Larvicidal activity of hexane extract, fatty acids, and methyl esters from Tecoma stans pericarps against Culex quinquefasciatus

Abstract Culex quinquefasciatus (Diptera: Culicidae) is responsible for the transmission of lymphatic filariasis. The search for natural sources of new insecticides to control mosquitoes has intensified in recent years. Tecoma stans is an exotic species in Brazil, known as ‘ipê de jardim’. T. stans pericarps were extracted with hexane in Soxhlet apparatus, and fatty acids (FA) and methyl esters (FAME) were obtained by transesterification reaction of hexane extract (HE). HE, FA, and FAME were evaluated against 3rd. and 4th. instar larvae of Culex quinquefasciatus. Analysis by gas chromatography-mass spectrometry (GC/MS) identified the hydrocarbon nonacosane (81.69%) as the major compound in HE, and linolenic (16.89%), linoleic (16.83%), and palmitic acids (21.00%) were predominant in FA. FA and HE, at a concentration of 250 µg/mL, promoted mortality of 81.67% and 68.66% of larvae, respectively. HE and FA obtained from T. stans pericarps have larvicidal potential for the control of C. quinquefasciatus. Graphical Abstract


Introduction
Culex spp., particularly Culex quinquefasciatus, are important vectors of arboviruses and filariasis, some of which are fatal in the absence of treatment and others can cause lifelong disability (Kauffman and Kramer 2017).Despite its important epidemiological role, there is still a lack of data on its bionomy (Talipouo et al. 2021).Culex quinquefasciatus is a vector with marked biological importance that feeds on the blood of both humans and animals (Farajollahi et al. 2011), increasing its involvement in the transmission of pathogens to both groups of hosts.These mosquitoes use a variety of breeding habitats, including swamps, drains, latrines, and permanent or semi-permanent stagnant bodies of water containing organic matter (Nchoutpouen et al. 2019), which are common environments in and around cities in countries with tropical climates.Another factor to consider is the prominent growth of regions under anthropogenic action, which favors the installation of mosquitoes (Antonio-Nkondjio et al. 2012).
The search for natural sources of new insecticides has intensified in recent years.In this context, bioactive substances from plant extracts are an important tool in controlling insect vectors, especially when they have some larvicidal or insecticidal activities.The botanical family Bignoniaceae has an expressive pantropical dispersion (Lohmann 2004), and is comprised of about 830 species, including Tecoma stans, which is considered an exotic species in Brazil and is commonly used as an ornamental plant.This species produces several classes of substances (monoterpene alkaloids, phenolic acids, flavonoids, carotenoids, terpenoids, saponins, glycosides, phytosterols, carbohydrates, proteins and resins, volatile oils, and unsaturated fatty acids) related to a wide range of biological activities such as cytotoxic, anti-inflammatory, analgesic, healing, antimicrobial, antidiabetic, sleep-inducing, gastroprotective, anti-obesity and insecticidal (Gonc¸alves et al. 2019).
It is undeniable that sustainable development strategies are essential for the insertion of emerging countries into the technological and economic vanguard.With this in mind, this work presents the beginning of the development of a methodology for the biological control of C. quinquefasciatus vectors and the creation of eco-friendly bioinsecticides using plant material that was once discarded, such as the pericarp of T. stans.
Fatty acids were identified by GC-MS in their methyl esters form.The sample exhibited a high percentage of saturated fatty acids (40.28%) as compared with unsaturated fatty acids (33.72%).Linolenic acid (16.89%), and linoleic acid (16.83%) were detected as the major unsaturated fatty acids, and palmitic acid (21.00%), stearic acid (5.92%), and eicosanoic acid (5.10%) were the most abundant among the saturated fatty acids in FA (Silva et al. 2022).
The results for larvicidal activity are shown in Table 1.Fatty acids promoted higher mortality of C. quinquefasciatus larvae at 1.67, 43.33 and 81.67% at concentrations of 62.5, 125, and 250 mg/mL, respectively.The larvae were also susceptible to hexane extract with mortality rates of 38.33 and 68.66% at concentrations of 125 and 250 mg/mL, respectively.FAME was not toxic to mosquito larvae.
After 24 h of exposure, the hexane extract and fatty acids exhibited LC 50 values of 174.18 and 148.56 mg/mL, respectively.The LC 90 values were 408.48 and 283.58 mg/mL for HE and FA, respectively, from pericarps of T. stans.There was no statistical difference between the data obtained at 24 h and 144 h after treatment.
Studies evaluating the larvicidal activity of T. stans on mosquitoes are scarce.The methanol (ME), chloroform (CH), and petroleum ether (PE) extracts of T. stans leaves obtained in Soxhlet extractor were evaluated for larvicidal effects against the larvae of Aedes aegypti and C. quinquefasciatus.The highest larvicidal activity was shown by the PE extract of T. stans on C. quinquefasciatus (LC 50 ¼ 19.26 mg/mL) and A. aegypti (LC 50 ¼ 55.41 mg/mL) (Hari and Mathew 2018).
Larvicidal activity of oils, fatty acids, and methyl esters on Culicidae have been reported.Commercial edible canola, corn, sunflower, and soybean oils and their methyl esters showed a larvicidal effect on C. quinquefasciatus after 144 h of treatment, with LC 50 values between 42.32 and 286.33 mg/mL (Ribeiro-Neto et al. 2017).Silva et al. (2015) tested the larvicidal effects of the ether extract, fatty acids, and methyl esters of unripe Solanum lycocarpum (Solanaceae) fruits on C. quinquefasciatus larvae, with LC 50 values of 0.70 and 27.54 mg/mL, demonstrating great larvicidal potential.The ether extract, fatty acids, and methyl esters obtained from the leaves of Smilax brasiliensis (Smilacaceae) were not toxic to C. quinquefasciatus larvae (Amado et al. 2019).Melo et al. (2018) tested the standards oleic, linoleic, linolenic, palmitic, and stearic acids, and their respective methyl esters against fourth instar of C. quinquefasciatus larvae; however, only oleic, linoleic and linolenic acids exhibited LC 50 values of 8.58, 10.04 and 19.78 mg/mL, respectively.Linoleic and linolenic acids were also identified in FA from T. stans pericarps.Thus, the observed larvicidal activity can be correlated to the presence of these compounds in the sample.
The leaf extract of Momordica charantia Linn.(Cucurbitaceae) showed activity on Anopheles stephensi and C. quinquefasciatus larvae, with LC 50 values of 52.86 and 42.63 mg/mL, respectively.GC-MS analysis identified the hydrocarbon nonacosane as a major compound in M. charantia leaf extract (Gandhi et al. 2017).This hydrocarbon was also identified as the major compound in the hexane extract of T. stans pericarps, and the larvicidal effect, at least in part, could be attributed to the presence of this compound in the sample.The hydrocarbon nonacosane (C 29 H 60 ) occurs naturally and has been reported to be a pheromone component, playing an important role in the chemical communication of various insects (Gandhi et al. 2017).

Conclusions
This is first report about larvicidal activity of T. stans pericarps.FA and HE exhibited a good larvicidal activity at concentrations of 125 and 250 mg/mL.The hydrocarbon nonacosane was identified as the major compound in HE, and linolenic, linoleic, and palmitic acids in FA.Thus, the larvicidal effect, at least in part, could be attributed to the presence of these compounds in the samples.HE and FA obtained from T. stans pericarps have larvicidal potential for the control of C. quinquefasciatus; however, more studies are needed to evaluate the activity of these samples in the environment in order to confirm the events observed in the laboratory.

Table 1 .
Larvicidal activity of different concentrations of hexane extract, fatty acids and methyl esters from pericarps of T. stans after 24 h post-treatment.