Introduction

The prevalence of mosquito-borne diseases is one of the world’s most important health problems. Mosquitoes are responsible for transmitting various infectious diseases; hence, the mosquito has been declared as ‘Public Enemy Number One’ (WHO 1996a). Mosquitoes belonging to the genera Anopheles, Culex and Aedes are the vectors for the pathogens of different diseases such as malaria, filariasis, Japanese encephalitis, dengue and dengue haemorrhagic fever, yellow fever and chickungunya (Rahuman et al. 2009; Borah et al. 2010). They cause allergic responses including local skin and systemic reactions, such as angioedema and urticaria (Peng et al. 1999). Tropical areas are more vulnerable to parasitic diseases, and the risk of contracting arthropod-borne illnesses has increased due to climate change and intensifying globalization (Karunamoorthy et al. 2010). It is necessary to prevent mosquito-borne diseases and improve public health by controlling mosquitoes.

Aedes aegypti is the vector of arboviruses responsible for major diseases like dengue, dengue haemorrhagic fever and chikungunya. Dengue fever is endemic in the Southeast Asia and India, Bangladesh and Pakistan (Akram and Ahmed 2005) and continues to increase with more severe forms of the disease such as dengue haemorrhagic fever and dengue shock syndrome or with unusual symptoms such as central nervous system involvement (Hendarto and Hadinegoro 1992; Pancharoen et al. 2002). Culex quinquefasciatus is an important vector of lymphatic filariasis in tropical and subtropical regions. It is a pantropical pest and urban vector of Wuchereria bancrofti (Holder 1999) and is possibly the most abundant house mosquito in towns and cities of tropical countries. Lymphatic filariasis is an arthropod-borne infectious disease affecting 128 million people over 78 countries around the world (WHO 2008). In India alone, 25 million people harbour microfilaria, and 19 million people suffer from filarial disease manifestations (NICD 1990).

In recent years, mosquito control programmes had a setback because of the ever-increasing insecticide resistance (WHO 1992; Liu et al. 2005). Besides insecticidal resistance in arthropod vectors, the increased cost of insecticides and increased public concern over environmental contamination have demanded a continued search for alternative vector control strategies which would be environmentally safer and specific in their action (Coats 1994; Khan and Selman 1996; Peng et al. 1998). Studies on the effectiveness of the plant products as larvicides indicated that they could be used as alternatives to synthetic pesticides, and many studies of phytochemicals have been done to test their potential as a source of commercial mosquito control agents or as lead molecules (Pitasawat et al. 2007; Arnason et al. 1989; Pavela 2008; Park et al. 2002; Maheswaran 2010; Arivoli et al. 2011).

Clerodendrum L. [Family Lamiaceae (Verbanaceae)] is widely distributed in tropical and subtropical regions of the world and is comprised of small trees, shrubs and herbs. Clerodendrum phlomidis is an important and well-known medicinal plant and one of the highly traded plants from tropical forests as the leaves and roots are widely used in Unani, Siddha, Ayurveda and folklore medicines for the treatment of various diseases (Ved and Goraya 2008; Murugadoss et al. 2010). In ethnomedicine, it is used to treat diabetes (Dhanapal et al. 2008), fever, malaria, stomachache, dyspepsia, digestive disorders, eye disorders, lung diseases, rheumatism, asthma, inflammatory diseases and swelling (Pandey et al. 2005; Anonymous 1992) and to treat guinea worms (Katewa et al. 2004). The alcoholic extract of C. phlomidis exhibited antimalarial activity against Plasmodium falciparum (Simonsen et al. 2001). Clerodendrum inerme also inhibited the growth of larvae of A. aegypti, C. quinquefasciatus and Culex pipiens (Gayar and Shazll 1968; Kalyanasundaram and Das 1985). Our preliminary studies had indicated that among hexane, chloroform and ethyl acetate crude extracts of C. phlomidis, only the chloroform extract had shown mosquito larvicidal activity (Maheswaran et al. 2008). Hence, in the present study, the isolated fractions and the compound (pectolinaringenin) from the chloroform extract of C. phlomidis were evaluated for their larvicidal effect against C. quinquefasciatus and A. aegypti.

Materials and methods

Plant collection and extraction

The matured leaves of C. phlomidis were collected from Alanthurai in Coimbatore district, Tamil Nadu, India. Leaves were shade-dried and powdered using an electric blender. One kilogram of leaf powder was soaked in a glass jar containing 3 L of hexane, chloroform and ethyl acetate with intermittent shaking. The leaves were sequentially extracted, and the extracts were condensed using a rotary vacuum evaporator at 40 °C. The condensed materials were placed in a glass bottle and stored in a refrigerator at 4 °C.

Isolation of the active principle using column chromatography

The chloroform extract was selected for the isolation of fractions and identification of the compound for further study on the basis of the biological activities (Maheswaran et al. 2008). The chloroform extract (30 g) was mixed with 120 g of silica gel (60–100 mesh), and the mixture was packed with 300 g of silica gel (100–200 mesh) in the column and successively eluted with individual and a combination of solvents as follows: (1) hexane (100 %), (2) hexane/ethyl acetate (75:25), (3) hexane/ethyl acetate (50:50), (4) hexane/ethyl acetate (25:75), (5) ethyl acetate (100 %), (6) acetone (100 %) and (7) methanol (100 %). A total of 206 fractions were collected. Based on the thin-layer chromatography (TLC) profile, 12 fractions were obtained. Fraction 5 was selected for purification of the compound on the basis of the bioassay and was purified further using preparatory high-performance liquid chromatography (HPLC). One pure compound was obtained, and the purity of the compound was checked using HPLC.

Identification of the active principle

Melting points were taken by the open capillary method on a heating block apparatus. The compound was subjected to UV–visible spectrophotometer (Model–Hitachi 2010), IR spectrum (Perkin-Elmer FT-IR instrument in KBr disc), proton nuclear magnetic resonance (1H NMR) spectrum (500 MHz) and carbon-13 nuclear magnetic resonance (13C NMR; 75 MHz) spectrum on a JEOL AL-300 instrument in CDCl3. Electron ionization mass spectrometry (EI-MS) was taken on a Shimadzu instrument at 70 ev by the direct inlet method. All the chemicals and solvents used were of analytical grade.

Insect rearing

C. quinquefasciatus eggs were collected from the Cooum river in Chennai. The eggs of A. aegypti were collected from water trays with filter paper lining the sides of the trays placed in and around the laboratory and in the mosquito breeding room for hatching. The hatched larvae were kept in enamel trays containing tap water. They were maintained at 27 ± 2 °C, 75–85 % RH with 14:10 L/D photoperiod cycle. The larvae were fed with dog biscuits and Brewer’s yeast in the ratio of 3:2. Pupae were transferred from the trays to a cup containing tap water and placed in screened cages (23 × 23 × 32 cm in dimension) for the adult emergence. Adults were provided with 10 % sucrose solution in a jar with a cotton wick. On the day of post-emergence, the adult females were deprived of sucrose for 12 h and then provided with a mouse in a resting cage overnight for blood feeding. After 3 days, the ovitrap was kept in the cages, the eggs were collected, transferred to the enamel trays and maintained for further experimentation.

Larvicidal bioassay

Larvicidal activity was evaluated using a modified method prescribed by the WHO (1996b). Twenty early fourth-instar larvae of C. quenquefasciatus and A. aegypti were introduced into the containers. Concentrations were 100, 50, 25 and 12.5 ppm for fractions and 5, 2.5, 1.0 and 0.5 ppm for compounds. The volume in each container was increased to 250 ml using tap water. The test materials were dissolved in acetone, and Tween 80 was used as an emulsifier. Acetone with Tween 80 was used as negative control, and azadirachtin (40.86 % purity) was used as positive control. Mortality and survival rate were registered after a 24-h exposure period. The moribund and dead larvae were collected, and larval mortality was calculated for each concentration. Five replicates were maintained. Percent mortality was calculated using the formula (1), and corrections for mortality when necessary were done using Abbot’s (1925) formula (2).

Percentage of mortality:

$$ \frac{{{\text{no}}.{\text{ of dead larvae}}}}{{{\text{no}}.{\text{ of larvae introduced}}}} \times 100 $$
(1)

Corrected percentage of mortality:

$$ \frac{{({1} - n\;{\text{in T after treatment}} \times {1}00}}{{n\;{\text{in C after treatment}}}} \times 100 $$
(2)

Where, n is the number of larvae, T the treated and C is the control.

The corrected percentage mortality value for each concentration was considered to estimate LC50 and LC90 values using probit analysis (Finney 1971). Statistical package, SPSS version 11.5, was used for statistical analysis.

Results

Isolation and characterization of the compound

The fraction eluted with the solvent hexane/ethyl acetate (1:1) gave a compound as a pale yellow crystal crystallized from methanol (melting point, 210 °C; Lit. mp, 210–212 °C). The yield was 850 mg. It gave a positive ferric reaction (green colour) with alcoholic ferric chloride for phenol. It also was positive for the Shinoda test for flavonoids by giving a reddish pink colour with Mg/HCl. TLC over silica gel G with chloroform/ethyl acetate (9:1) as the developing system produced a single spot, pale yellow turning dark yellow, on exposure to ammonia (Rf = 0.35). The purity of the compound was 98.2 % using HPLC. Spectral data using UV, IR, 1H NMR, 13C NMR and EI-MS were taken for structural elucidation of the isolated compound.

The 1H NMR spectrum showed absence of substituent at C-3, H-3 appearing as singlet at δ = 6.56. The lone aromatic proton H-8 appeared as singlet at δ = 6.54. The A2B2 system in ring B was shown by two sets of ortho coupled protons at δ 7.85 and 7.03 (J = 9.0 Hz) corresponding to H-2′, H-6′ and H-3′ and H-5′. 4′-OMe appeared as a three-proton singlet at δ 3.94, and 6-OMe appeared at 3.90. 7-OH appeared as a broad singlet at δ 9.39, and the chelated 5-OH appeared downfield at δ 12.96.

The 13C NMR also confirmed the structure of the compound as pectolinaringenin in the present investigation. The flavonoid carbonyl appeared much downfield at δ 182.33. This suggested that C-3 is unsubstituted and hydroxyl is present at C-5. The single peak at δ 93.09 was assigned to C-8, and the absence of a slightly downfield peak around δ 97.0 found for C-6 in 5,7-oxygenated flavones showed that C-6 was substituted. C-6 OMe appeared at δ 131.08. C-5 attached to chelated OH appeared at δ 152.72. The δc values in the ring B corresponded to pectolinaringenin.

The EI-MS gave M+, the molecular ion at m/z 314 corresponded to the molecular formula C17H14O6. The peak at m/z 296 corresponded to [M-H2O]+. The above data confirmed the structure as pectolinaringenin (Fig. 1).

Fig. 1
figure 1

Pectolinaringenin (5,7-dihydroxy-4′,6-dimethoxy-flavone)

Full size image

Larvicidal activity

All the 12 fractions eluted from the chloroform extract of C. phlomidis were evaluated for their larvicidal activity against C. quinquefasciatus and A. aegypti. Among the fractions, fraction 5 was found to be active against C. quinquefasciatus which recorded the lowest LC50 value of 5.02 ppm with the fiducial limits of 1.50 ppm (lower) and 12.67 ppm (upper) and the LC90 value of 61.63 ppm with the fiducial limits of 53.28 ppm (lower) and 74.56 ppm (upper). The chi-square values were significant in all the fractions except fractions 1, 8, 9 and 10. Fractions 4 and 6 also recorded good larvicidal activity with the LC50 and LC90 values of 13.0, 9.63 and 106.81, 113.95 ppm, respectively (Table 1).

Table 1 Larvicidal activity (in parts per million) of isolated fractions from chloroform extract of C. phlomidis against C. quinquefasciatus
Full size table

In the case of A. aegypti, fraction 5 recorded the lowest LC50 and LC90 values of 32.86 and 91.70 ppm, respectively. The fiducial limits were 28.06, 73.62 and 37.43, 92.76 ppm, for LC50 and LC90, respectively. Fractions 6 and 4 had also recorded a notable amount of larvicidal activity with the LC50 and LC90 values of 38.98, 50.35 ppm and 82.29, 133.63 ppm, respectively. Significant chi-square values were recorded in all the fractions (Table 2).

Table 2 Larvicidal activity (in parts per million) of isolated fractions from chloroform extract of C. phlomidis against A. aegypti
Full size table

The bioassay of the compound revealed that the effective LC50 concentrations to kill the larvae were 0.62 and 0.79 ppm with the fiducial limits of 0.28, 0.14 ppm (lower) and 0.88, 1.26 ppm (upper) for C. quinquefasciatus and A. aegypti, respectively. The LC90 values were 2.87 and 5.31 ppm with the fiducial limits of 2.50, 4.50 ppm (lower) and 3.44, 6.66 ppm (upper) for C. quinquefasciatus and A. aegypti, respectively. The chi-square values were significant as 64.91 and 83.61 against C. quinquefasciatus and A. aegypti, respectively (Table 3). The negative control acetone with Tween 80 showed no activity, and the positive control Temephos at 0.1 ppm showed 100 % mortality. C. quinquefasciatus was more susceptible to the treated compound pectolinaringenin than A. aegypti; the longevity of the larvae of A. aegypti was found to be extended more than 7 days.

Table 3 LC50 and LC90 values for the larvicidal activity (in parts per million) of pectolinaringenin (98.2 % purity) from chloroform extract of C. phlomidis against C. quinquefasciatus and A. aegypti
Full size table

Discussion

In the present study, chloroform extracts of C. phlomidis eluted with hexane/ethyl acetate (1:1) fractions recorded good larvicidal activity against C. quinquefasciatus and A. aegypti. Fraction 5 was the most effective followed by fractions 4 and 6. Earlier, Maheswaran et al. (2008) studied the mosquito larvicidal activity of different crude extracts of C. phlomidis and observed 72 % larval mortality in the chloroform extract against C. quinquefasciatus. Leaf powder of C. inerme (200 mg) exhibited 100 % larval mortality in A. aegypti (Patil et al. 2006). The LC50 values for fraction 5 were 5.02 and 32.86 ppm against C. quinquefasciatus and A. aegypti, respectively. This result corroborated with the findings of Katade et al. (2006) who studied different fractions isolated from the methanol extract of Sterculia guttata seeds. They observed that the LC50 values for an active fraction A1 were 21.552 and 35.520 ppm against C. quinquefasciatus and A. aegypti, respectively. They further reported that the larvicidal activity was due to the presence of compounds like flavonoids, alkaloids, etc. in the fraction.

Our study revealed that the compound pectolinaringenin belonged to flavonoid group; it exhibited the lowest LC50 values of 0.62 and 0.79 ppm with the fiducial limits of 0.28, 0.14 ppm (lower) and 0.88, 1.26 ppm (upper) for C. quinquefasciatus and A. aegypti, respectively and the LC90 values of 2.87 and 5.31 ppm with the fiducial limits of 2.50, 4.50 ppm (lower) and 3.44, 6.66 ppm (upper) for C. quinquefasciatus and A. aegypti, respectively. Many researchers have reported the effects of flavonoid group on C. quinquefasciatus and A. aegypti. Ho et al. (2003) reported that the isolated compounds such as meliternatin (3, 5-dimethoxy-3′, 4′, 6, 7-bismethylendioxyflavone) (6) and six other minor polyoxygenated flavones derived from Melicope subunifoliolata showed larvicidal activity against A. aegypti. Isolated compounds such as prenylated flavonoids derived from Dodonaea viscosa showed larvicidal activity against Aedes albopictus and C. pipiens (Niu et al. 2010). Isoflavonoids neotenone and neorautanone and the pterocarpans neoduline, nepseudin and 4-methoxyneoduline isolated from Neorautanenia mitis showed larvicidal activity against Anopheles gambiae and C. quinquefasciatus (Joseph et al. 2004). Larvicidal activity of Lantana camara was reported against C. quinquefasciatus and A. aegypti by Sathish Kumar and Maneemegalai (2008). They reported that methanol and ethanol flower extracts exhibited 96 and 100 % larval mortality against the third instar; 88 and 100 % against the fourth instar of A. aegypti, respectively, at 0.75 and 1.00-mg/ml concentrations. They further reported that the higher activity was due to the presence of flavonoid (2,3-dihydro-3,5-dihydroxy-6-methyl-4H-pyran-4-one). Similarly, Gopleshkhanna and Kannabiran (2007) reported the presence of flavonoids, tannins and carbohydrates in the plant extracts having larvicidal property against mosquitoes. Rajkumar and Jebanesan (2008) reported the larvicidal activity of flavonoid compounds poncirin, rhoifolin, naringin and marmesin against A. aegypti.

In the present study, the longevity of the larvae extended up to 7 days, after which they died. This may be due to the effect of juvenile hormonal activity of the tested materials. This finding coincides with the earlier findings of Pereira and Gurudutt (1990) who observed that (−)-3-epicaryoptin isolated from C. inerme inhibited the growth of C. quinquefasciatus. Sukumar et al. (1991) studied the effect of extracts of Cymbopogon citratus against A. aegypti. They observed significant growth inhibition and mortality of A. aegypti since C. citratus had high quantity of flavonoid. This is the first report on the larvicidal activity of fractions and compound pectolinaringenin from C. phlomidis against vector mosquitoes.

In summary, in this study, fractions 5, 4 and 6 showed significant mosquito larvicidal activity, and the isolated compound from fraction 5 showed the highest larval mortality at the lowest concentration. The study clearly demonstrates that the compound pectolinaringenin isolated from C. phlomidis could be used as an effective agent to develop mosquito larvicidal formulation for the control of disease-causing vector mosquitoes.