Insecticidal Activity of Spicy Plant Oils Against Sitophilus zeamais in Stored Maize in Cameroon
Ocimum gratissimum L. (Lamiaceae), Piper nigrum L. (Piperaceae) and Xylopia aethiopica Dunal A. Rich. (Annonaceae) are common spicy plant species in many recipes in Cameroon. A laboratory experiment was conducted to investigate the insecticidal activities of essential oil extracts from their fresh leaves or dried fruits against Sitophilus zeamais Motsch. (Coleoptera, Circulionidae), the principal weevil of stored maize and grain products in the tropics. The assays were carried out using the ingestion and contact techniques. After the extraction of essential oils by hydrodistillation using a Clevenger apparatus, formulations as dustable powder were prepared at 5 and 10%. Probit analysis was employed in evaluating the LD50 response. Insect mortality was recorded after 24, 48, 72 and 96 h. Results showed that, significant insect mortality was obtained with all the essential oils used and all the treatments had effective insecticidal activity by ingestion and contact after 96 h compared to untreated control and pirimiphos-methyl (actellic Super 2% dust) formulations. For all the formulations tested, a significant difference (p< 0.05) was observed between contact and ingestion assays, the contact test being the most active. The mortality rate of S. zeamais increased with increase in the concentration of essential oil of the three plants and the duration of exposure of the weevils on the treated substrates. By contact, the essential oil extract from P. nigrum fruits was the most effective insecticide (97.2±4.6%) whereas X. aethiopica’s (97.3±3.7%) was the most effective by ingestion. Results are discussed with regard to the use of the essential oils from the fruits of these plant species by the small-scale farmers as sustainable alternatives and maize grain preservatives against S. zeamais during storage to synthetic insecticides. The potential of these non toxic products to protect stored maize against attacks of S. zeamais is hereby appraised.