Studies On Bioactivities Of Essential Oils Against Culex Pipiens Pallens And The Development Of Natural Mosquito Control Agents

Mosquitoes could endanger human health by annoying, biting, sucking blood andspreading several diseases such as dengue, plague and malaria. Mosquito control is asystematic project, and the development and application of the mosquito control agents is animportant aspect. Mosquito control agents must be safe because they are applied in the humanliving environment and have in contact with skin. Control of mosquitoes relies heavily onsynthetic insecticides, and the overzealous use of them has led to problems such as widedevelopment of insecticide resistance, pesticide residues, and indoor environmental pollution,which has raised concerns about the safty of hygienic insecticides. Essential oils have theadvantages of renewability and low residue and toxicity, which are in line with therequirements of people for hygienic insecticides. However, they are less used in practiceowning to limited resources, high cost and difficulty in formulation processing. Thus,on thebasis of the previous research,25kinds of essential oils and35compounds were evaluatedfor their larvicidal, fumigant, repellent and the joint effect against Culex pipiens pallens inthis study, and several formulas of botanical agents with applied value in mosquito controlwere further developed. The main results and conclusions are shown as follows:1. The samples were evaluated for the larvicidal activity (via immersion method), thecombined effect, and the preliminarydevelopment of surface larvicidal agent.The resultsindicated that:(1) that seeds and leaf essential oils of star anise showed high larvicidaltoxicity against C. pipiens pallens larvae with95.0%and93.8%mortality after a24hexposure at80mg/L, respectively. trans-Anethole was the most toxic followed by farnesoland ρ-cymene among35compounds, with LC50values of16.1,21.5and25.9mg/L,respectively. trans-Anethole and farnesol displayed a synergistic effect at the ratio of1:1(w/w) with LC50value of9.5mg/L.(2) Oleic acid exhibited synergistic effect on thelarvicidal activities of trans-anethole and farnesol. The ternary system of trans-anethole, oleicand ethyl alcohol could form a layer of film and kill fourth instar larvae of C. pipiens pallenson the surface of water, and their efficacy was related to the surface area rather than thevolume of water. These results provide reference and evidence for the development of surfacelarvicidal agents based on essential oils. 2. The samples were tested for the knockdown and lethal activity (via airtightfumigation), the combined effect and the preliminarydevelopment ofvaporizing mat. Theresults demonstrated that:(1)Cinnamon oil and its major constituent, trans-cinnamaldehydeexhibited excellent lethal effects but low knockdown speed, and wintergreen oil and its majorconstituent, methyl salicylate had low lethal effects but fast knockdown speed at the sublethaldose againstC. pipiens pallens adults. The blend of methyl salicylate andtrans-cinnamaldehyde provided both fast knockdown and high acute toxicity.(2) Carvacrolexhibited the highest fumigant activity followed by thymol, with LC50values of0.26and0.28μL/L air, respectively, and their binary mixture (1:1, v/v) displayed a synergistic effectwith the co-toxicity coefficient (CTC) value of174.1and LC50value of0.16μL/L air.(3)Theactual efficacy of the binary mixture at300mg/mat was comparable with that of d-allethrin at30mg/mat tested in vaporizing mats by the glass chamber method (70×70×70cm).3. Repellent activities of the samples were investigated using “Y”-tube olfactometer onC. pipiens pallens adults. The results showed that:(1) Cinnamon oil and geranium oil exhibitgood repellent activity at the dosage of0.2μL (percent repellency>85%,20min) among25kinds of essential oils. trans-Cinnamaldehyde and geraniol exhibited the best repellentactivity at the same dosage (percent repellency=100%,20min).(2) The binary mixture oftrans-cinnamaldehyde and geraniol (1:1, v/v) displayed a synergistic effect. The preparationtechnology of a controlled-release hydrogel solid mosquito repellent was investigated basedon the the binary mixture of trans-cinnamaldehyde and geraniol (1:1, v/v) with the primaryadditions of carrageenan, tween-80and water.4. Seven compounds with preferable larvicidal, fumigant or repellent performance wereseparately combined with natural pyrethrin (1.0%+0.6%, wt) to evaluate their synergisticeffects in aerosols. The results showed that:(1)Geraniol and methyl salicylate exhibited thebest synergism with synergistic ratios (SR) of1.35and1.34, respectively. Furthermore, thebinary mixture of geraniol and methyl salicylate had a better synergistic effect at the ratio of7:3(w/w) screened from five different ratios (9:1,7:3,5:5,3:7,1:9, w/w) with SR value of1.58than that of piperonyl butoxide (PBO, SR=1.39).(2) The aerosol containing naturalpyrethrin (0.6%, wt) and synergist XS (1.0%, geraniol:methyl salicylate=7:3,wt) displayedpotent efficacy on C. pipiens pallens adults with100%knockdown (1h) and100%mortality(24h) in simulating house, reaching the “A” level of national standards. The test report fromthe relevant enterprise demonstrated that botanical synergist XS has potential ofmarketization for its significant synergywith natural pyrethrin.In conclusion，bioactivities of essential oils on mosquitoes can be classified into lethaleffects and sublethal effects, and both of them have developing potential and practical value. Joint effects between essential oil compounds provide the basis for developing safe, effectiveand natural mosquito control agents. The results of this study further confirm the technicalfeasibility and application possibility of developing natural mosquito control agents based onessential oils. Furthermore, these results have both theoretical significance and referencevalue in developing new technologies and products and establishing comprehensiveprevention systems for controlling mosquitoes.