Olfactory responses of Sitophilus zeamais L. to bushmint leaf powder and methanol extract on stored maize

Abstract

Bushmint (Hyptis suaveolens Poit) is a weed with pesticidal properties that have been explored in managing pests of agricultural and medicinal importance. The objective of this study was to evaluate the repellence ability of Hyptis suaveolens to prevent infestation of stored maize grain seed. The study involved evaluation of the olfactory responses of adult weevil to maize (Zea mays L.) grains treated with 5, 10, 15 and 20% concentrations of leaf powder (LP) and methanol extract (ME) of Hyptis suaveolens in the laboratory, with a static air four-chamber olfactometer. Also, the insecticidal effects of the substances on adult mortality and oviposition on maize grains treated with LP and ME were determined in a completely randomised design, with four replicates. The number of weevils in ME chambers decreased with increase in concentration of extracts; while the LP chambers had consistently fewer weevils with numbers, ranging from 2.5 to 3.75. Adult mortality in maize treated with LP was the least and varied between 0.00 (15% concentration) and 2.54% (20% concentration). Maize grains with ME caused significant mortalities that ranged from 65.50% at 15% concentration to 94.92% at 10% concentration. More eggs (3.5 to 5.75) were laid on maize with LP compared with ME treated maize. Generally, ME was richer in phytochemicals than LP. Hyptis suaveolens repellence manifested more in the form of LP than as ME; though ME caused more adult mortality and reduced oviposition than LP.