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Full PaperReceived: 11 May 2011 Revised: 21 July 2011 Accepted: 22 July 2011 Published online in Wiley Online Library: 9 September 2011

(wileyonlinelibrary.com) DOI 10.1002/aoc.1837

Synthesis, structural and larvicidal studiesof a series of triorganotin chrysanthemumatesAlejandra Zapataa, Diane P. Mcleana, Jose H. Delao Hernandeza,Angel C. de Diosb, Xueqing Songa and George Enga∗

A series of triorganotin chrysanthemumates (2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylates) (R3SnO2CC9H15)where R = methyl, ethyl, n-butyl and phenyl was synthesized. The solid state structures were deduced using infrared (IR) andMossbauer spectroscopies. The spectroscopic results indicated that all the compounds were found to be five-coordinated in thesolid state. Based on the NMR results, all the compounds are tetrahedral in solution. Larvicidal activities of the compounds wereevaluated against the second instar stage of Aedes aegypti, Anopheles stephensi and Culex pipiens quinquefasciatus mosquitoes.The toxicity results indicated that these compounds of triorganotins were effective larvicides against all three species of larvae.Copyright c© 2011 John Wiley & Sons, Ltd.

Keywords: Aedes aegypti; Anopheles stephensi; Culex pipiens quinquefasciatus; larvae; Mossbauer; mosquito; NMR (1H; 13C and 119Sn);toxicity; triorganotins

Introduction

The biocidal applications of triorganotins (R3SnX) are wellrecognized. A diverse collection of organisms are susceptibleto various triorganotin compounds, with the toxicity of thetriorganotins being species specific. The species specificity oftriorganotins is dependent on the organic R group attached to thetin atom and is well documented in the literature.[1,2] For example,Gram-negative bacteria are sensitive to tri-n-propyltins, whileaquatic species such as fish and mollusks are highly susceptibleto tri-n-butyl- and triphenyltins.[1,2] Three organic groups (n-butyl,phenyl and cyclohexyl) have been reported to be toxic againstvarious species of mosquitoes and their larvae.[3 – 5] The generallyaccepted view for the anionic X group in R3SnX compoundsis that they do not play a major role in the toxicity of thecompounds.[1,2] However, depending on the nature of the Xgroup, it has been reported to be both insignificant as well asimportant.[6 – 8]

Pyrethroids (synthetic pyrethrins) are another class of com-pounds that have been shown to be effective against mosquitolarvae as well as adult mosquitoes.[9] Several fragments inpyrethroids have been reported to have insecticidal activity;[9]

therefore incorporating a modified active component/fragmentof a pryethroid into a triorganotin moiety may increase theactivity of the triorganotin due to synergistic effects. Previ-ously, modified fragments of pyrethrin such as a methylbutyrateand/or cyclopropanecarboxylate group have been incorporatedinto various triorganotins and their activities were evaluatedagainst various mosquito larvae.[10,11] Chrysanthemic acid (2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylic acid) isanother active fragment in the first synthetic pyrethroid:allethrin.[9] Thus a series of modified triorganotin chrysan-themumates have been synthesized and their efficaciesagainst various species of mosquito larvae are reportedherein.

Experimental

Material and Elemental Analyses

2,2-Dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylicacid ethyl ester was obtained from Aldrich Chemical Co., Inc.(Milwaukee, WI, USA) and the triorganotin chloride, oxide andhydroxide were purchased from Gelest, Inc. (Tullytown, PA, USA).The starting materials were used as received. All the solvents wereobtained from Fisher Scientific Inc. (Pittsburgh, PA, USA) and usedwithout further purification.

Elemental analyses (C, H and Sn) were performed bySchwarzkopf Microanalytical Laboratory (Woodside, NY, USA).

Spectral Studies

The IR spectra in the 400–4000 cm−1 region were recorded asneat samples on a Nicolet Magna-IR 760 spectrometer equippedwith an ATR attachment. All NMR measurements were made on aVarian Unity Inova 500 MHz spectrometer with a carbon frequencyof 125.684 MHz in CDCl3. Sample and instrument temperatureswere controlled at 298 K. Proton-decoupled 13C and 119Sn spectrawere acquired with WALTZ decoupling. 1H and 13C chemicalshifts were referenced to internal TMS (tetramethylsilane), while119Sn chemical shifts were referenced to tetramethyltin externally.The Mossbauer spectra of the solid compounds were measuredat 80 K on a Ranger Mossbauer model MS-900 spectrometerin acceleration mode with a moving-source geometry using aliquid nitrogen cryostat. The source was 10 mCi Ca119mSnO3 and

∗ Correspondence to: George Eng, Department of Chemistry and Physics,University of the District of Columbia, Washington, DC 20008, USA.E-mail: Geng@udc.edu

a Department of Chemistry and Physics, University of the District of Columbia,Washington, DC 20008, USA

b Department of Chemistry, Georgetown University, Washington, DC 20057, USA

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the velocity was calibrated at ambient temperatures using acomposition of BaSnO3 and tin foil (splitting 2.52 mm s−1).

Synthesis of Chrysanthemic Acid

Chrysanthemic acid (2,2-dimethyl-3-(2-methyl-1-propenyl)cyclo-propanecarboxylic acid) was prepared by the saponification ofethyl chrysanthemumate in 5% NaOH aqueous solution. In a 250 mlround-bottomed flask were placed 19.6 g (0.1 mol) of 2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylic acid ethyl esterand 50 ml of 10% NaOH solution. The mixture was heated for 2 hwith stirring until the ester became emulsified. 50 ml water wasthen added and the heating was continued for another 30 min untila clear solution was obtained, indicating complete saponification.The solution was then poured into 100 ml of 10% hydrochloric acidwith stirring. The free acid was then extracted with ether and thechrysanthemic acid (9.6 g, 57%) was collected at 109 ◦C/1 mmHgvia vacuum distillation.[12] M.p. 50–52 ◦C (literature m.p. 54 ◦C).[13]

Synthesis of the Compounds

Compounds (I1 –I5) were synthesized using 2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylic acid and the appro-priate triorganotin according to the procedures given below.

Preparation of compounds I1 and I5 (trimethyltin- and triphenyltin-2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylates)

O

OH + R3SnOHReflux

Toluene

O

OSnR3 + H2O

R=Me (I1), Ph (I5)

Triorganotin hydroxide ((CH3)3SnOH (2 mmol, 0.361 g) for com-pound I1 and (C6H5)3SnOH (2 mmol, 0.733 g) for compound I5)was dissolved in 20 ml hot toluene in a 100 ml round-bottomedflask fitted with a Dean–Stark trap. To this was added, withstirring, an equal molar amount of 2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylic acid (0.337g) dissolved in 20 mltoluene. The mixture was refluxed for 2 h and, upon cooling, thereaction mixture was filtered. The solvent was removed using arotor evaporator, resulting in a crude oil and, upon refrigeration,a white solid formed. Recrystallization from 95% ethanol gave thedesired product.

Preparation of compounds I2 and I3 (triethyltin- and tri-n-propyltin-2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylates)

O

OH + R3SnClToluene

O

OSnR3 +

R = Et (I2), n-Pr (I3)

iBu2NH

iBu2NH2 Cl

Diisobutylamine (2 mmol, 0.26 g) in 10 ml toluene was addeddropwise to a mixture of the appropriate triorganotin chloride((C2H5)3SnCl (2 mmol, 0.413 g) for compound I2 or (n-C3H7)3SnOH(2 mmol, 0.480 g) for compound I3) and 2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylic acid (2 mmol, 0.337 g) dis-solved in 30 ml toluene under a nitrogen atmosphere. A cloudywhite solution formed immediately, which became clear uponrefluxing for 1 h. The insoluble ammonium salt which was formedupon cooling was removed by filtration. The solvent was removedpartially under reduced pressure until approximately 20 ml of solu-tion remained. A white solid formed upon refrigeration overnight.Recrystallization from a mixture of chloroform and petroleumether afforded fine crystals.

Preparation of compound I4 (tri-n-butyltin-2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylates)

O

OH + Reflux

TolueneO

OSnBun3+1/2 H2O

1/2 nBU3SnOSnBU3

I4

A suspension of bis(tri-n-butyltin) oxide and 2,2-dimethyl-3-(2-methyl-1-propenyl)cyclopropanecarboxylic acid in a 1 : 2 molarratio (1 mmol, 0.697 g; and 2 mmol, 0.337 g, respectively) wassuspended in 50 ml toluene in a 100 ml round-bottomed flaskfitted with a Dean–Stark moisture trap. The reaction mixture wasrefluxed for 4 h. The solution was then filtered and the clear filtratewas concentrated to dryness using a rotary evaporator, to givea white solid. Fine crystals of compound I4 were obtained byrecrystallizing the crude product in 95% ethanol.

The melting points and the elemental analyses for thecompounds are given in Table 1.

Toxicity studies

The mosquito larvae (Anolphes stephensi (An. stephensi), Aedesaegypti (Ae. aegypti) and Culex pipiens quinquefasciatus (Cx.p. quinquefasciatus)) were obtained from the Laboratory ofMalaria and Vector Research of the National Institutes of Health.The protocols for the larvicidal studies have been previouslyreported.[14]

Results and Discussion

Mossbauer Spectra

Solid state geometries of organotin compounds are commonly de-duced using 119Sn Mossbauer spectroscopy. Obtainable from the119Sn Mossbauer data are the isomer shift (δ) and quadrupolesplitting (�) values. Rho values (ρ = �/δ) have been re-lated to the coordination number of the tin atom.[15] Trigonalbipyramidal (tbp) structures tend to have higher � valuesthan tetrahedral structures.[16,17] In addition, trans tbp struc-tures have higher � values (3.00–4.00 mm s−1) than cis con-figurations (1.70–2.40 mm s−1) but less than the mer isomer(3.50–4.10 mm s−1).[2]

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Table 1. Melting points and elemental analysis of triorganotin chrysanthemumates (R3SnO2CC9H15)a

Elemental analysis

Compound Rb MP(◦C) C Found (calc.) H Found (calc.) Sn Found (calc.)

I1 Me 168–170 46.60 (47.17) 7.18 (7.31) 35.80 (35.86)

I2 Et 124–126 51.27 (51.50) 8.46 (8.10) 31.92 (31.82)

I3 n-Pr 103–105 55.14 (54.96) 8.75 (8.74) 28.39 (28.59)

I4 n-Bu 99–101 58.09 (57.78) 9.33 (9.26) 25.98 (25.96)

I5 Ph 123–125 65.05 (65.02) 5.58 (5.85) 23.15 (22.95)

a All the triorganotin chrysanthemumates were obtained as white powders with yields of 50–70%.b Me, methyl; Et, ethyl; n-Pr, n-propyl; n-Bu, n-butyl; Ph, phenyl.

Table 2. Selected IR (cm−1) and Mossbauer data (mm s−1) fortriorganotin chrysanthemumates (R3SnO2CC9H15)

IR Mossbauer

Compound Ra γ (COO)as γ (COO)s �ν δ � ρ = �/δ

I1 Me 1556 1377 179 1.33 3.59 2.70

I2 Et 1555 1376 179 1.45 3.61 2.49

I3 Pr 1557 1374 183 1.42 3.51 2.46

I4 Bu 1553 1374 179 1.44 3.55 2.47

I5 Ph 1552 1378 174 1.29 3.32 2.58

a Me, methyl; Et, ethyl; n-Pr, n-propyl; n-Bu, n-butyl; Ph, phenyl.

The Mossbauer parameters for the compounds are listed inTable 2. The rho (ρ) and � values for all the compounds rangedfrom 2.46 to 2.70 and from 3.32 to 3.61 mm s−1, respectively.The calculated ρ values indicate that the compounds aregreater than four coordinated, while the observed � valuesare indicative of five coordinated compounds with trans tbpstructures.

Infrared Spectra

The infrared spectra were recorded in the solid state and thedifferences of the asymmetric and symmetric OCO vibrations(�ν = νasy(OCO) − νsym(OCO)) have been used to determine themode of coordination of carboxylate groups to metals includingtin. Differences of less than 150 cm−1 have been assigned tocarboxylate groups that are chelated in the structures,[18] whiledifferences between 150 and 250 cm−1 have been assigned tocompounds with bridged carboxylate structures.[18] Values largerthan 250 cm−1 are indicative of compounds with tetrahedralstructures.[18,19] As is evident from Table 2, all the compoundshave differences between 174 and 183 cm−1, indicating thatthe compounds have bridged structures. Thus the results of

the infrared studies are in agreement with the Mossbauerdata.

In summary, in the solid state all the compounds are fivecoordinated polymers with a trans tbp geometry around the tinatom, as shown in Fig. 1(a).

NMR Spectra

All the NMR spectra were recorded in CDCl3 and the characteristic1H, 13C and 119Sn NMR resonance peaks are given in Tables 3and 4. The proton resonances were assigned based on theirmultiplicity, intensity and coupling constants. All the protonsin the compounds have been identified, and the total number ofprotons calculated agrees with the proposed molecular formulas.The resonances due to the trialkyltin protons were observed inthe region 0.51–1.71 ppm, while others were found to overlapwith the protons due to the ligand. The phenyl protons fromthe triphenyltin moieties were observed as two multiplets inthe regions 7.37–7.47 ppm and 7.66–7.78 ppm, respectively.The two methyl groups on the cyclopropane ring as well asthose on the double bond were observed as two singlets, asexpected.

The 2J(1H–119Sn) coupling constant has been used to esti-mate the C–Sn–C angle in trimethyltin chrysanthemumate usingthe Lockhart equation.[20] The calculated C–Sn–C bond anglebased on the observable 2J(1H–119Sn) value of 57.3 Hz was111◦, suggesting that the methyl derivative is four-coordinatedin solution. While triorganotin carboxylates are commonly pen-tacoordinated in the solid state, they have been reported todissociate, in solution, with tetrahedral structures.[21] Thus, whilethe trimethyltin derivative is five-coordinated in the solid state,it is dissociated into a four-coordinated structure in solution. Itwould be reasonable to assume that the other compounds wouldact similarly.

In the 13C NMR spectra, the carbon atoms of the alkyl (δ =−4.38–27.83 ppm) and phenyl (δ = 121.4–136.9 ppm) groupsattached to the tin atom are observed at positions comparable

Figure 1. Proposed structure for triorganotin chrysanthermumates.

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Table 3. 1H NMR chemical shifts (ppm) and coupling constants (Hz) of triorganotin chrysanthemumate (R3SnO2CC9H15)a,b,c

Compound (I) Rd CH CH3 CH3 R

I1 Me 1.39 (1H, d) [4.9] 1.68 (3H, s) 1.10 (3H, s) 0.51 (9H, s)

1.97 (1H, t) [6.5] 1.69 (3H, s) 1.23 (3H, s) {57.3}4.87 (1H, d) [7.8]

I2 Et 1.33 (1H, d) [4.9] 1.82 (3H, s) 1.02 (3H, d) [6.0] 1.19 (6H, t) [6.0]

2.17 (1H, m) 2.09 (3H, s) 1.24 (3H, d) [6.7] 1.25 (6H, q) [6.0]

4.86 (1H, d) [7.8]

I3 n-Pr 1.39 (1H, d) [5.2] 1.69 (3H, s) 1.10 (3H, s) 0.97 (9H, t) [7.3]

1.97 (1H, t) [6.5] 1.70 (3H, s) 1.24 (3H, s) 1.23 (6H, t) [8.0]

4.89 (1H, d) [7.5] 1.63–1.71 (6H, m)

I4 n-Bu 1.38 (1H, d) [5.3] 1.67 (3H, s) 1.09 (3H, s) 0.88 (9H, t) [7.3]

1.96 (1H, t) [6.5] 1.68 (3H, s) 1.23 (3H, s) 1.21 (6H, t) [7.3]

4.88 (1H, dd) [7.8/1.1] 1.26–1.36 (6H, m)

1.52–1.62 (6H, m)

I5 Ph 1.53 (1H, d) [5.3] 1.68 (3H, s) 1.12 (3H, s) 7.66–7.78 (6H, m)

2.10 (1H, t) [6.3] 1.70 (3H, s) 1.26 (3H, s) {57.1}4.89 (1H, d) [7.7] 7.37–7.47 (9H, m)

a s, singlet; t, triplet; q, quartet; m, multiplet.b Numbers in square brackets are the 2J(HH) coupling constants in Hz.c Numbers in curly brackets are the 2J(1H–119Sn)/2J (1H–117Sn) coupling constants in Hz.d Me, methyl; Et, ethyl; n-Pr, n-propyl; n-Bu, n-butyl; Ph, phenyl.

Table 4. 13C chemical shifts (ppm), coupling constants (Hz) and 119Sn NMR chemical shifts of triorganotin chrysanthemumate (R3SnO2CC9H15)a,b,c

Compound Rd C-1 C-2 C-3 C-4 C-5 C-6 C-7 C-8b C-9c C-10b C-11 C O 119Sn NMR

I1 Me 35.8 20.1 22.4 33.4 117.3 118.6 25.8 −4.38 179.3 123.3

21.6 27.3 [398/376]

I2 Et 36.0 19.3 22.3 33.2 117.5 118.7 26.3 9.56 20.3 178.9 106.1

21.4 27.3 [385/354] {27.2}I3 n-Pr 35.9 18.5 22.4 32.8 119.6 122.0 25.7 19.6 19.2 17.2 179.5 98.9

20.9 27.5 [356/340] {20.6} [64.0]

I4 n-Bu 35.7 18.3 22.3 32.7 119.3 121.8 25.5 16.41 27.83 27.00 13.6 177.9 99.1

20.6 27.7 [365] {20.4} [63.2]

I5 Ph 34.8 18.2 22.3 33.6 117.6 118.9 25.5 136.93 121.41 128.76 129.9 179.6 −122.1

20.7 28.9 [642/614] [63.4]

a Numbering scheme for carbons in the compounds:

C

O

O Sn C CC C1

2

3

5

4

118 9 102

67

7

b Numbers in square brackets are the nJ(119Sn–13C)/nJ(117Sn–13C) coupling constants in Hz.c Number in curly brackets are the averages of nJ(119Sn–13C) and nJ(117Sn–13C) coupling constants in Hz.d Me, methyl; Et, ethyl; n-Pr, n-propyl; n-Bu, n-butyl; Ph, phenyl.

with other similar compounds.[22 – 24] The 1J(119Sn–13C) couplingconstants in triorganotin compounds have been used to infer thecoordination number of the tin atom.[2,25] As can be seen in Table 4,the 1J(119Sn–13C) coupling constants range from 356 to 398 Hz forthe alkyl compounds and 642 Hz for the triphenyltin derivative.These values are consistent with values for other four-coordinatedtriorganotin compounds.[22 – 24]

119Sn chemical shifts have also been used to deduce thecoordination number of the tin atom. The 119Sn chemical shiftsrange for four-coordinated alkyltin compounds is approximately

between +200 and −60 ppm.[26] As shown in Table 4, the 119Snchemical shifts for the trialkyl compounds (98.9–123.3 ppm) arewithin the range for four-coordinated structures.[22 – 24] The 119Snchemical shift of the triphenyltin compound in CDCl3 solutionexhibited a single sharp resonance at −122.1 ppm, similar tothose values reported for other four-coordinated triphenyltincarboxylates.[21,23,27] Thus the 119Sn NMR results also indicate thatthe compounds are four-coordinated. In conclusion, multinuclearNMR results indicate that the compounds are four-coordinated insolution, as shown in Fig. 1(b).

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Table 5. LC50 values (ppm) of the triorganotin modified frag-ments (chrysanthemumates, methylbutyrates and cyclopropanecar-boxylates) against the second instar stage of An. stephensi, Ae. aegyptiand Cx. p. quinquefasciatus mosquito larvae

Ra An. stephensi Ae. aegypti Cx. p. quinquefasciatus

Chrysanthemumate compounds (I)

I1 Me 0.79 ± 0.01 0.82 ± 0.07 1.32 ± 0.04

I2 Et 4.25 ± 0.10 3.68 ± 0.10 0.81 ± 0.08

I3 n-Pr 1.53 ± 0.03 0.87 ± 0.04 0.74 ± 0.05

I4 n-Bu 2.80 ± 0.03 0.69 ± 0.02 1.36 ± 0.02

I5 Ph 1.34 ± 0.03 0.21 ± 0.01 0.52 ± 0.04

Methylbutyrate compounds[10] (II)

II1 Me 1.10 ± 0.02 3.13 ± 0.05 0.72 ± 0.07

II2 Et 5.87 ± 0.02 1.15 ± 0.04 3.21 ± 0.08

II3 n-Pr 1.99 ± 0.02 0.78 ± 0.04 0.68 ± 0.07

II4 n-Bu 0.31 ± 0.01 0.32 ± 0.01 0.39 ± 0.03

II5 Ph 0.33 ± 0.01 1.02 ± 0.03 0.43 ± 0.06

II6 Cy 0.16 ± 0.01 0.42 ± 0.01 0.57 ± 0.06

Cyclopropanecarboxylate compounds[11] (III)

III1 Me 3.21 ± 0.04 0.90 ± 0.02 1.09 ± 0.02

III2 Et 5.61 ± 0.08 1.23 ± 0.01 0.43 ± 0.01

III3 n-Pr 0.68 ± 0.02 0.27 ± 0.01 0.84 ± 0.08

III4 n-Bu 0.44 ± 0.01 0.20 ± 0.02 0.43 ± 0.01

III5 Ph 0.99 ± 0.06 0.14 ± 0.01 0.50 ± 0.06

III6 Cy 0.47 ± 0.03 0.15 ± 0.01 0.31 ± 0.01

a Me, methyl; Et, ethyl; n-Pr, n-propyl; n-Bu, n-butyl; Cy, cyclohexyl; Ph,phenyl; Ave, average.

Toxicity Results

The triorganotin chrysanthermumate compounds (I1 –I5) werescreened against the second larval stage of the An. stephensi, Ae.aegypti and Cx. p. quinquefasciatus mosquitoes. The individualtoxicity in parts per million, along with their standard deviations,are listed in Table 5. Also included in the table are resultsfrom earlier studies involving the methylbutyrates10 (II1 –II6)and cyclopropanecarboxylate[11] (III1 –III6) fragments. The toxicityresults in the current study are similar to other series oftriorganotins evaluated.[10,11,14] The results from the current studyindicate that there are no significant differences in the toxicityof the compounds towards the different species of mosquitolarvae based on a single-factor analysis of variance (ANOVA).Similar results were observed for the methylbutyrates[10] andcyclopropanecarboxylates[11] series. The results further indicatethat no common order of activity based on the organic groupattached to the tin atom was observed; however, the phenylderivatives were the most active in two cases (Ae. aegyptiand Cx. p. quinquefasciatus) and second most effective in theAn. stephensi case. Based on the ANOVA and the lack of anobservable order of activity of the organic group attached to thetin atom, this would suggest that the toxicity of the triorganotincompound towards these mosquito larvae is dependent on boththe compound and the species of mosquito larvae. This conclusionwas also reported for other series of triorganotins.[10,11,14] Owingto their high toxicity towards mosquito larvae, this series ofcompounds can be considered as potential larvicidal candidatesagainst these three species of mosquito larvae. An advantageof triorganotins as potential larvicides is that they are knownto biodegrade in the environment to non-toxic tin species.[21]

In addition, there has been no reported resistance of thesethree species of mosquitoes or their larvae towards triorganotincompounds.

A comparison of the averages of the three series of compounds(chrysanthemumates, methylbutyrates and cyclopropanecarboxy-lates) as a whole against the three species of mosquito larvaeusing ANOVA indicates that the An. stephensi larvae were moretolerant to the modified fragments of the pyrethroid than Cx.p. quinquefasciatus; however, there was no difference betweenAn. stephensi and Ae. aegypti nor between the Ae. aegyptiand the Cx. p. quinquefasciatus larvae. Based on this obser-vation it would indicate that modifying the fragments in apyrethroid molecule would affect the An. stephensi larvae butnot the other two. Thus future work involving modified frag-ments of pyrethroids should be limited to the An. stephensilarvae.

Acknowledgments

Financial support from the National Institutes of Health MinorityBiomedical Research Support Program (MBRS/SCORE, GM08005)and the Science, Technology, Engineering and Mathematics(STEM) Program are gratefully acknowledged. We are also gratefulto the Laboratory of Malaria and Vector Research of the NationalInstitutes of Health for supplying the mosquito larvae.

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