A comparative study was conducted to assess the contact and fumigant toxicities of eleven monoterpenes on two important stored products insects--, Sitophilus oryzae, the rice weevil, and Tribolium castaneum, the rust red flour beetle. The monoterpenes included: camphene, (+)-camphor, (-)-carvone, 1-8-cineole, cuminaldehyde, (L: )-fenchone, geraniol, (-)-limonene, (-)-linalool, (-)-menthol, and myrcene. The inhibitory effect of these compounds on acetylcholinesterase (AChE) activity also was examined to explore their possible mode(s) of toxic action. Although most of the compounds were toxic to S. oryzae and T. castaneum, their toxicity varied with insect species and with the bioassay test. In contact toxicity assays, (-)-carvone, geraniol, and cuminaldehyde showed the highest toxicity against S. oryzae with LC(50) values of 28.17, 28.76, and 42.08 microg/cm(2), respectively. (-)-Carvone (LC(50) = 19.80 microg/cm(2)) was the most effective compound against T. castaneum, followed by cuminaldehyde (LC(50) = 32.59 microg/cm(2)). In contrast, camphene, (+)-camphor, 1-8-cineole, and myrcene had weak activity against both insects (i.e., LC(50) values above 500 microg/cm(2)). In fumigant toxicity assays, 1-8-cineole was the most effective against S. oryzae and T. castaneum (LC(50) = 14.19 and 17.16 mg/l, respectively). Structure-toxicity investigations revealed that (-)-carvone--, a ketone--, had the highest contact toxicity against the both insects. 1-8-Cineole--, an ether--, was the most potent fumigant against both insects. In vitro inhibition studies of AChE from adults of S. oryzae showed that cuminaldehyde most effectively inhibited enzyme activity at the two tested concentrations (0.01 and 0.05 M) followed by 1-8-cineole, (-)-limonene, and (L)-fenchone. 1-8-Cineole was the most potent inhibitor of AChE activity from T. castaneum larvae followed by (-)-carvone and (-)-limonene. The results of the present study indicate that (-)-carvone, 1,8-cineole, cuminaldehyde, (L)-fenchone, and (-)-limonene could be effective biocontrol agents against S. oryzae and T. castaneum.

The acaricidal activity of 12 monoterpenes against the two-spotted spider mite, Tetranychus urticae Koch, was examined using fumigation and direct contact application methods. Cuminaldehyde and (-)-linalool showed the highest fumigant toxicity with LC(50) = 0.31 and 0.56 mg/l, respectively. The other monoterpenes exhibited a strong fumigant toxicity, the LC(50) values ranging from 1.28 to 8.09 mg/l, except camphene, which was the least effective (LC(50) = 61.45 mg/l). Based on contact activity, the results were rather different: menthol displayed the highest acaricidal activity (LC(50) = 128.53 mg/l) followed by thymol (172.0 mg/l), geraniol (219.69 mg/l) and (-)-limonene (255.44 mg/l); 1-8-cineole, cuminaldehyde and (-)-linalool showed moderate toxicity. At 125 mg/l, (-)-Limonene and (-)-carvone caused the highest egg mortality among the tested compounds (70.6 and 66.9% mortality, respectively). In addition, the effect of molecular descriptors was also analyzed using the quantitative structure activity relationship (QSAR) procedure. The QSAR model showed excellent agreement between the estimated and experimentally measured toxicity parameter (LC(50)) for the tested monoterpenes and the fumigant activity increased significantly with the vapor pressure. Comparing the results of the fumigant and contact toxicity assays of monoterpenes against T. urticae with the results of acetylcholinesterase (AChE) inhibitory effect revealed that some of the tested compounds showed a strong acaricidal activity and a potent AChE inhibitory activity, such as cuminaldehyde, (-)-linalool, (-)-limonene and menthol. However, other compounds such as (-)-carvone revealed a strong fumigant activity but a weak AChE inhibitory activity.