Utilisation des huiles essentielles pour la protection des grains contre les insectes ravageurs au Nord Cameroun

Les paysans stockent entre 60 et 80% de la production des grains qui est régulièrement infestée par les insectes ravageurs. Les plantes insecticides méritent d’être valorisées à travers notamment leur huile essentielle afin de limiter la dépendance des paysans aux insecticides chimiques, largement utilisés au Nord du Cameroun et dont les dangers sont bien connus. L’analyse chromatographique de quelques huiles essentielles locales, révèle que Cymbopogon citratus (DC.) Stapf. (Poacée), Ocimum gratissimum L. (Lamiacée) et Ocimum suave Willd (Lamiacée) sont constituées respectivement du citral, du thymol et de l’eugénol à plus de 50%. Les huiles essentielles de Annona senegalensis Pers. (Annonacée), Hyptis spicigera Lam. (Lamiacée), et Xylopia aethiopica (Dunal) A. Rich. (Annonacée), contiennent majoritairement des monoterpènes hydrocarbonés dont l’alpha-phellandrène, l’alpha-pinène, le sabinène et le beta;-pinène. H. spicigera et X. aethiopica, dont la composition varie suivant l’origine de la plante, sont les plus efficaces contre Callosobruchus maculatus (F.) et Sitophilus zeamais (Motsch.), principaux ravageurs primaires des greniers au Nord du Cameroun. L’activité insecticide de H. spicigera est marquée par sa teneur en alpha-pinène (39,0%) tandis que celle de X. aethiopica est due à l’effet synergique de ses composés. La volatilité des huiles essentielles nécessite des supports aptes à réguler le relarguage des molécules volatiles. Avec plus de 90% de molécules volatiles libérées en 7 jours, la poudre des infrutescences de X. aethiopica, qui contiennent plus de 5% d’huile essentielle, bien que toxique contre C. maculatus, ne permet pas de réguler et de prolonger la libération des molécules volatiles. Le kaolin testé dégrade l’huile essentielle X. aethiopica tandis que l’incorporation de l’amidon à la gomme arabique permet de libérer en 7 jours, 40,5 à 55,9% des molécules volatiles suivant la proportion d’amidon. Cette combinaison est un système qui autorégule le relarguage en deux phases. La première phase a un taux de relarguage élevé qui entraîne un effet « Knock down » sur C. maculatus, et la seconde phase présente un relarguage lent et prolongé qui pourrait, par un effet répulsif, éviter l’infestation des grains stockés. Cette étude est une contribution du projet « STOREPROTECT PIC-2003 » à la protection des denrées stockées par l’utilisation des ressources locales au Nord du Cameroun

Toxicity Of Some Terpenoids Of Essential Oils Of Xylopia Aethiopica From Cameroon Against Sitophilus Zeamais Motschulsky

The acute toxicity of essential oils from the whole fruit (EF) or from the fibres of the fruit (FF) of the local aromatic plants Xylopiα αethiopicα Dunal (Annonaceae) collected in north Cameroon was evaluated on Sitophilus zeαmαis adults. A concentration of 1 ml of essential oil per 100 g of maize seeds was tested to determine weevil mortality after 24 h of exposure. Under these conditions, the essential oil derived from both EF and FF of X. αethiopicα led to 100% mortality. In a second step, proportions of active compounds present in the oil of both EF and FF of X. αethiopicα were quantified. The toxicity of the four main compounds was tested against S. zeαmαis: α-pinene, β-pinene, -3-carene and terpinen-4-ol according to their proportion in the essential oil of the concerned plant part. β-pinene and terpinen-4-ol were responsible for 50% of the mortality at the proportion found in EF and FF essential oils respectively. When mixed, a synergic effect of the compounds was observed that restored the mortality percentage observed for the crude oil. It appears that X. αethiopicα essential oil could be a potential source of natural and low-cost insecticide to control storage pests

Persistent effect of a preparation of essential oil from Xylopia aethiopica against Callosobruchus maculatus (Coleoptera, Bruchidae)

Cowpeas, Vigna unguiculata (Walp), are commonly attacked by Callosobruchus maculatus (Coleoptera, Bruchidae) during storage. Current methods to prevent losses during storage involve synthetic insecticides. Although they have insecticidal efficiency, these products have adverse effects on consumers and the environment. Plant products such as those of Xylopia aethiopica (Annonaceae) are potential sources of natural insecticides. This study assessed the efficiency of a preparation of the essential oil of X. aethiopica mixed with its powdered fruits in order to increase the persistence of its effect against C. maculatus. The persistence relates to toxicity against adults, the prevention of F1 emergence, oviposition and the development of eggs laid by C. maculatus. Four treatments were compared: 1) control, consisting of 200 g of grain, 2) 200 g of grain mixed with 2 g of powder from X. aethiopica, 3) 200 g of grain mixed with 0.4 ml of essential oil, and finally 4) 200 g of grain mixed with 2 g of powder plus 0.4 ml of essential oil. The results showed that, with powder alone, crude essential oil, and powder imbibed with essential oil, the toxicity lasted respectively for 3, 9 and 12 days (p<0.05). The persistence of the effect of treatment with X. aethiopica powder plus essential oil on the mortality and oviposition of C. maculatus was better than that of the treatment with the crude essential oil (p<0.05 until day 18). Moreover, the X. aethiopica powder enriched with essential oil provoked, after 52 days, 100% of development inhibition. The hatching of eggs of C. maculatus was also inhibited by all the formulations except the control. The authors results indicate that protection of cowpeas against C. maculatus may be possible using a formulation based on the dry fruits of X. aethiopica; the powder may increase the persistence of the essential oil

Investigations on Both Chemical Composition and Insecticidal Activities on Essential Oils of Vepris heterophylla (Rutaceae) from Two Localities of Northern Cameroon Towards Tribolium castaneum (Herbst) (Coleoptera: Tenebrionidae)

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