Burkina Faso is one of West Africa’s leading producers of shea kernels, with an estimated annual production potential range from 70,000 to 300,000 tons (Bup et al., 2014) [1] . Bondé et al. (2019) [2] reported 506,717.56 tons of fresh fruit, corresponding to 69,067.13 tons of dried shea kernels, which have several uses. Indeed, the butter derived from processing shea kernels is used by local farmers as the main source of fat in the agro-food industries, for making soap and for cosmetic care (Ouédraogo et al., 2013) [3] . However, shea kernels are subject to insect attack during storage. The most significant damage caused by pests is perforation (Kambou et al., 2022) [4] . Faced with the threat of insect pests in stocks, the methods of control are essentially based on the use of synthetic pesticides (Guèye et al., 2011) [5] . Their effectiveness in controlling stock pests is well known. However, the use of synthetic pesticides has several drawbacks such as pollution of food chains and the environment, intoxication of producers and consumers and increased pest resistance to synthetic insecticides (Benhalima et al., 2004) [6] . These insecticides are also a source of ecological disorder (Regnault-Roger, 2002) [7] . The use of bio-pesticides could be an alternative way of reducing the use of synthetic insecticides. The use of plants with insecticidal properties is being promoted worldwide (Guèye et al., 2011) [5] . Indeed, the organs and parts of some plants are used to control pests (Guèye et al., 2011) [5] . Several laboratory tests were carried out with EOs, and the results indicate variable toxicities depending on the EOs used, the pest species present and, above all, the stage of development concerned by the tests (Togola et al., 2014 [8] ; Ilboudo et al., 2016 [9] ; Aouina and Khelifi, 2018 [10] ). The EOs of certain plants act by contact or inhalation, with varying degrees of effectiveness depending on the target insect. Increasingly, the leaves and kernels of neem (Azadirachta indica), with their anti-feeding, repellent, toxic and growth-inhibiting properties, have been used extensively to control insects on stored foodstuffs (Facknath, 2006 [11] ; Guèye et al., 2011 [5] ). Few scientific data exist on the use of insecticidal plants to control Ephestia cautella (Walker), the main insect predator of stored shea kernels in Burkina Faso (Kambou et al., 2023) [12] . Therefore, it is necessary to consider a trial to control the activity of E. cautella in order to prevent or limit its damage to shea kernel stocks. Hence, this study aimed to evaluate under laboratory conditions the efficiency of insecticidal plant extracts for controlling E. cautella populations in shea kernel stocks. This was done by evaluating the insecticidal properties of neem kernel powder on E. cautella adults and larvae, and assessing the susceptibility of E. cautella adults, larvae and eggs to Eucalyptus camaldulensis EOs.

Neem kernel powder used as a coating on shea kernels was less effective in controlling E. cautella adults after three days of exposure. The low toxicity observed in adults could be explained by the fact that adults did not feed on treated shea kernels. However, previous work has shown that neem kernel powder possessed insecticidal properties characterized by a decrease in female fecundity and a reduction in the number of insects emerging in the first generation (Koussoubé, 2018) [14] . The energy-intensive egg-laying activity of the females could explain the low mortality rate recorded in the control. In fact, the dead adults were all females. E. cautella larvae showed greater susceptibility to neem kernel powder. Indeed, this powder caused high mortality among the larvae. As a result, it is effective in controlling larvae associated with shea kernel stocks, with dose-dependent toxicity. This toxicity would be due to the high concentration of azadirachtin in neem seeds compared with other parts of the plant (Isman, 2006 [15] ; Singh et al., 2010 [16] ). Azadirachtin acts as a growth regulator by blocking the insect’s endocrine system, resulting in the slowing or total arrest of morphogenetic development and physiological disorders followed by behavioral disorders in the insect (Isman, 2006 [15] ; Petit, 2008 [17] ). Therefore, this chemical compound inhibited the development of the late larval instar of E. cautella. On the other hand, long larval and pupal development times may impact emergence rates. This could explain the mortalities recorded in the control.

E. camaldulensis EOs acts by fumigating exposed insects. It caused a dose-dependent mortality rate in adult E. cautella. With 15 µl/l, a 100% mortality in adults was recorded. LC₅₀ and LC₉₅ values were low for adults, attesting to the efficacy of E. camaldulensis EOs on E. cautella adults for stock protection. This efficacy was proven by Togola et al. (2014) [8] , who showed that impregnating jute bags and treating the bags with EOs of E. camaldulensis and Cymbopogon citratus is an effective method of protecting stored rice against Sitophilus oryzae and Sitotroga cerealella. The insecticidal effect of Eucalyptus saligna has also been reported on Callosobruchus maculatus in cowpea seeds protection trials (Tapondjou et al., 2003) [18] . The insecticidal activity of EOs of the genus Eucalyptus is linked to its main components (1, 8-cineole or eucalyptol, α-pinene and d-limonene) which proportions vary from one species to another (Erau, 2019) [19] . In general, the chemical compounds accountable for actions on stock pests are mainly monoterpenes (1 - 8 cineole, eugenol, camphor) with insecticidal effects (Kouninki et al., 2007 [20] ; Noudogbessi et al., 2009 [21] ) or alkaloids with inhibitory effects on pest larval development (Ngamo et al., 2001) [22] . However, the toxic and repellent effects of these plant materials are based on several factors, including their chemical composition and the level of susceptibility of the insects (Guèye et al., 2011) [5] .

Larvae exposed to E. camaldulensis EOs showed direct contact susceptibility and dose-dependent mortality rates. All larvae died at doses of 3 µl/larva. Therefore, E. camaldulensis EOs is highly effective on E. cautella larvae by direct contact.

E. cautella eggs 24 h old are less sensitive to the chemical compounds of E. camaldulensis EOs. After exposing eggs to increasing concentrations of oil for 24 h, hatching rates recorded were not significantly different from the control. However, marked ovicidal activity has been reported on C. maculatus and on Anopheles gambiae eggs with EOs of Ocimum americanum, Lippia multiflora, Hyptis suaveolens and Hyptis spicigera (Wangrawa et al., 2015 [23] ; Ilboudo et al., 2016 [9] ). The observed tolerance of E. cautella eggs may be due to the low penetrating power of the EO. According to Ilboudo et al. (2016) [9] , the ovicidal activity of EOs is based on their penetrating power or direct toxicity. Similarly, the ovicidal activity of oils can vary according to the age of the eggs and the method of use. Papachristos and Stamopoulos (2002) [24] demonstrated the toxicity of Lavandula hybrida, Rosmarinus officinalis and Eucalyptus globulus on Acanthoscelide obtectus eggs, with a difference in sensitivity significantly correlated with age. The authors found that sensitivity was highest after three days, probably due to greater permeability of the chorion or vitelline membrane, which facilitates the diffusion of volatile compounds.

The results obtained using neem kernel powder show that it could be used to control E. cautella larvae. The insecticidal effect of this powder is low in toxicity to adults. E. camaldulensis essential oil, tested for its contact and fumigant activity, is highly effective in controlling E. cautella adults, with mortality rates varying according to concentration. It inhibits larvae by direct contact. The difference in toxicity observed between the different developmental stages of the insect calls for integrated control of the two bio-pesticides to optimize their efficacy. Thus, the use of these insecticidal plants could be an interesting alternative to chemical control.

The authors are grateful to their respective institutions and the government of Burkina Faso which awarded a scholarship to Siébou François KAMBOU and supported the research carried out as part of his PhD activities.