Potential of Outdoor Ultra-Low-Volume Aerosol and Thermal Fog to Suppress the Dengue Vector, Aedes aegypti, Inside Dwellings

Mosquito Gut Microbiota: A Review

Mosquitoes are vectors of many important human diseases. The prolonged and widespread use of insecticides has led to the development of mosquito resistance to these insecticides. The gut microbiota is considered the master of host development and physiology; it influences mosquito biology, disease pathogen transmission, and resistance to insecticides. Understanding the role and mechanisms of mosquito gut microbiota in mosquito insecticide resistance is useful for developing new strategies for tackling mosquito insecticide resistance. We searched online databases, including PubMed, MEDLINE, SciELO, Web of Science, and the Chinese Science Citation Database. We searched all terms, including microbiota and mosquitoes, or any specific genera or species of mosquitoes. We reviewed the relationships between microbiota and mosquito growth, development, survival, reproduction, and disease pathogen transmission, as well as the interactions between microbiota and mosquito insecticide resistance. Overall, 429 studies were included in this review after filtering 8139 search results. Mosquito gut microbiota show a complex community structure with rich species diversity, dynamic changes in the species composition over time (season) and across space (environmental setting), and variation among mosquito species and mosquito developmental stages (larval vs. adult). The community composition of the microbiota plays profound roles in mosquito development, survival, and reproduction. There was a reciprocal interaction between the mosquito midgut microbiota and virus infection in mosquitoes. Wolbachia, Asaia, and Serratia are the three most studied bacteria that influence disease pathogen transmission. The insecticide resistance or exposure led to the enrichment or reduction in certain microorganisms in the resistant mosquitoes while enhancing the abundance of other microorganisms in insect-susceptible mosquitoes, and they involved many different species/genera/families of microorganisms. Conversely, microbiota can promote insecticide resistance in their hosts by isolating and degrading insecticidal compounds or altering the expression of host genes and metabolic detoxification enzymes. Currently, knowledge is scarce about the community structure of mosquito gut microbiota and its functionality in relation to mosquito pathogen transmission and insecticide resistance. The new multi-omics techniques should be adopted to find the links among environment, mosquito, and host and bring mosquito microbiota studies to the next level.

Evaluation of field resistance in field-collected mosquito Culex quinquefasciatus Say through quantification of ULV permethrin/PBO formulation in field bioassays

BACKGROUND

Pyrethroids are among the most applied adulticides worldwide to control mosquito vectors for prevention of arboviral diseases transmission. However, pesticide resistance development in a mosquito population could lead to decreased control efficacy. While most studies investigate the resistant genotype (i.e. kdr, CYP450, etc.) as explanatory variables, few field efficacy studies have measured pesticide quantities deposited at different distances from the sprayer in association with observed mosquito mortality. The current study determined field delivered amounts of an applied ULV permethrin/PBO formulation (31% permethrin + 66% piperonyl butoxide) by GC/MS and estimated practical resistance ratios using caged mosquito females.

RESULTS

For field samples, the extraction method recovered 78 ± 3.92-108 ± 8.97% of the permethrin/PBO formulation when utilizing the peaks of PBO from GC/MS to estimate the concentrations of adulticide deposited near the mosquito cages. The field bioassay showed that the spatial distribution of permethrin/PBO formulation was heterogeneous among three pseudo-replicates within the same distance. Within the quantifiable permethrin/PBO range of 15.7-51.4 ng/cm2 , field-collected mosquito mortalities started at 64% and linearly increased reaching 100% only in two areas, while all Sebring susceptible mosquitoes died. The field LC95 resistance ratio (RR) of F0 Cx. quinquefasciatus ranged from 2.65-3.51, falling within the 95% CI of RR95 estimated by laboratory vial assays. Tests with and without PBO indicated P450's enzymes contributed to field resistance.

CONCLUSION

Results showed the suitability of the collection and quantification method to estimate the field resistance ratio at the applied pesticide rate. Pesticide quantification would also allow the association of the known frequencies of resistance mechanisms (e.g. kdr, CYP450) with field mortalities to estimate the resistance level conferred by such mechanisms. © 2023 The Authors. Pest Management Science published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.

Semi-field evaluation of the space spray efficacy of Fludora Co-Max EW against wild insecticide-resistant Aedes aegypti and Culex quinquefasciatus mosquito populations from Abidjan, Côte d'Ivoire

BACKGROUND

Space spraying of insecticides is still an important means of controlling Aedes and Culex mosquitoes and arboviral diseases. This study evaluated the space spray efficacy of Fludora Co-Max EW, (water-based insecticide space spray combining flupyradifurone and transfluthrin with film forming aqueous spray technology (FFAST)), against wild insecticide-resistant Aedes aegypti and Culex quinquefasciatus mosquitoes from Abidjan, Côte d'Ivoire, compared with K-Othrine EC (deltamethrin-only product), in small-scale field trials.

METHODS

Wild Ae. aegypti and Cx. quinquefasciatus mosquito larvae were collected in Abidjan, Côte d'Ivoire from August to December 2020. Mosquito larvae were reared in the laboratory until the adult stage. Fludora Co-Max EW and K-Othrine EC were tested against emerged adult females (F₀ generation) using ultra-low volume cold fogging (ULV) and thermal fogging (TF) delivery technology, both outdoors and indoors in Agboville, Côte d'Ivoire. Specifically, cages containing 20 mosquitoes each were placed at distances of 10, 25, 50, 75 and 100 m from the spraying line for outdoor spraying, and at ceiling, mid-height and floor levels for indoor house spraying. Knockdown and mortality were recorded at each checkpoint and compared by treatments.

RESULTS

Overall, Fludora Co-Max EW induced significantly higher knockdown and mortality effects in the wild insecticide-resistant Ae. aegypti and Cx. quinquefasciatus compared with K-Othrine EC. In both species, mortality rates with Fludora Co-Max EW were > 80% (up to 100%) with the ULV spray outdoors at each distance checkpoint (i.e. 10-100 m), and 100% with the ULV and TF sprays indoors at all checkpoints (i.e. ceiling, mid-height and floor). K-Othrine EC induced high mortality indoors (97.9-100%), whereas mortality outdoors rapidly declined in Ae. aegypti from 96.7% (10 m) to 36.7% (100 m) with the ULV spray, and from 85.0% (10 m) to 38.3% (100 m) with the TF spray. Fludora Co-Max EW spray applied as ULV spray outdoors had higher knockdown and higher killing effects on Ae. aegypti and Cx. quinquefasciatus than when applied as TF spray. Fludora Co-Max EW performed better against Cx. quinquefasciatus than against Ae. aegypti.

CONCLUSIONS

Fludora Co-Max EW induced high mortality and knockdown effects against wild insecticide-resistant Ae. aegypti and Cx. quinquefasciatus Abidjan strains and performed better than K-Othrine EC. The presence of flupyradifurone and transfluthrin (with new and independent modes of action) and FFAST technology in the current Fludora Co-Max EW formulation appears to have broadened its killing capacity. Fludora Co-Max EW is thus an effective adulticide and may be a useful tool for Aedes and Culex mosquito and arbovirus control in endemic areas.

Emerging Mosquito Resistance to Piperonyl Butoxide-Synergized Pyrethroid Insecticide and Its Mechanism

Piperonyl butoxide (PBO)-synergized pyrethroid products are widely available for the control of pyrethroid-resistant mosquitoes. To date, no study has examined mosquito resistance after pre-exposure to PBO and subsequent enzymatic activity when exposed to PBO-synergized insecticides. We used Culex quinquefasciatus Say (Diptera: Culicidae), an important vector of arboviruses and lymphatic filariasis, as a model to examine the insecticide resistance mechanisms of mosquitoes to PBO-synergized pyrethroid using modified World Health Organization tube bioassays and biochemical analysis of metabolic enzyme expressions pre- and post-PBO exposure. Mosquito eggs and larvae were collected from three cities in Orange County in July 2020 and reared in insectary, and F0 adults were used in this study. A JHB susceptible strain was used as a control. Mosquito mortalities and metabolic enzyme expressions were examined in mosquitoes with/without pre-exposure to different PBO concentrations and exposure durations. Except for malathion, wild strain Cx quinquefasciatus mosquitoes were resistant to all insecticides tested, including PBO-synergized pyrethroids (mortality range 3.7 ± 4.7% to 66.7 ± 7.7%). Wild strain mosquitoes had elevated levels of carboxylesterase (COE, 3.8-fold) and monooxygenase (P450, 2.1-fold) but not glutathione S-transferase (GST) compared to susceptible mosquitoes. When wild strain mosquitoes were pre-exposed to 4% PBO, the 50% lethal concentration of deltamethrin was reduced from 0.22% to 0.10%, compared to 0.02% for a susceptible strain. The knockdown resistance gene mutation (L1014F) rate was 62% in wild strain mosquitoes. PBO pre-exposure suppressed P450 enzyme expression levels by 25~34% and GST by 11%, but had no impact on COE enzyme expression. Even with an optimal PBO concentration (7%) and exposure duration (3h), wild strain mosquitoes had significantly higher P450 enzyme expression levels after PBO exposure compared to the susceptible laboratory strain. These results further demonstrate other studies that PBO alone may not be enough to control highly pyrethroid-resistant mosquitoes due to multiple resistance mechanisms. Mosquito resistance to PBO-synergized insecticide should be closely monitored through a routine resistance management program for effective control of mosquitoes and the pathogens they transmit.