecticide Heatageing1 UVdegradation2 Humidity3
Organochlorine DDT <50h <100h lost
Organophosphate malathion <50h <50h -
fenitrothion <50h <50h -
pirimiphos-methyl <50h <50h lost
Carbamates bendiocarb <50h <50h lost
propoxur <50h <50h lost
Pyrethroids alphacypermethrin >250h >250h present
bifenthrin <50h <150h present
cyfluthrin >250h <150h present
deltamethrin >250h <150h present
lambdacyhalothrin <150h <150h present
Other etofenprox <200h <150h present
1Ovenageingat80°Cin50hintervals;2DrycycleUVdegradationinaQUVat63°Cand0.67W/m2in50hintervals;3168hHumidityageingat40°Cand90%
ℜH
peak characteristic of the carbamate carbonyl has com- Effect of mineral powder on insecticide hydrolysis rates
pletely disappeared after 50 h of irradiation. Figure 4 presents the effect of the mineral fillers on the
persistence of bendiocarb (Figure 4A) and alphacyper-
Humidity ageing methrin (Figure 4B) solutions in acetone containing 1
The FTIR spectra for the samples that were exposed to wt.% water. Adding catalytic amounts of sodium ethox-
a relative humidity of 90% at 60°C revealed that only the ide caused, as expected, rapid hydrolysis of both insecti-
pyrethroids were still present after 168 h. The results for cides. Bendiocarb was quite stable in this medium. No
alphacypermethrin, given in Figure 3, are typical. DDT, change in concentration was detected in this medium
pirimiphos-methyl and the carbamates were undetect- when aged at room temperature (Control sample). In
able after this ageing period. contrast, the alphacypermethrin Control sample
degraded over time. The concentration of both insecti-
cides decreased rapidly in the presence of the organo-
clay filler. This does not necessarily mean that it was
degraded in the presence of this substance. Co-intercala-
tion of the insecticides into the clay galleries can also
remove them from the liquid phase. Adding phospho-
gypsum appears to have improved the hydrolytic stabi-
lity of alphacypermethrin whereas the opposite appears
to be true for bendiocarb.
Bioassays
Exploratorybioassaytestswereperformedusinggypsum
“whitewash”withminimalacrylicbinderoptionsprayed
onmudandondungcoatedmudsurfaces.Propoxurper-
formed well on both surfaces achieving 100% mortality
after 24h.However,alphacypermethrinfailed theWHO
criterion with an initial mortality of only 25%. This is
attributed to a high solubility of this insecticide in the
resinsuchthat the surface concentrationfalls below the
effective level. Furthermore, spray tests showed that the
phosphogypsum adhered well to a variety of wall sur-
faces,i.e.additionoftheacrylicemulsionwasnotneces-
sary. These findings guided our selection of the
formulationsthatweresprayedandperformance-charac-
Figure 1 FTIR spectra of alphacypermethrin deposited on a
terized by bioassay testing. The results for insecticidal
Teflonsubstrateasafunctionofovenageingtimeat80°C. “paints” applied to mud surfaces and cattle manure
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A B
Phenol/─CH2 ─C≡C
250 h
150 h
Absorbance, - 112050000 hhh ─C═C bsorbance, - 100 h
A 50 h
50 h
═CH ─CH ─CH2 ─C═C
Neat ─C═O Neat
3400 3000 2600 2200 1800 1400 3400 3000 2600 2200 1800 1400
Wavenumber, cm-1 Wavenumber, cm-1
C D
─C═C─
─CH
ce, - P─O─R ce, -
Absorban O═P─OH/Phe─noClH O═P─O O═P─O Absorban ─NH ─CH
Neat Neat
─CH 50 h ─CH R─C
100 h 50
3400 3000 2600 2200 1800 1400 3400 3000 2600 2200 1800 1400
Wavenumber, cm-1 Wavenumber, cm-1
Figure2FTIRspectrafollowingQUVageing:A.Alphacypermethrin;B.DDT;C.Fenitrothion;D.Bendiocarb.
coatedmudsurfacesarepresentedinFigure5andFigure
6respectively.
The WHO effectiveness criterion for IRS is a mos-
quito mortality exceeding 80% after 24 h following a 30-
minute exposure. The data show considerable scatter
but it is clear that most formulations fail by the second
month of exposure to the accelerated laboratory ageing
conditions. The data were fitted using logistic regression
in order to allow statistically valid comparisons of the
time to failure for each treatment. The times to reach
80% mortality were estimated from the fitted equations.
The results are plotted in Figure 7 together with 95%
confidence intervals. The following ranking (estimated
time to failure in days indicated in brackets) was
obtained for paints applied to mud surfaces:
DDT (31.3) < bendiocarb in binder (38.3) ≈ alphacy-
Figure 3 FTIR spectra for neat alphacypermethrin and after permethrin on gypsum (38.7) ≈ bendiocarb on gyp-
ageinginahumiditychamberat60°Cand90%ℜHfor168h. sum (39.4) ≈ bendiocarb on gypsum with paint
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A
B
Figure6Bioassay(24 h Mortality) results for manure coated
soilsurfacestreatedwithinsecticidesandagedat40°Cand
90%ℜHforvaryingtimeperiods.
means that the difference in performance between DDT
and the other paints on mud surfaces is not statistically
significant. However, this outcome applies specifically to
Figure 4 Hydrolysis rates for: A) Bendiocarb and B) the procedure used for preparing the dispersion applied
Alphacypermethrinondifferentmineralpowdersand to the mud surfaces.
suspendedinacetonecontainingwater.
DDT showed similar performance on the dung coated
mud surfaces with an estimated active time of 29.3 days
binder (39.6) < propoxur in acrylic paint binder and a 95% confidence interval of 25.3 days to 33.4 days.
(50.3) The overall compositions of the insecticide-on-gypsum
slurriessprayedontothemudanddungcoatedmudsam-
However, the 95% confidence interval for DDT effec- ples were identical. However, the improved preparation
tive life time overlaps with that for all the other treat- methodwasusedinthelattercase.Thereforeitisnotpos-
ments except propoxur in the acrylic binder. This sible to make direct comparisons of the performance
achievedonthedungsurfacesurfaceswiththosefoundon
thecorrespondingmudsurfaces.However,mortalityrates
of100%wereachievedevenaftersevenmonthsofageing
for both the alphacypermethrin-on-gypsum and bendio-
carb-on-gypsumsprayedontomanure-coatedsurfaces.
Figure7WHOeffectivelifetime(>80%Mortalityafter24h)
Figure5Bioassay(Mortalityafter24h)resultsobtainedonsoil forvarioustreatmentsonmudsurfaces.Theerrorbars
samplestreatedwithinsecticidesandagedat40°Cand90% indicatethe95%confidenceintervalsdeterminedvialogistics
ℜHforvaryingtimeperiods. regression.
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Discussion were estimated using Fuller’s method [50]. Published
Loss of insecticide vapour pressure data at 25°C [8-10,51-59] are summar-
IRS relies on the fact that female mosquitos tend to set- ized in Figure 8. Assuming a diffusion distance of 2 m,
tle on the walls inside dwellings where they may rest for ambient temperature of 25°C, using the WHO IRS
a few days after they have taken a blood meal. The dosagerecommendationsandapplyingequation(1)leads
insecticides used in IRS are contact poisons that are tothefollowingestimatesforthebioactivelifeinyears:
applied to the inside walls of dwellings to kill the female
mosquitoes in this resting phase. The activity of the Fenitrothion, 136; pirimiphos-methyl 110; bendio-
insecticides deposited on such walls decreases over time. carb, 11; propoxur 96; malathion 457; DDT, 7726;
This may be due to volatilization or degradation pro- and alphacypermethrin 319.
cesses, e.g. biodegradation, photodecomposition and
alkaline hydrolysis. Volatilization occurs by evaporation Raisingtheambienttemperaturehasadramaticeffect.
for liquid insecticides and by sublimation for solid insec- For example, at 40°C the persistence of fenitrothion is
ticides. The rate of vaporization increases with the reducedto24years.Ventilationfurtherincreasestherate
ambient temperature and the degree of ventilation. In of volatilization. In order to get a rough estimate of the
general, it can be estimated from equation (1) [49]: impact of this factor, a flat plate approximation model
for the wall is assumed with the air flow parallel to the
n = ShDABPAMA (1) wallata velocityV in m.s-1. Forthis geometry the char-
A
zRT acteristiclengthzcorrespondstothelengthofthewallL
wheren ismassfluxinkg.m2.s-1;Shisthedimension- andtheSherwoodnumberisgivenby[60]:
A
lessSherwoodnumber;D isthediffusioncoefficientin √
AB
m2.s-1;P isthevapourpressureoftheinsecticideinPa; Sh = 0.667 Re Sc1/3 (2)
A
z is a characteristic length of the system measured in m
where Re is the Reynolds number (VL/v); Sc is the
the diffusion distance in m, taken here as half the width
of the house; R is the gas constant 8.314 J.mol-1.K; T is Schmidt number (v/DAB); v is the kinematic viscosity in
theabsolutetemperatureinK;andM isthemolarmass m2.s-1.
A
oftheinsecticideexpressedinkg.mol-1. Typical velocities that could be encountered inside a
house range from ca. 0.001 m.s-1 to about 1 m.s-1. Using
Therateoflossisleastintheabsenceofanyairmove-
these values and typical property values for air at ambi-
ment.Inthisscenariotherateofmasslossisdetermined
ent conditions reveals that the active life times indicated
by rate of diffusion of the insecticide through the stag-
above could be reduced by 4 to 100 times. This means
nant air layer and the Sherwood number assumes the
value of unity while z is the diffusion distance in meters that, with the possible exception of bendiocarb, the loss
of activity of the WHO insecticides experienced in the
normal to the wall surface. For preliminary estimation
field cannot be attributed to volatilization! The implica-
purpose it is taken as half the width of the house. The
tion is that the loss of activity over time must be asso-
diffusion coefficients in air of the present insecticides
ciated with degradation processes.
Ageing processes
Ingeneral organic insecticides degrade when exposed to
elevated temperatures (thermal degradation); sunlight
(photodegradation);moisture(e.g.hydrolysis),andmicro-
bial attack. The laboratory tests used in this study simu-
lated relatively extreme conditions of temperature, ultra
violetlightexposure,humidity,alkalinityandmicrobiolo-
gicallyactivity.Thetemporalperformanceofneatandsta-
bilized WHO-approved insecticides samples were
compared to that of neat DDT. It was found that DDT
alternativesstabilizedthroughincorporationinanacrylic
binderorprecipitatedonacidicfilleroutlastedDDTwhen
appliedtodungcoatedsurfaceandperformedatleastata
similarlevelwhenappliedtosoilsurfaces.
Figure8Logvapourpressureplotsoftheinsecticidesat25°C The loss of DDT and carbamate insecticide activity
butforpirimiphos-methylat20°C.Referencesaregivenin under the ageing condition of 80°C is attributed to a
bracketsaftereachinsecticidename.
combination of thermal degradation and volatilization.
Sibandaetal.MalariaJournal2011,10:307 Page10of12
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The latter speculation is based on the vapour pressure alphacypermethrin dissolved in acetone containing 1 wt.
data for the insecticides (Figure 8). The pyrethroid class % water. Similar results were obtained with bendiocarb
of insecticides feature the lowest vapour pressures whilst although in this case greater stabilities were achieved.
the organophosphates, carbamates and DDT have the The stabilizing effect appeared to correlate with the sur-
highest, second highest and third highest vapour pres- face pH of the mineral powders (indicated in brackets)
sure respectively. The vapour pressure data explains suggesting they acted as acidic pH buffers. Phosphogyp-
why the pyrethroids are generally less fugitive than the sum showed the best results. It is a cost-effective option
rest of the insecticides under oven ageing conditions and preliminary spray tests showed that it sticks well to
conducted at 80°C. The low vapour pressure makes the a variety of vertical surfaces that are encountered in
pyrethroid group of insecticide the least volatile as com- rural dwellings.
pared to the rest of the insecticides and hence evapora- This suggests that pH-catalysed hydrolysis is a degra-
tion is slower than the rest. The present observations, dation mechanism that is able to deactivate all the other
however, suggest that pyrethroids are more stable insecticide classes at a rate that is significantly faster
towards thermal exposure compared with DDT and that DDT. Malaria control authorities in South Africa
carbamates. prefer to spray DDT on mud walls commonly found in
The chemical transformation of insecticides subjected African rural rondavels whilst they spray pyrethroids on
to UV exposure is quite remarkable. Pyrethroids clearly western style painted cement walls [61]. This preference
exhibited significantly greater resilience to UV radiation is, in part, due to the fact DDT has the ability to persist
than the rest of the insecticide classes. Thus the acceler- longer on mud walls compared to pyrethroids. It is pos-
ated ageing tests conducted presently indicate that pyre- sible that in the field of application, surface chemical
throids were a far more stable class of insecticides when and biological properties of the walls being sprayed with
exposed to typical environmental elements of degrada- the insecticides play a significant role in the breakdown
tion compared with the other classes of insecticides. of the insecticides through water mediated hydrolysis.
This strongly suggests that high temperatures, ultra vio-
let light and humidity are not the dominant mechanisms Bioassays
of oxidative/hydrolytic degradation of insecticides in the Dwellings in deep rural areas commonly feature mud
field. and dung coated walls. Bioassays were used to follow
It has been shown that, except for DDT, the other the time-dependent efficacy of the stabilized insecticides
insecticides tested are very sensitive to pH-mediated applied onto such surfaces under accelerated laboratory
hydrolysis. This is especially true at elevated pH values. ageing conditions of high humidity and elevated tem-
The plots of the published half-life (DT50) values perature. The considerable variability in the data makes
[14,51-53,57] against pH presented in Figure 9 show statistically reliable comparisons difficult. However, the
that the WHO-approved insecticides are more stable at results suggest that the carbamates bendiocarb and pro-
lower pH values. The data presented in Figure 4 showed poxur perform well when incorporated into an ordinary
that some selected fillers had a stabilizing effect on paint binder, while bendiocarb and alphacypermethrin
precipitated on phosphogypsum also performed well in
the accelerated ageing test conducted at 40°C and 90%
relative humidity.
Conclusions and recommendations
DDT has a significantly longer persistence than other
WHO approved insecticides under IRS field conditions.
Comparative tests were performed to track the stability
of DDT and alternative insecticides under accelerated
laboratory ageing conditions of elevated temperature,
ultraviolet light exposure and high humidity. The results
suggest that these factors are unlikely causes of the
lower persistence of IRS insecticides in the environment.
It is likely that water mediated hydrolysis, facilitated by
a suitable environmental pH values and/or high bioac-
Figure 9 Effect of pH on the rate of hydrolysis of WHO tivity, are more important. To test this hypothesis, bio-
approvedinsecticides.Therateofhydrolysisisexpressedashalf- assay experiments were conducted on mud and dung
lifeDT50,ameasureofthetimeittakesforhalftheparent coated samples and aged at high humidity conditions
compoundtotransform.
that should favour rapid hydrolysis of the insecticides.