Chemical analysis and insecticidal activity of Ocimum gratissimum essential oil and its major constituent against Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) Análise química e atividade inseticida do óleo essencial de Ocimum gratissimum e de seu constituinte majoritário contra Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae) Análisis químico y actividad insecticida del aceite esencial de Ocimum gratissimum y su principal constituyente contra Spodoptera frugiperda (Smith, 1797) (Lepidoptera: Noctuidae)

The fall armyworm, Spodoptera frugiperda, causes damage at several stages of the maize crop cycle. Due to its resistance to synthetic insecticides and the high costs of pest control, there is an ever-increasing amount of research on alternative or complementary products that have a minor environmental and financial impact on agriculture. Therefore, the aim of this study was to evaluate the chemical composition and insecticidal potential of Ocimum gratissimum (african basil) leaves essential oil and the effect of its major component, thymol, on S. frugiperda control. Gas Chromatography (GC) and Gas Chromatography-Mass Spectrometry (GC-MS) analysis identified p-cymene, γ-terpinene, and thymol compounds as the main constituents of the oil, which presented a yield of 4.75%. Among the 30 identified compounds, thymol (33.2%) was the major constituent, representing 97.8% of the total oil. The efficacy of both the oil and thymol standard (Sigma-Aldrich) was evaluated against S. frugiperda using topical acute toxicity and contact surface tests at different concentrations. The oil was more active than thymol standard, with topical acute toxicity of LD50 at 0.020 μl/insect and LC50 at 0.171 μl/cm 2 for contact surface toxicity. The oil proved to be superior to the thymol standard, offering an effective and promising alternative for the control of S. frugiperda, which is most likely due to the contribution of other oil components that acted Research, Society and Development, v. 9, n. 11, e4999119787, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9787 3 synergistically. Consequently, this result provides an opportunity for further study and the development of an effective fall armyworm control system.


Introduction
Fall armyworm, Spodoptera frugiperda (Smith 1797) (Lepidoptera: Noctuidae), is considered a key crop pest of maize because it attacks all stages of plant development and different species of food plants (I. Cruz, 2008;Negrini et al., 2019). Corn, together with soy, is the crop that most demands the use of agrochemicals in the handling of the caterpillar (SINDAG, 2012). This insect comes from the tropics, and the climatic conditions in Brazilian corn-producing regions are favorable to its establishment and development (Campos & Boiça Junior, 2012).
The corn crop (Zea mays L.) (Poaceae) has always been notable in Brazil for its importance in human and animal nutrition. However, several factors hamper its production chain and, among the technical aspects, the elimination of insect pests is especially important, as in specific cases when there is no efficient and safe control, it interferes in grain yield (Figueiredo et al., 2006;Michelotto et al., 2013).
Synthetic insecticides are mainly used to control the fall armyworm, and they are effective and quick-acting. However, the emergence of populations resistant to these chemical groups and their toxicity to natural enemies led to the search for biodegradable insecticides that are specific to target insects (Felix et al., 2019;Lima et al., 2013;Niculau et al., 2013). Development, v. 9, n. 11, e4999119787, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9787 5 In this context, essential oils, which have a complex mixture of volatile substances and act synergistically in the control of various insect pests, have been shown as an alternative strategy within Integrated Pest Management. This occurs not only because they do not leave residues in food or the environment, but also because they are generally rapidly degraded, which mitigates environmental contamination (Azevedo et al., 2013;Costa et al., 2017;Isman, 2006;Lima et al., 2013). Other characteristics include their action on mortality, deformations of the different stages of development, repellency and deterrence (Castro et al., 2006;G. S. Cruz et al., 2014;R. K. Lima et al., 2009). Their toxic substances can penetrate the insects through the airways, contact or ingestion. This insecticidal action is due to the diversity of their monoterpenes and sesquiterpenes, which can be identified and quantified through chromatographic analysis coupled with mass spectrometry (Ootani et al., 2013).
Several essential oils with insecticidal activity have been studied. They are suitable for dealing with S. frugiperda, which is easy to handle and reproduce in the laboratory.
Research has shown that these oils influence the behavioral and biological parameters of the pest, which attacks various crops. For example, a previous study carried out with O. gratissimum demonstrated its action as an insecticide and repellent by fumigation for the control of Sythofilus zeamais, a corn crop insect pest, reducing the number of adults and indicating a promising and possibly curative effect of this essential oil (Araújo et al., 2019;Cruz et al., 2016).
A number of biological applications of essential oils of the genus Ocimum have been reported, including insecticidal action of their chemical components, which may act in isolation or in association (synergistic or antagonistic form), thus causing a greater or minor reaction. Therefore, the objective of this study was to evaluate the chemical composition and insecticidal activity of Ocimum gratissimum (African basil) leaves essential oil and its major component in the control of Spodoptera frugiperda through acute topical and surface contact toxicity tests.

Extractions of essential oil and its major compounds
Ocimum gratissimum leaves were dried at room temperature (26 ± 3 °C) for three days, with loss of 75% of water content. After grinding, 25 g were subjected to hydrodistillation using a Clevenger type apparatus (3 h). After drying with anhydrous sodium sulfate, the oil was packed into amber glass ampules at 5 ± 2 °C in a freezer (Teles et al., 2012). The extraction process was carried out at the UFMA Natural Products Laboratory.
The obtained yield was calculated by the dry weight of the plant and the moisture content of the sample, through an infrared and sample moisture analyzer (IV2500, GEHAKA), to measure the loss of water.

Oil composition analysis
The qualitative analysis of the oil was performed by gas chromatography coupled with mass spectrometry (GC-MS), FOCUS equipment (Thermo Electron) DSQ II, heliumentrained gas, equipped with a DB-5 MS capillary column (30 m X 0.25 mm X 0.25 μm). The injection mode was splitless (Split flow 20:1). For each sample, 0.1 μL of oil in hexane (400 ηg in the column) was injected. The column temperature was 60 to 240 °C, ranging from 3 °C/min. Ion source: 70 eV (electronic impact), ion source temperature and transfer line 200 °C.
The quantitative analysis was performed under the same conditions by gas chromatography coupled to a flame ionization detector (GC/FID), FOCUS equipment (Thermo Electron), except for the drag gas, which was nitrogen. Retention rates of all volatile constituents were calculated using a homologous series of n-alkanes (C8-C20, Sigma-Aldrich, USA), according to van Den Dool and Kratz (1963). The oils components were identified by Research, Society and Development, v. 9, n. 11, e4999119787, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9787 7 comparing their retention indices and mass spectra (fragmentation standard and molecular mass), with the spectra existing in the libraries and in the GC/MS system literature (Adams & Sparkman, 2007;NIST, 2011).

Insects
The bioassays were carried out at the Laboratory of Entomology Research, Agrarian Unit, Anhanguera-Uniderp University, Campo Grande, Mato Grosso do Sul. The temperature was 27 ± 2 ºC; relative humidity 70 ± 5%, and photoperiod of 12 hours. After pupation, the insects were sexed, and eight couples were placed in cages to perform the egg-laying. Eggs were kept in Petri dishes in a climate-controlled room under the same conditions until larval hatching (Panizzi & Parra, 2009).

Acute topical toxicity test
Tested concentrations were 0.0144; 0.0173; 0.0208; 0.0250; and 0.0312 µL/insect for the oil and thymol standard, having been determined in the light of data obtained from a previous study (Favero & Conte, 2008). The third instar larvae were used in this experiment and were fed with an artificial diet based on beans and wheat germ. Larvae were treated individually by adding the concentrations to the prothoracic region using a micropipette (1 μL). After treatment, each larva was kept in Petri dishes (60 mm). Control group contained 10 larvae were treated with the solvent (acetone) and each concentration was replicated tree times. Mortality was recorded at 1.0, 6.0, 12.0, 24.0, and 36.0 Hatching After Time (HAT), and dead larvae were removed and counted.

Contact surface toxicity
Tested concentrations (0.127; 0.152; 0.183; 0.220; and 0.265 µL.cm -2 ) were established as described in Favero & Conte (2008). Also, the larvae were fed with an artificial diet based on beans and wheat germ. Petri dishes (60 mm) were treated with 0.3 mL of the solutions and evaporated after 20 min. After evaporation, the armyworm larvae were placed on the plates, and the plates were closed. The treatment was identical, 10 armyworm larvae for concentration and each concentration with 3 repetitions. Dead individuals were counted after 24 hours. Research, Society and Development, v. 9, n. 11, e4999119787, 2020 (CC BY 4.

Data analysis
The tabulated data from both acute and contact toxicity experiments were converted to percentages and then analyzed by ANOVA, after were submitted to PROBIT analysis using the SAS PROC PROBIT program (Sas, 2002) to determine the dose-mortality curve and the sublethal and lethal doses 50% and 90% (LD50 and DL90) (Finney, 1971). The results are presented as the mean ± SE.
The armyworm larvae were found dead after 24 hours, and the dead larvae were removed, and mortality compared to control was calculated using the following equation by Abbott (1925). The LT50 results of essential oil and thymol are presented as the slope ± SE, LC50, hour (at 95% confidence interval), v2 (degree of freedom) and P-value.

Characterization of essential oil
Ocimum gratissimum essential oil produced a liquid yield of 4.75% and, according to the chromatographic analysis, 30 compounds, corresponding to 97.8%, of their total composition ( Figure 1 and Table 1) were identified. The compounds consisted of 92.1% of monoterpenes, with predominance of 55.1% of monoterpene hydrocarbons and 37% of oxygenated monoterpenes, as well as 5.7% of sesquiterpenes, distributed in 5.1% of sesquiterpene hydrocarbons and 0.6% of oxygenated sesquiterpenes.
Following the order of elution, the chromatogram shows that the peaks with retention time 8.64, 9.85 and 19.51 were highlighted, the last being the one with the highest intensity.
By mass spectrometry and comparison with the spectra from data libraries, it was possible to identify the peaks as being the p-cymene, γ-terpinene, and thymol compounds, respectively.

Acute Topical Toxicity
Leaf essential oil from O. gratissimum showed a higher topical acute toxicity with 0.020 (0.017-0.022) μL/insect LD50 (Table 2) when compared to the commercial standard thymol, with LD50 0.329 (0.302-0.356) μL/insect. By comparison of LD50 and LC50 it's possible observe the oil was more active than the major component thymol. The essential oil was 16.5 times more efficient than thymol alone, with a LD50 of 0.020 and 0.329 μL/insect (Table 2).

Discussion
Previous studies have demonstrated quantitative variations in the chemical composition of O. gratissimum essential oil (Benelli et al., 2019;Kumar et al., 2019). This may be associated with the genetic difference between Ocimum populations, in addition to the stage of development and the plant age, the part of the plant from which the oil was extracted, collection season, analysis conditions and also factors such as precipitation, temperature, duration of daily brightness, soil characteristics, and geographic location (Benelli et al., 2016;Blank, 2013).
For both tests, the oil was more active than when its major component thymol was used alone, a fact observed by the comparisons of LD50 and LC50 and their respective toxicity ratios. These results indicate that the mixture of components of the oil acts synergistically with thymol, contributing to greater activity.
The effect of essential oils on insects has been extensively studied, given that they contain combinations of natural and complex volatiles, characterized by strong odor and formed by a diversity of constituents (Bakkali et al., 2008). In the present study, it can be inferred that the synergism of the monoterpenes (92.1%) and sesquiterpenes (5.7%) of O.
gratissimum essential oil was more effective than commercial thymol for both tests.
Since p-cymene and γ-terpinene compounds have greater permeability in the insect cuticle because they are hydrocarbons of low polarity, they have a more hydrophobic character. Therefore, it can be deduced that the insect cuticle permeability potential can increase the toxicity of these two hydrocarbons. On the other hand, thymol, a compound found in greater quantity, belongs to a phenolic hydroxyl group (hydrophilic polar group). It also interacts with the insect cuticle, since the intracellular medium is predominantly aqueous, thus presenting good solubility to the hydrophilic substances. Research, Society and Development, v. 9, n. 11, e4999119787, 2020 (CC BY 4 Cruz et al. (2016) and presented trans-anethole (34.9%), limonene (15.6%) and eugenol (9.1%) as major components. These data can be justified by the influence of the seasons on the plant. Plants under climate stress can present variability in the metabolite production, which can interfere in the increase or decrease of the biological response (Sampaio et al., 2016). For example, the rainy season affects the essential oil production by Lamiaceae plants, since the structures that accumulate these oils are located on the surface of the leaf (epidermal cells) (Blank et al., 2011). Crambidae) (Melo et al., 2018), Azadirachta indica A. Juss (control of Brevicoryne brassicae L., 1758) (Hemiptera:Aphididae) (Carvalho et al., 2008) and Piper hispidinervum C. DC.
(control of S. frugiperda Smith 1797) (Lepdoptera: Owlet moths) (Alves et al., 2014). Thus, it is justifiable to search for other plant species and their essential oils in the control of S. frugiperda to minimize the use of chemical control (Fonseca et al. 2012).
Ocimum gratissimum essential oil indicated toxicity to S. frugiperda, exhibiting a LD50 of 0.020 μL/insect. Another study has also demonstrated similar results, although the concentrations were different, as in the work by Cruz et al. (2016) Research, Society and Development, v. 9, n. 11, e4999119787, 2020 (CC BY 4.0) | ISSN 2525-3409 | DOI: http://dx.doi.org/10.33448/rsd-v9i11.9787 13 and reproductive parameters. It is of note that the essential oils obtained from our work showed greater toxicity than those presented by Cruz et al. (2016).
Ocimum gratissimum was toxic to the S. frugiperda caterpillar, with thymol, which is a monoterpenoid, aromatic phenol, biosynthesized in plants from γ-terpinene and p-cymene as a major component (Franco et al., 2014). Thymol is present in 60% of essential oils from native species in northeastern Brazil (Fontenelle et al., 2007). This compound is found in the essential oil of species of the genera Thymus and Origanum, Lamiaceae, such as T. vulgaris and O. vulgare (Hudaib et al., 2002). It can also be detected in species of the genus Lippia, Verbenaceae, such as L. gracilis and L. sidoides (Franco et al., 2014). The synergism that occurs among the compounds present in the essential oils of plants can result in a greater bioactivity than that observed for the isolated compounds. These effects are common in terpenes, hydrophobic compounds that present synergistic actions in relation to other compounds, solubilizing them and facilitating their passage through the membranes (Rattan, 2010). It also allows the use of smaller dosages of the mixture, thus reducing costs with pest management and environmental risks (Ribeiro et al., 2002).
Studies have shown the relationship between the chemical structure and the biological activity of the compounds present in the essential oils, since the higher the hydrophobicity is, the greater the penetration in the insect's integument (Rattan, 2010). The essential oils insecticidal properties depends on these factors to achieve pest insects susceptibility to synthetic compounds (Mairesse et al., 2007). Resistance to pyrethroids and organophosphates is common among S. frugiperda populations (Poletti & Alves, 2013). Due to its activity against S. frugiperda, the essential oil extracted in this study presents the possibility of controlling this fall armyworm resistance.

Final Considerations
According to results obtained, the tests indicated that O. gratissimum essential oil showed a topical and contact toxic effect on S. frugiperda caterpillars and was more active than the commercial thymol standard. Therefore, the use of essential oils is a promising option to reduce the use of synthetic insecticides against this pest.