Editorial Board

Today, increasing public awareness of the environmental effects of chemical pesticides, health hazards, and adverse effects on non-target organisms becomes more critical on continued use of synthetic nematicides (Pandey et al., 2000; Hong et al., 2010). These undesirable effects highlight the need for application of selective nematode control alternatives, because chemical nematicides such as 1,2-dibromo-3-chloropropane (DBCP) and 1,2-dibromoethane (ethylene dibromide) have been reserved from the agricultural chemical market due to their harmful effects on human health (Oka et al., 2000; Hong et al., 2010). It is necessary to develop new alternative control strategies for plant-parasitic nematodes, especially those with fumigant action in soil, because powders formulation are usually less effective and do not adequately penetrate deep layers (Hong et al., 2010). One possible alternative is utilization of natural pesticides based on plant-derived extracts such as essential oils or other nematicidal sources from plants (Chitwood, 2002; Nguyen et al., 2013; Kim et al., 2014). Consequently a large number of plants (flowers, leaves, and rhizomes) have been experimented for their nematicidal effects (Pandey et al., 2000). However, nematicidal effects of essential oils and its volatile constituents against Meloidogyne hapla have not been reported. In our present study, plant essential oils from nine species were evaluated for their nematicidal and hatching inhibition activities against M. hapla second-stage juveniles (J2) and eggs under laboratory conditions. We also analyze the volatile compounds of essential oil extracted from Alpinia galanga rhizomes.

Nematicidal activities of nine essential oils against M. hapla J2 were evaluated by 24-well culture plate bioassay (Table 1). Based on the mortality of juveniles at a concentration of 125 μg/mL, the most active essential oil was A. galanga (100%), followed by C. carbi (22.3%), E. caryophyllata (9.4%), C. zeylanicum (7.2%), M. pulegium (2.4%), and F. vulgare (2.1%). However, L. intermedia cv. Grosso, M. viridiflora, and M. fragrans had no activity at the same concentration against M. hapla juveniles. Because of potent activity of six essential oils (A. galanga, C. carbi, E. caryophyllata, C. zeylanicum, M. pulegium, and F. vulgare) against M. hapla juveniles, the hatching inhibition of selected oils to M. hapla eggs was investigated using 24-well culture plate bioassay at a concentration of 500 μg/mL (Fig. 1). A. galanga was also effective at hatching inhibition, followed by C. zeylanicum, E. caryophyllata, F. vulgare, M. pulegium, and C. carbi. These results indicate that toxicity differences were related to that of susceptibility varies with the developmental stage. According to Wharton (2002), egg stage is the most resistant stage in the nematode developmental stages, due to its three-layer shell (vitelline layer, chitinous layer, and lipid layer).

Fig. 1. 
Inhibitory effect of 500 μg/mL essential oils on hatching of Meloidogyne hapla eggs.

Volatile constituents accounting 98.1% of A. galanga rhizomes oil were identified by GC/MS (Table 2). The identified compounds in A. galanga oil were methyl cinnamate (87.39%), 1,8-cineole (4.37%), β-pinene (2.54%), α-pinene (2.23%), p-cymene (1.05%), and fenchone (0.53%). The volatile constituents from A. galanga rhizomes oil were grouped as monoterpene ester (methyl cinnamate), monoterpene hydrocarbon (α-pinene, β-pinene, and p-cymene), monoterpene oxide (1,8-cineole), and monoterpene ketone (fenchone). Compared with the previous studies (Raina et al., 2014), the major components of A. galanga oil were 1.8-cineole (63.4%), (E)-β-farnesene (8.4%), eugenol acetate (3.3%), α-terpineol (2.8%), terpinen-4-ol (2.8%), and (E)-methyl cinnamate (0.2%). However, the occurrence of methyl cinnamate as major component in the A. galanga oil in the present study is not matched those of previous reports. The quantitative and compositional differences of chemical composition of plants are attributed to geographical origin, extraction methods, parts of herbs, chemotypes, and harvest conditions (Raina et al., 2002; Yang and Lee, 2013). Essential oil from various plants is used commercially sources of pesticides for insect control (Oka et al., 2000; Jeon et al., 2015). Several essential oils and their constituents have been reported to their nematicidal effect, such as Mentha spp., Origanum spp., Eucalypus spp., and Tagetes spp., and their components (alcohols, alkaloids, phenolics, and terpenoids) (Uhlenbroek et al., 1958; Belcher and Hussey, 1977; Oka et al., 2000; Pandey et al., 2000; Lee et al., 2011). Our study also demonstrated that essential oil from A. galanga rhizome had nematicidal effective against M. hapla juveniles and eggs. Therefore, natural nematicides based on plant extracts, essential oils, and phytochemicals could be beneficial as alternatives to pesticides for control nematodes. Further experiments are needed to evaluate the nematicidal efficacy under greenhouse and field conditions for the practical use of A. galanga rhizome-derived oil and its volatile constituents. Additionally, nematicidal constituents derived from A. galanga oil and mode of action need to be conducted.