Efficiency of Plant Oils to Control Lesser Grain Borer [Rhyzopertha dominica (Fabricius)] and Maize Weevil (Sitophilus zeamais Motschulsky) in Rough Rice

Abstract

Lesser grain borer (Rhyzopertha dominica (Fabricius)] and maize weevil (Sitophilus zeamais Motschulsky) are major insect pests leading to qualitative and quantitative damages on rough rice during storage. Chemical seed treatment and fumigation have been used for controlling these insect pests, resulting in adverse effects such as resistant to insecticides and toxic to non-target organisms. The objectives of this study were to screen plant oils and find their appropriate application in a laboratory in order to apply for controlling these pests under warehouse conditions. Six plant oils were extracted from turmeric (Curcuma longa), black pepper (Piper nigrum), clove (Syzygium aromaticum), citronella (Cymbopogon nadus) by water distillation, wild spider flower (Cleome viscose) and thiam (Azadirachta excelsa) by maceration with n-hexane. Their repelling, killing and anti-feeding activities were evaluated against R. dominica and S. zeamais adults in laboratory. The most toxic plant oil was selected to further study for chemical composition analysis, application methods by seed treatment, fumigation and sack coating as well as effect on seed germination as compared to chlorpyrifos and phosphine under laboratory and warehouse conditions. Cost of plant oil application was also compared with chlorpyrifos and phosphine under warehouse conditions. Black pepper oil was the most effective to repel both insect species with 100% repellency at the concentration of 0.47 μL/cm2 after 48 h of treatment. Clove oil was the most toxic due to the lowest LC50 value. Dermal LC50 at 72 h were 6.13 μL/L and 3.52 μL/L and inhalation LCso at 48 h were 92.95 μL/L air and 77.63 μL/L air for R. dominica and S. zeamais, respectively. In addition, clove oil inhibited feeding action with 97.12% and 95.91% of inhibition against R. dominica and S. zeamais, respectively. Major chemical components in clove oil included eugenol with 65.83%, p-caryophyllene with 13.54% and eugenol acetate 8.29%, respectively. Effectiveness of clove oil was not different between applications by seed coating and fumigation. At 200 uL/L after 7 d of application, the mortality of R. dominica and S. zeamais were 99.33% and 95.33% for seed coating, and were 98.67% and 100% for fumigation, respectively. Sack coating method was the least effective with the penetration inhibition through the sack of 38.33% and 46.67% for R. dominica and S. zeamais, respectively. At the same time of application, the mortality of R. dominica and S. zeamais were 66.00% and 100%, after seed coating with chlorpyrifos, and were 62.00% and 98.67% after fumigation with phosphine, respectively. It suggests that these two insecticides were lower effective to kill R. dominica than clove oil, but all of them were still effective to kill S. zeamais. Under warehouse conditions for six months at Suratthani Rice Seed Center (SRSC) and Phatthalung Rice Seed Center (PRSC), the clove oil application by seed coating combined with fumigation was the most effective to control R. dominica with 85.96% and 90.31% as compared to control in SRSC and PRSC, respectively. The application by seeds coating with chlorpyrifos and fumigation with phosphine could control R. dominica 40.09% and 40.92% in SRSC and PRSC, respectively. Clove oil and these two insecticides had no affect rice seed germination after six months of storage. However, cost of clove oil application was higher than that of chemical application. Costs of seed coating with chlorpyrifos and fumigation with phosphine were 6.90 Bath/ton and 7.50 Bath/ton, whereas those of seed coating with clove oil and fumigation with clove oil were 2,568.00 Baht/ton and 1,945.97 Baht /ton, respectively. In conclusions, chlorpyrifos and phosphine were low effective to control R. dominica, but still effective to control S. zeamais. Therefore, clove oil is an alternative method for controlling R. dominica under a reasonable cost of application.