The development of pest management and control is striving toward a future of sustainable agriculture. Insect pests caused serious problems in agricultural ecosystems during cropping or after harvest. Pesticides have been a major contributor to the increase of agricultural productivity and food supply. Nevertheless, they are a source of concern because of human and environmental health side effects [1]. The undesirable side effects include: target pest resistance and/or resurgence, secondary pest outbreaks, food residue problems, environmental pollution, human toxicity and ecotoxicological risks. Consequently, growing concern about environmental protection, human health, and food safety has brought renewed interest in pesticide use in agriculture. The search for solutions to these problems led to the search and development of more effective alternatives, friendly to environment and respectful to human health. Subsequently, researches has been concentrated on plants for solutions leading to the production of a multitude of secondary compounds that can have toxic, growth reducing, and antifeedant properties against insects [2]. The use of plant extracts (botanical insecticides) to protect crops and stored products is as old as crop protection. Indeed, prior to the development and commercial success of synthetic insecticides beginning in the 1940s, botanical insecticides were major weapons in the farmer’s arsenal against crop pests [3]. Four major types of botanical insecticides are being used for insect control including pyrethrum, rotenone, neem, and essential oils [4].

Aromatic plants and their essential oils have been used since antiquity in flavor and fragrances, as spice, in medicines, as antimicrobial/insecticidal agents, and to repel insect or protect stored products [5-7]. Presence of volatile monoterpenes or essential oils in the plants provides an important defense strategy to the plants, particularly against herbivorous insect pests and pathogenic fungi [8]. These constitute effective alternatives to synthetic pesticides without producing adverse effects on the environment [5,9].

The interest in essential oils has regained momentum during the last decade, primarily due to their fumigant and contact insecticidal activities and the less stringent regulatory approval mechanisms for their exploration due to long history of use [4].

Several studies suggest that essential oils may help fend off insect pests [7,10-13]. In fact, essential oils may provide protection comparable to several widely used synthetic insecticides. Furthermore, essential oils from various plant species show important biological effects that include: cytotoxicity [14], phototoxicity [15], nuclear mutagenicity [16,17], cytoplasmic mutagenicity [18], carcinogenicity [19] and antimutagenic properties [20-22].

This mini-review presents researches conduced in Tunisia for assessing insecticidal effects of some essential oils extracted from various aromatic and medicinal plants. It also provides an overview of their chemical composition and their main bioactive components. This mini-review covers major research that analyzes essential oils as alternative approach to address issues of chemicals use replacement, as well as potential considerations of effective control using essential oils. The mini-review summarizes existing and ongoing research works on the valorization of essential oils against insect pests in Tunisia. Analysis is provided to relate essential oils applications to the larger context of Integrated Pest Management (IPM) strategies. This mini-review will recapitulate all essential oils achievement in terms of insect pests control; it will mainly focus on stored product pests.

This mini-review will focus on essential oils extracted from different Artemisia and Eucalyptus species. The choice was made on the base that the genus Artemisia is native to temperate and Northern Africa regions including Tunisia while Eucalyptus, Australian native genus, was introduced and well adapted in Tunisia. The white wormwood Artemisia herba-alba Asso. The absinthe wormwood Artemisia absinthium L. are native to northern Africa. They are common species widely distributed in the Mediterranean basin and in Tunisia. These species are valued because they are used in folk medicine, as spices and as flavoring agents. Moreover, Eucalyptus is Australian native genus that had been introduced and adapted in Tunisia since 1957. It had been used as fire wood, for the production of mine wood and against erosion. Furthermore, in this mini-review, insecticidal proprieties of essential oils were assessed against beetles and moths species of economic importance causing quantitative and qualitative losses to various stored products including dates, cereal products and various others food stuffs. On the other hand, the dried-fruit beetle, Carpophilus hemipterus L., the sawtoothed grain beetle Oryzaephilus surinamensis L., the red flour beetle Tribolium castaneum Herbst., the tropical warehouse moth Ephestia cautella Walker, the Mediterranean flour moth Ephestia kuehniella Zeller, the carob moth Ectomyelois ceratoniae Zeller and the Indian meal moth Plodia interpunctella (Hübner) are causing significant economic losses during storage in Tunisia. These insects are reported as major pests of stored commodities in Tunisia. These devastating insects cause loss of weight and downgrading of the commercial value of the products.

Bioactive essential oils from Eucalyptus species

This section highlights the chemical composition of essential oils as a source of variation of anti-insect properties. The main investigated oils were those from the genus Eucalyptus.

Table 1 reported chemical composition of respectively 10 Eucalyptus species namely: E. astringens, E. leucoxylon, E. lehmani, E. rudis, E. camaldulensis, E. dumosa collected from the arboretum of Korbous (north East Tunisia); E. transcontinentalis and E. dumosa collected from the arboretum of Sidi Ismail (Sahel of Tunisia); E. cinerea, E. maidenii and E. viminalis collected from the arboretum of Souiniet, Aïn Draham (north West of Tunisia). Results reported in Table 1 indicated that major and predominant components of their essential oil were: α-pinene, Phellandrene, β-pinene, 1,8-cineole, Cis-Ocimene, γ-terpinene, α-terpinolene, Linalool, Dehydro-pcymene, Isoamyl isovalerate, β-Thujone, Cis-Geraniol, 3-Methylene cycloheptane, Trans-pinocarveol, Trans-pinocarveol, Pinocarvone, Borneol, Terpinene-4-ol, Cyclohexanol, Linalyl propionate, α-terpineol, β-fenchyl alcohol, Trans-Piperitol, Cuminic aldehyde, Geraniol, Piperitone, Isothymol, Ethyl mesylate, Cuminyl alcohol, Methyl geranate, Bicycloelemene, Camphene, α-terpinene, Geranyl acetate, α-gurjunene, Bicyclogermacrene, Trans-caryophyllene, β-caryophyllene, Aromadendrene, γ-gurjunene, β-eudesmene, 1,4-Dimethyltertraline, Epiglobulol, Viridiflorol, γ-cadinene, Globulol, Ledene, β-eudesmol, Spathulenol, β-selinene, Isospathulenol, γ-selinene, t-muurolol, 4-Tert-Butylphenyl methyl ether, α-cadinol, Allospathulenol, Isospathulol, -costal, β-Oplopenone, β-Elemenone, Vitrenal, Isoaromadendrene epoxide, Acide n-Hexadecanoique, Acide 9-Octadecenoique and Heptacosane. Generally, these major components determine the biological properties of the essential oils. Additionally, results indicated that these major identified components in the 10 essential oils with pesticidal activity were in accordance with those extracted from various Eucalyptus species throughout the word [11,23-29]. Furthermore, our data provides a clear evidence that 1,8 cineole is the major constituent of the ten essential oils. Its relative proportions reached 82.82% for E. transcantinentalis, 79.44% for E. dumosa, 75.79% for E. viminalis, 75.64% for E. maidenii and 67.22% for E. cinerea. Such results were reported by Duke [30] who indicated that among the various components of Eucalyptus oil, 1,8-cineole is the most important one and is a characteristic compound of the genus Eucalyptus, and is largely responsible for a variety of its pesticidal properties.

Table 1: Percentage composition of essential oils from ten Eucalyptus species collected from different arboreta in Tunisia (Major and predominant compounds marked in bold form).
RI, KI were respectively Retention Index and Kováts Index calculated on a HP-5MS capillary column (30 m × 0.25 mm × 0.25 µm), tr: traces

Bioassays were designed to assess median lethal concentration LC₅₀ (dose that kills 50% of the exposed insects). Probit analysis [31] was used to estimate LC₅₀ values. Table 2 illustrated results of the ten essential oils reported on adults of different Tunisian strains of stored product insect pests.

Table 2: LC₅₀ values (µl/l air) of different essential oils extracted from different arboreta in Tunisia against various stored product insects [28,29,32,33].
LC₅₀ (µl/ air) values calculated for mortality within 24 h of exposure at 25°C (95% lower and upper confidence limits are shown in parenthesis).

Bioactive essential oils from Artemisia species

The Genus Artemisia L. comprises important medicinal plants which are currently the subject of phytochemical attention due to their biological and chemical diversity [34]. This genus contains over 500 species, which are mainly found in Asia, Europe and North America [35]. Le Floc’h, [36] reported that in Tunisia, the genus Artemisia contains six species: Artemisia arborescens, Artemisia atlantica, Artemisia campestris Artemisia herba alba, Artemisia inculta, and Artemisia vulgaris. The white wormwood, Artemisia herba-alba Asso is dwarf shrub that grows wild in arid areas of the Mediterranean basin, extending into northern Himalayas [37]. In Tunisia, A. herba-alba is growing wild in the southern arid zone [38]. Moreover, the wormwood or absinthe wormwood, Artemisia absinthium L., is a medicinal and aromatic bitter herb, which has been used as a medicine from ancient times [39]. Artemisia species essential oils have insecticidal or repellent properties [40-44].

In this section, we investigated the chemical composition of the essential oil from Tunisian A. herba-alba and A. absinthium (Table 3). In addition, we evaluated their fumigant efficacy, contact toxicity and repellency against the red flour beetle Tribolium castaneum (Herbst) and the sawtoothed grain beetle, Oryzaephilus surinamensis (L.) that are two of the most important beetle pests of stored products in Tunisia (Tables 4 and 5).

Table 3: Percentage composition of essential oils from two Artemisia species collected from north Tunisia (Major and predominant compounds marked in bold form).

Table 4: Lethal Concentrations LC₅₀ and LC95 (µl/l air) of two A. herba-alba and A. absinthium essential oils against adults of T. castaneum and O. surinamensis [47].
LC₅₀ and LC₉₅ (µl/ air) values calculated for mortality within 24 h of exposure at 25°C.

Table 5: Repellency (%) of A. herba-alba and A. absinthium essential oils against adults of T. castaneum and O. surinamensis after 24 h of exposure [47].

Major and predominant components from A. absinthium essential oil were: β-thujone (22.72%), Camphor (16.71%), 1,8 cineole (5.47%), Camphene (2. 37%), Borneol (1.77%), Pinocarvone (0.94%), Benzyl bromoacetate (0.71), m-cymene (0.63%) and Sabinyl acetate (0.43%). While, for A. herba-alba essential oil, major and predominant components were: 1,8-cineole (19.59%), Camphor (11.48%), α-pinene (5.4%), Camphene (2.63%), Borneol (2.29%), β-pinene (1.64%), α-terpineol (1.09%), Caryophyllene (0.99%), Bornyl acetate (0.72%) and m-cymene (0.48%).

Results reported in Table 3 showed similarity regarding chemical composition of the two essential oils. Results indicated quantitative and qualitative variations in the composition of the essential oils were observed. Results in Table 3 clearly demonstrated that both oils were rich in compounds known to possess insecticidal activity. Moreover, our data confirms previous works related to chemical composition of both essential oils [39,45,46].

Results demonstrated that for fumigant and contact tests, essential oils were toxic against the two beetles. Regarding, fumigant toxicity, results indicated that A. herba-alba oil was more effective against both insects compared to A. absinthium oil. This result could be attributed to its chemical composition. Indeed, highest percentages of α-pinene, β-pinene, 1.8-cineole and α-terpineol in A. herba-alba oil compared to A. absinthium oil (Table 3) conferred it best insecticidal potential against the two tested insect species. Indeed, the pesticidal activity essential oils has been due to various components such as 1,8-cineole, α-pinene and α-terpineol [27,48,49]. Concerning contact toxicity, results showed that the sawtoothed grain beetle O. surinamensis was more susceptible than the red flour beetle T. castaneum. Additionally, A. herba-alba oil was also more effective than A. absinthium oil. To summarize, results indicated that both A. herba-alba and A. absinthium essential oils were toxic against adults of the two beetles. Our data support the use of Tunisian Artemisia essential oils as grain protectants for the management of stored insect pests.

Results reported in Table 5 showed that the highest repellent activity was recorded with A. absinthium essential oil. Indeed, repellency reached respectively 92.5 and 53.09% for T. castaneum and O. surinamensis at the dose 0.09 μl/cm² after 24 h of exposure. This important repellency could be attributed to chemical composition of this oil. In this respect, the components as β-thujone, chrysanthenyl acetate and sabinyl acetate were responsible for the repellent activity of A. absinthium essential oil [50]. In addition, several researches reported the repellent activity of A. absinthium oil various insects such as fleas, flies and mosquitoes [51,52]. All those results valorize Tunisian A. herba-alba and A. absinthium essential oils as sources of biological active compounds. Thus, these species might be good candidates for further investigations in developing new control approaches and can be used as natural bio-insecticides instead of more toxic synthetic pesticides. Moreover, further investigations on other Artemisia species would be of interest.

The biological proprieties of the tested essential oils were consistent with results from other scientific studies. Eucalyptus and Artemisia essential oils from Tunisia were successfully applied against various stored product insect pests. As indicated above, some changes in chemical composition occur, these changes were directly linked to plant factors (genotype, climate, age of plantation, soil,…). However, standard protocols for bioassays and a database for collecting and organizing bioactivity data would be useful for more comprehensive study on bioactive component distribution through the word.