Repellency and toxicity of essential oils from Ocimum gratissimum (Lamiaceae) and Laurus nobilis (Lauraceae) from Georgia against the rust-red flour beetle (Tribolium castaneum Herbst) (Coleoptera: Tenebrionidae).

Abstract

Six essential oils, three of which were obtained from Laurus nobilis (leaves, branches and stems) and three from Ocimum gratissimum (whole plant) samples from Georgia, USA, were evaluated for repellency and toxicity properties against T. castaneum. The content of the oils in 500 µlitre with acetone as the solvent varied from 0.0625 to 5.0000 µlitre/ml. Repellency tests on impregnated filter paper demonstrated pronounced differences in the number of insects repelled with the six samples of L. nobilis and O. gratissimum oils. The degree of repellency of all test compounds followed the following pattern: O. gratissimum (3) > L. nobilis (3) > O. gratissimum (2) > O. gratissimum (1) > L. nobilis (1) > L. nobilis (2). All samples of both plant species showed high repellency class (IV-V). A contact toxicity bioassay on treated filter paper discs revealed differences between the respective oils from O. gratissimum 1, 2 and 3. Test solutions were prepared by dissolving essential oils in 1 ml acetone. The concentration ranged from 1 to 300 µlitre/ml. Acute adult mortality was determined after 3, 5 and 7 days of treatment. In experiments with concentrations of less than 10 µlitre/ml, mortality was not observed. Toxicity was positively correlated with the concentration up to 50-80 µlitre/ml and negatively correlated with concentrations of more than 80-100 µlitre/ml. At 60 µlitre/ml, 76 and 88% mortality were obtained within 7 days of application of the essential oils from O. gratissimum (1) and O. gratissimum (2), respectively. At 80-100 µlitre/ml, mortality of adult beetles did not increase further. The beetles released an odoriferous substance (quinone) from their defence glands. Weak insecticidal property was recorded for the other essential oil from O. gratissimum (3) and L. nobilis (2). Within 7 days, concentrations of less than 50 µlitre/ml did not result in insect mortality. The fecundity of the surviving insects was reduced. The composition of the essential oils significantly varied with the raw material from which the oil was obtained.