Plant essential oils have been reported to show repellent, insecticidal and growth-reducing effect on many insect herbivores. Recently, it has been reported that plants synthesize secondary metabolites regulating insect juvenile hormone (JH) receptor complex as a part of their defense mechanisms. We tested 180 plant essential oil compounds using a yeast two-hybrid system transformed with the Aedes aegypti JH receptor as a reporter system. Among them, we identified 17 compounds that showing high JH agonist (JHA) or antagonist (JHAN) activities. They were grouped into 4 groups by their structural similarity and their insecticidal activity was tested against larvae of Aedes albopictus, Plutella xylostella, Plodia interpunctella and Laodelphax striatellus. These results could provide insights on the plant-insect coevolution and may be useful for the development of insect specific and safe pesticides.

Dental caries is the most common chronic disease in the dental field. Streptococcus mutans (S. mutans) is the most important bacteria in the formation of dental plaque and dental caries. In a previous study, we confirmed that the essential oil of Chrysanthemum boreale has antibacterial activity against S. mutans. Alpha-pinene is one of the major chemical components of Chrysanthemum boreale essential oil. In the present study, we investigated the inhibitory effects of ɑ-pinene on cariogenic properties such as growth, acid production, biofilm formation, and bactericidal activity on S. mutans. Alpha-pinene at a concentration range of 0.25-0.5 mg/mL significantly inhibited the growth of S. mutans and acid production of S. mutans. Biofilm formation was significantly inhibited at > 0.0625 mg/mL ɑ-pinene, similar to the data from scanning electronic microscopy. Under confocal laser scanning microscopy, the bacterial viability was decreased by ɑ-pinene in a dose-dependent manner. These results suggested that ɑ-pinene may be a useful agent for inhibiting the cariogenic properties of S. mutans.

The penetration-enhancing effect of a binary mixture of rosemary essential oil constituents, and its correlation to increased toxicity were investigated. GC-MS analyses revealed that the major constituents of rosemary essential oil were 1,8-cineole and camphor, and a binary mixture of them showed synergistic insecticidal activity via topical application in third instar larvae of the cabbage looper, Trichoplusia ni. In the mixture, penetration of camphor was significantly enhanced both in vivo and in vitro, whereas that of 1,8-cineole did not change much compared to individual application. Injection into hemocoel of fifth instar larvae indicated that camphor was more toxic than 1,8-cineole, although it was less toxic than 1,8-cineole when applied topically. A possible mechanism for enhanced penetration of camphor by 1,8-cineole is reduced surface tension of the mixture. Contact angles of 50% acetonic solutions on a layer of beeswax showed that the surface tension of 1,8-cineole was lower than that of camphor. Another probable reason is a change of camphor from solid to liquid status, when mixed with 1,8-cineole. This is the first report attempting to explain the synergistic insecticidal interaction of essential oil components, from a physiochemical perspective.

The larvicidal activity of 104 plant essential oils and their major constituents against third instar larvae of mushroom fly Camptomyia corticalis was examined using a vapor-phase mortality bioassay. Results were compared with those of the conventional insecticides dichlorvos. Over ten plant essential oils exhibited showed more than 90% mortality. At a rate of 0.2 mg/cm3 air, pulegone, and thujone, exhibited 100% morality whereas camphor, 1,8-cineole, α-pinene, thymol and menthol showed 90% mortality at 0.6 mg/cm3 air. Global efforts to reduce the level of highly toxic synthetic insecticides in the agricultural environment justify further studies on the essential oils and their constituents described as potential insecticides for the control of the mushroom fly as fumigants with contact action.

The repellency to female Stomoxys calcitrans (L.) of 21 essential oils alone or in combination with Calophyllum inophyllum nut oil (tamanu oil) was examined using a skin bioassay. Results were compared with those following treatment with the commonly used repellent DEET (N,N-diethyl-3-methylbenzamide). As judged by the protection time (PT) to first bite at 0.5 mg cm-2, patchouli (3.67 h) was the most effective essential oil, followed by clove bud, lovage root, clove leaf and thyme white essential oils (3.50-2.12 h). Thyme red, oregano and geranium essential oils exhibited moderate protection time (PT, 1.24-1.11 h). At 0.25 mg cm-2, effective protection time of clove bud, clove leaf and lovage root essential oils was about 1 h. The protection times of DEET were 4.47 and 2.17h at 0.5 and 0.25 mg cm-2 respectively. The remarkable increase in the protection time were produced by binary mixtures of five essential oils (clove bud, clove leaf, thyme white, patchouli and savory) and tamanu oil (0.25:2.0 mg cm-2) compared with those of either the constituted essential oil, tamanu oil or DEET alone, indicating the involvement in synergy. These essential oils, tamanu oil and binary mixtures did not cause any adverse effects on the human volunteers at 0.5 mg cm-2 except savory oil. Thus binary mixtures of essential oils and tamanu oil described merit further study as potential insect repellents for protection from humans and domestic animals from biting and nuisance caused by S. calcitrans.

This study was carried out to investigate the effect of forms and levels of nitrogen fertilizer on plant growth and essential oil production of Agastache rugosa. Calcium nitrate had more influenced on length and width of leaves and lateral branch length than did urea. When nitrogen fertilizer level was increased from 12 kgN/I0a to 24kgN/I0a, plant growth was stimulated and dry matter of leaf and inflorescence were increased. Top dry matter of plant with calcium nitrate treatment (38.4 g) was heavier than that of urea treatment (32.8 g). Interactions among accession and nitrogen form and nitrogen rate were not significantly different for top dry matter. The forms and rate of nitrogen fertilizer did not affect estragole content. The estragole contents was higher in leaf (91.8%) than that of inflorescence (81.3%). While the essential oil content was not affected by different nitrogen forms, nitrogen level affected the essential oil contents positively by increasing dry matter. Essential oil yield was not affected by accession or nitrogen form, but by nitrogen rate. With increasing N application from 12kgN/I0a to 24 kgN/I0a, essential oil yield was increased by 95.8 %.

This expriment was carried out to get basic information on composition and yield of aromatic constituents in leaves of four medicinal plants, Angelica tenuissima, Chrysanthemum zawadskii. ssp. latilobum, Artemisia iwayomogi and Artemisia capillaris. Volatile aromatic constituents, 28 compounds in Angelica tenuissima were identified and 19 compounds were indentified in Chrysanthemum zawadskii ssp. latilobum. Volatile aromatic constituents, 23 compounds in Artemisia iwayomogi and Artemisia capillaris were identified. Major volatile aromatic consitiuents analyzed by GC/MS in four plants were α-pinene, camphene, sabinene, cis-2-hexanol, and camphor etc. Content of essential oils in Angelica tenuissima, Chrysanthemum zawadskii ssp. latilobum, Artemisia iwayomogi and Artemisia capillaris were 0.014, 0.275, 0.785, and 0.452%, respectively. As a result, it was suggested that a medicinal plant, Artemisia iwayomogi, was worthy of using as a useful material of perfume.