Effects and use of some Ocimum Plant Species and their essential Oils on some Storage Insect Pests

Abstract

The bioactivity of 0. suave, 0. kenyense and 0. kilimandscharicum against three storage insects (Sitophilus zeamais, Rhizoperta dominica and Sitotroga cerealella) was evaluated in the laboratory. Fresh, dry, ground plant materials, and essential oils of the three Ocimum plant species at three levels of applications were mixed with 250 g of disinfested maize and sorghum seeds in one liter volume glass jars. The effects of treatments on insect mortality, survival, reproduction and feeding were assessed. The repellent action of the materials and their effect on seed germination were also evaluated. Chemical analysis of the essential oils of the three plants was done using GC and GC-MS. The identity of a total of 28 constituent compounds of the oils of the three plants were confirmed and their relative proportion determined. Toxicity tests of the essential oils and their constituent compounds (at their natural relative amounts) was conducted on filter paper discs. Similarly, source of toxicity of the ground plant materials of 0. kilimandscharicum and O. suave was studied. The toxic effect of the three Ocimum plant species was observed only in the treatments involving ground plant materials and essential oils. All other treatments showed no significant level of toxicity to the insects. Of the three Ocimum plant species tested, 0. kilimandscharicum was the most toxic (inducing 100% mortality) and 0. kenyense the least. Toxicity of O. suave essential oil was attributable mainly to eugenol, that of 0. kilimandscharicum to camphor and that of 0. kenyense to 1,8-cineole and iso-eugenol. Toxicity of the major compounds in the essential oils were dose-dependent. Toxicity of the ground plant materials was attributed to the essential oils of the plants as no mortality was observed in treatments involving ground plant materials free of essential oils. The highest dose of fresh plant materials of the succulent 0. kenyense and 0. kilimadscharicum increased reproduction and feeding of the insects, which could be due to the high humidity maintained in the glass jars. No significant effect of the other treatments on reproduction and feeding was observed. 0. suave repelled S. zeamais in all forms, while fresh plant materials at all levels of application were not repellent to R. dominica and S. cerealella. S. cerealella was significantly repelled only by the highest doses (level 3) of dry and ground plant materials and essential oil of the plant. Similarly, all materials of 0. kilimandscharicum repelled S. zeamais and R. dominica, with the exception of fresh plant materials (levels 2 and 3). Treatments involving fresh, ground plant materials and essential oil of O. kilimandscharicum at level 3 were significantly repellent to S. cerealella. 0. kenyense was repellent to 5. zeamais in all forms of treatment. However, the highest repellent activity of the plant was obtained with the highest doses of fresh, dry and ground plant materials and essential oil treatments. All plant materials and essential oil of 0. kenyense with the exception of dry plant materials levels 1 and 2 were also repellent to R. dominica. Significant repellent activity of 0. kenyense against S. cerealella was obtained with only fresh plant materials level 3 treatment. Essential oil of 0. suave reduced the germination of maize seeds while dry plant materials of the same plant enhanced the germination of the seeds. None of the other treatments had any significant effect on the germination of maize or sorghum seeds. Large scale evaluation of the most effective treatment (ground plant material of 0. kilimandscharicum) was carried out by mixing the powder with 25 kg of disinfested maize and sorghum seeds. The effectiveness of the treatment was evaluated by sieve sampling, grain probe and pheromone traps. 100% mortality of R. dominica and S. zeamais at 2% and 10% levels of treatments respectively, was observed 48 h after treatment. Pheromone baited grain probes were the most effective in sampling live adults of R. dominica. Grain probe was more effective compared to sieve sampling in sampling adults of S. zeamais . Under small scale farmers conditions the most effective and practical treatment could be one based on the use of ground plant materials.