Persistent susceptibility of Aedes aegypti to eugenol

[Effects of larval feeding amount on development and deltamethrin resistance in Aedes albopictus]

OBJECTIVES

To investigate how larval feeding regimens influence development and deltamethrin resistance of Aedes albopictus to provide evidence for standardizing larval feeding protocols in studies of insecticide resistance.

METHODS

Aedes albopictus larvae of a laboratory resistant strain were divided into 3 groups (n=500) and reared with high, medium, and low food availability (100, 50, or 25 mg daily for the 1st and 2nd instars, and 500 mg 250, or 125 mg daily for 3rd and 4th instars). The developmental time, pupation rate, adult emergence rate, adult body weight, and wing length were recorded in each group, and deltamethrin resistance of the mosquitoes was assessed using larval bioassays and contact tube tests for adults.

RESULTS

Significant developmental differences were observed across the 3 feeding groups. Larval development time decreased as the food availability increased, and both high- and low-food groups showed reduced pupation rates (χ²=16.282, 7.440) and emergence rates (χ²=4.093, 6.977) compared to the medium-food group. Adult body weight and wing length were positively correlated with the amount of larval food intake (P<0.05). In high, medium and low food intake groups, larval LC50 values for deltamethrin were 0.110, 0.072 and 0.064 mg/L, adult KDT50 values were 97.404, 68.964 and 65.005 min, and adult mosquitoe mortality rates at 24 h after deltamethrin exposure were 12%, 16% and 19%, respectively.

CONCLUSIONS

The feeding amount during larval stage significantly impacts the development and deltamethrin resistance of Aedes albopictus, suggesting the importance of standardization of larval nutrition for ensuring comparability of resistance test data across laboratories.

Essential Oil and Waste Hydrosol of Ocimum Tenuiflorum L.: A Low-Cost Raw Material Source of Eugenol, Botanical Pesticides, and Therapeutic Potentiality

In this study, essential oils and waste hydrosols of leaves of Ocimum tenuiflorum in four different geographical locations were extracted by hydrodistillation method and using gas chromatography/mass spectrometry (GC/MS) for chemical composition analysis. All four essential oil samples contained the main components (E)-β-caryophyllene (27.8-49.0 %), trans-β-elemene (20.3-37.1 %) and eugenol (9.0-44.0 %). Three of the four hydrosol samples had eugenol in absolute content (94.5-98.6 %), while the remaining hydrosol sample had two main components, elemicin (77.8 %) and eugenol (14.2 %). Essential oils and hydrosols demonstrated larvicidal activities against four important disease-transmitting mosquito species including Aedes aegypti, Aedes albopictus, Culex quinquefasciatus, and Culex fuscocephala with 24-h LC50 values in the range 15.42-56.01 μg/mL and 53.88-97.80 μg/mL for the essential oils and the hydrosols, respectively. Essential oils and hydrosols strongly inhibited the acetylcholinesterase (AChE) enzyme of electric eels with IC50 values in the range of 25.35-107.19 μg/mL. Microemulsion (ME) can be considered as a sustainable pesticide formulation over 300 days and has improved larvicidal activity compared to free essential oil. The O. tenuiflorum in Vietnam can be considered a low-cost source of eugenol, botanical pesticides that control disease-transmitting mosquitoes, as well as having therapeutic potential to be further investigated.

Larvicidal activity of β-Citral: An In-vitro and In-silico study to understand its potential against mosquito

Tropical and subtropical regions face millions of deaths from mosquito-borne illnesses yearly. Insecticides prevent transmission but pose health risks like dermatitis and allergies. The primary objective was to mitigate the recurring dependence on synthetic insecticides, thereby curbing the development of mosquito resistance. Leaves of Cymbopogon flexuosus (lemongrass) was collected from Mayurbhanj, India, processed, then extracted by steam distillation for essential oils & analyzed spectroscopically. Larvicidal assays were performed across varying concentrations, revealing the significant mortality induced by the Cymbopogon flexuosus extract against Anopheles stephensi larvae. 3D structure was modelled by using G protein-coupled receptors (GPCR) sequence and structural stability was also validated. After docking the binding free energy was determined from GPCR protein with β-citral complex. Molecular dynamics (MD) study was conducted on the docked pose that displayed an optimal interactome profile. The larvicidal assay at the 12th and 24th hour revealed the highest LC50 (lethal concentration) of 23.493 ppm and 19.664 ppm . β-Citral has a high binding affinity and an identifiable binding site, which suggests that it may play a larvicidal role in regulating the receptor's function by creating stable complexes with it. β-Citral from lemongrass oils has potential larvicidal activity and effective against GPCR family 1 of mosquito and highly effective repellents against mosquito-borne diseases.

The low-lethal concentrations of rotenone and pyrethrins suppress the population growth of Rhopalosiphum padi

As an important pest on winter wheat, Rhopalosiphum padi (L.) causes damage to the wheat yield by sucking plant nutrients, transmitting plant viruses and producing mildew. R. padi has been reported to develop resistance to pyrethroids and neonicotinoids. To explore potential alternative approaches for R. padi control, the activity of 10 botanical insecticides was evaluated. Results suggested that the toxicity of rotenone and pyrethrins to R. padi were the highest and near to the commonly used chemical insecticides. When exposed to the low-lethal concentrations (LC10, LC30) of rotenone or pyrethrins for 24 h, the lifespan and fecundity of adults in F0 generation decreased significantly compared to control. The negative effect could also be observed in the F1 generation, including the decreased average offspring, longevity of adult, and prolonged nymph period. The population parameters in F1 generation of R. padi were also inhibited by exposing to the low-lethal concentrations of rotenone or pyrethrins, including the decreased net reproductive rate, intrinsic rate of natural increase, finite rate of population increase, and gross reproduction rate. Co-toxocity factor results showed that mixtures of rotenone and thiamethoxam, pyrethrins and thiamethoxam showed synergistic effect. Our work suggested that rotenone and pyrethrins showed negative effect on the population growth under low-lethal concentrations. They are suitable for R. padi control as foliar spraying without causing population resurgence.

Bioactivity of Eugenol: A Potential Antibiotic Adjuvant with Minimal Ecotoxicological Impact

Combining commercial antibiotics with adjuvants to lower their minimum inhibitory concentration (MIC) is vital in combating antimicrobial resistance. Evaluating the ecotoxicity of such compounds is crucial due to environmental and health risks. Here, eugenol was assessed as an adjuvant for 7 commercial antibiotics against 14 pathogenic bacteria in vitro, also examining its acute ecotoxicity on various soil and water organisms (microbiota, Vibrio fischeri, Daphnia magna, Eisenia foetida, and Allium cepa). Using microdilution methods, checkerboard assays, and kinetic studies, the MICs for eugenol were determined together with the nature of its combinations with antibiotics against bacteria, some unexposed to eugenol previously. The lethal dose for the non-target organisms was also determined, as well as the Average Well Color Development and the Community-Level Physiological Profiling for soil and water microbiota. Our findings indicate that eugenol significantly reduces MICs by 75 to 98%, which means that it could be a potent adjuvant. Ecotoxicological assessments showed eugenol to be less harmful to water and soil microbiota compared to studied antibiotics. While Vibrio fischeri and Daphnia magna were susceptible, Allium cepa and Eisenia foetida were minimally affected. Given that only 0.1% of eugenol is excreted by humans without metabolism, its environmental risk when used with antibiotics appears minimal.

Insecticidal activity of essential oil from leaves of Eugenia stipitata McVaugh against Aedes aegypti

Aedes aegypti, a mosquito, is responsible for the spread of many diseases, including dengue, zika, and chikungunya. However, due to this mosquito's developed resistance to conventional pesticides, effectively controlling it has proven to be challenging. This study aimed to evaluate the insecticidal potential of the essential oil from the leaves of Eugenia stipitata against Ae. aegypti, offering a natural and sustainable alternative for mosquito control. Tests were conducted using third-stage larvae to evaluate larvicidal activity and pupae collected up to 14 h after transformation to investigate pupicidal activity. Throughout the bioassays, the organisms were exposed to various essential oil concentrations. The findings demonstrated that the essential oil of E. stipitata exhibited larvicidal action, resulting in 100% larval mortality after 24 h and an LC50 value of 0.34 mg/mL. The effectiveness of essential oil as a pupicidal agent was also demonstrated by its LC50 value of 2.33 mg/mL and 100% larval mortality in 24 h. It can be concluded that the essential oil of E. stipitata holds promise as a natural pest control agent. Its use may reduce the reliance on conventional chemical pesticides, providing a more sustainable and effective strategy to combat diseases spread by mosquitoes.

Urbanization and Water Containers Influence the Mosquito Community with Consequences for Aedes aegypti

We aimed to determine how the degree of urbanization in a Neotropical city influences Aedes aegypti (L.), a pantropical vector of urban yellow fever, dengue, Zika and Chikungunia, via other mosquito species, whether they are competitors or predators, native to the area or invasive. We conducted experiments twice a month during one year in the city of Kourou, French Guiana, on three sites characterized by increasing percentages of imperviousness (i.e., 0.65%, 33.80% and 86.60%). These sites were located in a ≈5 ha forest fragment, a residential area with gardens, and in the older part of the city, respectively, and correspond to slightly, moderately and highly urbanized sites. There, we monitored twice a month during one year a total of 108 mosquito communities inhabiting four types of containers (i.e., a tank bromeliad, dry stumps of bamboo, ovitraps and car tires) installed in a random block design. In the tanks of the bromeliad, likely due to the acidity of the water, the immatures of native mosquito species prevailed, particularly Wyeomyia pertinans (Williston) in the slightly urbanized site. The general pattern was very similar in the three other types of containers where Limatus durhamii Théobald dominated in the slightly urbanized site, so that the abundance of Ae. aegypti immatures was low compared to those of native species. Yet, Ae. aegypti strongly dominated in the two more urbanized sites. These findings open up perspectives for vector management, including the conservation and/or the augmentation of natural enemies through modifications to landscape features.

Do combinations of fipronil, eugenol and carvacrol have synergistic effects against Rhipicephalus sanguineus?

The tick Rhipicephalus sanguineus is one of the main ectoparasites that affects dogs, causing direct and indirect damage to parasitized animals. Currently, infestation control is mainly carried out by using synthetic acaricidal drugs. However, a decrease in efficacy and an increase in resistance to the main therapeutic protocols against tick infestations have been increasingly reported and confirmed, a factor that has driven research into the potential acaricide activity of natural compounds, including in association with synthetic molecules. The aim of this work was to evaluate whether the combinations of fipronil (FIP) and eugenol (EUG), FIP and carvacrol (CAR), and EUG and CAR would have synergistic effects against immature and unfed adult stages of R. sanguineus through in vitro bioassays. Bioassays were carried out using the larval packet test (FAO 2004) adapted for nymphs and adults. The synergistic activity was explored by combining each solution, based on the estimated LC50, in a 1:1 ratio (FIP: EUG, FIP: CAR and EUG: CAR). CompuSyn software was used to evaluate the various pairwise combinations of FIP, EUG and CAR, checking if there was synergism or antagonism between them. FIP and EUG and FIP and CAR showed combination index (CIn) values above 1.45, indicating antagonism. The synergistic activity between EUG and CAR was verified against all unfed phases of R. sanguineus, since the CIn was below 0.70, a value that indicates synergism. The combination of fipronil with either eugenol or carvacrol presented antagonistic effects against R. sanguineus larvae. On the other hand, carvacrol and eugenol had excellent pharmacological synergism against all tick stages with mortality values in the range of 80 to 100%, including the adult stage, which is less susceptible than immature stages.

Nanoliposomal Trachyspermum ammi (L) sprague essential oil for effective control of malaria mosquito larvae, Anopheles stephensi Liston

Controlling mosquito vectors at immature stages using larvicides is a practical strategy to stave off mosquito-borne diseases such as malaria. Developing nanoliposomes bearing essential oil is a promising approach to improving the efficacy and stability of EOs-derived larvicides. The main aim of this investigation was to assess the efficacy of nanoliposome containing Trachyspermum ammi L. EO (TAEO-NL) as a new potential formulation to control Anopheles stephensi Liston (Diptera, Culicidae) mosquito larvae. The chemical constituents of T. ammi L. essential oil (TAEO) were first investigated using gas chromatography-mass spectrometry (GC-MS) analysis; its dominant component (48.22%) was thymol. TAEO-NL with a particle size of 54.6 ± 5 nm and zeta potential of -18 ± 0.5 mV were then prepared using the ethanol injection method. Besides, the successful loading of TAEO was confirmed using Attenuated Total Reflection-Fourier Transform Infra-Red (ATR-FTIR) spectroscopy analysis. A significant difference (P < 0.05) was observed in the efficacy of TAEO-NL and TAEO with lethal concentration 50% (LC50) values of 14.09 and 59.47 μg/mL against An. stephensi larvae. However, free nanoliposomes show negligible larvicidal effects (<5%). This nano-formulation could thus be suggested as a green product against insects to impede transmission of deadly infectious diseases with possible field applicability scope.