Assessment of the Potential of the Invasive Arboreal Plant Ailanthus altissima (Simaroubaceae) as an Economically Prospective Source of Natural Pesticides

Abstract

The extensive use of pesticides may negatively affect human health. Additionally, it is one of the main reasons for the decline of pollinators and is thus a hazard for most crops and biodiversity as a whole. Good candidates for the replacement of pesticides with ones less toxic to humans and pollinators are natural products (bioactive compounds extracted from plants), even though it should be kept in mind that some of them can be toxic too. Ailanthus altissima (Mill.), swingle, known also as tree of heaven, (Simaroubaceae) is one of the most aggressive alien invasive plants. It demonstrates a high tolerance to various habitat conditions and a potent propagation ability. This plant has a prominent ability to suppress the seed development of local vegetation. The aim of this review study is to summarize the potential of this plant for use as a natural pesticide, starting with ethnobotanical information. The essential oils extracted from A. altissima with its main components α-curcumene α-gurjunene, γ-cadinene, α-humulene, β-caryophyllene, caryophyllene oxide, germacrene D, etc., have been reported to possess different activities such as insect repellent, insecticidal, and herbicidal activity. Additionally, polar extracts and particularly quassinoids, the phenolic constituents of A. altissima leaves, are potent phytotoxins and fumigants. The basic extraction protocols are also summarized.

1. Introduction

Pesticides are a broad group of heterogeneous chemicals. They are toxic substances used to kill, prevent, or control pests such as insects and other animals, plants/weeds or fungi that harm crops, ornamental plants, stock, or, humans. In addition, they are considered to have public health benefits by increasing food productivity and decreasing food-borne and vector-borne diseases/infections caused by bacteria, fungi, or other pathogens [1,2]. All pesticides interfere with normal metabolic processes in the pest organism and are often classified according to the type of organism they are intended to control (e.g., herbicides; insecticide; fungicide; fumigant) [2]. However acute, high-dose pesticide exposures have been known for decades to cause clinically obvious and sometimes fatal poisoning. Moreover, the subclinical toxicity with a wide range of asymptomatic effects at levels of exposure too low to produce overt signs and symptoms should not be underestimated—they can cause cancer, cardiovascular dysfunctions, neurodegenerative disorders, etc. [1,3,4,5,6,7,8,9], and children are particularly at risk [1,8,10,11].

According to The Food and Agriculture Organization, “it is estimated that the value of pollination services to global food production is worth up to USD 600 billion annually” [12]. However, there is a great deal of evidence for pollinators’ global decline [13,14,15,16,17,18,19,20,21,22,23,24,25,26,27]. One of the biggest issues besides habitat destruction, the loss of floral resources, and emerging diseases is the negative impact of pesticides, particularly neonicotinoids, with more than 19,000 scientific references addressing these environmental threats [19,20,28,29,30,31,32,33,34,35]. Herbicides and fungicides such as glyphosate, metolachlor, oxadiazon, prochloraz, propiconazole, etc., have been found to harm pollinators [29,32,36,37,38,39,40,41]. The essential elements of an effective pollinator conservation policy have been summarized and the approach is holistic and based on scientific knowledge [42,43].

To reduce the harm of the pesticides in use, it is necessary to find a way to replace them with ones less toxic to humans and pollinators. Good candidates for this are natural products—bioactive compounds obtained from plants. Of course, this should be approached with caution. It is well known that many poisons have a vegetal origin. It is important to discover the ones that have selective activity. This requires an approach in two steps. The first step is finding the pesticide activity of the natural products. The second step involves tests for safety.

Two groups of natural products deserve attention for their possible roles as biopesticides. Some plant essential oils (e.g., Thymus serpyllum, Origanum majorama, Alpinia conchigera, Zingiber zerumbet, Curcuma zedoaria, Achillea vermicularis, and A. teretifolia) repel insects and have contact and fumigant insecticidal actions against specific pests [44,45,46]. These actions are attributed to the compounds amphene, camphor, 1,8-cineole (eucalyptol), terpinen-4-ol, isoborneol, α-humulene, α-pinene, β-pinene, and (−)-α-bisabolol [44,46,47,48]. Additionally, essential oils are considered potential bio-herbicides, with different and selective herbicidal mechanisms in comparison to the synthetic herbicides [49,50,51,52,53,54,55] as they are active against germination and early radicle growth at different levels [55]. The high presence of oxygenated monoterpenes (β-pinene, limonene, p-cymene, carvone, carvacrol, etc.) is related to potent phytotoxic activity [55] as well as to the α-pinene and 1,8-cineole [47,51,56]. In addition, the active phenolic monoterpenoids carvacrol and thymol have been suggested as alternative pesticides, herbicides, and insecticides [52]. Many studies on the various quassinoids (isolated compounds) from different genera have revealed the promising pesticide potential of this class of compounds [57,58,59].

The search for a replacement of pesticides is worth being conducted among alien invasive plants firstly because they are inexpensive and abundant sources of bioactive compounds, and secondly because they obviously have the phytochemical equipment to suppress the local vegetation and resist pests. Ailanthus altissima (Mill.) Swingle, the tree of heaven (Simaroubaceae), is a hard to control alien, aggressive, and invasive woody plant species [60,61,62,63,64,65,66,67,68,69,70]. The plant is native to northern and central China and has turned into a noxious weed in Europe, America, Australia, and other parts of the world where it has been introduced. [61]. Particularly, A. altissima is considered the most invasive alien species in Europe together with Ambrosia artemisiifolia L. and Robinia pseudacacia L. [61], which negatively affects the local biodiversity [68,70]. The tree of heaven not only outcompetes the local plants but also suppresses their seed germination and seedling development [71]. Additionally, it is less attacked by herbivorous insects [61,62,66].

The aim of this review study is to summarize the potential of A. altissima for use in natural pesticides through the following methods: (1) by summarizing the ethnobotanical data for pesticide activity reports, (2) by identifying the groups of compounds with pesticide potential, and (3) by summarizing the extraction protocols for each of the compounds’ groups in order to further enhance the optimal extraction protocols’ designs.

2. Material and Methods

In 2019–2022, we accessed Google Scholar, Web of Science, and PubMed to identify publications with the search strings: “Ailanthus altissima”, “ethnobotany”, “traditional”, “quassinoids”, “essential oil(s)”, “fumigant”, “insect repellent”, “juglone index”, “phytotoxic”, “insecticide”, “insecticidal”, “herbicide”, “herbicidal”, “fungicide”, and “antifungal”. No particular restriction was considered for the search strategy, such as publication language or publication year. The results of the search were publications primarily in the English language, and they covered the period from 1980 to 2021. Following the PRISMA 2000 guidelines, the records were assessed for eligibility and the inappropriate ones were excluded (namely, 160 studies were included in the review; the excluded ones were 35 studies that did not fit to the review topic and 2 that were not reliable).

We focused on the quassinoids and the essential oils for two reasons: firstly, these groups of compounds are known for their insect repellent, fumigant, fungicidal, and phytotoxic potential, and secondly, quassinoids are the most prevalent constituents in genus Ailanthus [72,73,74,75,76,77,78]. The aggressively invasive behavior of A. altissima suggests the promising potential of this plant for future pesticide formulations.

3. Results and Discussion

3.1. Ethnobotanical Data about Ailanthus altissima (Mill.) Swingle

Ethnobotanical information is usually focused on the medicinal properties of plants. Therefore, information regarding the pesticide potentials of plants is valuable but scarce. For invasive plant species, ethnobotanical records are collected in their native ranges of distribution. The local human populations in these regions have established traditions in the application of such plants. The bark of Ailanthus altissima (臭椿 chou chun) was initially recorded in Xin Xiu Ben Cao, a renowned traditional Chinese medicine monograph [79]. The information within this book relates that besides the many others therapeutic effects of A. altissima, the bark of the plant was used as an insecticide [79]. A. altissima plant materials were often used in ancient China against insect predators of stored grains [80]. The traditional use of A. altissima in Chinese medicine represents the starting point for scientific research seeking evidence of such pharmacological activities, and in this particular case, its potential pesticidal effects.

3.2. Chemical Constituents of Ailanthus altissima and Extraction Methods

A. altissima contains various secondary metabolites such as alkaloids, terpenoids, flavonoids, essential oil, etc., with a wide range of pharmacological effects such as anti-cancer, anti-inflammatory, anti-protozoal, etc. [79,81,82,83,84,85,86,87,88,89,90,91,92,93]. For instance, extracts of A. altissima stems containing ailanthone possess antiplasmodial activity against Plasmodium falciparum P. berghei [94,95]. An interesting new discovery is the antifungal effect of the alkaloid canthin-6-one isolated from A. altissima against Fusarium oxysporum f. sp. cucumerinum [96].

Here we focus on the quassinoids and essential oils as potential biopesticides since there is an indication that these groups of compounds have such effects [9,10,11,12,13,14,15,16,17,18].

3.3. Essential Oil of Ailanthus altissima: Composition and Extraction Overview

The qualitative and quantitative compositions of A. altissima essential oil vary considerably. This variability depends on the plant populations/ecological factors, the extractable parts, the ontogenesis stage, and the drying process. The main components are α-curcumene, α-gurjunene, γ-cadinene, α-humulene, β-caryophyllene, caryophyllene oxide, germacrene D, etc. [83,97,98,99].

The extraction methods are summarized here. The collection of the materials for A. altissima essential oil extraction may take place in the summer in Tunisia [97,98] or in September in Croatia [83]. The extraction of essential oil is a technological challenge as our own experience revealed (unpublished data). Basically, the essential oil of different plant parts (roots, stems, leaves/young and old plants, flowers, and ripe fruits, all cut into small pieces) is extracted by hydrodistillation for 3–4 h using a Clevenger-type apparatus [83,98] or a simple laboratory Quick-fit apparatus [97]. The identification of the components is performed by GC-FID and GC/MS analyses.

Additionally, the essential oil of A. altissima bark was extracted by the Soxhlet method with anhydrous diethyl ether until the distilled liquid became colorless. The solvent was evaporated under a vacuum in a rotary evaporator and the fumigant activity was tested against four major stored grain insects [100].

3.4. Quassinoids Extraction, Fractionation, and Isolation Overview

Quassinoids are all-chair cyclic and highly oxygenated derivatives of squalene. Biogenetically, they can be regarded as the degraded triterpenoids, which are isolated exclusively as bitter principles from plants of the Simaroubaceae family [101].

A. altissima is rich in quassinoids (

Table 1. List of isolated quassinoids from A. altissima.

Compound		CAS Registry Number	Plant Material	Contents or Obtained Amount mg/g Dry Weight	Ref.
1	2-dihydroailanthone	not assigned	Bark	0.027	[103]
2	6α-tigloyloxychaparrin	75144-71-7	Root bark	0.003	[104]
3	6α-tigloyloxychaparrinone	69423-70-7	Seedling	0.017	[104]
4	11-acetylamarolide	29913-88-0	Bark, seed	0.018	[104]
5	12-dihydroisoailanthone	n. a	Bark	0.080	[103]
6	13,18-dehydroglaucarubinone	68703-94-6	Root bark	0.124	[104]
7	Ailanthone	981-15-7	Root, seed, leaves	0.003–0.05	[103,105]
8	Ailantinol A	176181-83-2	Aerial parts	0.007	[72]
9	Ailantinol B	177794-39-7	Stem bark	0.002	[72]
10	Ailantinol C	n. a	Stem bark	0.002	[73]
11	Ailantinol D	n. a	Stem bark	0.0005	[73]
12	Ailantinol E	n. a	Root bark	0.0004	[74]
13	Ailantinol F	n. a	Aerial parts	0.0004	[74]
14	Ailantinol G	n. a	Aerial parts	0.0007	[74]
15	Ailantinol H	n. a	Aerial parts	0.0002	[106]
16	Altissinol A	n. a	Bark	0.001	[104]
17	Altissinol B	n. a	Bark	0.003	[104]
18	Amarolide	29913-86-8	Bark, seed	0.001	[104]
19	Chaparrinone	22611-34-3	Root bark	0.002	[104]
20	Chaparrolide	33512-38-8	Bark	0.003	[104]
21	Δ13−18-dehydroglaucarubolone	n. a	Seed	0.0002	[72,104]
22	Glaucarubin	1448-23-3	Stem bark	0.003	[104]
23	Glaucarubinone	1259-86-5	Seed	n. a	-
24	Glaucarubol	1448-22-2	Stem bark	n. a	-
25	Isoailanthone	n. a	Root bark	0.0002	[103]
26	Shinjudilactone	80180-30-9	Seed	0.003	[107]
27	Shinjuglycoside A	n. a	Seed	0.012	[108]
28	Shinjuglycoside B	n. a	Seed	0.044	[108]
29	Shinjuglycoside C	n. a	Seed	0.005	[108]
30	Shinjuglycoside D	n. a	Seed	0.002	[108]
31	Shinjuglycoside E	112667-45-5	Root bark	0.0002	[109]
32	Shinjuglycoside F	112667-46-6	Root bark	0.00005	[109]
33	Shinjulactone A	89353-91-3	Seed	0.002	[105]
34	Shinjulactone B	80648-28-8	Aerial parts	0.001–0.004	[110]
35	Shinjulactone C	82470-74-4	Root bark	0.001	[107]
36	Shinjulactone F	n. a	Root bark	0.003	[111]
37	Shinjulactone G	n. a	Root bark	0.0003	[112]
38	Shinjulactone H	n. a	Root bark	0.001	[112]
39	Shinjulactone I	n. a	Root bark	0.0002	[111]
40	Shinjulactone J	n. a	Root bark	0.0001	[111]
41	Shinjulactone K	94451-22-6	Root bark	0.0005	[111]
42	Shinjulactone L	n. a	Root bark	0.0005	[113]
43	Shinjulactone M	n. a	Root bark	0.0005	[114]
44	Shinjulactone N	n. a	Root bark	0.0002	[114]
45	Shinjulactone O	n. a	Root bark	0.001	[115]
46	Chuglycoside A	n. a	Seed (samara)	0.003	[116]
47	Chuglycoside B	n. a	Seed (samara)	0.014	[116]
48	Chuglycoside C	n. a	Seed (samara)	0.024	[116]
49	Chuglycoside D	n. a	Seed (samara)	0.001	[116]
50	Chuglycoside E	n. a	Seed (samara)	0.145	[116]
51	Chuglycoside F	n. a	Seed (samara)	0.002	[116]
52	Chuglycoside G	n. a	Seed (samara)	0.001	[116]
53	Chuglycoside H	n. a	Seed (samara)	0.0005	[116]
54	Chuglycoside I	n. a	Seed (samara)	0.032	[116]
55	Chouchunlactone A	n. a	Root bark	0.0001	[90]
56	Chouchunlactone B	n. a	Root bark	0.0003	[90]
57	Chouchunlactone C	n. a	Root bark	0.0007	[90]
58	Chouchunlactone D	n. a	Root bark	0.0002	[90]
59	Chouchunlactone E	n. a	Root bark	0.0002	[90]

, Figure 1) and the process of the identification of new quassinoids is still progressing [90]. The concentration of ailanthone, one of the main quassinoids, may range from 6.44 µg/mL to 825 µg/mL, depending on the source locality in China [102].

Different extraction and isolation procedures have been developed according to the chemical nature and class of the quassinoids. Many of the quassinoids are categorized as non-polar or low polar compounds. However, a significant number of polar quassinoids have been reported as well. The extraction approach is performed either using polar, semi-polar, or non-polar solvents. The polar group of solvents includes hot methanol, hot water, ethanol, or similar ones [72,73,74,103,104,105]. As example of a non-polar solvent that is used is hexane [106,107]. Generally, the procedures include the solvent partitioning and solid-phase fractionation. In most cases, during the solvent partitioning, quassinoids concentrate in the semi-polar solvent (e.g., dichloromethane, chloroform, and ethyl acetate) [105,108]. In the solid-phase extraction and isolation methods, various stationary phases such as silica gel or/and C-18, C-8 (reverse phase) are used [109,110]. A wide range of solvent mixtures have been used as mobile phases with varying polarities, although a rising polarity gradient was often considered for future separation. Methanol in ethyl acetate (with an increasing methanol percentage), methanol in chloroform, and methanol in acetone are some of the most popular eluents [57,114].

Many of the quassinoids could be formed as crystalline matters. Hence, quassinoids are purifiable phytochemicals [111,112,113].

For the phytotoxicity and larvicidal tests, fresh leaves were cut into pieces, soaked in methanol in a glass container, kept at room temperature (25 °C) for 72 h, were filtered, and then the methanol was evaporated [117]. For the fumigant and phytotoxicity bioassays of the quassinoids, the extracts are prepared as follows: the roots and leaves are extracted separately at room temperature—at a dose of 10% w/v—successively with solvents of increasing polarity [petroleum ether, chloroform, chloroform: methanol (9:1), methanol and water]. The aqueous leaf extract, more active in bioassays, is fractionated in H₂O:BuOH. The n-butanol extract, which shows activity in the preliminary bioassays, is dissolved in methanol and 2 g of this extract is fractionated by gel-permeation chromatography on a Sephadex LH-20 column, eluting with MeOH [57].

3.5. Biopesticide Potential of Ailanthus altissima and Tests’ Design

3.5.1. Phytotoxicity Assay of Ailanthus altissima

Essential Oil Phytotoxicity

The essential oils of A. altissima negatively affect the seed germination and early-stage development of the seedlings of the target species. The effect is dose-dependent and is greater in the light than in the dark. In addition, the phytotoxic effect depends on the origin of the essential oil, as the oil extracted from flowers is the most phytotoxic [97,98]. The caryophyllene oxide, b-caryophyllene, germacrene D, and hexahydrofarnesyl acetone presented in the essential oil may be responsible for such a phytotoxic effect [98,118,119]. Additionally, the complete inhibition of the germination of target plants is achieved after the application of 400 to 600 μg/mL hydrodistilled leaf residues [97].

Phytotoxicity of Polar Ailanthus altissima Extracts

The juglone index [120] of A. altissima has been assessed as very high (0.80–1.40 depending on the extract concentration [121,122,123]. The plant produces allelopathic substances that inhibit the seed germination and seedling growth of competing species. They are located mostly in the bark and the roots, but also occur in the leaves, seeds, and wood. The inhibitor(s) can readily be extracted from A. altissima with methanol, but not dichloromethane, indicating the plant’s polar characteristics. The experimental tests show “striking” postemergence effects, with a nearly complete mortality of all the receiver plant species [124].

The compounds of the methanolic extracts from A. altissima’s fresh leaves and some sub-fractions have strong inhibitory effects on plant growth. Some fractions show a regulatory effect on plant by inhibiting the growth of radicles at higher concentrations and enhancing their growth at lower concentrations [117]. The compounds of the aqueous extracts from A. altissima’s fresh leaves and bark negatively influence the growth of the treated seedlings of Sinapis alba L. and Brassica napus L. regardless of the dilution [125]. The aqueous extracts of A. altissima leaves have a concentration-dependent herbicidal effect on Medicago sativa L. seed germination [126].

Ailanthone is highly phytotoxic, with concentrations of 0.7 mL/L causing 50% inhibition of radicle elongation in a standardized bioassay with garden cress (Lepidium sativum L.) seeds [127]. The quassinoids (from the root bark of A. altissima), e.g., ailanthone, ailanthinone, and ailanthinol; the alkaloids such as 1-methoxycanthin-6-one; and the phenolic constituents of the leaves are potent phytotoxins [57,97,128,129,130,131,132]. A significant pre-emergence herbicide activity is found for most of the bark dichloromethane extracts, which is directly correlated with the ailanthone concentration. A remarkable combined pre- and post-emergence herbicidal activity was found for a specific fraction. These results indicate that the bark of A. altissima is a potential source for the production of natural herbicides for use in agriculture [133]. Methanol bark extract with the main component ailanthone was tested for herbicidal effects under field conditions. The results show that it was quite efficient against the weeds but also caused serious injuries to the crops. Thus, a weakness of ailanthone is its non-selectivity, but a positive feature lies in its ephemeral effects. Ailanthone is easily degradable by soil microorganisms [126,134]. It is necessary to note, however, that ailanthone is an acute toxic triterpene and should be used with caution [135].

3.5.2. Antifungal Activity

The antifungal activity test results are contradictory and depend on the extraction methods and reagents. The methanol and ethanol A. altissima leaves’ extracts have fungicidal activity only against Cladosporium cladosporioides of all the tested nine species belonging to Fusarium, Penicillium, Aspergillus, and Giberella—the toxic microfungi found in cereals used for livestock and human food. However, this activity is weaker compared to the Juglans regia leaves’ extracts [136]. Ethanol, methanol, and aqueous extracts of A. altissima were tested against Ceratocystis manginecans (the causal agent of Mango Sudden Death) using a poisoned food technique and the treatments result in thin, collapsed/damaged hyphae compared to the control. Phytochemical profiling of the most effective extracts revealed that 9-octadecanoic acid and I-(+)- ascorbic acid 2, 6-hexadecanoate possibly contribute to the antifungal effect [137]. Both acetone and methanol from the leaves’ extracts have activity against Candida albicans, which is higher than amphotericin B, a gold standard in antifungal therapy [87]. Although C. albicans is not a crop pathogen, the result shows that further antifungal activity is worth testing. The chloroform extract of Ailanthus excelsa stem bark shows fungistatic and fungicidal activity against Aspergillus niger, A. fumigatus, Penicillium frequentence, P. notatum, and Botrytis cinerea [138]. It is the quassinoids that have been found to have inhibitory activities against plant fungal pathogens [139].

3.5.3. Fumigant and Insect Repellent Activity

Essential Oil Fumigant and Insect Repellent Activity

The essential oil of A. altissima bark has a fumigant activity against some pest beetles. One possible application of A. altissima bark essential oil is for killing insects that damage stored foods or seeds, as it causes 99.3 and 81.9% mortality to Oryzaephilus surinamensis (Linnaeus) (Coleoptera: Silvanidae) and Sitophilus oryzae (Linnaeus) (Coleoptera: Curculionidae) with within 24 h, respectively [80,140,141]. In addition, Lü and his co-workers revealed that despite its weak fumigant activity against Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae) and Liposcelis paeta Pearman (Psocoptera: Liposcelididae) adults, it notably repels T. castaneum adults and L. paeta nymphaea [80,140,141]. Additionally, A. altissima bark oil possesses high fumigant activity against Lasioderma serricorne (Fabricius 1792) (Coleoptera: Anobiidae) adults with a mortality of 100% at 8 µL/L air within 48 h of exposure; thus, it is obviously a strong repellent of these pests [142]. (Z)-3-hexen-l-ol, which is one of the main components of the essential oil extracted from A. altissima stems [97], is known as a key herbivore-induced plant volatile. There is no doubting its role as an indirect defense and this compound is a good candidate for novel insect pest control strategies [143]. Additionally, caryophyllene and caryophyllene oxide, which are the main constituents of the essential oil of A. altissima leaves and samara [97], are attractive to green lacewings [144]. Green lacewing larvae are predators of many soft-bodied insect pests such as: aphids, thrips, whiteflies, leafhoppers, spider mites (especially red mites), and mealybugs, and consequently they participate in biological control [145]. Caryophyllene and caryophyllene oxide stimulate oviposition in green lacewings, which leads to increased larval predation against pest insects [144]. A. altissima contains compounds with strong acaricidal activity against the parasitic mites that cause skin disease, namely, Psoroptes cuniculi and Sarcoptes scabiei var. cuniculi [146]. It was also found to have activity towards nematodes of the Meloidogyne genus [147].

Polar Extracts’ Fumigant and Insect-Repellent Activity

The methanol extracts of A. altissima fresh leaves are practically non-toxic to the mosquito Aedes aegypti larvae [117] and the leaves are even used for feeding silkworms [148]. However, the methanolic extract of A. altissima leaves causes the malformation and mortality of the larvae of the moth Agrotis ipsilon, (Lepidoptera: Noctuidae), which are known to cause considerable damage to crops by severing young plants at the ground level. Aqueus extracts of A. altissima leaves have oviposition-deterrence effects against Spodoptera frugiperda (Smith) (Noctuidae), causing delays in the time to pupation and emergence in addition to reduced larval and pupal biomasses [149,150]. This moth is considered a noxious pest because the larvae cause massive damage to various crops; consequently, insecticide sprays are employed against it [151]. In addition, 0.5, 1, and 2% ethanol (70%) extracts of A. altissima bark and leaves have strong antifeeding activity against and significant insecticidal effects on gypsy moth (Lymantria dispar (L.)) larvae—insects known as voracious defoliating pests of deciduous trees.

The diethyl ether extract of A. altissima possesses an extremely strong repellent effect and to a certain extent a contact-killing effect on Oryzaephilus surinamensis (Linnaeus), the saw-toothed grain beetle [152]. The ethanol extract of A. altissima leaves possess strong acaricidal activity (97.4%) against the spider mite, Tetranychus urticae (Koch), a plant-feeding mite generally considered to be a pest [153]. The extract has no direct toxic effect on the pest but reduces its fertility about threefold and suppresses the development of larvae from eggs. The maximum efficiency of the extract was observed after 7–10 days when a filial generation of the spider mites started developing [154].

Quassinoids extracted both from leaves and roots have insecticidal, antifeedant, and insect-growth-regulatory activity, and ailanthone, in particular, was found to be efficient against the aphid Acyrtosiphon pisum [57]. There is a high mortality rate of aphids, pests of peas, when treated with ailanthone [155]. Methanol extracts or active substances such as ailanthone, chaparinone, glaucarubinone, and 13 (18)-dehydroglaucarubinone obtained from A. altissima leaves can be recommended for the development of new botanical insecticides targeted against the phytophagous larvae of Spodoptera littoralis, a moth referred to as the African cotton leafworm [156]. At the same time, quassinoids seems to be nontoxic for bees as they are found in propolis [57,134,157,158,159,160]. In addition, A. altissima bark-based hexane and methanol extracts do not possess any genotoxic, mutagenic, or carcinogenic effects on Saccharomyces cerevisiae, which was used as a test object to evaluate the potential harm to human health [161].

4. Conclusions

The essential oil and other extracts from A. altissima are quite promising as natural herbicides. Additionally, the essential oil and other tree-of-heaven compounds have potent fumigant activity. The essential oil and other extracts from A. altissima—as natural products—are biodegradable and possibly less harmful to human health and to pollinators. Of course, one should keep in mind that even natural products may have some toxicity; for instance, carvacrol and thymol aside from their efficacy cannot be considered completely safe. Even though the hexane and methanol extracts of A. altissima do not possess in vitro any genotoxic, mutagenic, or carcinogenic effects, further well-designed tests for both the pesticidal efficiency and toxicity in humans and pollinators of the essential oils and quassinoids obtained from this plant are required.

Ideally, effective extraction protocols for industrial yield should be developed so that both essential oils and quassinoids from A. altissima can be obtained as natural pesticides. They can help to reduce the use of synthetic pesticides and thereby their negative effects on wild pollinators and honeybees. Additionally, the intensified harvesting of this aggressive invasive plant species might contribute to decreasing their populations and reducing their destructive impact on natural habitats.