The efficacy of two plant essential oils and their ground powders(Lippia javanica and Microglossa pyrifolia) in reducing post-harvest loss caused by Sitophilus zeamais in maize was determined in the laboratory. Each plant powder and essential oil was tested by exposing adult weevils of mixed sex to two levels of 2 and 4 g/20 g of grains and 0.5 and 2.0 ml/100 g of grains for plant powders and essential oils respectively each in five replications. Each replication had 20 adult weevils of mixed age and sex. Both the essential oils individually and in mixtures recorded significantly (P<0.05) high mortality of adult weevils (70-100%) at 4 days after treatment at the highest dose of treatment. The percentage weevil mortality in the treated grains was significantly higher than in the controls. The mixture of essential oils caused the highest mortality against the adult S. zeamais weevils and was significantly higher than the two essential oils individually. Ground plant powders of the two plants were tested at two levels of 10 and 20% treatment, but minimal mortality but significantly higher than in the control against the weevil was recorded at 14 days after treatment with powder from L. javanica inducing 33.67% at the highest dose applied. Repellency test of essential oils from the two plants was contacted on filter paper discs (50, 75, 100 and 125 mg doses) and using choice bioassay system (50,150 and 300 mg/50 g grains dose). Both the oils were significantly repellent to the weevil with oil from Lippia javanica repellency exhibiting the best activity in the two tests (>90%) 30 min after exposure. LD50 for the essential oils individually and blend was 0.147, 1.019 and 0.398 µl/100 g of grains for L. javanica, Microglossa pyrifolia and then blend respectively. The toxicant LD50 was 0.199 and co-toxicity coefficient 178.94 for the blend implying the oils were synergistic to each other. Chemical analysis of the essential oils of the two plants using GC, GC-MS and GC-co injection with digitized standards identified 18 constituent compounds. The major components in L. javanica were, 1-imidazol-1-yl-3-methyl-but-2-en-1-one (38.356%), 4-tertbutylphenol (10.597%) and 2-tert-butylphenol (23.453%) and from M. pyrifolia, (1Z,7Z)-3-isopropyl-2-methylene-cyclodeca-1,7-diene(α-humulene)(29.576%),5-isopropyl-2-methyl-bicyclo[3.1.0]hex-3-ene(7.088%)and1,1,7,7a-tetramethyl-2,3,5,6,7,7b-hexahydro-1aH-cyclopropa[a]naphthalene (6.472%). Progeny assessment in the treated grains revealed the number of emerged weevils after 62 days was insignificant (P>0.05) but significant in the controls (P<0.05). The germination potential for the treated grains is significantly higher than the untreated grains (p<0.05) and plant oils treatment did not significantly affect the viability of the grains (p >0.05).
Keywords: Sitophilus zeamais, Microglossa pyrifolia, Lippia javanica, mortality, Progeny assessment, essential oil.