Comparative sensitivity of maize weevil to essential oil of Hoslundia opposita Vahl leaves subjected to different drying regimes

Abstract

Botanical essential oils (EOs) are effective alternatives to over-dependence on synthetic insecticides in stored product protection. However, the burden of handling bulky fresh botanicals needed for EO extraction and the tendency of the freshly harvested botanical to rot in transit, if wrongly handled, can be bottlenecks to pilot scale application of EO. Therefore, this study evaluates the comparative efficacy of the EOs obtained from freshly harvested Hoslundia opposita Vahl (Lamiaceae) and plants exposed to different shade-drying regimes (1-5 days) against maize weevil, Sitophilus zeamais Motschulsky, under laboratory conditions. Evaluation included fumigant toxicity and repellence bioassays. At 2 hours after exposure (HAE), EO of freshly harvested H. opposita leaves caused significantly (P <0.001) lower mortality (53.30%) than the value observed in H. opposita leaves dried for 5 days (90.00%). The lethal time for 90.00% of the weevils (LT 90 ) obtained in EO from H. opposita leaves dried for 5 days (2.00 h) was significantly lower than 2.80 and 7.14 h obtained in EOs from the fresh and 1 day-dried leaves, respectively. At 1 HAE, EOs obtained from H. opposita leaves dried for 4 and 5 days caused 60.00% repellence which was significantly higher than 20.00% observed in fresh leaves. At 2 HAE, EOs obtained from H. opposita dried for 4 and 5 days caused significantly higher repellence (100.00 and 80.00%, respectively) than what was observed in fresh leaves (70.00%). The results imply that shade drying freshly harvested H. opposita leaves has no deleterious effects on the efficacy of its EO against maize weevil.