Synergistic interaction of thymol with Piper ribesioides (Piperales: Piperaceae) extracts and isolated active compounds for enhanced insecticidal activity against Spodoptera exigua (Lepidoptera: Noctuidae)

Background: Plant secondary metabolites or mixtures in extracts or essential oils are well known to enhance the activity in binary mixtures. The present study is the first to report that thymol synergistically or additively enhances the activity of P. ribesioides extracts and isolated compounds against S. exigua larvae at sublethal doses. Results: Thymol was synergistic when are mixed with hexane extract; however, if the hexane extract level was higher (LD 30 ) than the thymol level (LD 10 ), the reaction was antagonistic. CH 2 Cl 2 extract and thymol were more toxic than the extract or thymol alone, and EtOAc extract was synergized by thymol if the components were combined at similar levels (1:1 thymol:EtOAc extract at the LD 10 or LD 30 ). MeOH extract individually had moderate insecticidal activity, but all combinations with thymol were synergistic as binary mixtures. Isolated compounds, piperine, phenethyl cinnamamide and cinnamic acid represented synergistic, additive, and antagonistic action after combining with thymol (1:1 at the LD 10 or LD 30 ). Detoxification enzymes after exposure of insects to treatments showed isolated compounds + thymol could inhibit CE, GST and AChE reaction of S. exigua exceptional being piperine + thymol, which induced detoxification enzyme activity. Conclusion: The synergistic activity was extract- and dose-specific. The impact on detoxification enzymes was variable and dependent on the composition of the extract and the doses of extract and thymol used in a binary mixture. In this metabolic model, the major insect compound in an extract may become detoxified, whereas a minor compound will act unimpeded, showing a lower LD 50 than acting alone. This model suggests that thymol synergizes with extract components differently, which could depend on the specific metabolites in the extract and the dose applied. Such studies will help design effective insecticides based on natural plant mixtures and a synergistic compound.


Introduction
Conventional insecticides such as chlorantraniliprole, tetraniliprole, methoxyfenozide, indoxacarb, chlorfenapyr, emamectin benzoate and β-cypermethrin are used to prevent herbivory by Spodoptera exigua, which is a polyphagous insect infesting cereals, legumes, cotton, tobacco and cannabis that causes severe economic losses worldwide [1]. The use of such hazardous chemicals has resulted in resistance to these insecticides, environmental contamination, and mortality of beneficial insects [1,2]. In contrast, botanical insecticides such as pyrethrum, neem, rotenone, nicotine and plant essential oils protect crops and stored products to avoid such hazards of chemical insecticides [3][4][5][6]. Several recent studies have demonstrated the efficacy of essential oil compounds against lepidopterans [7,8]. Thymol is a well-known monoterpenoid found in the essential oil of Thymus vulgaris L., commonly known as thyme. It also occurs in several other plant genera, such as Ocimum, Origanum, Carum, Satureja, and Oliveria. It has been used in the medical, dental, veterinary, food, and agrochemical industries [9]. Its agricultural applications as an insecticide, acaricide, and repellent have also been reported [10].
Commonly, botanical insecticides, including volatile oils, are considered safer than conventional insecticides due to their biodegradability; variable modes of action; and low toxicity to mammals, birds and fish [11]. While the biodegradable nature of botanical pesticides is advantageous for the environment and human health, the short shelf lives of these pesticides make developing appropriate biopesticide formulations difficult. One of the approaches in the recent past has been to use binary mixtures with synergistic activity against insects that can solve both problems, increasing the shelf life of a formulation while minimizing the environmental impact [5,12].
Piper ribesioides is a Piperaceae plant that is widely distributed in tropical and subtropical regions and is now known to have insecticidal and antifeedant properties [8]. Similarly, thymol is obtained from Thymus vulgaris (Labiatae) and some other aromatic plants and is reported to have insecticidal, acaricidal and antifeedant activities [13,14]. One objective of this research was to determine whether S. exigua larval control efficiency could be increased while reducing synthetic pesticide usage by using a mixture of compounds from plant extracts such as Piper ribesioides extracts and thymol.
Our previous research found that both thymol and P. ribesioides extract [15][16][17] has insecticidal activity. However, farmers always prefers have quick pest control efficiency. Thus, the increase strategies of botanical extract need to be developed. Our present study is the first to establish the synergistic effects of such binary mixtures based on a combination of P. ribesioides extracts and the botanical insecticide thymol. The main hypothesis goal is to increase control efficiency of this mixture product compare to alone compound.
In addition, as the control of these pests requires the massive use of insecticides, they have developed resistance to all chemical families. This Spodoptera species present in the Arthropod Pesticide Resistance Database are in the top 15 most resistant arthropods on the planet: S. litura, S. frugiperda and S. exigua (Hübner) [18,19]. Since knowledge of the stability of resistance is a crucial aspect when recommending rotation of insecticides with different modes of action [20], here, we examined beet armyworm stability of resistance to our mixed compounds. Our other purposes were to determine the impact of such mixtures on detoxification and neural enzyme activities and to investigate whether such combinations would inhibit these enzymes and prevent the possibility of developing resistance.

Plant extraction
Twigs (4 kg) of P. ribesioides were dried under shade and then powdered using a commercial blender. The obtained powder (1 kg each) was extracted separately with four organic solvents, hexane, dichloromethane (CH 2 Cl 2 ), ethyl acetate (EtOAc), and methanol (MeOH), for 7 days each using a soaking procedure [8]. Each extract was filtered through a suction filter (Whatman no. 0) with a vacuum pump and then concentrated using a rotary evaporator (BUCHI R-215). The quantitative yield of each extract was recorded, and the extract was then stored at 4 °C until further use in biological assays.
Melting points (uncorrected) were recorded in °C and were determined on a digital Electrothermal IA 9000 series apparatus for all the compounds. The FT-IR and UV spectra were recorded on a spectrum GX-FT-IR system (Perkin Elmer) and JASCO V-530 spectrophotometers, respectively. NMR spectra were recorded in deuterated chloroform and deuterated methanol on Bruker Ascend TM 400 spectrometer using TMS. HRMS-TOF on Micro mass model VQ-Tof2. Low-resolution EI mass spectra were recorded on a Thermo Finnigan Polaris Q mass spectrometer at 70 eV (probe). Silica gel 60 (Merck, 70e230 mesh) and silica gel plates (Merck, Silica gel 60 PF254) were used for CC and preparative TLC, respectively.

Insects
S. exigua larval colonies obtained from the National Center for Genetic Engineering and Biotechnology, Bangkok, were transferred to an insect rearing room and maintained on an artificial diet. The protocol was approved by the committee overseeing animal care and use in scientific research under approval no. ACKU64-SCI-019. The diet comprised commercial green bean powder (110 g) (Raithip ® ), agar (11 g), ascorbic acid (1.5 g), sorbic acid (1 g), methylparaben (2 g), yeast (8.5 g), mixed vitamins (17 mL), formaldehyde (1 mL), and distilled water (680 mL) and was used with a procedure modified from that of Nobsathian et al. [16]. Net cages (50 × 50 cm, 50 cm high) were the arenas for adult emergence from developed pupae. Cotton swabs dipped in 10% sugar solutions placed in a small plastic dish (5 cm in diameter, 15 cm high) were used as food for the adults in net cages. The cages were also lined on two sides with wax sheets for the oviposition of moths. Egg colonies were collected daily and sterilized by dipping in 10% formaldehyde for 10 s to prevent infections. All stages of S. exigua were maintained under laboratory conditions of 27 ± 2 °C, 60-70% relative humidity and a 14:10 h (light:dark) photoperiod.

Larvicidal bioassay
The thymol used in the current study was obtained commercially (99% purity) from Sigma-Aldrich (Bangkok). Thymol and P. ribesioides extracts were applied topically to 2nd instar S. exigua in the thorax region (2 µL/larva). For each treatment, 30 larvae were used in 3 replicates (n = 90). Treated larvae were placed in sealed plastic petri dishes and fed an artificial diet. The petri dishes were covered with black paper to prevent phototaxis and maintained under the laboratory conditions described above. Acute sublethal toxicity due to each treatment was assessed after 24 h, and the mortalities were used to calculate the LD 10 and LD 30 using a probit analysis program (STATPLUS, version 2019).

Synergistic assay
For the preparation of binary mixtures, the LD 10 and LD 30 were used as the sublethal doses of various treatments. Combinations of thymol and extracts were made by mixing thymol with individual extracts prepared in hexane, CH 2 Cl 2 , EtOAc, and MeOH in a 1:1 ratio at the specified sublethal doses. Thus, four binary mixtures with thymol (one for each extract) were assayed for acute toxicity as described above for the individual extracts. After treatment, the mortalities due to the mixtures used were compared to the expected mortalities based on the formula [21] described below: where E is the expected mortality and O a and O b are the observed mortalities of individual treatments at the given concentration. The efficacies of the binary mixtures were determined to be antagonistic, additive, or synergistic by χ 2 comparisons: where O m is the observed mortality due to the binary mixtures, E is the expected mortality, and χ 2 with df = 1 and α = 0.05 is 3.84. χ 2 values > 3.84 and greater-thanexpected mortality were considered to indicate synergistic effects, while χ 2 values < 3.84 indicated additive effects. Similarly, lower-than-expected observed mortality rates were taken to indicate an antagonistic effect of a mixture.

Detoxification enzymes
Detoxification enzyme reactions were determined only for binary mixtures that were synergistic in bioassays.

Enzyme source
The enzyme was prepared from S. exigua larvae treated with synergistic mixtures using a modified procedure from Kumrungsee et al. [7] and Nobsathian et al. [8]. Surviving 2nd instars of S. exigua were taken for this study (5 larvae/replicate) and homogenized by using a pestle in a microtube in 100 mM phosphate buffer (pH 7.2) and Triton X-100 (0.5%). All homogenates were centrifuged for 15 min at 12,000 rpm at 4 °C. The supernatants were collected carefully and stored at −20 °C for further use as enzyme sources.

Carboxylesterase (CE)
The reaction of CE was analyzed by a procedure modified from those of Kumrungsee et al. [7] and Nobsathian et al. [8]. The enzyme (50 μL) from the enzyme source was preincubated at 30 °C for 30 min. The substrate was prepared, which contained 10 mM paranitrophenylacetate (pNPA) (50 μL) and 50 mM phosphate buffer (pH 7.2, 2.9 mL). The enzyme and substrate solutions were homogenized and transferred to 96-well microreader plates to measure the enzyme activity (at 400 nm, 25 °C, 3 min) in the kinetic mode under an extinction coefficient 176.4705 for pNPA. Three biological replicates per treatment were analyzed.

Glutathione-s-transferase (GST)
The procedures of Kumrungsee et al. [7] and Nobsathian et al. [8] were again followed to measure GST activity. Enzyme solution (20 μL) was mixed thoroughly with a substrate mixture of 0.1 phosphate buffer (0.1 M, pH 7.2, 1150 μL) and 1-chloro-2,4-dinitrobenzene (CDNB) (150 mM, 10 μL). The glutathione-s-transferase reaction was measured by using a 96-well microplate reader at 25 °C for 3 min, and the optical density was measured at 340 nm. The CDNB level was determined from the extinction coefficient of 0.000137 for the GST reaction. Three replicates per treatment were analyzed.

Plant extracts
The extracts obtained from the twigs of P. ribesioides varied quantitatively. The maximum yield recorded was for the hexane fraction, followed by the EtOAC, and MeOH fractions (Table 1).

Isolated compounds
The active compounds were isolated from ethyl acetate crude extracts of P. ribesioides. All pure compounds were verified by comparing their physical properties and spectroscopic data with those reported in the literature [23][24][25]. The compounds ( Fig. 1) were isolated and identified by spectral analysis as follows: Piperine (1)

Toxicity at sublethal doses
Analysis of the toxicity of thymol at sublethal doses revealed an LD 10 of 1.21 μg/larva and an LD 30 of 9.54 μg/ larva. The methanol extract was less active at the LD 10 dose than the hexane, CH 2 Cl 2 , and EtOAc extracts, which were not significantly different, as was evident due to the overlap of the fiducial limits (Additional file 1: Data S1). However, to obtain 30% mortality with the CH 2 Cl 2 and EtOAc extracts, the LD 30 doses required were similar. In the case of hexane treatment, this dose was higher than that needed for the CH 2 Cl 2 and EtOAc extracts (Additional file 1: Data S1). Overall, the MeOH extract was the least active for both sublethal applications.
When used as binary mixtures, the combinations of thymol with hexane extract at sublethal doses were generally synergistic; however, if the amount of the hexane extract was higher (LD 30 level) than that of thymol (LD 10 level), the interaction was antagonistic ( Table 2). Although moderately toxic toward S. exigua by itself, the CH 2 Cl 2 extract showed enhanced toxicity after the addition of thymol at both the LD 10 and LD 30 doses. The EtOAc extract, significantly more active than the other P. ribesioides extracts, showed enhanced activity if combined with thymol at a similar level (1:1 thymol:ethyl extract at the LD 10 or LD 30 ). The effects were antagonistic or additive if the components were combined at different doses ( Table 2). MeOH extract individually had moderate insecticidal activity, but binary mixtures of the  The piperine with thymol was synergistic when combined at LD 10 and LD 30 doses or vice versa. However, if the combination was at similar sublethal amounts, the activity was either additive or antagonistic (Table 3). In the case of phenethylcinnamamide and thymol, the synergistic action was seen when combined at LD 30 and LD 10 doses (1:1), respectively (Table 3). Similarly, thymol was synergistic at the LD 10 level for cinnamic acid, independent of the cinnamic acid dose of LD 10 or LD 30 . If the thymol dose was increased to LD 30 in the mixture, the activity changed to additive or antagonistic (Table 3).

Detoxification enzymes
The enzyme activity of the extracts with thymol was determined only for those that were synergistic in toxicity assays. The impacts of treatment with binary mixtures of thymol and P. ribesioides extracts on 2nd instar S. exigua carboxylesterase were variable. Thymol and hexane extract inhibited CE, while thymol + CH 2 Cl 2 and thymol + EtOAc extracts induced the enzymatic reactions.
Similarly, thymol + MeOH extract had both inhibitory and inducing actions dependent on the dose of thymol used in the mixture; i.e., if thymol was used at the LD 10 , inhibition of the enzyme was observed, whereas if it was used at the LD 30 , induction was recorded (Table 4). Similar significant inhibition and induction compared to those in the control group were observed for GST and AChE reactions after binary mixture treatment of S. exigua larvae ( Table 4).
The reaction of the isolated compounds with thymol was determined, cinnamic acid (LD 10 ) and thymol represented the highest impacts of inhibited CE, while piperine (LD 10 ) + thymol (LD 30 ) induced the enzyme activities. Thymol also significantly inhibited/induced GST and AChE reactions (Table 5).

Discussion
The roles of phytochemicals in integrated pest management strategies for sustainable food safety and agricultural production are well documented [12,26] because of the challenges we have faced in terms of the human and environmental hazards caused by conventional chemical pesticides via the food chain, water sources, pesticide resistance and impacts on natural enemies [27]. Therefore, the current study was an attempt to show that the toxicity of P. ribesioides extracts, which have been reported to have insecticidal effects on S. exigua larvae [7], can be further enhanced by the addition of the known natural insecticidal compound thymol from a  [10]. Such combinations work in two potential ways: first by exerting synergistic and additive effects and second by reducing the chance of resistance development due to the multicomponent nature of the product. Previous reports have suggested that combining two compounds, thymol and pulegone (0.22 + 20 µg/larva), to combat Plutella xylostella larvae enhances toxicity by 2to 3-fold compared to that of thymol or pulegone alone [7]. Binary mixtures of piperine and β-asarone improve feeding deterrence in Spodoptera litura and Mythimna separata, while the individual compounds [28,29] causes inhibition.
Combinations of extracts and essential oils have also been reported to be synergistic. For instance, several combinations of Allium sativum and Citrus paradisi extracts are synergistic and are most potent against adult mosquitoes when applied in 1:1 ratios [31]; furthermore, some combination formulations of Cymbopogon citratus, Illecium verum, and Myristica fragrans essential oils are strongly synergistic against Musca domestica and are more effective than cypermethrin [32]. Similarly, the insecticidal activity of rosemary oil appears to be a consequence of the synergistic interaction between 1,8-cineole and ( ±)-camphor, where ( ±)-camphor is considered a promising synergizing agent against the cabbage looper, Trichoplusia ni [30].
The objective of the present study was to determine how an individual compound could enhance the activity of a mixture of compounds in a plant extract. We first report that blends of P. ribesioides extracts and thymol do exhibit enhanced activity that is synergistic or additive against S. exigua larvae. However, the activity is an extract-and dose-specific. For instance, thymol generally enhanced the activity of hexane extract; however, the reaction was antagonistic if the hexane extract level was higher (LD 30 ) than the thymol level (LD 10 ). In contrast, although CH 2 Cl 2 extract was moderately toxic, the combination of CH 2 Cl 2 extract with thymol was more toxic than either component alone.
Similarly, ethyl EtOAc extract has been reported to be the most toxic fraction against S. exigua larvae [8], which was also found in the present study; synergistic activity was observed if the extract and thymol were combined at similar levels (1:1 thymol:EtOAc extract at the LD 10 or LD 30 ). MeOH extract alone had moderate insecticidal activity, but all combinations with thymol were synergistic as binary mixtures. Moreover, this study found active ingredients of EtOAc extract as piperine, phenethylcinnamamide and cinnamic acid.
When combined with thymol, these compounds showed synergistic action at specific combination doses. It shows that the interactions between different compounds from a plant source can be synergistic or antagonistic, and the interaction could depend on the content, concentration, interaction between the compounds, and susceptibility of the target organism [33]. When a mixture of compounds is combined with a secondary metabolite, inactive compounds can influence the biological  [34] that showed the binary mixtures (deltamethrin and linalool) caused mortality higher than the products applied alone at 100% LD 50, except to 75% LD 50 deltamethrin added to 25% LD 50 linalool, whose mortality did not differ the products alone which mean  linalool is a potential insecticidal and can be associated with pyrethroids to control of S. frugiperda. Thus, the combination of major and minor compounds in an extract with thymol in the present case may have modified the activity by exerting significant synergistic or antagonistic effects. In conclusion, the synergism of a plant extract and a secondary metabolite depends on the composition of the extract and the specific doses used. This results will be an useful data for develop the highest efficiency in commercial level which is the main goal for this research. Further studies are required evaluate the synergistic combinations against field populations of S. exigua.
Xenobiotics are recognized by a variety of membranebound or nuclear receptors [35]. Detoxification enzymes, Cytochrome P 450, CE or GST in insects is generally demonstrated as the enzymatic defense against xenobiotic compounds and play significant roles in maintaining their normal physiological function. For instance, CE detoxify xenobiotics and contribute to several metabolic reactions, such as neurogenesis and developmental regulation induced by organophosphates, carbamates, and pyrethroids, which in turn are associated with insecticide resistance [36]. However, it is well known that multicomponent allelochemical mixtures can negate such impacts in extracts or in binary mixtures with almost no chance of resistance development [5].
Our present study showed that the combinations of thymol + MeOH or thymol + hexane extract act synergistically compared to the compound alone can also inhibit CE and GST suggesting that resistance was significantly delayed. Especially, thymol + hexane extract mostly inhibited for almost combination rate.
Moreover, our study found that that when used isolated compounds at LD30 dose with thymol, the results showed significantly inhibited all CE and GST level after binary mixture treatment of S. exigua larvae.
This again suggests that the impacts of the various treatments were both extract-compound-and dosedependent, which further support the idea that the interaction depends on the content, concentration, interaction between the compounds and susceptibility of the target organism. The variability could also have been due to the configurations of GST, which comprise six major subclasses: sigma, omega, zeta, theta, delta, and epsilon. These enzymes play a crucial role in detoxification in innate cells and artificial molecules in culture systems [37].
AChE is an enzyme that catalyzes the hydrolysis reaction of acetylcholine. Xenobiotics and chemical pesticides inhibit these enzymes and cause intolerable accumulation of acetylcholine, resulting in acute and chronic neurotoxic symptoms [38,39]. However, the extracts alone did not significantly affect AChE activity in the present study, but combinations with thymol moderately inhibited AChE. However, the combination of isolated compounds showed a significant inhibited AChE level; exceptional piperine + thymol (LD 30 ) induced the enzymatic activity. This finding further supports the idea that the possibility of resistance is reduced to a large extent when plant products are used in binary mixtures. It is known that essential oils that are mixtures of terpenes show intense insecticidal activity [10]. Therefore, there must be synergistic effects among these terpenes. However, the possibility of an impact on detoxification enzymes is variable. In other words, an insect is expected to detoxify a fraction of the LD 50 of the major terpene in a mixture; in contrast, a minor terpene should act unimpeded, showing a lower LD 50 than when acting alone. This metabolic model [40] has been applied to M. domestica, in which the synergism of piperonyl butoxide (PBO) with lethal concentrations of terpenes as fumigants has been established. The model indicates that the insect detoxifies the major component in a mixture. At the same time, the compounds in lesser proportions act as toxicants with higher toxicity than when assayed alone. This hypothesis seems especially pertinent in the present case because thymol synergized differently with the different extracts, which could have depended on the metabolite in the extract and the application dose.
From previous research, CE are one of detoxification enzymes are involved in resistance to pyrethroids and organophosphate in many insects [41][42][43]. Delorme et al [41] described that the S. exigua resistant strain has an enhanced esterase activity toward chromogenic substrates, such as naphthyl acetate or methylumbelliferyl acetate, the level depending on the substrate used. It is likely that activity toward chromogenic substrates and the hydrolysis of deltamethrin are related. Thus, CE play an important role in insect defense through catabolism of the esters of higher fatty acids that influence flight as well in the degradation of inert metabolic esters and various xenobiotics, including insecticides [43]. Changes in the activities of these enzymes are reflected in insect resistance to insecticides as well as in degradation of toxic molecules produced. GST plays a pivotal role in detoxification and cellular antioxidant defenses against oxidative stress by conjugating reduced glutathione to the electrophilic centers of natural and synthetic exogenous xenobiotics, including insecticides, allelochemicals, and endogenously activated compounds [43].
However, this research did not analyze CYP450 which is one of other important detoxification enzymes. This need to be concerned for further analysis. Additional similar studies will be necessary to design effective insecticides based on natural plant mixtures and a synergistic compound.

Conclusion
When used at sublethal doses, binary mixtures of P. ribesioides extracts and thymol showed enhanced activity against S. exigua larvae. A metabolic model is suggested in which an insect may detoxify the major component in a mixture. At the same time, minor compounds could act as potential toxicants with higher toxicity than when assayed alone. The basis for this hypothesis is that the synergistic effects of thymol with the components in the extracts were variable. They depended on the metabolite stereochemistry in the extract and the dose of application. Such studies will help design effective insecticides based on combinations of natural plant mixtures with a synergistic compound.