Efficacies of essential oils (EOs) of Vetiveria zizanioides (L.) Nash. (Poales: Poaceae) (VZ EO), Cananga odorata (Lam) Hook. F. & Thomson (Magnoliales: Annonaceae) (CO EO), and crude extract (CE) of Andrographis paniculata (Burm.F.) Wall ex. Nees (Lamiales: Acanthaceae) (AP CE), against laboratory (lab) and field strains of Culex quinquefasciatus Say were investigated. Irritant and repellent activities of individual and binary mixtures of plant extracts were compared with N,N-diethyl-m-toluamide (DEET) using an excito-repellency system. The irritant activity (direct tarsal contact), the mean percent escape response of VZ EO (91.67%, 83.33%), and CO EO (80%, 88.33%) were not significantly different compared with DEET (88.33%, 95%) against lab and field strains, respectively. Similarly, irritant responses in combinations (1:1 and 1:2, v:v) of either VZ EO or CO EO with AP CE were not significantly different from DEET against both strains (P > 0.001). The repellent activity (no tarsal contact), the mean percent escape response of VZ EO (68.33%), CO EO (61.67%), and VZ EO+AP CE (1:1, v:v) (81.67%) against lab strain and CO EO (85%) against field strain were not significantly different from that of DEET (P > 0.001). Interestingly, the greatest contact irritancy of VZ EO+AP CE (1:1, v:v) (96.67%) (P = 0.0026) and a stronger repellency response of CO EO (85%) (P = 0.0055) produced significantly different patterns of escape response compared with DEET against both lab and field strains, respectively. The EOs of VZ EO and CO EO or their mixture with AP CE showed potential as plant-based active ingredients for mosquito repellents. In addition, the major chemical constituents of VZ EO were β-vetivone (6.4%), khusimol (2.96%), and α-vetivone (2.94%) by gas chromatograpy–mass spectrometry.
Grain storage plays a crucial role in ensuring food security to Thai farmers so sustainable protection methods against deleterious microorganisms, such as fungi, are necessary. Essential oils (EOs) have demonstrated broad-spectrum fumigant antifungal activity against most storage fungi that are problematic in Thailand. Four storage fungi (Aspergillus flavus, A. niger, Curvularia lunata and Fusarium proliferatum) were isolated from dried rice and corn grains (stored for at least six months). EOs were extracted by hydrodistillation from clove buds (Syzygium aromaticum), fruit peel and leaves of makrut lime (Citrus hystrix), eucalyptus leaves (Eucalyptus sp.) and lemongrass stems (Cymbopogon citratus). The fungi inoculated on PDA in plastic cups were exposed to each EO vapour originating from paper disc attached in the lids in an inverted position. The minimum inhibitory concentration (MIC) for each EO was determined. Selected MICs were combined in a binary manner and similarly tested against the fungi. Fractional inhibitory concentration indices (FICI) were determined for each combination. Lemongrass and makrut lime leaf EOs were the most effective with MICs of 0.09 µL/mL against Curvularia lunata and 0.19-0.28 µL/mL against A. flavus, A. niger and F. proliferatum. Eucalyptus oil produced the least effective vapour (MIC 0.56-0.74 µL/mL) against all tested pathogens. A combination of lemongrass and makrut lime leaf EOs was partially synergistic against A. niger (FICI=0.75) but was fully synergistic against the other three fungi tested (FICI=0.5). The EOs from lemongrass and makrut lime leaf have potential to suppress the growth of the four grain-storage fungi tested.
We evaluated the behavioral responses of Aedes albopictus and Anopheles minimus to 3 isovaleric acid and lactic acid-based chemical lure blends and 2 individual alcohols, using Spatial Repellency Assay in a high-throughput screening system (HITSS). Five doses of 0.0002, 0.001, 0.0025, 0.005, and 0.01 g were tested per lure. A BG-lure was used as a reference standard. After 10-min exposure, the number of mosquitoes moving toward or away from the treated chamber was calculated. The results showed that all lures were repellent against Ae. albopictus except for Lure-4 (4% w/v isovaleric acid + 2% w/v lactic acid + 0.0025% w/v myristic acid + 2.5% w/v ammonium hydroxide) which showed a nonsignificant attractancy at the lowest dose. Significantly high spatial repellency was observed at the highest dose of all the tested lures including BG-lure. Lure-2 (isoamyl alcohol) was significantly repellent at all the tested doses. Against An. minimus, Lure-5 (0.02% w/v isovaleric acid + 2% w/v lactic acid) showed significant spatial repellency while Lure-4 was significantly attractant, at all the tested doses. All lures, except Lure-4, showed strong spatial repellency at high doses and attractancy or weak spatial repellency at the lowest dose of 0.0002 g. In summary, our study demonstrated that spatial repellency and attractancy of the tested lures were influenced by both the dose tested and the mosquito species. Lure-2 and Lure-4 are potential spatial repellents and attractants, respectively, for malaria and dengue vectors. However, further studies are necessary to confirm these results at a semifield and open field level.
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