Efficacy of Mentha aquatica L. Essential Oil (Linalool/Linalool Acetate Chemotype) against Insect Vectors and Agricultural Pests

In recent years, agrochemical industries have been focused on the development of essential oil (EO)-based biopesticides, which can be considered valuable alternatives to traditional chemical products. The genus Mentha (Lamiaceae) comprises 30 species characterized by a wide range of biological activities, and some of their EOs showed good potential as pesticidal agents. In this regard, the aim of this study was to evaluate the insecticidal activity of the EO obtained from a rare linalool/linalool acetate chemotype of Mentha aquatica L. The EO was found to be highly effective against Culex quinquefasciatus (Say) 2nd instar larvae, Metopolophium dirhodum (Walker) adults, Spodoptera littoralis (Boisduval) 2nd instar larvae, and Tetranychus urticae (Koch) adults, showing lethal concentrations (LC50) or doses (LD50) of 31.5 ± 2.2 µL L−1, 4.9 ± 0.8 mL L−1, 18.5 ± 2.1 µg larvae−1, and 3.3 ± 0.5 mL L−1, respectively. On the contrary, Musca domestica L. adults and 3rd instar larvae of C. quinquefasciatus and S. littoralis were moderately affected by the treatment (LC50 or LD50: 71.4 ± 7.2 µg adult−1, 79.4 ± 5.2 µL L−1, 44.2 ± 5.8 µg larvae−1, respectively). The results obtained in this work demonstrated that various insects and pests could be differently sensible to the same EO and may lead to the exploitation of this plant or its major volatile compounds as novel ingredients of botanical insecticides and pesticides.


Introduction
The genus Mentha belongs to the Lamiaceae family and comprises approximately 30 species distributed all over the world [1]. Plants of this genus have been widely used for several purposes by the pharmaceutical, nutraceutical, food, beverage, and tobacco industries [2]. They represent the most exploited sources for the extraction of essential oils (EOs), which are produced at a rate of 23,000 metric tons every year for a value of $400 million [3].
In addition, several species of this genus have been proven as good sources for botanical insecticide ingredients due to their capacity to produce EOs equipped with contact toxicity, fumigant, and repellent effects against a wide spectrum of target insects, such as storage pests, vectors, and larvae [4]. The most investigated species of this genus are Mentha x piperita L., Mentha spicata L., and Mentha pulegium L.
Mentha aquatica L., also known as 'water mint', is a member of this genus growing in wet environments of Europe, North Africa, and West Asia. Moreover, it has been recently introduced into America and Australia [5,6]. The ethnobotanical uses reported for this plant have been mainly associated with its medicinal value, as M. aquatica is currently employed as a remedy for colds, respiratory, and gastrointestinal problems. Particularly, the gastrointestinal effect depends on the modulation of non-protein sulfhydryl substances, nitric oxide, and gastric secretion [7]. In addition, the leaves of the plant are smoked in South Africa to treat mental diseases [8], and the central nervous system activity has been associated with a strong affinity to the GABA-benzodiazepine receptor [6]. Mentha aquatica also showed butyrylcholinesterase inhibitory activity and antioxidant, antimicrobial, catalytic, and cytoprotective actions [2,9,10]. However, most of the available studies on M. aquatica mainly focus on the chemical variability of its EO, which is in turn related to its geographic origins and to the agronomic treatments applied when it is cultivated as a crop. Currently, the reported chemotypes of M. aquatica are dominated by menthofuran, pulegone, menthol, piperitone oxide, or linalool [3,[11][12][13][14][15]. To the best of our knowledge, the M. aquatica EO has been poorly explored for potential insecticidal effects when compared with other representatives of the genus Mentha.
In recent decades, the exploitation of botanical products capable of replacing traditional chemical pesticides has exponentially increased [16][17][18][19]. Indeed, problems related to food safety and environmental pollution have led to greater attention to sustainability, also in the agrochemical sector [20,21]. Although pesticides are essential for crop protection and, consequently, for food production, chemical residues can be toxic to other non-target organisms and have a negative impact on various environmental media such as air, soil, and water [22]. Therefore, replacing chemical substances with botanical products results in a good compromise to guarantee the protection of crops without causing damage to the environment, humans, or non-target species [23,24]. Among botanical products, EOs could be potential candidates for the development of novel biopesticides and insecticides. In previous studies, we showed that numerous EOs could display their insecticidal and pesticidal potential towards different insect species depending on the synergistic or antagonistic effect of their components that revealed suitable LC 50 and LC 90 values [25][26][27].

Insecticidal and Acaricidal Efficacy
Regarding the insecticidal and acaricidal efficacy of M. aquatica EO, it was found to cause relatively good acute toxicity to all the target species. The estimated lethal doses (LD 50 ) or concentrations (LC 50 ) are shown in Table 2. The EO was more effective on younger larval instars. For instance, the LC 50 for C. quinquefasciatus was estimated at 31.5 µL L −1 for the 2nd instar and at 79.4 µL L −1 for the 3rd larval instar. The same trend was found for 2nd and 3rd instar larvae of S. littoralis (LD 50 = 18.5 and 44.2 µg larva −1 , respectively). Very good effectiveness of the EO was found for small pests, such as the adults of M. dirhodum and T. urticae tested by us, for which the LC 50 was estimated at 4.9 and 3.3 mL L −1 , respectively. Conversely, low efficacy was found for M. domestica adults (LD 50 = 71.4 and 50.5 µg adult −1 ; LD 90 = 329.8 and 462.6 µg adult −1 , for females and males, respectively).

Discussion
It is well known that the chemical composition of EOs is linked to several endogenous and exogenous factors, such as chemotypes, geographical distribution, growing conditions and climate, time of collection, and extracting techniques [31]. This chemical variability has also been reported for M. aquatica EO, for which the main varieties described in the literature are reported in Table 3. The composition found in our study is similar to that of other cultivated populations of M. aquatica, being linalool and linalool acetate the main compounds, even if at different ratios. For instance, for plants cultivated in Iran, the EO was mainly characterized by the presence of linalool (37.8%) and linalool acetate (30.6%) [32], as well as for species collected in India, for which the amount of these two compounds varied according to the season of collection. In fact, linalool was the dominant compound for plants collected from April to September (25.2-48.4%), while linalool acetate was the dominant compound for those collected from October to December (42.1-48.0%). A similar chemical constitution was also found for M. aquatica var. citrata, for which linalool and linalool acetate were the most representative compounds [14]. On the other hand, the chemical composition herein described contrasts with those reported from other studies. In fact, menthofuran has sometimes been reported as the most abundant compound. This is the case for the EO obtained from wild-growing plants in Vojvodina (16.9%), as well as the ones obtained from wild populations in Ethiopia (70.5%) and Romania (51.3-58.6%) [3,5,36]. The predominance of menthofuran in the EO seems to be also linked to other growing conditions, as in the case of M. aquatica plants growing in presence of Chrysolina herbacea (Duftschmid 1825) (Coleoptera: Chrysomelidae). In these conditions, the plant activates some genes involved in the biosynthesis of terpenoids and redirects them to the production of menthofuran, which was demonstrated to repel C. herbacea [37]. The preponderance of menthofuran was also correlated with genetic factors [38,39].
The genus Mentha has been extensively studied for its insecticidal and acaricidal activity against agricultural pests and insect vectors, and some species have shown great efficacy [28,29]. For example, Mentha longifolia (L.) and Mentha suaveolens (Ehrh.) have demonstrated high larvicidal activity against third instar larvae of C. quinquefasciatus after 24 h of exposure with LC 50 values of 17 mg L −1 for both EOs, which were characterized by the main compound piperitone oxide [40]. In a study, among 34 EOs from different Mentha species, M. pulegium was found to be the most effective against M. domestica adults under laboratory conditions in fumigant and topical bioassays, with LD 50 values of 13 µg fly −1 and 4.7 µg cm −1 , respectively [41]. Its EO was dominated by pulegone, the main responsible for the biological activity. Moreover, M. piperita caused >90% mortality, while M. spicata caused 81-82% mortality at 14 × 10 −3 µL mL −1 , demonstrating a significant acaricidal effect against T. urticae; in this case, menthol and carvone usually represent the main compounds of the EOs for the two species, respectively [42].
Despite the large body of relevant literature regarding the potential of the Mentha species to be used for the control of several vectors and pests, studies concerning the linalool/linalool acetate chemotype's insecticidal activity have not yet been reported. This is the first study recording useful information for the potential development of biopesticides exploiting the rare chemotype of this species from Lebanon. However, both linalool and linalool acetate have been revealed to be effective pesticides in several studies [4,[43][44][45][46]. Linalool has been demonstrated to be a competitive acetylcholinesterase inhibitor [46,47], and both linalool and linalool acetate seem to interfere with the insect central nervous system, in particular interacting with glutamatergic transmission and the GABA A receptor [48][49][50]. Indeed, EOs containing linalool and/or linalool acetate have been reported as effective insecticidal agents. For instance, basil EO showed a promising insecticidal potential on targets such as Rhyzopertha dominica L. (75.0% mortality at 4% of EO) [51], Sitophilus oryzae L. (LC 50 of 4.9 µL mL −1 ) [52], Ceratitis capitata Wiedemann (LT 90 of 17.0 min), Bactrocera dorsalis (Hendel) (LT 90 of 26.0 min), and B. cucurbitae Coquillett (LT 90 of 32.0 min) [53]. This effect has been mainly linked to the high levels of linalool in the EO. In the same way, the EO from Cinnamomum camphora Ness and Eberm var. linaloolifera Fujita, which is characterized by linalool as the main compound, has been reported for its insecticidal properties against Anticarsia gemmatalis Hübner (LC 50 of 0.908% v/v) [54] and Trialeurodes vaporariorum Westwood (nymph mortality of 88.5% at 2.0% v/v) [55]. Similarly, Coriandrum sativum L. seeds' EO displayed an insecticidal potential on adults of Tribolium confusum Duval (LC 50 of 1.34 µL L −1 air) and Callosobruchus maculatus Fabricius (LC 50 of 318.02 µL L −1 air), and this action was correlated to the predominant presence of linalool [56]. On the other hand, the EO from Myrtus communis L., mainly characterized by linalool and linalool acetate, displayed insecticidal action on three stored-product insects, namely Ephestia kuehniella Zeller (LC 50 of 12.7 µL L −1 air), Plodia interpunctella Hübner (LC 50 of 22.6 µL L −1 air), and Acanthoscelides obtectus Say (LC 50 of 49.6 µL L −1 air) [57]. In addition, Cananga odorata (Lam.) Hook. f. and Thomson EO showed marked contact toxicity against Sitophilus zeamais Motschulsky with an LD 50 value of 33.1 µg adult −1 and fumigant toxicity with an LC 50 value of 14.8 mg L −1 [58]. In our work, we did not have a positive control available; however, we can compare the effectiveness of EO with the positive control of previously published works in which the same insect species were used in the same developmental stages and the application was carried out in a similar way with the same genetic material of the target organisms and under similar post-application conditions. Regarding the herein presented study, M. aquatica EO was found to be more effective than Rock Effect (a commercial biopesticide based on Pongamia pinnata L. oil), which was used as a positive control by Pavela et al. [59] and tested against the same targets. Specifically, the LD 50 or LC 50 values were higher for the positive control (>500 µg adult −1 , 275.4 µg mL −1 , 12.5 mL L −1 , 5.8 mL L −1 , 3.3 ± 0.5 mL L −1 , respectively) than for the M. aquatica EO (71.4 ± 7.2 µg adult −1 , 79.4 ± 5.2 µg mL −1 , 4.9 ± 0.8 mL L −1 , respectively) when both were tested against M. domestica female adults, C. quinquefasciatus 3rd instar larvae, and M. dirhodum adults. On the other hand, their activity is quite comparable in the test against S. littoralis (LD 50 of 18.2 and 18.5 µg larva −1 for the positive control and M. aquatica EO, respectively). The effectiveness of M. aquatica EO was of varying degrees of intensity, as the different species of insects tested were differently sensitive to the same EO.

Plant Material and EO Extraction
Leaves of cultivated M. aquatica were manually collected in Kafarkela (33 •

GC-MS Analysis of Essential Oils
The GC-MS analysis was carried out with an Agilent 6890N-5973N GC-MS system (Santa Clara, CA, USA) on a sample of M. aquatica EO prepared by dilution to 1:100 with n-hexane. The instrument was operating in the EI mode at 70 eV and using a HP-5MS (5% phenylmethylpolysiloxane, 30 m, 0.25 mm i.d., film thickness 0.1 µm) (J&W Scientific, Folsom, CA, USA) capillary column.

Target Insects and Mites
As target arthropod species, we tested C. quinquefasciatus, M. domestica, M. dirhodum, S. littoralis, and T. urticae. These species have been reared under controlled laboratory conditions at the Crop Research Institute (Prague, Czech Republic) for more than 20 generations.
Arthropod mass rearing in brief: C. quinquefasciatus larvae were fed with dry dog biscuits; adults were allowed to mate; females were fed with blood in order to complete their egg development. Eggs were laid in unprepared containers of water. M. domestica larvae were fed a diet developed at the Crop Research Institute (Prague, Czech Republic), which was composed of sawdust, milk, and agar. Housefly adults were fed sugar solutions and powdered milk. Eggs were laid on cotton wool dipped in sweet milk. Wheat plants in pots with ordinary substrate were selected to rear M. dirhodum. S. littoralis larvae were fed with agar, soybean meal, and vitamins; adults, fed with honey solution, mated and laid eggs on filter paper previously prepared. Bean plants grown in a common garden substrate were selected to rear T. urticae in a growth chamber. All arthropod target species were maintained at 25 ± 1 • C, 70 ± 3% R.H., and 16:8 h (L:D). Experiments described thereafter were carried out under the same conditions [59].

Insecticidal and Acaricidal Activity
The M. aquatica EO was diluted in acetone (p.a., Sigma Aldrich, Prague, Czech Republic) to obtain various concentrations (applied at 1 µL): for S. littoralis larvae, 10,20,30,40,50,60,70,80,90, and 100 µg larva −1 ; and for M. domestica adults, 50,80,100,150,200,250, and 300 µg adult −1 . Before application, the arthropods were anesthetized with CO 2 . Acetone was the negative control. After treatment, the tested organisms were placed into the rearing containers (15 × 12 × 8 cm) equipped with a perforated lid and fed with the aforementioned diet. The experiments were replicated four times; each replicate was performed with 20 individuals. For C. quinquefasciatus larvae, EO was dissolved in DMSO (dimethyl sulfoxide, Merck, Prague, Czech Republic) and tested according to the WHO (1996) procedure [60] with minor modifications. Each time, 1 mL of DMSO, which contained a defined amount of EO, was thoroughly mixed in 99 mL of chlorine-free standing water. In this way, a concentration series containing 20, 40, 60, 80, and 100 mg mL −1 of mint EO was obtained. DMSO was used as a negative control. For each replicate, 20 larvae were used, and the experiment was repeated four times. For experiments with M. dirhodum and T. urticae, first, the EO was emulsified using Tween 80 (Sigma-Aldrich, Prague, Czech Republic) in a 1:1 (v:v) ratio. Afterwards, different concentrations were prepared (for M. dirhodum adults, 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0 mL L −1 , and for T. urticae adults, 1.0, 2.0, 3.0, 4.0, 5.0, and 6.0 mL L −1 ) by thoroughly mixing the modified EO in water. Always, 20 adults of M. dirhodum or T. urticae (for each replication) were transferred to wheat or bean leaves, respectively, using a fine brush. The plants were located in a flowerpot with a diameter of 9 cm. An electric applicator was used to spray the plants (5 mL of solution per plant) in five replicates.
All experiments were conducted in an air-conditioned room at a temperature of 25 • C, a photoperiod of 16 h of light, and 70-80% relative humidity. Twenty-four hours after the application, the number of dead individuals was determined. All individuals that did not show any movement in response to a mechanical stimulus were considered dead.
For the calculation of lethal doses or concentrations, at least five concentrations or doses for which mortality was found to be in the range of 20-90% were always selected. After correction of mortality by Abbott [61], LD(LC) 50(90) were estimated using Probit analysis [62].

Conclusions
In this work, a linalool acetate/linalool chemotype of M. aquatica was tested on C. quinquefasciatus, M. domestica, M. dirhodum, S. littoralis, and T. urticae, showing a relatively good acute toxicity on most of the tested targets. In detail, for C. quinquefasciatus and S. littoralis, a higher efficacy of the EO was found on the lower larval stages, while moderate activity was detected on M. dirhodum and T. urticae. Conversely, the EO was less effective on M. domestica adults. The different results obtained in the reported study suggest that various mechanisms of action, likely ascribable to the EO main constituents linalool and linalool acetate, could be involved in the different targets effects, and more studies should be performed to deepen this aspect.
Even though the genus Mentha has been widely reported for its insecticidal and acaricidal potential, this is the first study evaluating the above-mentioned properties of M. aquatica EO, namely the linalool acetate/linalool chemotype from Lebanon. The results herein presented could represent the starting point for a further exploration of this plant EO and/or its two major constituents as a botanical insecticide and pesticide ingredient.