Léonard S.T. Ngamo

Toxicity of some terpenoids of essential oils of Xylopia aethiopica from Cameroon against Sitophilus zeamais Motschulsky

Abstract:  The acute toxicity of essential oils from the whole fruit (EF) or from the fibres of the fruit (FF) of the local aromatic plants Xylopia aethiopica Dunal (Annonaceae) collected in north Cameroon was evaluated on Sitophilus zeamais 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. aethiopica led to 100% mortality. In a second step, proportions of active compounds present in the oil of both EF and FF of X. aethiopica were quantified. The toxicity of the four main compounds was tested against S. zeamais: α‐pinene, β‐pinene, Δ‐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. aethiopica essential oil could be a potential source of natural and low‐cost insecticide to control storage pests.

Insecticidal activities of essential oil of Callistemon viminalis applied as fumigant and powder against two bruchids

The fumigant and contact toxicity of essential oil (EO) extracted from the leaves of Callistemon viminalis and its aromatized clay powder (ACP) was evaluated against adults of Acanthoscelides obtectus and Callosobruchus maculatus (Coleoptera: Bruchidae). The results obtained for fumigation assays showed that C. maculatus seems to be more susceptible (LC50 = 0.019 μl/cm3) to the vapours of the essential oil than A. obtectus (LC50 = 0.011 μl/cm3) after 12 h exposure. On the other hand, A. obtectus seems to be more susceptible (LD50 = 0.133 μl/g) to the essential oil applied by contact on grains than C. maculatus (LD50 = 0.170 μl/g) after 2 days exposure. The ACP was also very toxic towards the adults of A. obtectus (LD50 = 0.100 μl/g) and C. maculatus (LD50 = 0.098 μl/g) by contact on grains. At the doses of 0.133 μl/g and 0.266 μl/g, mortalities caused by ACP on grains were higher than those caused by the same dose of EO against the two bruchids. It is also established that both the EO and the ACP caused higher inhibition of F1 progeny production of A. obtectus than that of C. maculatus. The loss of insecticidal activity of the two materials in the course of time has been observed; however, the toxicity of the ACP was more persistent than that of the oil in the course of time when applied on grains. These results suggest that EO from the leaves of C. viminalis can be used as fumigant agent against A. obtectus and C. maculatus. In addition, it could be advisable to use an adsorbent mineral material as carrier of this EO for the prolongation of its insecticidal activity in the course of time.

Effect of Site Location and Collecting Period on the Chemical Composition of Hyptis Spicigera Lam. an Insecticidal Essential Oil from North-Cameroon

Abstract Hyptis spicigera essential oils from seven localities in the North-Cameroon (Ngaoundere, Guirvidig, Kodeck, Lara, Toloum, Kaele, Tchecal-baila) were investigated by GC and GC/MS. Results showed differences within harvesting sites and between the different sites of collection but did not revealed clear tendencies in the evolution of the oil composition with regard to the sampling period. The main group of compounds in all the analyzed samples were: α-pinene (11.9%-42.1%), β-pinene + sabinene (6.0%-39.8%) and β-phellandrene + 1,8-cineole (8.8%-27.4%) except in one oil where β-caryophyllene (23.4%) was the principal component. The insecticidal activity of H. spicigera and its principal terpenic components was evaluated against the cowpea weevil Callosobruchus maculatus F., the major cause of damages of cowpea (Vigna unguiculata (L.) Walp) in North Cameroon.

Antioxidant activities of essential oil of Bidens pilosa (Linn. Var. Radita) used for the preservation of food qualities in North Cameroon

Abstract This study aimed to determine the total antioxidant capacity of the essential oil (EO) of leaves of Bidens pilosa (Linn. Var. Radita) used as protectant of stored grains in Northern Cameroon. EO was characterized by GC‐FID, antioxidant activity (AA) was determined by combining: evaluation of radical‐scavenging activity, reducing power (RP) and co‐oxidation of β‐carotene methods. Tests were carried out on crude and stored EO kept for two weeks at 31.48 ± 2.88°C and 58.56 ± 6.78% relative humidity. These conditions are the same as those of grain storage. GC analyses enabled the identification of 27 compounds, representing around 97.57% of the total oil contents. The major constituents of the EO were α‐pinene (14.7%), ε‐caryophyllene (13.5), and β‐ocimene (12.8%). The AA of the crude and stored EO are proportional to the concentrations and time of exposition. Exposed at the day light, this EO inhibit 77.4–18.69% for the DPPH system, 59.55–19.14% for RP method and 91.88–21.8% for β‐carotene‐linoleate model system, respectively, from crude and 15 days storage EO at 20 mg L−1. For the EC50 values, β‐carotene method is excellent and in the decreasing order of DPPH method, PR with 2.52 mg L−1, 2.77 mg L−1 and 4.13 mg L−1, respectively, for the crude oil. The ET50 were 1.59 days for the RP method and 2.88 days DPPH system and β‐carotene‐linoleate model system at 20 mg L−1. These results showed that the EO of B. pilosa leaves exhibits AA that might be an added value for this EO preventing stored products from pest attacks.

Diversity and feeding behaviour of insects visiting Brachiaria ruziziensis (Poaceae)

The diversity of visitors to flowers of the grass Brachiaria ruziziensis R. Germ and Evrard is very low in Ngaoundéré, Cameroon. Bees (Hymenoptera) and syrphids (Diptera) were observed gathering pollen, and each insect group had its own strategy depending on the prevailing environmental conditions. Syrphids are early-morning flower visitors and take time to feed on wet pollen. They visit flowers from 0630 h when the relative humidity is higher than 80% and temperature lower than 25°C Bees gather dry pollen from 0930 h when the relative humidity is < 50% and temperature > 30°C, and carry it to the nests for feeding.RésuméLa diversité de l’entomofaune floricole de Brachiaria ruziziensis R. Germ et Evrard est pauvre à Ngaoundéré au Cameroun. Les abeilles (Hymenoptera) et les syrphes (Diptera) exploitent le pollen de cette graminée. Les syrphes sont des visiteurs du point du jour se nourrissant du pollen humide et ils sont plus actifs à 6h30 quand l’humidité relative est supérieure à 80% et la température moins de 25°C Les abeilles qui sont actives après 9h30, (humidité relative < 50% et température > 30°C), collectent du pollen sec et l’emportent.