Species abundance and density of malaria vectors in Western Thailand and implications for disease transmission.

Understanding the dynamics of malaria vectors and their interactions with environmental factors is crucial for effective malaria control. This study investigated the abundance, species composition, seasonal variations, and malaria infection status of female mosquitoes in malaria transmission and non-transmission areas in Western Thailand. Additionally, the susceptibility of malaria vectors to pyrethroid insecticides was assessed. Entomological field surveys were conducted during the hot, wet, and cold seasons in both malaria transmission areas (TA) and non-transmission areas (NTA). The abundance and species composition of malaria vectors were compared between TA and NTA. The availability of larval habitats and the impact of seasonality on vector abundance were analyzed. Infection with Plasmodium spp. in primary malaria vectors was determined using molecular techniques. Furthermore, the susceptibility of malaria vectors to pyrethroids was evaluated using the World Health Organization (WHO) susceptibility test. A total of 9799 female mosquitoes belonging to 54 species and 11 genera were collected using various trapping methods. The number of malaria vectors was significantly higher in TA compared to NTA (P < 0.001). Anopheles minimus and An. aconitus were the predominant species in TA, comprising over 50% and 30% of the total mosquitoes collected, respectively. Seasonality had a significant effect on the availability of larval habitats in both areas (P < 0.05) but did not impact the abundance of adult vectors (P > 0.05). The primary malaria vectors tested were not infected with Plasmodium spp. The WHO susceptibility test revealed high susceptibility of malaria vectors to pyrethroids, with mortality rates of 99-100% at discriminating concentrations. The higher abundance of malaria vectors in the transmission areas underscores the need for targeted control measures in these regions. The susceptibility of malaria vectors to pyrethroids suggests the continued effectiveness of this class of insecticides for vector control interventions. Other factors influencing malaria transmission risk in the study areas are discussed. These findings contribute to our understanding of malaria vectors and can inform evidence-based strategies for malaria control and elimination efforts in Western Thailand.

The impact of insect growth regulators on adult emergence inhibition and the fitness of Aedes aegypti field populations in Thailand.

BACKGROUND: The frequent use of insecticides in vector control causes the development of insecticide resistance. Insect growth regulators (IGRs), which effect insect development, are used as a promising alternative to control resistant insect vectors. This study aimed to develop novel effective tools for Aedes aegypti control by evaluating the efficacy of different IGRs on larval development, blood feeding capacity, fecundity, and fertility in females and sperm productivity in males across geographical regions of Thailand. METHODS: The efficacy of 16 technical grade IGRs were evaluated against laboratory strain Ae. aegypti larvae in order to determine their emergence inhibition (EI) at 50% and 95% under laboratory conditions. Six IGRs were selected for fecundity, fertility, and sperm productivity studies using feed-through treatments at EI95 concentration levels against adult Ae. aegypti field strains. RESULTS: The results from larval bioassay tests indicate that juvenile hormone mimics (EI50 = 0.010-0.229 ppb; EI95 = 0.066-1.118 ppb) and chitin synthesis inhibitors affecting CHS1 (EI50 = 0.240-2.412 ppb; EI95 = 0.444-4.040 ppb) groups effectively inhibited adult Ae. aegypti emergence. Methoprene and fenoxycarb significantly reduced blood feeding capacity. Egg production was comparable among strains while methoprene, pyriproxyfen and diflubenzuron induced egg production. Egg retention was detected in females fed on diflubenzuron. Methoprene, fenoxycarb, diflubenzuron, and teflubenzuron reduced egg hatching rates in mosquito field strains compared to laboratory strain. Male mosquitoes fed on fenoxycarb showed significantly lower sperm production compared to other treatments. CONCLUSION: Juvenile hormone analogues and chitin synthesis inhibitors affecting CHS1 groups showed excellent results in adult emergence inhibition in this study. They also disrupted reproductive systems in both adult males and females. This study suggested that they can be used as an alternative larvicide in mosquito control programs.

Too "sexy" for the field? Paired measures of laboratory and semi-field performance highlight variability in the apparent mating fitness of Aedes aegypti transgenic strains.

BACKGROUND: Evaluating and improving mating success and competitive ability of laboratory-reared transgenic mosquito strains will enhance the effectiveness of proposed disease-control strategies that involve deployment of transgenic strains. Two components of the mosquito rearing process, larval diet quantity and aquatic environment - which are linked to physiological and behavioural differences in adults - are both relatively easy to manipulate. In mosquitoes, as for many other arthropod species, the quality of the juvenile habitat is strongly associated with adult fitness characteristics, such as longevity and fecundity. However, the influence of larval conditioning on mating performance is poorly understood. Here, we investigated the combined effects of larval diet amount and environmental water source on adult male mating success in a genetically modified strain of Aedes aegypti mosquitoes in competition with wild-type conspecifics. Importantly, this research was conducted in a field setting using low generation laboratory and wild-type lines. RESULTS: By controlling larval diet (high and low) and rearing water source (field-collected and laboratory water), we generated four treatment lines of a genetically modified strain of Ae. aegypti tagged with fluorescent sperm. Laboratory reared mosquitoes were then competed against a low generation wild-type colony in a series of laboratory and semi-field mating experiments. While neither food quantity nor larval aquatic environment were found to affect male mating fitness, the transgenic lines consistently outperformed wild-types in laboratory competition assays, an advantage that was not conferred to semi-field tests. CONCLUSIONS: Using a model transgenic system, our results indicate that differences in the experimental conditions of laboratory- and field-based measures of mating success can lead to variation in the perceived performance ability of modified strains if they are only tested in certain environments. While there are many potential sources of variation between laboratory and field lines, laboratory adaptation - which may occur over relatively few generations in this species - may directly impact mating ability depending on the context in which it is measured. We suggest that colony-hybridization with field material can potentially be used to mitigate these effects in a field setting. Release programs utilising mass-produced modified laboratory strains should incorporate comparative assessments of quality in candidate lines.