In this study, the four essential oils tested showed remarkable insecticidal activity against adults of Sitophilus oryzae. And this varied between spontaneous and cultivated plants. The insecticidal activity of essential oils extracted from aromatic plants is the subject of much research with a view to reducing losses caused by insect pests on stored seeds.
According to Derradji-Heffaf (2013) 9, essential oils of Artemisia campestris and Thymus algeriensis tested on Sitophilus oryzae recorded 100% mortality rates after 72 h, with the largest dose being 16 µl, while Teucrium polium oil caused no mortality. This ineffectiveness is due to the low monoterpene content of this essential oil. The results obtained for Satureja essential oil showed interesting activity against the insects tested, and the strong insecticidal activity observed in Satureja EO is also confirmed by the work of Abbad et al. (2023)10. The same authors reported that EO obtained from wild Satureja was significantly more toxic, with LD50 = 1,988 µL/L and LD90 = 4,106 µL/L, than that obtained from cultivated plants (LD50 = 6,453 µL/L and LD90 = 17,625 µL/L). This result may be explained by the high sensitivity of the Sitophilus oryzae insect to the high concentration of the cyclic monoterpene pulegone present in wild plant oil (Lee et al., 2003, 11).
Mustafa ALKAN (2020) 12 reports that essential oils of all species except O. vulgare x O. onites (F = 150; Df = 4.24; P < 0.001) resulted in > 90% mortality. And that Sitophilus oryzae is highly sensitive to applied essential oils, with > 90% mortality recorded with essential oils of all species except O. vulgare x O. onites.
Benazzeddine (2010)13 reports that rosemary essential oil was the most effective of the oils tested. It produced a 100% mortality rate after 48 h of exposure to toxic vapors, followed by spearmint, which produced a 69.62% mortality rate after 24 h, reaching 100% mortality after 96 h of exposure. On the fifth day, 100% mortality was reached, whereas for thyme, this same rate was recorded only after 6 days of treatment. Thyme essential oil had a significant effect, with 65.38% mortality recorded after 48 h. In the same context.
Kordali et al. (2008) tested the insecticidal properties of essential oil isolated from Origanum acutidens on S. granarius and Tribolium confusum. This oil caused 68.3% and 36.7% mortality in S. granarius and T. confusum adults, respectively, after 96 hours of exposure. Thus, EO obtained from wild Satureja calamintha was significantly more toxic (LD50 = 1,988 µL/L and LD90 = 4,106 µL/L) than that obtained from cultivated plants. (LD50 = 6.453 µL/L and LD90 = 17.625 µL/L), this result may be explained by the high sensitivity of the Sitophilus oryzae insect to the high concentration of the cyclic monoterpene pulegone present in wild plant oil, as reported by Origanum acutidens on S. granarius and Tribolium confusum. This oil caused 68.3% and 36.7% mortality in S. granarius and T. confusum adults, respectively, after 96 hours of exposure.
Regnault-Roger and Hamraoui (1994) tested the efficacy of essential oils extracted from twenty-four aromatic plants of several families on the bruchid A. obtectus. The results showed that oils from seven plants of the Lamiaceae family (Thymus serpyllum, Origanum vulgare, Satureia hortensis, Lavandula angustifolia, Rosmarinus officinalis,Origanum majorana, and Ocimum basilicum) and Petroselinum sativum of the Apiaceae family were the most toxic, causing 100% mortality after 1 to 4 days' exposure and at low doses of 10-² µL/cm3.
As for the essential oils of Thymus vulgaris and Salvia officinalis (Lamiaceae), Laurus nobilis, and Cinnamomum verum (Lauraceae), they cause 100% mortality after 2–6 days' exposure at a dose of 5.10 µL/cm3. Whereas of the twenty-four essential oils tested, Citrus limon essential oil (Rutaceae) is the least toxic, causing 43% and 67% mortality after 8 days' exposure at doses of 10² µL/cm³ and 5.10² µL/cm³, respectively.
Similarly, the biological efficacy of essential oils from Laurus nobilis (Lauraceae), Citrus bergamia (Rutaceae), and Lavandula hybrida (Lamiaceae) has been demonstrated by Cosimi et al. (2009) 16 on adult corn weevils Sitophilus semés (Coleoptera: Curculionidae), wheat weevils Cryptolestes ferrugineus (Coleoptera: Cucujidae), and rice weevil larvae Tenebrio molitor (Coleoptera: Tenebrionidae). According to Righi (2010)17, thyme essential oil causes 100% mortality in Callosobruchus chinensis bruchid adults after one hour's exposure with a dose of 10 µL.
The different susceptibility of Sitophilus oryzae to essential oils may be due to several factors, including the species' resistance to certain essential oil compounds. Indeed, recent studies (Shaaya et al., 1997; Sékou Moussa et al., 2000; Kim et al., 2003; Ngassoum et al., 2003; Ngamo et al., 2001; Lee, 2002) 18–20 reveal that the effect of oils on insects is not systematic, as different responses are observed depending on the insect species and the essential oil. According to Shaaya et al. (1997), 18 T. castaneum (LC50 = 11.1 µl.l-1) is more resistant than S. oryzae (LC50 = 7, 5 µl.l-1) or Rhyzopertha dominica (LC50 = 9, 6 µl.l-1) to the fumigant effect of mint essential oil, while it is twice as sensitive to Eucalyptus nicholiic essential oil as S. oryzae (Lee, 2002)21
Furthermore, according to Ngamo and Hance (2007)20, an essential oil does not necessarily exert the same activity at different stages of an insect's life cycle, as there is great variation in the sensitivity of insect species to the same essential oil.
According to Bostanian et al. (2005), 22 essential oils act directly on the cuticle of soft-bodied insects and mites and are less effective with hard-shelled insects such as adult Coleoptera and Hymenoptera. Insecticide effectiveness may also depend on the sex of the insect. Indeed, several authors have demonstrated that males of Acanthoscelides obtectus are more sensitive to essential oils than females (Regnault-Roger and Hamraoui, 1994; Papachristos and Stamopoulos, 2002)15.
The toxicity of essential oils on insects is induced by the action of their major compounds (Seri-Kouassi et al., 2004)23. These have insecticidal efficacy either singularly or when combined (Ngamo or Hance, 2007). 20. Asawalam et al. (2008) 24 tested the toxicity of the major components of essential oils extracted from Vernonia amygdalina (Asteraceae) on Sitophilus zeamais (Coleoptera: Curculionidae), which are 1, 8-cineole, β-pinene, α-pinene, myrtenal, pinanol, L-carveol, transpinocarveol, and linalool. At the minimum lethal dose that causes 100% mortality in S. zeamais adults by V. amygdalina essential oils, it causes 63 ± 2.1% mortality by 1,8-cineole, 37 ± 2.4% by β-pinene, 00 ± 00% by each of the other components, and 100% by the mixture of the eight major components.
The results show that the combined toxic action of the major components is more remarkable than the individual action of these components. Indeed, recent work shows that monoterpenes have neurotoxic physiological effects and act on different targets depending on their chemical nature (Huignard 2013). Regnault-Roger et al. (2008) 15 report that monoterpenes have a rapid inhalation-type toxic action on adults. In addition, monoterpenoids are generally volatile and more lipophilic compounds that can enter insects rapidly and interfere with their physiological functions. (Lee et al., 2002) 11. Because of their high volatility, they have a fumigant effect that could be of importance in controlling insects in stored foodstuffs (Ahn et al., 1998). 25