Effect of leaf essential oil of Coccinia indica on egg hatchability and different larval instars of malarial mosquito Anopheles stephensi

Chemical analysis, repellent, larvicidal, and oviposition deterrent activities of plant essential oils against Aedes aegypti, Anopheles gambiae, and Culex quinquefasciatus

Plant-based essential oils have gained attention as a natural alternative for controlling mosquitoes due to their repellent, larvicidal and oviposition deterrent properties. We tested repellent, larvicidal, and oviposition deterrent effects of essential oils (EOs) of Mentha spicata (L.), Ocimum basilicum (L.), and Abutilon indicum (L.) against three mosquito species (Diptera: Culicidae) including Aedes aegypti (L.), Anopheles gambiae s. l. Giles, and Culex quinquefasciatus Say by using contact-based technique. In screening bioassays, M. spicata I, M. spicata II, O. basilicum I, O. basilicum II, and A. indicum EOs showed higher repellency against Cx. quinquefasciatus as compared to Ae. aegypti and An. gambiae when tested at 33.3 μg/cm2. In time-span bioassays performed at 33.3 μg/cm2, EO of M. spicata I exhibited 100% repellence up to 45, 30, and 75 min against Ae. aegypti, An. gambiae, and Cx. quinquefasciatus, respectively. Interestingly, at this tested dose, M. spicata I and M. spicata II showed higher repellence compared to DEET against Ae. aegypti and Cx. quinquefasciatus after 45 and 75 min, respectively. Their repellency was observed up to 150 and 210 min against Ae. aegypti and Cx. quinquefasciatus, respectively. In larvicidal bioassays, M. spicata I EO proved more toxic against 2nd instar larvae of Ae. aegypti, An. gambiae, and Cx. quinquefasciatus (LC50 = 11.0, 42.9, and 12.6 mg/L, respectively) compared to other tested EOs. In oviposition bioassays, M. spicata I exhibited the highest activity, showing 60%, 46%, and 79% oviposition deterrence against Ae. aegypti, An. gambiae, and Cx. quinquefasciatus, respectively, tested at a dose of 600 µg/cm2. Major compounds of M. spicata I, M. spicata II, O. basilicum I, and O. basilicum II EOs were piperitenone oxide (38.8%), piperitone oxide (35.4%), estragole (55.3%), and linalool (43.8%), respectively. In conclusion, M. spicata EO could be used to control mosquitoes and their bites.

Bioactivity of select essential oil constituents against life stages of Anopheles arabiensis (Diptera: Culicidae)

Malaria is transmitted by infected female Anopheles mosquitoes, and An. arabiensis is a main malaria vector in arid African countries. Like other anophelines, its life cycle comprises of three aquatic stages; egg, larva, and pupa, followed by a free flying adult stage. Current vector control interventions using synthetic insecticides target these stages using adulticides or less commonly, larvicides. With escalating insecticide resistance against almost all conventional insecticides, identification of agents that simultaneously act at multiple stages of Anopheles life cycle presents a cost-effective opportunity. A further cost-effective approach would be the discovery of such insecticides from natural origin. Interestingly, essential oils present as potential sources of cost-effective and eco-friendly bioinsecticides. This study aimed to identify essential oil constituents (EOCs) with potential toxic effects against multiple stages of An. arabiensis life cycle. Five EOCs were assessed for inhibition of Anopheles egg hatching and ability to kill larvae, pupae and adult mosquitoes of An. arabiensis species. One of these EOCs, namely methyleugenol, exhibited potent Anopheles egg hatchability inhibition with an IC₅₀ value of 0.51 ± 0.03 μM compared to propoxur (IC₅₀: 5.13 ± 0.62 μM). Structure-activity relationship study revealed that methyleugenol and propoxur share a 1,2-dimethoxybenze moiety that may be responsible for the observed egg-hatchability inhibition. On the other hand, all five EOCs exhibited potent larvicidal activity with LC₅₀ values less than 5 μM, with four of them; cis-nerolidol, trans-nerolidol, (-)-α-bisabolol, and farnesol, also possessing potent pupicidal effects (LC₅₀ < 5 μM). Finally, all EOCs showed only moderate lethality against adult mosquitoes. This study reports for the first time, methyleugenol, (-)-α-bisabolol and farnesol as potent bioinsecticides against early life stages of An. arabiensis. This synchronized activity against Anopheles aquatic stages shows a prospect to integrate EOCs into existing adulticide-based vector control interventions.

Ovicidal toxicity of plant essential oils and their major constituents against two mosquito vectors and their non-target aquatic predators

Plant essential oil (EO) is a natural alternative to synthetic chemical insecticides for mosquito control. EOs from Citrus aurantium L., Cymbopogon citratus (Stapf.), and Cinnamomum verum (J. Presl.) were selected for topical assay of their ovicidal activity against Aedes aegypti (Linnaeus) and Aedes albopictus (Skuse). Their efficacy was compared to that of 1% (w/w) temephos. In addition, their non-toxicity against aquatic mosquito predators, Poecilia latipinna and Poecilia reticulata, was tested. Found by GC-MS analysis, the major constituent of C. verum EO was trans-cinnamaldehyde, of C. aurantium EO was D-limonene, and of C. citratus EO was geranial. Both C. verum EO and trans-cinnamaldehyde at a high concentration (30,000 ppm) exhibited high ovicidal activity against Ae. aegypti and Ae. albopictus eggs after 48 h of incubation with an inhibition rate of 91.0-93.0% for C. verum EO and 96.7-95.2% for trans-cinnamaldehyde. The combination of C. verum EO + geranial exhibited the strongest synergistic inhibition activity (100%) against the two mosquito vectors and was five times more effective than temephos. Moreover, they were not toxic to the non-target fishes. As a safe ovicidal agent for mosquito egg control, the combination of C. verum EO + geranial has excellent potential.

Insecticidal activities of Streptomyces sp. KSF103 ethyl acetate extract against medically important mosquitoes and non-target organisms

A potentially novel actinobacterium isolated from forest soil, Streptomyces sp. KSF103 was evaluated for its insecticidal effect against several mosquito species namely Aedes aegypti, Aedes albopictus, Anopheles cracens and Culex quinquefasciatus. Mosquito larvae and adults were exposed to various concentrations of the ethyl acetate (EA) extract for 24 h. Considerable mortality was evident after the EA extract treatment for all four important vector mosquitoes. Larvicidal activity of the EA extract resulted in LC₅₀ at 0.045 mg/mL and LC₉₀ at 0.080 mg/mL for Ae. aegypti; LC₅₀ at 0.060 mg/mL and LC₉₀ at 0.247 mg/mL for Ae. albopictus; LC₅₀ at 2.141 mg/mL and LC₉₀ at 6.345 mg/mL for An. cracens; and LC₅₀ at 0.272 mg/mL and LC₉₀ at 0.980 mg/mL for Cx. quinquefasciatus. In adulticidal tests, the EA extract was the most toxic to Ae. albopictus adults (LD₅₀ = 2.445 mg/mL; LD₉₀ = 20.004 mg/mL), followed by An. cracens (LD₅₀ = 5.121 mg/mL; LD₉₀ = 147.854 mg/mL) and then Ae. aegypti (LD₅₀ = 28.873 mg/mL; LD₉₀ = 274.823 mg/mL). Additionally, the EA extract exhibited ovicidal activity against Ae. aegypti (LC₅₀ = 0.715 mg/mL; LC₉₀ = 6.956 mg/mL), Ae. albopictus (LC₅₀ = 0.715 mg/mL; LC₉₀ = 6.956 mg/mL), and An. cracens (LC₅₀ = 0.715 mg/mL; LC₉₀ = 6.956 mg/mL), evaluated up to 168 h post-treatment. It displayed no toxicity on the freshwater microalga Chlorella sp. Beijerinck UMACC 313, marine microalga Chlorella sp. Beijerinck UMACC 258 and the ant Odontoponera denticulata. In conclusion, the EA extract showed promising larvicidal, adulticidal and ovicidal activity against Ae. aegypti, Ae. albopictus, An. cracens, and Cx. quinquefasciatus (larvae only). The results suggest that the EA extract of Streptomyces sp. KSF103 has the potential to be used as an environmental-friendly approach in mosquito control. The current study would serve as an initial step toward complementing microbe-based bioinsecticides for synthetic insecticides against medically important mosquitoes.

Essential oil of Piper purusanum C.DC (Piperaceae) and its main sesquiterpenes: biodefensives against malaria and dengue vectors, without lethal effect on non-target aquatic fauna

The mosquito vectors of the genera Aedes and Anopheles present resistance to several commercial insecticides, which are also toxic to non-predator targets. On the other hand, essential oils are a promising source of insecticides. Thus, in this work, the essential oil from the leaves of Piper purusanum was characterized by gas chromatography-based approaches and evaluated as biodefensive against malaria and dengue vectors. The main compounds of P. purusanum essential oil were β-caryophyllene (57.05%), α-humulene (14.50%), and germacrene D (8.20%). The essential oil inhibited egg hatching (7.6 ± 1.5 to 95.6 ± 4.5%), caused larval death (LC₅₀ from 49.84 to 51.60 ppm), and inhibited the action of acetylcholinesterase (IC₅₀ of 2.29 µg/mL), which can be related to the mechanisms of action. On the other hand, the biological activities of β-caryophyllene, α-humulene, and germacrene D were higher than that of essential oil. In addition, these sesquiterpenes and essential oil did not show a lethal effect on Toxorhynchites splendens, Anisops bouvieri, Gambusia affinis, and Diplonychus indicus (LC₅₀ from 2098.80 to 7707.13 ppm), although D. indicus is more sensitive (SI/PSF from 48.56 to 252.02 ppm) to essential oil, representing a natural alternative against these relevant vectors.

Impact of Cymbopogon flexuosus (Poaceae) essential oil and primary components on the eclosion and larval development of Aedes aegypti

The current study describes the effects of sub-lethal concentrations and constituent compounds (citral and geranyl acetate) of Cymbopogon flexuosus essential oil (EO) on the development of Aedes aegypti. We treated eggs with 6, 18, or 30 mg L^-1 and larvae with 3 or 6 mg L^-1 of EO and its major compounds (citral and geranyl acetate). Citral and geranyl acetate were evaluated at 18, 30, and 42 mg L^-1 and compared with commercial growth inhibitors (diflubenzuron and methoprene). We measured larval head diameter, siphon length, and larval length. Finally, we examined concentrations of molt hormone (MH) and juvenile hormone III (JH III) using high-performance liquid chromatography coupled to mass spectrometry. All geranyl acetate concentrations decreased egg hatching, while EO altered molting among larval instars and between larvae and pupae, with an increase in the larval length (3 mg L^-1: 6 ± 0.0 mm; 6 mg L^-1: 6 ± 0.7 mm) and head width (3 mg L^-1: 0.8 ± 0 mm; 6 mg L^-1: 0.8 ± 0.0 mm) compared with the control group. We did not detect chromatographic signals of MH and JH III in larvae treated with C. flexuosus EO or their major compounds. The sub-lethal concentrations C. flexuosus EO caused a similar effect to diflubenzuron, namely decreased hormone concentrations, an extended larval period, and death.

Plant-Derived Essential Oils; Their Larvicidal Properties and Potential Application for Control of Mosquito-Borne Diseases

Mosquito-borne diseases are currently considered as important threats to human health in subtropical and tropical regions. Resistance to synthetic larvicides in different species of mosquitoes, as well as environmental pollution, are the most common adverse effects of excessive use of such agents. Plant-derived essential oils (EOs) with various chemical entities have a lower chance of developing resistance. So far, no proper classification based on lethal concentration at 50% (LC₅₀) has been made for the larvicidal activity of EOs against different species of Aedes, Anopheles and Culex mosquitoes. To better understand the problem, a summary of the most common mosquito-borne diseases have been made. Related articles were gathered, and required information such as scientific name, used part(s) of plant, target species and LC₅₀ values were extracted. 411 LC₅₀ values were found about the larvicidal activity of EOs against different species of mosquitoes. Depending on the obtained results in each species, LC₅₀ values were summarized as follows: 24 EOs with LC₅₀ < 10 µg/mL, 149 EOs with LC₅₀ in range of 10- 50 µg/mL, 143 EOs having LC₅₀ within 50- 100 µg/mL and 95 EOs showing LC₅₀ > 100 µg/mL. EOs of Callitris glaucophylla and Piper betle against Ae. aegypti, Tagetes minuta against An. gambiae, and Cananga odorata against Cx. quinquefasciatus and An. dirus having LC₅₀ of ~ 1 µg/mL were potentially comparable to synthetic larvicides. It appears that these plants could be considered as candidates for botanical larvicides.

Laboratory Evaluation of Larvicidal and Oviposition Deterrent Properties of Edible Plant Oils for Potential Management of Aedes aegypti (Diptera: Culicidae) in Drinking Water Containers

The yellow fever mosquito, Aedes aegypti (Diptera: Culicidae) transmits several devastating arboviruses, including dengue, chikungunya, and Zika virus, making development of inexpensive and eco-friendly strategies for its control an urgent priority. We evaluated the lethality of 13 commonly used plant-derived edible oils against late-third instar Ae. aegypti and then tested the three most lethal oils for stage-specific differences in lethality. We also examined the effects of the most lethal (hempseed), moderately lethal (sunflower and peanut), and least lethal (olive) oils on survival to adulthood and oviposition behavior of gravid females. We hypothesized that the insecticidal activity of edible oils is a function of the content of their linoleic acid, a key fatty acid component with film-forming properties. Among the 13 oils tested, hempseed oil was the most lethal, with an LC50 of 348.25 ppm, followed by sesame (670.44 ppm) and pumpkinseed (826.91 ppm) oils. Oils with higher linoleic acid content were more lethal to larvae than those with low linoleic acid content. Furthermore, pure concentrated linoleic acid was more lethal to larvae compared to any edible oil. In comparison to early instars, late instars were more susceptible to hempseed, sunflower, peanut, and olive oils; these oils also acted as oviposition deterrents, with effective repellency ≥63%. The proportion of larvae surviving to adulthood was significantly reduced in hempseed, sunflower, peanut, and olive oil treatments relative to controls. Our results suggest that some edible plant oils have potential as effective, eco-friendly larvicides, and oviposition deterrents for controlling container-dwelling mosquitoes, especially in resource-limited settings.

Environmentally benign nanometric neem-laced urea emulsion for controlling mosquito population in environment

The increasing risk of vector-borne diseases and the environmental pollution in the day-to-day life due to the usage of the conventional pesticides makes the role of nanotechnology to come into the action. The current study deals with one of the applications of nanotechnology through the formulation of neem urea nanoemulsion (NUNE). NUNE was formulated using neem oil, Tween 20, and urea using the microfluidization method. Prior to the development of nanoemulsion, the ratio of oil/surfactant/urea was optimized using the response surface modeling method. The mean droplet size of the nanoemulsion was found to be 19.3 ± 1.34 nm. The nanoemulsion was found to be stable for the period of 4 days in the field conditions which aids to its mosquitocidal activity. The nanoemulsion exhibited a potent ovicidal and larvicidal activity against A. aegypti and C. tritaeniorhynchus vectors. This result was corroborated with the histopathological analysis of the NUNE-treated larvae. Further, the effect of NUNE on the biochemical profile of the target host was assessed and was found to be efficacious compared to the bulk counterpart. The nanoemulsion was then checked for its biosafety towards the non-target species like plant beneficial bacterium (E. ludwigii), and phytotoxicity was assessed towards the paddy plant (O. sativa). Nanometric emulsion at the concentration used for the mosquitocidal application was found to be potentially safe towards the environment. Therefore, the nanometric neem-laced urea emulsion tends to be an efficient mosquito control agent with an environmentally benign property.