Piper aduncum essential oil: a promising insecticide, acaricide and antiparasitic. A review

Several studies have assessed the potential of essential oils as substitutes for synthetic pesticides, in order to counter insect resistance to commercial pesticides. Piper aduncum L. is a very common shrub in the Amazon Rainforest and in other subtropical areas. The objective of this review was to analyse the existing information on P. aduncum essential oil as a raw material for new bioproducts for sustainable pest disease management. With this review, we collected and critically analysed 59 papers, representing all the studies that aimed to evaluate the essential oil properties of this species as an insecticide, acaricide and antiparasitic. The chemical composition differs depending on the origin, although phenylpropanoid dillapiole is the most cited component, followed by myristicin, 1,8-cineole and β-ocimene. Between the acaricidal, antiparasitic and synergistic activities, the insecticidal effects are highly promising, with optimal results against the malaria vector Aedes aegypti, with an LC50 that ranges between 57 and 200μg/mL. Acaricidal activity has mainly been reported against Tetranychus urticae, showing an LC50 that ranges between 5.83 and 7.17μg/mL. Antiparasitic activity has predominately been found on Leishmania amazonensis, and antipromastigote activity has been found to be between 23.8 and 25.9μg/mL. Concerning the synergistic effect between dillapiole and synthetic insecticides, four studies on Spodoptera frugiperda found promising results with cypermethrin. In this review, we highlighted the potential of P. aduncum essential oil as a biopesticide, also focusing on the lack of information about applied research. We also provide suggestions for future investigations.


Introduction
About 80% of the world population relies predominantly on plants and plant extracts for health care [81]. Starting from the Orient, mainly from Egypt, Persia and India, through to the Western World, from the ancient Greeks to the Romans, and finding a clear definition in the Middle Ages, Essentials Oils (EOs) are a fundamental part of the history of our civilization [37].
After World War II, there was new trend to mostly study synthetic chemical substances, which led to a decrease in the use of botanical extracts. Clearly, synthetic chemical compounds are more effective compared to natural extracts, but the lack of knowledge about natural substances is too great to compare them fairly: only a small fraction of the 250,000 plant species has been studied properly [91,102].
Every day, there is increasing evidence about the consequences of using synthetic antiparasitics and insecticides, including their role in environmental pollution, their residual presence in foods and feeds, and most of all their function in developing resistance in parasites and insects [10,33,[39][40][41]85].
The Amazon Rain Forest is one of the most megadiverse places in the world, with more than 50,000 plant species, including at least 14,000 seed plants [13]. For instance, more than 250 species are used by indigenous communities as medicinal treatments in just a small corner of the forest [101].
For the same reason, there are thousands of studies that aim to define the proprieties of the EOs derived from this megadiversity, such as Viola surinamensis [57], Guatteriopsis species [20] and Lippia grandis [89].
Piper species belong to one of the largest genera of basal angiosperm [97] and are widespread in the tropical and subtropical regions of the world. They have a long history of use in traditional medicine and many studies have aimed to prove their efficacy. For instance, EOs from P. auritum showed promising results in the inhibition of promastigote proliferation in different species of the Leishmania genus [67], and EOs from P. hispidum showed high antileishmanial activity [43]. Also, P. cubeba EOs have anti-trypomastigote and anti-amastigote activity in Trypanosoma cruzi [24]. Various compounds from different species have been studied to assess these activities: dihydrochalcones from P. longicaudatum, where asebogenin (2 0 ,6 0 ,4-trihydroxy-4 0 -methoxydihydrochalcone) showed inhibitory effects against Staphylococcus aureus and methicillinresistant S. aureus (MRSA) [48]; prenylated hydroquinone from P. crassinervium with trypanocidal activity [56]; and eupomatenoid-5 isolated from leaves of P. regnellii which induces apoptosis in Leishmania amazonensis [36].
In this genus, we also find P. aduncum, which has a geographic range that extends mainly through the Neotropics, Southern Asia and the South Pacific [47]. It is a shrub or small tree up to 2-5 (À8) m tall with pubescent stems. Leaves up to 20 cm long and 5-9 wide, oblong-elliptic or lanceolate, scabrous on the upper surface and pubescent underneath, acuminate, base rounded or slightly lobed, alternate, distichous. Short petioles rarely up to 8 cm, pubescent. Prophylls up to 25 mm present. Inflorescence 5-17 cm, forming arching cream to green spikes on peduncles 8-15 mm long, sparsely pubescent. Floral perianth absent. Androecium with four stamens, anthers 0.2-0.3 mm long. Floral bracts 0.4-0.7 mm wide, triangular-round, densely yellow-white ciliate. Ovary with three stigmas. One-seeded berries 0.8-1 mm wide, obovoid, round from above, glabrous. Seeds reticulated [100] (Fig. 1).
Several ethnobotanical uses have been reported, such as the treatment of inflammation and cuts, skin irritations, "bone pains" and nasal haemorrhage. A tea made from its leaves is used to stop pulmonary haemorrhage, to treat tenesmus in women in labor, or to relieve menstrual colic. It is also mentioned as a folk remedy against kidney disorders, stomach ache, ulcers, rheumatism and skin infections; it is externally applied to relieve skin eruptions in babies. This same tea was described as a diuretic when mixed with corn silk tea. The ethanolic extract obtained from dry leaves was traditionally used to relieve headache and water infusion of buds to lower cholesterol. The entire plant has been mentioned for hot baths in order to help patients during their convalescence; moreover, the tea has been described as a traditional antihemorrhagic and a health tonic [6,22].
De Castro et al. provided evidence of the effectiveness of cardamonin as a schistosomicidal chalcone from P. aduncum extracts, inhibiting the ATP diphosphohydrolase of Schistosoma mansoni, the major aetiological agent of human schistosomiasis [14].
Many of the biological proprieties of P. aduncum EO have been studied, for instance its antibacterial and antifungal activity [68], which showed good results against the problematic agents of nosocomial infections such as Staphylococcus aureus, S. epidermidis and S. lentus [9]. Similarly, there are promising results in the prevention of infection in immunocompetent or immunocompromised patients for its activity against Cryptococcus neoformans [69]. There are also antioxidant [45], anti-inflammatory [74] and antiplatelet [38] activities, among others.
In order to carry out a systematic review, we collected all the studies regarding insecticidal, acaricidal and antiparasitic activity, mostly aiming to find a way to describe results which can be useful for an alternative or complementary strategy against leishmaniasis and malaria, considering that these diseases are still a great challenge in many countries from both the Eastern and Western tropical and subtropical regions [51].

Materials and methods
To assess the compound's activities, we selected articles from the electronic databases PubMed (https://pubmed.ncbi. nlm.nih.gov/), SciFinder (https://scifinder.cas.org), Science-Direct (https://www.sciencedirect.com/), ISI-Web of Science (http://apps.webofknowledge.com), SciELO (https://scielo.org/) and Google Scholar (https://scholar.google.com/). To manage bibliographic references, we used Mendeley software (https:// www.mendeley.com/). Most of the papers date from the last 20 years, but we also included some key data starting from 1948 to develop the introduction. To gather information, we only considered those articles regarding the use of the essential oil in its totality or the compounds derived from it: no semi-synthetic element's activity is shown in this review.
We collected all articles related, on the one hand to antiparasitic activity mainly focusing on the Leishmania genus, and on the other, to insecticide properties, most of all regarding the multiple studies on controlling malaria vectors, among others. And finally, we collected articles on acaricidal activity and the applications of this volatile oil in synergy with already known chemical insecticides.
To form the tables, we selected the following criteria: geographic distribution of the plant used for the extraction, parts of the plant used, method of extraction, main compounds found in the oil, type of application, and organism in which it was applied and activity that it produced in that organism, specifying the concentration used to obtain different ranges of the same activity (when not specified, we reported the effect of the minor time experiments).

Results and discussion
Geography and aim of the studies Using these criteria, we were able to collect 59 papers, in English, Portuguese and Spanish. As shown in Figure 2, most of the articles aimed to analyse the properties of the EOs of P. aduncum plants taken from the Amazon Rain Forest. We counted 39 from the Amazon, whereas only 8 were from the Atlantic Forest, and just 6 from both the Central America region (Cuba) and the Asian Tropics (Malaysia).
Most of these articles focused on insecticidal activity: we considered 43 experiments (60%) which aimed to determine this type of activity. We also counted 5, 19 and 4 experiments (7, 27 and 6%) regarding the acaricidal and antiparasitic activities and the synergistic effect as an insecticide, respectively (Fig. 3).

Extraction of the EO and its components
To obtain the essential oil, the leaves were the part of the plant mostly used in the experiments, sometimes together with fine stems. Regarding extraction methods, hydro-distillation (HD) by the conventional method using a Clevenger type apparatus was the most common, except in the experiments that defined the synergistic effect of the EO, which used steamdistillation (SD) to extract the oil. Gas chromatography was   the basic procedure used, mainly together with mass spectrometry, to analyse and determine its chemical properties.

Compounds of the essential oil
The chemical composition of the EO in the different studies mainly shows two large groups: phenylpropanoids and monoterpenes (Table 1).
According to Salehi et al. (2019) [84], the Piper genus is known to be a source of essential oils and Piper plants may contain different EOs from several organs and tissues such as seeds, leaves, fruits, branches, stems, and roots; more than 270 identified compounds have been found in Piper species. However, phytochemical studies on essential oils of different Piper species have pointed out high variability in chemical composition. As mentioned by Silva et al. (2017) [93], studies regarding essential oil composition of different Piper species found nine chemotypes characterised by 1,8-cineole, (E)-nerolidol, dillapiole and asaricin. Additionally, due to the resilient nature of the plants, the direct consequence of this biological approach is that the variability of secondary metabolism reflects the adaptation capacity of the plant to all external factors [16,61]. This chemical evidence concerning essential oil variability is highly relevant to identify specific chemotypes that could be useful in the modern food and health products market, even though variable composition is a frequent obstacle to their use. The present review, pointing out the main constituents reported by different research, sheds light on these aspects of variability within the species.
Dillapiole was almost always the main component characterising the oil, followed by myristicin. However, the articles highlight variable abundance mainly due to the different growth conditions and geographical origins, which inevitably affect the qualitative and quantitative profile of the phytocomplex. Dillapiole was the most cited component and the one that had the most promising proprieties, but importantly dillapiole showed better activities as a component of the entire EO than as an isolated compound. Due to this variability, dillapiole amounts ranged from 9.20% [94] to 94.84% [87], even though several studies reported that dillapiole represents more than the 70% of total essential oil composition, as reported in Tables 2-5. For EOs, variable composition is a frequent condition that makes it difficult to develop standardised batches. The choice and selection of specific chemotypes, the study of the balsamic period of the species and rigorous quality control on extraction, can partially mitigate the problem of variability, allowing production with reproducible quantities of the main components.
Like other phenylpropanoids, dillapiole is a result of the shikimic acid pathway [11] and several authors found biological activities of the isolated molecule or EOs rich in dillapiole. It can be found in a variety of plants, and various studies have aimed to define its uses or functions. These include its gastroprotective function when extracted from Peperomia pellucida [82], and its toxicity against the fungus Leucoagaricus gongylophorus in the control of fungus-feeding ants (Tribe Attini), an agricultural pest in the Neotropics [83]. It is a phenylpropanoid constituted by a benzodioxole with a methyl group in the aromatic ring and an alkyl group in the side chain. The structure of this organic compound is strictly related to its function and activities [74], and when we take a wider view on its synergistic function with some other compounds (Table 1), we observe that this same structure helps to amplify the effects of these chemical substances. This supports its potential when mixed, and not isolated. Finally, other articles cite the activity of semisynthetic derivatives of dillapiole against certain species of the genus Aedes [23,34], and show its cytotoxic effects against a variety of tumour cells [32].

Insecticidal activity
In Table 2, we show the results obtained in experiments on the insecticidal properties of the EO. We indicate the scientific names and how the insect is commonly classified. When specified, the concentration of dillapiole ranged between the 8.43%  (Continued on next page)  (Continued on next page)    [15] (Continued on next page)   to the 85.4% if the EO was used in its totality. Only four experiments did not show the presence of dillapiole: one of them used dried fruits to extract the EO, differing from most of the other experiments that mainly used leaves. Many papers focused on activity against Aedes aegypti, a mosquito vector of several viruses.
Concerning use of the EO, or its main compound in these experiments that isolated it from the rest of the components, a variety of methods were used: topical, with direct application to the insect using a specialized instrument regarding the type or stage of the insect; contact, allowing the insect to move through a surface where the EO was placed, or through the substance diluted in water; fumigation, mostly obtained by evaporation of the solution containing diluted EO; immersion, where the organism was directly submerged in the solution; and spraying. For the fumigation and contact experiments, residual effects were also often determined. Finally, activity was expressed in multiple forms, for instance adulticide, larvicide and ovicide effects, or repellency to oviposition. Almost all results expressed the lethal concentration (LC) of the EO that affected at least 50% of the insects tested. This concentration was also expressed as lethal dose (LD), lethal time (LT) or effective dose (ED). ED was related to the efficiency of repelling mosquitos from the human body. These measurements depended on how the researcher wished to express EO lethality. Considering for example the larvicidal effect in A. aegypti and where dillapiole was the main compound of the EO applied through contact or fumigation, we find that the LC 50 ranged between 57 and 200 lg/mL.
Finally, when not specified, we reported only experiments with a lowest duration as there was no significant difference between them: generally, the longer the experiment duration, the lower the concentration needed to obtain the same efficiency.
Piper aduncum EO appears to have promising properties in terms of insecticidal activity as it has been tested in a wide range of different insects and provided positive results. On the other hand, there are many studies that report this property for others EOs, for instance recently described activity of Foeniculum vulgare EO extracts against certain aphid species [75]. These findings support the possibility of discovering suitable substitutes for chemical insecticides. Table 3 shows the properties of the EO as an acaricidal and, like in Table 1, we report both classification and scientific name. We were able to collect only five experiments, three of them regarding the effects against Tetranychus urticae, a species of plant-feeding mite that is considered as a pest.

Acaricidal activity
The concentration of dillapiole ranged between 94.84% and 28%, and in just one of the experiments using plants of P. aduncum from Cuba, dillapiole was not present as a component of the EO. The application methods were similar to those used in the insecticide experiments.
There are fewer data on the acaricidal effects, but they still show promise regarding the control of pests in a wide variety of crops and cattle. In this field, we also found many studies on the use of the EOs as acaricides, for instance extracts of Lippia gracilis against Tetranychus urticae [8] or Ocimum gratissimum against Rhipicephalus microplus [95].

Antiparasitic activity
In Table 4, we report antiparasitic activities, specifying the scientific name of each organism. Articles have been published concerning the toxicity of the EO against various species, such as Haemonchus contortus and Hysterothylacium sp., whereas most of the experiments aimed to determine activity against the genus Leishmania, the protozoa responsible for leishmaniasis, followed by Trypanosoma, which causes Chagas disease. Lastly, a number of studies were related to Plasmodium falciparum, suggesting that it may be more likely to overcome malaria using essential oils against its vector (Table 2), rather than approaches against the protozoa that causes the disease.
When specified, the concentration of dillapiole was always higher than 75%. In this case, the EO was mostly applied through incubation in which it was added to the infected cell cultivation and placed in the best conditions (usually 37°C) for protozoan growth, for at least 24 h. Efficiency was mostly expressed as activity against promastigotes. For instance, in the case of Leishmania, to inhibit the growth of at least 50% (IC 50 ) of promastigotes, about 15 lg/mL of the EO with dillapiole are needed as the main component.
Antiparasitic activities, mostly antileishmanial effects, represent the second largest group of results, and are supported by other studies showing these properties of various EOs, such as Cryptocarya aschersoniana EO [2] and others. Table 5 shows the activity of dillapiole in synergy with other chemical substances used as insecticides to determine potential efficacy against Spodoptera frugiperda, a caterpillar known as the fall armyworm, considered a pest due to the damage it causes to a wide variety of crops and the associated economic losses.

Synergistic activity
In these experiments, two methods of application were used to determine the larvicidal effect: in the first case, they applied the solutions topically to the dorsal side of the larvae, whereas in the second, to verify the residual potential of the synergistic combination, they soaked filter paper in the substance, let it dry and placed it in a Petri dish where the larvae were then situated for 24 h. The concentration of dillapiole was always 71.9% and one of the chemical compounds that was used by three of the four experiments was cypermethrin and its different forms. Larvicidal activity was systematically tested. These results reinforce the conclusions pointed out by various authors [5,12] concerning synergistic activity.

Conclusions
Clearly, the Amazon Rain Forest still harbours a huge amount of knowledge. Articles have described the proprieties of just some of the species from this region, and of these, thousands concerned EO properties.
In this review, the authors aim to increase interest in this useful species and its EO, and in a wider context, to highlight the possibility of finding natural substitutes for chemical biopesticides.
EOs are a promising natural alternative as insecticides, acaricides and antiparasitic products, but researchers and industry must address several challenges in order to obtain new commercial products. EOs are recognised as ecofriendly, biodegradable and cost effective raw materials [17] and generally present very low mammalian toxicity and short environmental persistence [80]. Unfortunately, precisely due to their short persistence, EOs exert a short duration of action and require several applications or additional efforts on formulation strategies. The most common formulation strategy is to add chemical fixatives or stabilizers to the EO, in order to prolong effects. Fixatives make it possible to slow evaporation of EOs due to their lower volatility in comparison with EOs compounds. Promising results with natural (vanillin) and synthetic fixatives (Glucam P-20, Fixolide) have been reported by Songkro et al. (2012) [98], with several repellent mosquito formulations. The presence of the fixatives was able to improve repellent activity and the longevity of formulation efficacy.
Concerning the mode of action, EOs are generally able to affect insects and mites through neurotoxic effects involving the inhibition of acetylcholinesterase and an effect on the octopamine synapses and GABA receptors. In particular, due to the presence of methylene dioxy rings, dillapiole may inhibit insect P450 cytochrome activity. This mechanism affects the phase I metabolism of xenobiotics, which is responsible for inactivating the insecticides. EOs also exert repellent activity, blocking the odour receptor neurons (ORNs) of the insects [17,33,80].
As reported by Misni et al. (2011) [63], a general approach to the understanding of the insecticidal mode of action of EOs should be similar to that for pyrethroids. After initial excitability, the insects lose the control of their movements and present convulsions, paralysis and ultimately, death.
Dillapiole seems to be the most valuable compound among those forming this EO and for the same reason should be extensively studied. Even though essential oils may be a source of allergy in humans, dillapiole has not been added to the list of 26 fragrance ingredients listed as allergens in Annex III of the European Union's Scientific Committee on Consumer Safety Opinion on Fragrance allergens in cosmetic products (SCCS/1459/11). Interestingly, a study performed by Aciole et al. (2013) [1], on dillapiole identified genetic toxicity on somatic cells of Drosophila melanogaster. Hsuuw and Chan (2015) [44] investigated the effects of dillapiole on mouse oocyte maturation, showing a potential teratogenic effect. The authors suggest that the effect of dillapiole on human oocytes should also be investigated in order to assess the safety of its application as a drug or biocide. These findings indicate that dillapiole should be studied further to determine its possible toxicity in humans. For instance, it would be useful to evaluate its effects in synergy with other compounds to find the most useful match, as it has been shown that the oils are more effective when used in their totality than just in main component form. Other compounds are also of interest based on the data shown in the tables, as in some cases they are even the main compound instead of dillapiole, for example myristicin or 1,8-cineole. Therefore, a more specific study of these isolated components is required to have a wider understanding of the entire EO.
In addition, it is important to define the best way to investigate P. aduncum EO or dillapiole in open field studies or infested areas, in order to develop new phytoiatric formulations, such as nanoemulsions or insecticidal sugar baits, which can be successfully adopted as alternative commercial biopesticides. With the same approach, deeper pharmacological investigations that include formulation development should be performed in order to obtain new plant-based antiparasitic drugs.
We also need to understand the consequences of using P. aduncum EO as a biopesticide, such as its effect on the environment and its residual phytotoxicity, among others.
The proprieties and effects of this EO must be better analysed and clarified, as the results highlighted in this review are quite promising. Finally, further studies could focus on finding a semisynthetic product from this EO, which would be more economically practical and, at the same time, place lower demands on natural resources.