Encapsulation of Basil Essential Oil by Paste Method and Combined Application with Mechanical Trap for Oriental Fruit Fly Control

Simple Summary Essential oils of the Ocimum spp. comprised of the volatile phenylpropanoids known for the Oriental fruit fly attractant property. However, fully exposing the essential oils in the field limited their ability. Therefore, we proposed a cheap yet effective paste encapsulation technique combined with a plastic trap for controlling tropical fruit fly in tropical fruit orchard. Abstract In this work, the chemical compositions of basils oils, including those of lemon basil, white holy basil, Thai basil, tree basil and red holy basil, were analysed. Methyl eugenol was detected in all types of basils. The essential oils of red and white holy basils possessed a comparable ability (~25%) to attract male Oriental fruit fly to the synthesised fruit fly attractant in the laboratory experiment. To control the release of the active ingredients, the white holly basil oil (WBO) was encapsulated with maltodextrin (MD) and gum arabic (GA) by paste method. The essential oil is retained in the wall complex much longer with the addition of MD. The results also revealed that the combination of the MD:GA (25:75) had the highest loading efficiency of the oil (9.40%) as observed by the numerous porous structures by scanning electron microscopy. Fourier-transform infrared spectra of the encapsulated polymer confirmed traces of essential oil functional groups. The field test study advised that WBO-encapsulated products improved fruit fly attractive efficiency by maintaining the release rate of basil essential oil.


Introduction
Tropical fruit production along with postharvest handling chains, particularly those of soft skin types, such as avocado, mango and papaya, have been severely desecrating by the invasion of Oriental fruit fly (ORF; Bactrocera dorsalis Hendel). In Thailand, for instance, the estimated loss from this pest alone is roughly over 100-million US$ per year, and the most vulnerable crop being mango [1,2]. The infestation leads to a vast reduction of the consumption demand and has raised quarantine restrictions of the importing countries. Many

Insects
ORF pupae were obtained from Plant Protection Research and Development, Department of Agriculture, Bangkok, Thailand. The adult ORFs were fed daily with an artificial diet (yeast extract and sugar with 1:3 ratio) and water (wet sponge). ORFs were between 15-30-day-old (days after emerging from pupae) upon the attractiveness laboratory testing. , were collected fresh from the local contract farmer and brought to the laboratory immediately. The plant species and types were confirmed by their taxonomy [34]. The leaves were separated from the stems and shade-dried at room temperature (27)(28)(29)(30)(31)(32) • C) until a constant weight was reached. Dried leaves (~300 g) were used for essential oil extraction in a 5 L Clevenger hydro-distillation apparatus containing 2.5 L of distilled water. The extraction was for 2 h at 120 • C. After cooling down at room temperature, the essential oil was removed and treated with anhydrous sodium sulphate to eliminate the remaining water [41].

Volatile Analyses
Gas Chromatography-Mass Spectrometry (GC-MS) analyses were accomplished with a Bruker-scion 436 GC-MS equipped with 30 m × 0.25 mm, Rxi-5Sil MS column (Restek, Bellefonte, PA, USA). Essential oil (2 µL at a dilution of 1% (v/v) in dichloromethane with a presence of 0.015 mg/mL toluene as an internal standard) was injected in a split mode (1:20). The oven temperature was set at 60 • C for 3 min and increased by 3 • C/min until 240 • C, where it was held for 6 min. The carrier gas was helium with a flow rate of 1 mL /min [34]. The interface with MS was at 200 • C, and mass spectra were taken at 70 eV in electron impact ionisation mode, with a scanning speed of 0.5 scans/s from m/z 20-350 [42]. The identification of the volatile compositions was by comparison with mass spectra in NIST 05.L and NIST 98.L libraries. The amount in µg/mL of essential oil was calculated relative to the amount of the internal standard.

Attractiveness
Attractiveness was performed in a mesh cage (50 × 40 × 40 cm) in a controlled room (23-25 • C; RH 40-70%) as modified from Biasazin et al. [43]. Two bottle-trap was placed inside the cage. The air was pumped in and filtered through the activated charcoal and distilled water then essential oil vapour was released through the top of the cone on bottle traps (Scheme 1). The airflow was adjusted to 0.50 L/min constantly, and all pieces of the experimental apparatus were joined by Teflon tubing. The concentration of the test solutions was 1% (v/v) of essential oil in acetone and acetone as a control solution. The test solution 10 µL was applied to 2 cm 2 of filter paper 5 min before testing. One hundred ORFs (50 males and 50 females) were released between two bottles (control and treatment). The number of ORFs trapped in the bottle was recorded (male and female) after ORFs were released for 1 h. The bottles containing the test material were replaced and cleaned every single test. Four replications were determined for each test solution.
acetone as a control solution. The test solution 10 µL was applied to 2 cm 2 of filter paper 5 min before testing. One hundred ORFs (50 males and 50 females) were released between two bottles (control and treatment). The number of ORFs trapped in the bottle was recorded (male and female) after ORFs were released for 1 h. The bottles containing the test material were replaced and cleaned every single test. Four replications were determined for each test solution.

Insect Olfactory-Chemosensory Responses
To investigate insect behaviour response to the essential oils, male and female flies were mounted alive with glue in the position that their frontal parts (heads, wings, and forelegs) could move freely. A 20 µL drop of the 1% essential oils or ethanol as the control was pipetted onto the filter paper located ~10 cm underneath the ORF approximately every 30-60 secs apart. The movement of the antennae and the forelegs were observed [44].

Experiment 2: Paste matrix complexation with basil essential oil encapsulation
The highly effective essential oil type was chosen to conduct this experiment.

Paste Matrix Complexation
Wall materials were made according to Shrestha, Ho, and Bhandari [40], Bhandari et al. [45], and Carneiro et al. [46]. In the preparation, the mixture of the wall material (Table  1) was mixed with distilled water (40 °C) and stirred until completely dissolved. After that, the essential oil was added to the mixture solution at a concentration of 20% to total solids (wall materials). Emulsions were formed using a homogeniser operating at 18,000 rpm for 5 min, and the emulsion characteristic was recorded.

Insect Olfactory-Chemosensory Responses
To investigate insect behaviour response to the essential oils, male and female flies were mounted alive with glue in the position that their frontal parts (heads, wings, and forelegs) could move freely. A 20 µL drop of the 1% essential oils or ethanol as the control was pipetted onto the filter paper located~10 cm underneath the ORF approximately every 30-60 secs apart. The movement of the antennae and the forelegs were observed [44].

Experiment 2: Paste matrix complexation with basil essential oil encapsulation
The highly effective essential oil type was chosen to conduct this experiment.

Paste Matrix Complexation
Wall materials were made according to Shrestha, Ho, and Bhandari [40], Bhandari et al. [45], and Carneiro et al. [46]. In the preparation, the mixture of the wall material (Table 1) was mixed with distilled water (40 • C) and stirred until completely dissolved. After that, the essential oil was added to the mixture solution at a concentration of 20% to total solids (wall materials). Emulsions were formed using a homogeniser operating at 18,000 rpm for 5 min, and the emulsion characteristic was recorded. Emulsion viscosity and stability were analysed according to Carneiro, Tonon, Grosso, and Hubinger [46]. Immediately after emulsion, the viscosity of the paste solution was Insects 2021, 12, 633 5 of 18 measured by determining steady-shear flow curves using a Rheometer. Emulsion stability was analysed using 25 mL-aliquots of each sample transferred to graduated cylinders of 25 mL, sealed, stored at room temperature for one day, and the upper phase's volume measured after 24 h. The stability was measured by the percentage of separation and expressed as: where H 0 represents the initial emulsion height, and H 1 is the upper phase height.

Encapsulation Characteristics
The emulsion paste was dried using a vacuum oven at 60 • C for 4 h and then ground to a powder using a grinder. The encapsulation efficiency was quantitatively determined by a UV-visible spectrophotometer [47]. The capsule morphology was examined using Scanning Electron Microscopy (SEM). The capsule powder was mounted on double-sided tape attached to the specimen stub and then subjected to metallisation (sputtering) with a thin layer of gold. The images were viewed at magnifications of ×2000 using SEM (JEOL JSM-5910, Japan Electron Optics Laboratory Co., Ltd., Tokyo, Japan) with an accelerating voltage of 5 kV.

Fourier-Transform Infrared Spectroscopy (FTIR)
FTIR analysis was conducted using a DairyGuard TM Milk Powder Analyser (PerkinElmer). Each sample was scanned by placing the sample on the ATR diamond crystal and applying the pressure tower. The spectrum was verified in the transparent mode from 450-4000 cm −1 .

Controlled Release of the Capsules
One gram of each capsule was placed into the 3 mL vial without sealing. The vials were then left at 25-30 • C (capsules were strictly maintained in dry condition), and weight loss was monitored in an analytical balance as a function of time for 3 h. The percentage of release was calculated in relation to the initial mass of the products [48].

The Experiment Site
The experiment was conducted in a multi-crop orchard (Dusanee orchard) located in Mae Taeng District, Chiang Mai, Thailand (19 • 5 51.08 , 98 • 53 55.54 ) in which mango and santol were cultivated 3 × 3 m alternately to one another. The test experiment was conducted during the peak harvesting of mango var. nam dok mai. The maintenance of the orchard was in accordant with Good Agricultural Practice (GAP). The orchard was in a healthy condition comprising a compact load of mango trees (five-year-old, small-medium tree size of~3 m tall).

Trap
The trap was made from a plastic coffee cup (250 mL) with a round lid. Six holes were drilled on the cup (3 × Ø 1 cm on the lid and 3 × Ø 0.3 cm on the cup) (Scheme 2a). The capsule (1 g) was placed inside the lid, capped with the cup, and immediately tided onto the mango tree by upside-down position. The traps were hung up-side-down where natural air was able to flow naturally through holes. The efficacy of the encapsulated products was compared by placing traps inside the canopy at the height of ca 1.5-2 m (Scheme 2b). Insects caught on or in the traps were generally counted between 2.30-5.00 pm for 30, 60, 90, and 120 min, and thereafter 24, 48, and 72 h. A completely randomised design (CRD) was used, with eight replicates (Scheme 2c) [49]. The controls were the essential oil (5 µL) on a cotton ball and trap without the product. The product efficiency test was compared with traps with 5 µL on a cotton ball of the commercial fruit fly attractant. They were placed two rows away from the experiment. ucts was compared by placing traps inside the canopy at the height of ca 1.5-2 m (Scheme 2 (b)). Insects caught on or in the traps were generally counted between 2.30-5.00 pm for 30, 60, 90, and 120 min, and thereafter 24, 48, and 72 h. A completely randomised design (CRD) was used, with eight replicates (Scheme 2 (c)) [49]. The controls were the essential oil (5 µL) on a cotton ball and trap without the product. The product efficiency test was compared with traps with 5 µL on a cotton ball of the commercial fruit fly attractant. They were placed two rows away from the experiment.

Statistical Analysis
The averages of all the experiments' data were subjected to analysis of variance (ANOVA) at the significance level p = 0.05 using the IBM ® SPSS ® Statistics version 23. In addition, principal component analysis (PCA) to summarise graphical differences of volatile components of the essential oil was adopted using Minitab Statistical Software (Minitab ® ).

Chemical Compositions
The VOCs and the relative amount of essential oil from five basils are illustrated in Table 2. Overall, a total of 65 volatile compounds was identified, by which estragole, eu-

Statistical Analysis
The averages of all the experiments' data were subjected to analysis of variance (ANOVA) at the significance level p = 0.05 using the IBM ® SPSS ® Statistics version 23. In addition, principal component analysis (PCA) to summarise graphical differences of volatile components of the essential oil was adopted using Minitab Statistical Software (Minitab ® ).
The relationship of the VOCs and types of basil essential oil were elucidated using principal component analysis in Figure 1c. The biplots analysis depicted a total of 78.93% across the PCA range, which accounted for 45.44% in PC1 and 33.49% in PC2. The volatile compositions of all essential oil were similar from the analysed diagram, but TrB and LB oils were separated from the others by the lower content of estragole. Methyl eugenol was dominant in essential oils of RB, WB and TrB, while contents of eugenol, trans-caryophyllene, β-elemene and germacrene were variable among these oils. The volatiles were also categorised according to their chemical structure types (Figure 1b). It was apparent that the majority of the VOCs was of phenylpropanoids, followed by the sesquiterpene hydrocarbon and oxygenated terpenes. The free volatiles of the monoterpene were detected only in TrB and TB oils.

Attractiveness
The basil essential oils were able to attract ORF differently, and the vast majority of responsive ORF was male, as shown in Table 3. Although the efficiency of the essential oil was less than commercial ME, the essential oils of the RB (~25 flies) and WB (~19 flies) were comparable to the ME (28 flies). However, the numbers of responses were low for LB, TB, and TrB oils.

Insect Olfactory Chemosensory Responses
Overall, the ORFs of both sexes responded to the volatile substances by giving more movement frequency of the antenna and forelegs than when treated with ethanol ( Table 4). The highest response was with ME, following by the WB, TB, and RB, respectively, and the degree of response was stronger in male ORFs than from the female ORFs. By looking only at the male ORFs, the antennae movement was more frequent when applied with the essential oils than the ME, but a high frequency of foreleg movement was observed with the ME (Supplementary Materials Video S1). These, however, were less obvious in the female ORFs.

Experiment 2: Paste matrix complexation with basil essential oil encapsulation
From the first experiment, we found that the essential oils that were effective in attracting ORF were of the holy basils of red and white types, in which the finding was in correspondent with its high content of methyl eugenol. In addition, the percentage yield of these essential oil types was considerably higher (~1%) compared with other basil types. Considering the material availability and yield, we then selected white holy basil oil (WBO) for the substantial studies.

Emulsion Characteristic and Encapsulation Characteristics
The viscosity of different wall material pastes is illustrated in Table 5. The result showed that the paste solution of GA gave high viscosity of 2976 cP. However, when mixed with MD, the viscosities decreased to 1348.0, 631.0, and 422.0, with 75%, 50%, and 25% MD content, respectively. 422.0 >1 0.235 ± 0.0493 ab 6.14 * The difference of mean ± SE (standard error) of three replications in the oil loading column was analysed using one-way ANOVA. The letters (a,b) indicated the significant difference at 95 confidence level.
After loading with WBO, the loading capacity and encapsulation efficiency were also shown in the same table. The GA complexation gave a higher loading capacity (~0.3 µL/0.2 g) than that of the absolute MD. In addition, the GA-consisting products also gave the high WBO encapsulation efficiency. The 25:75 ratio of MD:GA had the highest efficiency of containing WBO at~9.4%.
The SEM images illustrate surface characteristics of each processed capsule ( Table 6). The WBO loaded products exhibited tiny pore characteristics on the surface complexation.  422.0 >1 0.235 ±0.0493 ab 6.14 * The difference of mean ± SE (standard error) of three replications in the oil loading column was analysed using one-way ANOVA. The letters (a-b) indicated the significant difference at 95 confidence level.
The SEM images illustrate surface characteristics of each processed capsule ( Table 6). The WBO loaded products exhibited tiny pore characteristics on the surface complexation.  422.0 >1 0.235 ±0.0493 ab 6.14 * The difference of mean ± SE (standard error) of three replications in the oil loading column was analysed using one-way ANOVA. The letters (a-b) indicated the significant difference at 95 confidence level.
The SEM images illustrate surface characteristics of each processed capsule ( Table 6). The WBO loaded products exhibited tiny pore characteristics on the surface complexation.

The Structure of Processed Maltodextrin and Gum Arabic
MD100 GA100 The Structure of White Holy Basil Essential Oil (WBO) Loaded Capsules WBO-MD100 WBO-GA100

FTIR
The FTIR patterns of the WBO-encapsulated products and wall material (MD25GA75) are illustrated in Figure 2. We were interested mainly in the FTIR characteristics of holy basil essential oil previously described by Sutaphanit and Chitprasert [50] and Agatonovic-Kustrin et al. [51]. The C-H bending peaks of the alkene/aromatic groups were detected in the FTIR spectra at ~550 and ~995 cm −1 , and at ~1123 cm −1 represented C-O stretching vibrations. The spectrum at ~1369 and ~1451 cm −1 was assigned to the symmetric and asymmetric C-H bending vibrations in -CH3, respectively. The bands observed at ~1513, ~1608, and ~1638 cm −1 indicated the -C=C-stretching vibrations of alkene/aromatic, symmetric and asymmetric C-H stretching vibrations in -CH2-are present in the bands at ~2860 and ~2928 cm −1 and asymmetric C-H stretching vibration in =CH2 at the band of ~3079 cm −1 . The FTIR spectrum peaks of WBO-encapsulated product showed evidence of the functional groups of those detected in WBO marked at ~995, ~1369, ~1451, ~1608, and ~2928 cm −1 . This implied that WBO was successfully loaded and remained in the wall material. It is also worth highlighting that there is no significant chemical interaction between WBO and the wall material based on the detected spectra.

FTIR
The FTIR patterns of the WBO-encapsulated products and wall material (MD25GA75) are illustrated in Figure 2. We were interested mainly in the FTIR characteristics of holy basil essential oil previously described by Sutaphanit and Chitprasert [50] and Agatonovic-Kustrin et al. [51]. The C-H bending peaks of the alkene/aromatic groups were detected in the FTIR spectra at ~550 and ~995 cm −1 , and at ~1123 cm −1 represented C-O stretching vibrations. The spectrum at ~1369 and ~1451 cm −1 was assigned to the symmetric and asymmetric C-H bending vibrations in -CH3, respectively. The bands observed at ~1513, ~1608, and ~1638 cm −1 indicated the -C=C-stretching vibrations of alkene/aromatic, symmetric and asymmetric C-H stretching vibrations in -CH2-are present in the bands at ~2860 and ~2928 cm −1 and asymmetric C-H stretching vibration in =CH2 at the band of ~3079 cm −1 . The FTIR spectrum peaks of WBO-encapsulated product showed evidence of the functional groups of those detected in WBO marked at ~995, ~1369, ~1451, ~1608, and ~2928 cm −1 . This implied that WBO was successfully loaded and remained in the wall material. It is also worth highlighting that there is no significant chemical interaction between WBO and the wall material based on the detected spectra.

FTIR
The FTIR patterns of the WBO-encapsulated products and wall material (MD25GA75) are illustrated in Figure 2. We were interested mainly in the FTIR characteristics of holy basil essential oil previously described by Sutaphanit and Chitprasert [50] and Agatonovic-Kustrin et al. [51]. The C-H bending peaks of the alkene/aromatic groups were detected in the FTIR spectra at ~550 and ~995 cm −1 , and at ~1123 cm −1 represented C-O stretching vibrations. The spectrum at ~1369 and ~1451 cm −1 was assigned to the symmetric and asymmetric C-H bending vibrations in -CH3, respectively. The bands observed at ~1513, ~1608, and ~1638 cm −1 indicated the -C=C-stretching vibrations of alkene/aromatic, symmetric and asymmetric C-H stretching vibrations in -CH2-are present in the bands at ~2860 and ~2928 cm −1 and asymmetric C-H stretching vibration in =CH2 at the band of ~3079 cm −1 . The FTIR spectrum peaks of WBO-encapsulated product showed evidence of the functional groups of those detected in WBO marked at ~995, ~1369, ~1451, ~1608, and ~2928 cm −1 . This implied that WBO was successfully loaded and remained in the wall material. It is also worth highlighting that there is no significant chemical interaction between WBO and the wall material based on the detected spectra.

FTIR
The FTIR patterns of the WBO-encapsulated products and wall material (MD25GA75) are illustrated in Figure 2. We were interested mainly in the FTIR characteristics of holy basil essential oil previously described by Sutaphanit and Chitprasert [50] and Agatonovic-Kustrin et al. [51]. The C-H bending peaks of the alkene/aromatic groups were detected in the FTIR spectra at~550 and~995 cm −1 , and at~1123 cm −1 represented C-O stretching vibrations. The spectrum at~1369 and~1451 cm −1 was assigned to the symmetric and asymmetric C-H bending vibrations in -CH 3 , respectively. The bands observed at~1513, 1608, and~1638 cm −1 indicated the -C=C-stretching vibrations of alkene/aromatic, symmetric and asymmetric C-H stretching vibrations in -CH 2 -are present in the bands at 2860 and~2928 cm −1 and asymmetric C-H stretching vibration in =CH 2 at the band of 3079 cm −1 . The FTIR spectrum peaks of WBO-encapsulated product showed evidence of the functional groups of those detected in WBO marked at~995,~1369,~1451,~1608, and~2928 cm −1 . This implied that WBO was successfully loaded and remained in the wall material. It is also worth highlighting that there is no significant chemical interaction between WBO and the wall material based on the detected spectra.

Controlled Release of the White Holy Basil-Encapsulate Product
The volatile release characteristic of encapsulated products was given as in Figure 3. The results were reported according to the percentage of weight loss after exposure in the ambient condition. The releasing of all capsules during the first 120 min was initially subtle. However, after 120 min, capsules containing high GA (GA100 and MD25GA75) gave a remarkable increase in the release. In contrast, high doses of MD-containing capsules remained relatively stable.

Controlled Release of the White Holy Basil-Encapsulate Product
The volatile release characteristic of encapsulated products was given as in Figure 3. The results were reported according to the percentage of weight loss after exposure in the ambient condition. The releasing of all capsules during the first 120 min was initially subtle. However, after 120 min, capsules containing high GA (GA100 and MD25GA75) gave a remarkable increase in the release. In contrast, high doses of MD-containing capsules remained relatively stable.

Controlled Release of the White Holy Basil-Encapsulate Product
The volatile release characteristic of encapsulated products was given as in Figure 3. The results were reported according to the percentage of weight loss after exposure in the ambient condition. The releasing of all capsules during the first 120 min was initially subtle. However, after 120 min, capsules containing high GA (GA100 and MD25GA75) gave a remarkable increase in the release. In contrast, high doses of MD-containing capsules remained relatively stable.

Product Efficacy
The essential oil capsules portrayed different attraction capacities at different times (Table 7). At the initial 30 min, MD75GA25 attracted the most fruit flies among all other products and increased proportionately thereafter. The WBO encapsulated in the MD25GA75 formula was able to trap flies constantly (at about 0.25 files per trap) at intervals of 30-120 min and increased after 24 and 72 h exposure in the field. As for the GA100 product, the capacity was approximately 0.25 and 0.50 per trap at 24 and 72 h, respectively. Whereas WBO alone resulted in a rapid loss of the volatile active ingredients to the environment, thereby reducing the attractant ability. 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 0.00 ± 0.00 The difference of mean ± SE (standard error) of eight replications in the same column was analysed by one-way ANOVA and the letters (a-c) are indicated the significantly different at 95 confidence level.
The efficiency of the capsules combined with the fruit fly traps was by potting response curves of the efficacy data ( Figure 4). The results showed that the encapsulation by paste method could progressively increase fruit fly attraction at later hours, especially in formulations with a mixture of GA and MD. Meanwhile, the product's efficiency of the commercial ME trended to decrease over time.

Product Efficacy
The essential oil capsules portrayed different attraction capacities at different times (Table 7). At the initial 30 min, MD75GA25 attracted the most fruit flies among all other products and increased proportionately thereafter. The WBO encapsulated in the MD25GA75 formula was able to trap flies constantly (at about 0.25 files per trap) at intervals of 30-120 min and increased after 24 and 72 h exposure in the field. As for the GA100 product, the capacity was approximately 0.25 and 0.50 per trap at 24 and 72 h, respectively. Whereas WBO alone resulted in a rapid loss of the volatile active ingredients to the environment, thereby reducing the attractant ability. 0.00 ±0.00 0.00 ±0.00 0.00 ±0.00 0.00 ±0.00 0.00 ±0.00 * The difference of mean ±SE (standard error) of eight replications in the same column was analysed by one-way ANOVA and the letters (a-c) are indicated the significantly different at 95 confidence level.
The efficiency of the capsules combined with the fruit fly traps was by potting response curves of the efficacy data ( Figure 4). The results showed that the encapsulation by paste method could progressively increase fruit fly attraction at later hours, especially in formulations with a mixture of GA and MD. Meanwhile, the product's efficiency of the commercial ME trended to decrease over time.

Discussion
The volatile compositions from the essential oils of all basil types comprised the phenylpropanoid group, including methyl chavicol (estragole), eugenol and methyl eugenol. The others belong to the sesquiterpenes and monoterpenes. These compounds contribute to the unique sweet-anise aroma, which is the aroma characteristic of the essential oils from these species [28]. Moreover, the influence of gene expression for the biosynthesis of the phenylpropanoids is able to confer variation of the chemotypes within the same species. Thus, the altered levels between methyl chavicol (estragole) and linalool can be used to classified subspecies of the sweet basil [28,[52][53][54]. Similarly, the constituents of the eugenol and methyl eugenol can be used to discriminate sub-varieties of the holy basil (O. sanctum) [30]. This is in agreement with our finding where the distinct levels of these phenylpropanoids were observed in both WB and RB. In addition, the content of methyl eugenol also depended upon the cultivation climate in which the intensity was with the Ocimum spp. grown in sub-tropical areas of North Africa, Eastern Europe, and parts of Asia [32,55]. Methyl eugenol is the primary volatile active ingredient in holy basils and is also known as a highly potent attractant for the ORF and other species in the Tephritidae family [56]. Male ORFs are attracted to this substance because it acts as a hormonal precursor and booster of male ORF's sex pheromone [16,57]. Insect antennae is an essential organ in the olfactory system that escorts insect behavioural expression such as food hunting, mating, and finding appropriate sites for laying eggs [58]. First, the semiochemicals are detected by sensilla on the antennae, bound to its transmembrane odorant receptors (ORs) that generate the chemical signals transmitted into nerve impulses to the brain and control movement organs, such as forelegs [58,59]. In our study, insects responded to the VOCs with the increase in the movement frequency of the antennae and forelegs. The antennae's higher frequency move was with the essential oils indicating the sensitivity of natural VOCs to its olfactory organs. Gomez-Diaz, Martin, Garcia-Fernandez, and Alcorta [58], Leal [60] described that the insect sensilla comprises pores in the cuticle that allow odorants to diffuse into the nerve system. The located ORs are also described as odour-specific, especially natural products such as essential oils [61,62]. In the ORF, in particular, its ORs are able to bind structurally with all portions of methyl eugenol molecule and respond to ortho-substituted benzenes with adjacent oxygen atoms or isoelectronic equivalents [63]. Our study conveyed that male B. dorsalis responded to various forms of VOCs in the essential oils. However, only methyl eugenol composition rendered the movement of the forelegs towards the odorant sources.
The WBO was encapsulated to improve the volatility of the active compounds. Maltodextrin and gum arabic are affordable polysaccharides that are often used in encapsulations [64,65]. Maltodextrin is a water-soluble substance of low viscosity, low sugar content and free from colour and smell [66]. Likewise, GA is an emulsifier and enveloping material [67]. The viscosity of the MD was less but increased thereafter mixing with the GA. The efficiency of WBO loading also increased from the higher GA ratio in wall material formulation. Akhavan Mahdavi et al. [68] suggested that the encapsulation properties of the compounds were improved when the encapsulation mixture was composed of more than one type of wall material. The spectrum patterns were able to characterise the functional groups of the substance by the absorption or transmission, and the appearance of the vibration peak indicated [51]. The FTIR and SEM results indicated that the encapsulation characteristics of the wall materials were structurally formed. This was, however, not a chemical bond encapsulation as the volatile organics were not soluble in the water-soluble wall materials. The finding was in line with Sutaphanit and Chitprasert [50], in which the holy basil oil was not fully dissolved in the gelatine wall.
Our field study was conducted during the sunlight to test the ability of the product as an attractant and the efficiency compared to the methyl eugenol. The temporal pattern of the mating behaviour of fruit flies: courtship episodes of the flies were usually between 0-9 h, and the striking peak of the activity was before noon [69,70]. The decision of female flies to mate with male fruit flies depends on long-range attraction signals, willingness to copulate, and male-male competition [71]. On the other hand, males decide to mate upon their libido and external stimuli, such as methyl eugenol [72]. Courtship usually takes place on fruits and fruit trees, and the estimated male success rate of mating is only around 0.6 per female per day [70]. Without mechanical traps, delivery of methyl eugenol fumigation enhances the mating success of B. carambolae and B. dorsalis males [73], which is not recommended for the non-sterile insect approach. Therefore, it is crucial to maintain the release of VOCs with semiochemical properties during the courtship episodes to be successful in the mechanical trap. Our finding demonstrated that paste encapsulation with MD combined with GA could maintain the release rate of the essential oil.