Efficiency of two larval diets for mass-rearing of the mosquito Aedes aegypti

Context-specific variation in life history traits and behavior of Aedes aegypti mosquitoes

Aedes aegypti, the vector for dengue, chikungunya, yellow fever, and Zika, poses a growing global epidemiological risk. Despite extensive research on Ae. aegypti's life history traits and behavior, critical knowledge gaps persist, particularly in integrating these findings across varied experimental contexts. The plasticity of Ae. aegypti's traits throughout its life cycle allows dynamic responses to environmental changes, yet understanding these variations within heterogeneous study designs remains challenging. A critical aspect often overlooked is the impact of using lab-adapted lines of Ae. aegypti, which may have evolved under laboratory conditions, potentially altering their life history traits and behavioral responses compared to wild populations. Therefore, incorporating field-derived populations in experimental designs is essential to capture the natural variability and adaptability of Ae. aegypti. The relationship between larval growing conditions and adult traits and behavior is significantly influenced by the specific context in which mosquitoes are studied. Laboratory conditions may not replicate the ecological complexities faced by wild populations, leading to discrepancies in observed traits and behavior. These discrepancies highlight the need for ecologically relevant experimental conditions, allowing mosquito traits and behavior to reflect field distributions. One effective approach is semi-field studies involving field-collected mosquitoes housed for fewer generations in the lab under ecologically relevant conditions. This growing trend provides researchers with the desired control over experimental conditions while maintaining the genetic diversity of field populations. By focusing on variations in life history traits and behavioral plasticity within these varied contexts, this review highlights the intricate relationship between larval growing conditions and adult traits and behavior. It underscores the significance of transstadial effects and the necessity of adopting study designs and reporting practices that acknowledge plasticity in adult traits and behavior, considering variations due to larval rearing conditions. Embracing such approaches paves the way for a comprehensive understanding of contextual variations in mosquito life history traits and behavior. This integrated perspective enables the synthesis of research findings across laboratory, semi-field, and field-based investigations, which is crucial for devising targeted intervention strategies tailored to specific ecological contexts to combat the health threat posed by this formidable disease vector effectively.

The mosquito Aedes aegypti requires a gut microbiota for normal fecundity, longevity and vector competence

Mosquitoes shift from detritus-feeding larvae to blood-feeding adults that can vector pathogens to humans and other vertebrates. The sugar and blood meals adults consume are rich in carbohydrates and protein but are deficient in other nutrients including B vitamins. Facultatively hematophagous insects like mosquitoes have been hypothesized to avoid B vitamin deficiencies by carryover of resources from the larval stage. However, prior experimental studies have also used adults with a gut microbiota that could provision B vitamins. Here, we used Aedes aegypti, which is the primary vector of dengue virus (DENV), to ask if carryover effects enable normal function in adults with no microbiota. We show that adults with no gut microbiota produce fewer eggs, live longer with lower metabolic rates, and exhibit reduced DENV vector competence but are rescued by provisioning B vitamins or recolonizing the gut with B vitamin autotrophs. We conclude carryover effects do not enable normal function.

Effects of bacterial composition and aquatic habitat metabolites on malaria vector larval availability in irrigated and non-irrigated sites of Homa Bay county, western Kenya

Gravid Anopheles malaria vectors depend on both chemical and physical (including microbial) cues for selection of preferred habitats for oviposition. This study focused on assessing the effects of bacterial composition and habitat metabolites on malaria vector larval availability in irrigated and non-irrigated potential larval sources. Water samples were collected from larval positive and negative habitats in the irrigated and non- irrigated areas of Homa Bay county. Bacteria cultured from the water samples were subjected to Matrix Assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) for species identification. DNA was extracted from the colonies and polymerase chain reaction (PCR) and sequencing done. Finally, the metabolite composition of larval positive and negative habitats was determined. MALDI-TOF MS results revealed that Bacillus was the only genera identified from larval sources in the non-irrigated zone. In the irrigated area, Shigella was the dominant genera (47%) while Escherichia coli was the abundant species (13/51). Of the sequenced isolates, 65% were Bacillus. Larvicidal isolates Brevibacillus brevis, Bacillus subtilis, and Exiguobacterium profundum were isolated and grouped with Bacillus mojavensis, Bacillus tequilensis, Bacillus stercoris, and Brevibacillus agri. Irrigated areas with larvae had reduced crude fat (0.01%) and protein content (0.13%) in comparison to those without larvae. In irrigated and non- irrigated areas, larval presence was evident in habitats with high total chlorophyll content (1.12 μg/g vs 0.81μg/g and 3.37 μg/g vs 0.82). Aquatic habitats with larvae in both irrigated and non-irrigated areas exhibited higher sugar concentration than habitats without larvae; however, when compared, non-irrigated areas with larvae had higher sugar concentration than similar habitats in irrigated areas. In addition, substantial concentrations of Manganese, Calcium, and Copper were found in aquatic habitats containing larvae in both irrigated and non-irrigated areas. These results allow for prospective examination as potential larvicidal or adulticidal agents and could be considered when designing potential vector control interventions.

Release of sterile Aedes aegypti mosquitoes: chilling effect on mass-reared males survival and escape ability and on irradiated males sexual competitiveness

In the sterile insect technique, it is important to measure the impact of mass-rearing and handling of sterile males to allow a successful control of the target wild population. This study evaluates the effect of pre-release chilling on the survival, escape ability, and sexual competitiveness of male Aedes aegypti. To determine survival and escape ability, mosquitoes were chilled at 4 °C using four different treatments of either one exposure (25 min) or two consecutive exposures (25 + 25 min, 25 + 50 min, 25 + 100 min). For sexual competitiveness, two different treatments were evaluated, chilling for 25 min once and twice. Results showed that the longest exposure to chilling caused a significant reduction of survival time, from 67 to 54 days. Escape ability was reduced by the first chilling from 25 to 7% and with the second chilling, it was reduced from 30 to 24% in the control to 4.9, 2.0 and 0.5% for 25, 50 and 100 min, respectively. Sexual competitiveness index was reduced from 1.16 in the control, to 0.32 and - 0.11 for treatments involving one and two chilling periods, respectively. It is recommended to increase the chilling temperature and reduce the exposure time to reduce the harmful effects on sterile males.

Bovine pancreatic trypsin inhibitor and soybean Kunitz trypsin inhibitor: Differential effects on proteases and larval development of the soybean pest Anticarsia gemmatalis (Lepidoptera: Noctuidae)

Pest management is challenged with resistant herbivores and problems regarding human health and environmental issues. Indeed, the greatest challenge to modern agriculture is to protect crops from pests and still maintain environmental quality. This study aimed to analyze by in silico, in vitro, and in vivo approaches to the feasibility of using the inhibitory protein extracted from mammals - Bovine Pancreatic Trypsin Inhibitor (BPTI) as a potential inhibitor of digestive trypsins from the pest Anticarsia gemmatalis and comparing the results with the host-plant inhibitor - Soybean Kunitz Trypsin Inhibitor (SKTI). BPTI and SKTI interacts with A. gemmatalis trypsin-like enzyme competitively, through hydrogen and hydrophobic bonds. A. gemmatalis larvae exposed to BPTI did not show two common adaptative mechanisms i.e., proteolytic degradation and overproduction of proteases, presenting highly reduced trypsin-like activity. On the other hand, SKTI-fed larvae did not show reduced trypsin-like activity, presenting overproduction of proteases and SKTI digestion. In addition, the larval survival was reduced by BPTI similarly to SKTI, and additionally caused a decrease in pupal weight. The non-plant protease inhibitor BPTI presents intriguing element to compose biopesticide formulations to help decrease the use of conventional refractory pesticides into integrated pest management programs.

Differential Hatching, Development, Oviposition, and Longevity Patterns among Colombian Aedes aegypti Populations

Simple Summary

Aedes aegypti is a mosquito that transmits viruses responsible for several diseases in humans, such as dengue, Zika, and chikungunya. It is crucial to study mosquito populations from different countries and regions because control of disease transmission with insecticides can be more effective if adjusted to each population’s characteristics. For this reason, we determined several features of mosquitoes captured in different cities of Colombia: Neiva, Bello, Itagüí, and Riohacha. These included the length of their lifespan, the number of eggs they lay, and the stages in which they die. We found specific patterns for each population. This knowledge will help control programs determine the optimal times to apply insecticides and make surveillance, as well as the type of insecticide used.

Abstract

Dengue, Zika, and chikungunya are arboviral diseases for which there are no effective therapies or vaccines. The only way to avoid their transmission is by controlling the vector Aedes aegypti, but insecticide resistance limits this strategy. To generate relevant information for surveillance and control mechanisms, we determined life cycle parameters, including longevity, fecundity, and mortality, of Colombian Ae. aegypti populations from four different geographical regions: Neiva, Bello, Itagüí, and Riohacha. When reared at 28 °C, Bello had the shortest development time, and Riohacha had the longest. Each mosquito population had its own characteristic fecundity pattern during four gonotrophic cycles. The survival curves of each population were significantly different, with Riohacha having the longest survival in both males and females and Bello the shortest. High mortality was observed in mosquitoes from Neiva in the egg stage and for Bello in the pupae stage. Finally, when mosquitoes from Neiva and Bello were reared at 35 °C, development times and mortality were severely affected. In conclusion, each population has a unique development pattern with an innate trace in their biological characteristics that confers vulnerability in specific stages of development.

Optimization of Aedes albopictus (Diptera: Culicidae) Mass Rearing through Cost-Effective Larval Feeding

Simple Summary

The Asian tiger mosquito (Aedes albopictus) is an important invasive species of medical concern, which could be successfully suppressed by including the sterile insect technique (SIT) in integrated mosquito management. This technique is based on the mass rearing of males, and their sterilization and release into the habitats to compete with wild males in the mating process. Our research compared the effectiveness of three larval diet recipes (IAEA-BY, BCWPRL, and MIX-14) in the rearing of Ae. albopictus males in order to evaluate the available economical feeding alternatives. The separation of male pupae was done by the sieving method, and reared adult males were tested for flight capacity and longevity. The application of BCWPRL resulted in a higher portion of sieved male pupae than females, but the obtained number of both pupae and adult males was lower and the development was slower than the other two diets. The adult mean survival time was the highest in males fed with MIX-14 and the lowest in males fed with IAEA-BY. Males fed by IAEA-BY also demonstrated higher initial mortality in the adult stage. The diets BCWPRL and MIX-14 are cheaper than IAEA-BY (2.28 and 5.30 times, respectively). The diet MIX-14 represents a candidate for replacing the effective but still expensive IAEA-BY diet.

Abstract

Aedes (Stegomyia) albopictus (Skuse, 1895) is an invasive important medical and veterinary pest species. The sterile insect technique (SIT) involves the mass rearing of males, and their sterilization and release into the habitat to compete with wild males. Our research objective was to compare the effectiveness of three larval diet recipes (IAEA-BY, BCWPRL, and MIX-14) in the laboratory rearing of Ae. albopictus males to evaluate the available economical feeding alternatives. The separation of sexes was done in the pupal stage by sieving. Reared males were tested for flight capacity and longevity. The application of the BCWPRL diet resulted in a higher portion of sieved male pupae than females, but the development of males was the slowest, and the number of obtained males (pupae and adults) was lower compared to the other two diets. The adult mean survival time was the highest in males fed with MIX-14 and the lowest in males fed with IAEA-BY. Males fed by IAEA-BY also demonstrated higher initial mortality in the adult stage. The diets BCWPRL and MIX-14 are economically more convenient than IAEA-BY (2.28 and 5.30 times cheaper, respectively). The cheapest diet, MIX-14, might represent a candidate for replacing the effective but still expensive IAEA-BY larval diet, providing lower costs of sterile male production.

Osorio A, Valle J, Angulo-Kladt R, Gómez-Simuta Y, Fernández-Salas I, Dor A, Williams T. Comparison of Ground Release and Drone-Mediated Aerial Release of Aedes aegypti Sterile Males in Southern Mexico: Efficacy and Challenges

Sterile males of Aedes aegypti were released once a week for 8 weeks to evaluate the dispersal efficiency of ground and aerial drone release methods in a rural village of 26 Ha in southern Mexico. Indoor and outdoor BG-Sentinel traps were placed in 13−16 houses distributed throughout the village. The BG traps were activated 48 h after the release of the sterile males and functioned for a 24 h period following each release. Over the 8-week period of simultaneous ground and aerial releases, an average of 85,117 ± 6457 sterile males/week were released at ground level and 86,724 ± 6474 sterile males/week were released using an aerial drone. The ground release method resulted in higher numbers of captured males (mean = 5.1 ± 1.4, range 1.1−15.7 sterile males/trap) compared with the aerial release method (mean = 2.6 ± 0.8, range 0.5−7.3 sterile males/trap) (p < 0.05). Similarly, the prevalence of traps that captured at least one sterile male was significantly higher for ground release compared to the aerial release method (p < 0.01). The lower numbers of sterile males captured in the aerial release method could be due to mortality or physical injury caused by the chilling process for immobilization, or the compaction of these insects during transport and release. However, aerial releases by a two-person team distributed insects over the entire village in just 20 min, compared to ~90 min of work for a five-person team during the ground release method. Ground release also resulted in higher aggregations of males and some villagers reported feeling discomfort from the presence of large numbers of mosquitoes in and around their houses. We conclude that modifications to the handling and transport of sterile males and the design of containers used to store males are required to avoid injury and to improve the efficiency of aerial releases for area-wide SIT-based population suppression programs targeted at mosquito vectors of human disease.

Effects of elevated temperatures on the development of immature stages of Anopheles gambiae (s.l.) mosquitoes

OBJECTIVE

This study investigated the effects of temperature on the development of the immature stages of An. gambiae (s.l.) mosquitoes.

METHODS

Mosquito eggs were obtained from laboratory established colonies and reared under eight temperature regimes (25, 28, 30, 32, 34, 36, 38 and 40 °C), and 80 ± 10% relative humidity. Larvae were checked daily for development to the next stage and for mortality. Pupation success, number of adults produced, and sex ratio of the newly emerged adults were recorded. Larval survival was monitored every 24 hours, and data were analyzed using Kaplan Meier survival analysis. Analysis of variance was used where data followed normal distribution, and a Kruskal-Wallis test where data were not normally distributed. Larval and pupal measurements were log-transformed and analyzed using ordinary least square regression with robust standard errors.

RESULTS

Increasing the temperature from 25 to 36 °C decreased the development time by 10.57 days. Larval survival (X² (6) = 5353.12, P < 0.001) and the number of adults produced (X² (5) = 28.16, P < 0.001) decreased with increasing temperature. Increasing temperatures also resulted in significantly smaller larvae and pupae (P < 0.001). At higher temperatures disproportionately more male than female mosquitoes were produced.

CONCLUSIONS

Increased temperature affected different developmental stages in the life cycle of An. gambiae (s.l.) mosquitoes, from larval to adult emergence. This study contributes to the knowledge on the relationship between temperature and Anopheles mosquitoes and provides useful information for modelling vector population dynamics in the light of climate change.

Host-seeking behavior and fecundity of the female Aedes aegypti to human blood types

BACKGROUND

Mosquitoes express different degrees of preference towards hosts depending on behavioral, ecological, and physiological factors. These preferences have implications for mosquito-borne disease risk. This study investigated the effect of human blood groups on the behavior and fecundity of the female Aedes aegypti (Diptera: Culicidae) from March 2018 to August 2019. In laboratory tests, mosquitoes were fed artificial membrane feeders on A, AB, B, and O blood groups. The level of attraction to different blood groups was tested under controlled conditions with a wind tunnel bioassay.

RESULTS

The responses of Ae. aegypti to the blood groups treatments in the five-choice tunnel chambers showed a significant preference (Kruskal-Wallis (2 = 85.772, df = 4, P < 0.0001) for favor blood group B. Also, the response of Ae. aegypti to olfactory cues (filth) derived from a pool of volunteers cutting across the blood groups showed a similar preference for pattern towards the blood group B. The percentage rate of digestibility in Ae. aegypti was highest in those fed on blood group O, while individuals fed on the AB blood group had the lowest digestion rate. Thus, the rate of digestibility highly varied significantly (P < 0.0001) between blood groups. Overall, Ae. aegypti fed on blood group B had the highest average feeding rate, number of females with eggs, and fecundity level, which showed a significant difference (P < 0.0001) on preferred.

CONCLUSION

This study provides novel insight into the ABO host choice of Ae. aegypti and reinforces the need for personal protection against dengue virus transmission in light of the increased risk of exposure for individuals with B blood type. © 2021 Society of Chemical Industry.

Temporal Viability of Aedes aegypti and Aedes albopictus Eggs Using Two Hygroscopic Substances as Preservatives under a Sterile Insect Technique (SIT) Program in Southern Mexico

Simple Summary

Diseases transmitted by Aedes mosquitoes, such as dengue, Zika, and chikungunya, are a public health problem of growing global concern. There are several strategies for mosquito control. One of them is the Sterile Insect Technique, which is a method of breeding millions of mosquitoes, where radiation is used in males to sterilize them. Then, the males are released into the wild. These sterile males mate with wild females without having offspring, thus decreasing field populations. However, one of the problems is being able to have a large number of viable eggs for these field operations. Therefore, this study evaluates the temporal viability of Aedes mosquito eggs employing two substances, such as hydrolyzed collagen and Hyalurosmooth^®. These two moisturizing substances maintained viable Aedes. aegypti eggs for up to eight weeks.

Abstract

Dengue and other Aedes-borne diseases have dramatically increased over the last decades. The Sterile Insect Technique (SIT) has been successfully used as part of integrated pest strategies to control populations of insect-plant and livestock pests and is currently being tested as a potential method to reduce mosquito populations in an environmentally friendly approach. However, during the mass rearing steps needed to produce millions of mosquitoes, egg storage and preservation are essential for a certain amount of time. Eggs of Aedes aegypti have a chorionic pad that functions as a sticky substance to glue them onto the inner walls of larval breeding sites. The chorionic pad is chemically made of hyaluronic acid, a hygroscopic compound, responsible to protect them from desiccation over time. Two commercial products with hygroscopic properties, hydrolyzed collagen, and Hyalurosmooth^®, both were tested to assess their ability to prolong egg life storage for A. aegypti and A. albopictus. Results showed that 85–95% of Ae. aegypti eggs were able to hatch up to week 8 after being treated with both hydrophilic compounds, compared with the control 66.3%. These two substances showed promising effects for keeping Ae. aegypti eggs viable during prolonged storage in mass rearing insect production focused on vector control SIT programs.

Human blood type influences the host-seeking behavior and fecundity of the Asian malaria vector Anopheles stephensi

The nutritional requirements of mosquitoes include both sugar (generally derived from the nectar of flowers) and blood (humans or animals). Mosquitoes express different degrees of preferences towards hosts depending on behavioral, ecological, and physiological factors. These preferences have implications for mosquito-borne disease risk. The present study is directed to reveal the effect of the human blood groups on the fecundity and fertility of the malaria vector Anopheles stephensi. In laboratory tests, mosquitoes were fed on ABO blood groups via artificial membrane feeders, and the level of attraction against different blood groups was tested by the electroantennogram and wind tunnel bioassay under control conditions. Results indicate that the female mosquitoes had a strong preference towards the blood group B, while in the case of females fed on O blood group had the highest digestibility rate. Overall, the human blood type had a significant impact on the fecundity and fertility of female An. stephensi. The highest numbers of eggs are laid, in the case of blood group B, (mean (± SD)) 216.3 (8.81) followed by the AB, 104.06 (7.67), and O, 98.01 (7.04). In the case of blood group B, females attain the highest fertility of about 92.1 (9.98). This study provides novel insight into the ABO blood type host choice of the mosquitoes that are still partially unknown and suggests encouraging personal protection for relevant individuals within communities at risk, which is a useful tool for preventing malaria where the An. stephensi is present as a dominant vector.

Diet-Microbiota Interactions Alter Mosquito Development

Gut microbes and diet can both strongly affect the biology of multicellular animals, but it is often difficult to disentangle microbiota–diet interactions due to the complex microbial communities many animals harbor and the nutritionally variable diets they consume. While theoretical and empirical studies indicate that greater microbiota diversity is beneficial for many animal hosts, there have been few tests performed in aquatic invertebrates. Most mosquito species are aquatic detritivores during their juvenile stages that harbor variable microbiotas and consume diets that range from nutrient rich to nutrient poor. In this study, we produced a gnotobiotic model that allowed us to examine how interactions between specific gut microbes and diets affect the fitness of Aedes aegypti, the yellow fever mosquito. Using a simplified seven-member community of bacteria (ALL7) and various laboratory and natural mosquito diets, we allowed larval mosquitoes to develop under different microbial and dietary conditions and measured the resulting time to adulthood and adult size. Larvae inoculated with the ALL7 or a more complex community developed similarly when fed nutrient-rich rat chow or fish food laboratory diets, whereas larvae inoculated with individual bacterial members of the ALL7 community exhibited few differences in development when fed a rat chow diet but exhibited large differences in performance when fed a fish food diet. In contrast, the ALL7 community largely failed to support the growth of larvae fed field-collected detritus diets unless supplemented with additional protein or yeast. Collectively, our results indicate that mosquito development and fitness are strongly contingent on both diet and microbial community composition.

The Insect Pest Control Laboratory of the Joint FAO/IAEA Programme: Ten Years (2010-2020) of Research and Development, Achievements and Challenges in Support of the Sterile Insect Technique

The Joint FAO/IAEA Centre (formerly called Division) of Nuclear Techniques in Food and Agriculture was established in 1964 and its accompanying laboratories in 1961. One of its subprograms deals with insect pest control, and has the mandate to develop and implement the sterile insect technique (SIT) for selected key insect pests, with the goal of reducing the use of insecticides, reducing animal and crop losses, protecting the environment, facilitating international trade in agricultural commodities and improving human health. Since its inception, the Insect Pest Control Laboratory (IPCL) (formerly named Entomology Unit) has been implementing research in relation to the development of the SIT package for insect pests of crops, livestock and human health. This paper provides a review of research carried out between 2010 and 2020 at the IPCL. Research on plant pests has focused on the development of genetic sexing strains, characterizing and assessing the performance of these strains (e.g., Ceratitis capitata), elucidation of the taxonomic status of several members of the Bactrocera dorsalis and Anastrepha fraterculus complexes, the use of microbiota as probiotics, genomics, supplements to improve the performance of the reared insects, and the development of the SIT package for fruit fly species such as Bactrocera oleae and Drosophila suzukii. Research on livestock pests has focused on colony maintenance and establishment, tsetse symbionts and pathogens, sex separation, morphology, sterile male quality, radiation biology, mating behavior and transportation and release systems. Research with human disease vectors has focused on the development of genetic sexing strains (Anopheles arabiensis, Aedes aegypti and Aedes albopictus), the development of a more cost-effective larvae and adult rearing system, assessing various aspects of radiation biology, characterizing symbionts and pathogens, studying mating behavior and the development of quality control procedures, and handling and release methods. During the review period, 13 coordinated research projects (CRPs) were completed and six are still being implemented. At the end of each CRP, the results were published in a special issue of a peer-reviewed journal. The review concludes with an overview of future challenges, such as the need to adhere to a phased conditional approach for the implementation of operational SIT programs, the need to make the SIT more cost effective, to respond with demand driven research to solve the problems faced by the operational SIT programs and the use of the SIT to address a multitude of exotic species that are being introduced, due to globalization, and established in areas where they could not survive before, due to climate change.

Riboflavin instability is a key factor underlying the requirement of a gut microbiota for mosquito development

We previously determined that several diets used to rear Aedes aegypti and other mosquito species support the development of larvae with a gut microbiota but do not support the development of axenic larvae. In contrast, axenic larvae have been shown to develop when fed other diets. To understand the mechanisms underlying this dichotomy, we developed a defined diet that could be manipulated in concert with microbiota composition and environmental conditions. Initial studies showed that axenic larvae could not grow under standard rearing conditions (27 °C, 16-h light: 8-h dark photoperiod) when fed a defined diet but could develop when maintained in darkness. Downstream assays identified riboflavin decay to lumichrome as the key factor that prevented axenic larvae from growing under standard conditions, while gut community members like Escherichia coli rescued development by being able to synthesize riboflavin. Earlier results showed that conventional and gnotobiotic but not axenic larvae exhibit midgut hypoxia under standard rearing conditions, which correlated with activation of several pathways with essential growth functions. In this study, axenic larvae in darkness also exhibited midgut hypoxia and activation of growth signaling but rapidly shifted to midgut normoxia and arrested growth in light, which indicated that gut hypoxia was not due to aerobic respiration by the gut microbiota but did depend on riboflavin that only resident microbes could provide under standard conditions. Overall, our results identify riboflavin provisioning as an essential function for the gut microbiota under most conditions A. aegypti larvae experience in the laboratory and field.

Effect of Larval Food Availability on Adult Aedes Aegypti (Diptera: Culicidae) Fitness and Susceptibility to Zika Infection

Aedes (Stegomyia) aegypti (Linnaeus, 1762) is a mosquito species of significant medical importance. The use of this vector in research studies usually requires a large number of mosquitoes as well as rearing and maintenance in a laboratory-controlled environment. However, laboratory conditions may be different from field environments, presenting stressful challenges such as low food concentration, especially during larval stages, which may, in turn, impair vector biology. Therefore, we tested herein if larval food availability (0.004, 0.009, 0.020, and 0.070% diets) would affect overall adult insect fitness. We observed slower development in mosquitoes fed a 0.004% diet 15 d post-eclosion (DPE) and shorter mean time in mosquitoes fed a 0.020% diet (7 DPE). Larval diet and adult mosquito weight were positively correlated, and heavier females fed higher larval diets exhibited greater blood feeding capacity and oviposition. In addition, larval diet concentrations led to median adult lifespan variations (male/female in days-0.004%: 30 ± 1.41, 45 ± 1.3; 0.009%: 31.5 ± 1.33, 41 ± 1.43; 0.020%: 26 ± 1.18, 41 ± 1.45; 0.070%: 29 ± 1.07, 44 ± 1.34), reduced tolerance to deltamethrin (1 mg/m2) and changes in detoxification enzyme activities. Moreover, in the larval 0.070% diet, females presented higher Zika susceptibility (plaque-forming unit [PFU]: 1.218 × 106) compared with other diets (0.004%: 1.31 × 105; 0.009%: 2.0 × 105; 0.020%: 1.25 × 105 PFU). Altogether, our study demonstrates that larval diet restriction results not only in larval developmental arrest but also in adult fitness impairment, which must be considered in future assessments.

Assessment of Developmental and Reproductive Fitness of Dengue-Resistant Transgenic Aedes aegypti and Improvement of Fitness Using Antibiotics

Background

Genetic modification offers opportunities to introduce artificially created molecular defence mechanisms to vector mosquitoes to counter diseases causing pathogens such as the dengue virus, malaria parasite, and Zika virus. RNA interference is such a molecular defence mechanism that could be used for this purpose to block the transmission of pathogens among human and animal populations. In our previous study, we engineered a dengue-resistant transgenic Ae. aegypti using RNAi to turn off the expression of dengue virus serotype genomes to reduce virus transmission, requiring assessment of the fitness of this mosquito with respect to its wild counterpart in the laboratory and semifield conditions.

Method

Developmental and reproductive fitness parameters of TM and WM have assessed under the Arthropod Containment Level 2 conditions, and the antibiotic treatment assays were conducted using co-trimoxazole, amoxicillin, and doxycycline to assess the developmental and reproductive fitness parameters.

Results

A significant reduction of developmental and reproductive fitness parameters was observed in transgenic mosquito compared to wild mosquitoes. However, it was seen in laboratory-scale studies that the fitness of this mosquito has improved significantly in the presence of antibiotics such as co-trimoxazole, amoxicillin, and doxycycline in their feed.

Conclusion

Our data indicate that the transgenic mosquito produced had a reduction of the fitness parameters and it may lead to a subsequent reduction of transgenic vector density over the generations in field applications. However, antibiotics of co-trimoxazole, amoxicillin, and doxycycline have shown the improvement of fitness parameters indicating the usefulness in field release of transgenic mosquitoes.

Life-History Traits and Flight Capacity of Grapholita molesta (Lepidoptera: Tortricidae) Using Artificial Diets With Varying Sugar Content

Successful application of the sterile insect technique (SIT), an environmentally friendly control technology, mainly depends on mass-rearing of high-quality and high-performance insects. For mass-rearing of insects, the development of artificial diets is a key component. For optimal insect growth and development, sugar is an essential nutrient as it provides energy for flight. To date, few studies have analyzed the effects of different sugar contents on the biological parameters, including the flight capacity of Grapholita molesta, a globally important economic pest. Artificial diets with different sucrose contents (0, 15, and 30 g) were evaluated in two consecutive generations. The insect flight mill was used to study the G. molesta flight capacity. The larval and pupal periods, adult longevity and pupal weight of the first-generation of G. molesta reared on artificial diets with different sucrose contents were significantly different. Insects of the second-generation had a shorter larval period, greater adult longevity, and heavier larvae and pupae in the treatment with 30 g of sucrose than using 15 g. Among the males, strong, medium, and weak flight capacities were recorded and the weakest one was observed in the diet without sucrose. Results showed that the proportion of insects with highest flight capacity increased with increasing sucrose content in insects of the second generation. It can be concluded that sucrose content is a key determinant in the biological traits, including flight capacity of G. molesta, and should be taken into consideration during the mass-rearing of the pest for SIT.

Sexual Competitiveness and Induced Egg Sterility by Aedes aegypti and Aedes albopictus Gamma-Irradiated Males: A Laboratory and Field Study in Mexico

Simple Summary

The sterile insect technique (SIT) involves the release of massive numbers of male insects that have been sterilized by irradiation treatment during their development. Wild females that mate with sterilized males are not able to produce offspring, resulting in rapid decline in the target insect population over a large area. The success of this technique depends on the ratio of wild:sterile males achieved following male releases and the ability of sterile males to mate with wild females, i.e., their sexual competitiveness compared to fertile wild male insects. There is growing interest in applying SIT to the area-wide control of mosquitoes, such as Aedes aegypti and Aedes albopictus, that transmit important human diseases caused by dengue, chikungunya, and Zika viruses. In the present study, the sexual competitiveness of both mosquito species was affected by irradiation treatments but did not vary greatly with different ratios of fertile:sterile males in mating cages. Most importantly, the fertility of eggs was greatly reduced when more sterile males were present in mating cages, resulting in an 88% decrease in the production of fertile eggs by both species of mosquitoes in some experiments. We will use these results to perform small-scale trials in rural villages frequently affected by outbreaks of mosquito-borne diseases in southern Mexico.

Abstract

The sterile insect technique may prove useful for the suppression of mosquito vectors of medical importance in regions where arboviruses pose a serious public health threat. In the present study, we examined the effects of sterilizing irradiation doses across different ratios of fertile:irradiated males on the mating competitiveness of Ae. aegypti and Ae. albopictus under laboratory and field-cage conditions. For both species, the percentage of females inseminated and the number of eggs laid over two gonotrophic cycles varied significantly in mating treatments involving 1:1, 1:5, and 1:10 fertile:irradiated males compared to controls of entirely fertile or entirely irradiated males but was not generally affected by the irradiation dose. Egg hatching was negatively affected in females exposed to increasing proportions of irradiated males in both laboratory and field cages. Male competitiveness (Fried’s index) values varied from 0.19 to 0.58 in the laboratory and were between 0.09 and 1.0 in field cages, depending on th species. Competitiveness values were negatively affected by th eirradiation dose in both species under field-cage conditions, whereas in the laboratory, Ae. albopictus was sensitive to the dose but Ae. aegypti was not. In general, male competitiveness was similar across all mating regimes. Most importantly, induced egg sterility was positively correlated with the proportion of irradiated males present in the mating treatments, reaching a maximum of 88% under field-cage conditions for both Ae. aegypti and Ae. albopictus males treated with 50 and 40 Gy irradiation, respectively. These results indicate that sterile males produced at our facility are suitable and competitive enough for field pilot SIT projects and provide guidance to decide the optimal sterile:fertile ratios.

Towards a method for cryopreservation of mosquito vectors of human pathogens

Mosquito-borne diseases are responsible for millions of human deaths every year, posing a massive burden on global public health. Mosquitoes transmit a variety of bacteria, parasites and viruses. Mosquito control efforts such as insecticide spraying can reduce mosquito populations, but they must be sustained in order to have long term impacts, can result in the evolution of insecticide resistance, are costly, and can have adverse human and environmental effects. Technological advances have allowed genetic manipulation of mosquitoes, including generation of those that are still susceptible to insecticides, which has greatly increased the number of mosquito strains and lines available to the scientific research community. This generates an associated challenge, because rearing and maintaining unique mosquito lines requires time, money and facilities, and long-term maintenance can lead to adaptation to specific laboratory conditions, resulting in mosquito lines that are distinct from their wild-type counterparts. Additionally, continuous rearing of transgenic lines can lead to loss of genetic markers, genes and/or phenotypes. Cryopreservation of valuable mosquito lines could help circumvent these limitations and allow researchers to reduce the cost of rearing multiple lines simultaneously, maintain low passage number transgenic mosquitoes, and bank lines not currently being used. Additionally, mosquito cryopreservation could allow researchers to access the same mosquito lines, limiting the impact of unique laboratory or field conditions. Successful cryopreservation of mosquitoes would expand the field of mosquito research and could ultimately lead to advances that would reduce the burden of mosquito-borne diseases, possibly through rear-and-release strategies to overcome mosquito insecticide resistance. Cryopreservation techniques have been developed for some insect groups, including but not limited to fruit flies, silkworms and other moth species, and honeybees. Recent advances within the cryopreservation field, along with success with other insects suggest that cryopreservation of mosquitoes may be a feasible method for preserving valuable scientific and public health resources. In this review, we will provide an overview of basic mosquito biology, the current state of and advances within insect cryopreservation, and a proposed approach toward cryopreservation of Anopheles stephensi mosquitoes.

Population Dynamics of Aedes aegypti and Aedes albopictus in Two Rural Villages in Southern Mexico: Baseline Data for an Evaluation of the Sterile Insect Technique

Indoor and outdoor ovitraps were placed in 15 randomly selected houses in two rural villages in Chiapas, southern Mexico. In addition, ovitraps were placed in five transects surrounding each village, with three traps per transect, one at the edge, one at 50 m, and another at 100 m from the edge of the village. All traps were inspected weekly. A transect with eight traps along a road between the two villages was also included. Population fluctuations of Aedes aegypti and Ae. albopictus were examined during 2016-2018 by counting egg numbers. A higher number of Aedes spp. eggs was recorded at Hidalgo village with 257,712 eggs (60.9%), of which 58.1% were present in outdoor ovitraps and 41.9% in indoor ovitraps, compared with 165,623 eggs (39.1%) collected in the village of Río Florido, 49.0% in outdoor and 51.0% in indoor ovitraps. A total of 84,047 eggs was collected from ovitraps placed along transects around Río Florido, compared to 67,542 eggs recorded from transects around Hidalgo. Fluctuations in egg counts were associated with annual variation in precipitation, with 2.3 to 3.2-fold more eggs collected from ovitraps placed in houses and 4.8 to 5.1-fold more eggs in ovitraps from the surrounding transects during the rainy season than in the dry season, respectively. Aedes aegypti was the dominant species during the dry season and at the start of the rainy season in both villages. Aedes albopictus populations were lower for most of the dry season, but increased during the rainy season and predominated at the end of the rainy season in both villages. Aedes albopictus was also the dominant species in the zones surrounding both villages. The numbers of eggs collected from intradomiciliary ovitraps were strongly correlated with the numbers of eggs in peridomiciliary ovitraps in both Río Florido (R2 adj = 0.92) and Hidalgo (R2 adj = 0.94), suggesting that peridomiciliary sampling could provide an accurate estimate of intradomiciliary oviposition by Aedes spp. in future studies in these villages. We conclude that the feasibility of sterile insect technique (SIT)-based program of vector control could be evaluated in the isolated Ae. aegypti populations in the rural villages of our baseline study.

Development of an Alternative Low-Cost Larval Diet for Mass Rearing of Aedes aegypti Mosquitoes

BACKGROUND

Aedes aegypti is a major vector of arboviruses that may be controlled on an area-wide basis, using novel approaches such as Sterile Insect Technique (SIT) and Incompatible Insect Technique (IIT). Larval diet is a critical factor to be considered in mass rearing of Aedes mosquitoes for SIT and IIT programs. Therefore, the current study is aimed at evaluating the effects of two novel diets developed from dry fish powder on the growth and development of immature stages and adult fitness-related characteristics of Ae. aegypti in Sri Lanka.

METHOD

Three batches of the first instar Ae. aegypti larva, each containing 250 larvae, were exposed to three different larval diets as standard dry fish powder (D1), dry fish powder meal and brewer's yeast (D2), and International Atomic Energy Agency- (IAEA-) recommended diet (D3), separately. Morphometric and developmental parameters of the 4th instar larvae, pupae, and adult mosquitoes reared under different dietary treatments were measured. The entire experimental setup was replicated thrice. A General Linear Model (GLM) in the form of two-way ANOVA was used for the statistical analysis.

RESULTS

Significant diet-based variations were observed in the head length, head width, thoracic length, thoracic width, abdominal length, abdominal width, and total length (F 2,87 > 4.811; P < 0.05) of Ae. aegypti larvae. The highest pupation success and the larval size were observed from the larvae fed the D2 diet, while the lowest was reported from D1. All adult morphometric parameters of adult male and female Ae. aegypti mosquitoes also denoted significant dietary variations, reporting the best-sized adults from the D2 diet (F 2,87 > 3.54; P < 0.05). Further, significantly higher fecundity and male longevity were also shown by the adult Ae. aegypti (F 2,6 > 7.897; P < 0.01) mosquitoes reared under diet D2.

CONCLUSION

Based on all the growth and developmental parameters, the D2 diet tends to perform similar to the IAEA-recommended diet in mass rearing of Ae. aegypti mosquitoes, while being more inexpensive. Therefore, larval diet D2 could be suggested as the ideal diet for mass rearing of Ae. aegypti for IIT and SIT-based vector control in Sri Lanka.

Collection and Rearing of Container Mosquitoes and a 24-h Addition to the CDC Bottle Bioassay

Container mosquitoes (Diptera: Culicidae) oviposit their eggs in both natural and artificial containers. Many container mosquito species also serve as important vectors of disease-causing pathogens including Aedes aegypti, Ae. albopictus, and Ae. triseriatus. Control of these species can be done through the use of adulticide sprays. The efficacy of these treatments is highly dependent on the insecticide susceptibility status of the local mosquito populations. This paper provides protocols on collecting and rearing container mosquitoes for use in the Centers for Disease Control and Prevention (CDC) bottle bioassay. A brief description of the CDC bottle bioassay is provided as well as a standardized protocol for the incorporation of a 24-h mortality to the CDC bottle bioassay. Results from this 24-h holding addition to the CDC bottle bioassay reveal that some forms of resistance may be missed without the incorporation of the additional mortality reading. These protocols provide a foundation for new laboratories to establish rearing protocols and begin conducting resistance monitoring.

Competition and growth among Aedes aegypti larvae: Effects of distributing food inputs over time

Male and female mosquito larvae compete for different subsets of the yeast food resource in laboratory microcosms. Males compete more intensely with males, and females with females. The amount and timing of food inputs alters both growth and competition, but the effects are different between sexes. Increased density increases competition among males. Among females, density operates primarily by changing the food/larva or total food; this affects competition in some interactions and growth in others. Food added earlier in the life span contributes more to mass than the same quantity added later. After a period of starvation larvae appear to use some of the subsequent food input to rebuild physiological reserves in addition to building mass. The timing of pupation is affected by the independent factors and competition, but not in the same way for the two sexes, and not in the same way as mass at pupation for the two sexes. There is an effect of density on the timing of pupation for females independent of competition or changes in food/larva or total food. Male and female larvae have different larval life history strategies. Males grow quickly to a minimum size, then pupate, depending on the amount of food available. Males that do not grow quickly enough may delay pupation further to grow larger, resulting in a bimodal distribution of sizes and ages. Males appear to have a maximum size determined by the early food level. Females grow faster than males and grow larger than males on the same food inputs. Females affect the growth and competition among males by manipulating the number of particles in the microcosm through changes in feeding behavior. Mosquito larvae appear to have evolved to survive periods of starvation and take advantage of intermittent inputs of food into containers.

Accelerating the Morphogenetic Cycle of the Viral Vector Aedes aegypti Larvae for Faster Larvicidal Bioassays

Any bioassay to test new chemically synthesized larvicides or phytolarvicides against Culicidae and more harmful mosquito species, such as Aedes aegypti and Aedes albopictus, which specifically transmit dengue, yellow fever, chikungunya viral fevers as well as Zika virus, or Anopheles gambiae, a vector for malaria and philariasis, requires thousands of well-developed larvae, preferably at the fourth instar stage. The natural morphogenetic cycle of Aedes spp., in the field or in the laboratory, may extend to 19 days at room temperature (e.g., 25°C) from the first permanent contact between viable eggs and water and the last stage of larval growth or metamorphosis into flying adults. Thus, accelerated sequential molting is desirable for swifter bioassays of larvicides. We achieved this goal in Aedes aegypti with very limited strategic and low-cost additions to food, such as coconut water, milk or its casein, yeast extract, and to a lesser extent, glycerol. The naturally rich coconut water was excellent for quickly attaining the population of instar IV larvae, the most advanced one before pupation, saving about a week, for subsequent larvicidal bioassays. Diluted milk, as another food source, allowed an even faster final ecdysis and adults are useful for mosquito taxonomical purpose.

Impacts of Dietary Nutritional Composition on Larval Development and Adult Body Composition in the Yellow Fever Mosquito (Aedes aegypti)

Simple Summary

The mosquito Aedes aegypti (Ae. aegypti) is responsible for the spread of viruses such as Zika and Dengue. The nutritional environment of immature Ae. aegypti is important for development of larvae and resulting adult mosquitoes. Larval mosquitoes with inadequate nutrition can result in developmental failure or impact the size and reproductive ability of adults. Understanding the nutritional requirements of larval mosquitoes allows us to optimize lab reared mosquitoes and identify new targets for mosquito control. We tested the effect of diets with different ratios of protein to carbohydrates on the life history traits of Ae. aegypti. Each diet was composed of autolyzed Brewer’s yeast (protein), and/or rice flour (carbohydrates). Larvae fed a medium-low protein diet had the shortest pupation time. As adults, the medium-low protein dietary group also had the longest wing lengths, highest weights, and increased lipid stores compared to the adults in all other dietary groups. These findings indicate that both carbohydrates and protein are essential components of Aedes aegypti larval diets. However, Ae. aegypti larvae fed a diet rich in carbohydrates and lower in protein seem to flourish as long as they receive enough dietary protein to fulfill basic biochemical requirements for growth and development.

Abstract

Background: the mosquito Aedes aegypti (Ae. aegypti) is an important vector of arboviruses, including Zika, Dengue, and Chikungunya. The dietary requirements of larval Ae. aegypti are not well understood and likely impact developmental and physiological parameters knowledge of which could be important for vector control. This study examines the effects nutrition has on growth and development of larval Ae. aegypti of laboratory-reared Rockefeller strain mosquitoes. Methods: mosquito larvae were split into five feeding groups with diets providing different ratios of protein and carbohydrates. Each group received autolyzed Brewer’s yeast (AY - high-protein), and/or rice flour (RF—high-carbohydrate). The groups were monitored to record larval developmental times, adult sizes and nutritional stores. Results: the 100% AY group failed to pupate, suggesting the AY alone is either lacking in critical nutrients or is toxic at higher concentrations. The 100% RF group resulted in the smallest adults that took the longest time to reach pupation. Of the remaining groups, the 25% AY/75% RF (Med–low) diet yielded adult mosquitoes with highest average weight, wing length, and lipid stores relative to the other diets. Conclusions: the dietary requirements for development, body size, and nutrient stores of Ae. aegypti mosquitoes appear to be dependent on a relatively low but essential proportion of dietary protein to carbohydrates to achieve optimal developmental outcomes.

Assessment of Aedes aegypti (Diptera: Culicidae) Males Flight Ability for SIT Application: Effect of Device Design, Duration of Test, and Male Age

The Sterile Insect Technique (SIT) is a pest control method where large numbers of sterile males are released to induce sterility in wild populations. Since a successful SIT application depends on the released sterile males being competitive with wild males, standard quality control tests are a necessary component of any SIT program. Flight ability (ability to fly out from a device) is a reliable indicator of insect quality. Based on previous studies, we developed four new tubular devices constructed with locally available materials to explore their potential as flight test devices for Aedes aegypti (L.) mass-reared males. Males were allowed to fly upwards through a vertical tube, the ones that flew out were considered successful. The effect of male age (0 to 21 d old), test time interval (30 min to 24 h), and the design of the device (40 and 80 cm height and 2 and 3.5 cm diameter) were evaluated. Our devices determined differences in the flight ability of Ae. aegypti males of different ages. During the first minutes, more old males escaped than young males in three out of four types of devices. However, young males reached higher rates of escape in all cases after 24 h. For standard quality control tests, we recommend testing 2- to 3-d-old sexually mature males in the high and narrow device (80 × 2 cm). Further observations for time intervals between 1 and 5 h might be performed to decide the shortest and more representative interval to use.

Effects of larval diets and temperature regimes on life history traits, energy reserves and temperature tolerance of male Aedes aegypti (Diptera: Culicidae): optimizing rearing techniques for the sterile insect programmes

Background

Producing high quality sterile males is vital in Aedes aegypti rear-and-release birth control strategies. Larval diets, rearing temperatures, and their interactions determine the accumulation rates of essential nutrients in larvae, but these factors have been understudied in relation to mass-rearing techniques for producing eminent males.

Methods

We compared the effects of two larval diets, a cereal-legume-based diet (Khan’s diet) and a standard larval diet developed in the FAO/IAEA Insect Pest Control Laboratory (IAEA 2 diet). Diets were tested at selected temperatures for both larval and male adult life history traits, adult extreme temperature tolerance, and mating capacity relative to energy reserves of reared male adult Ae. aegypti.

Results

Khan’s diet resulted in shorter immature development time at each test temperature (except for 25 °C) than an IAEA 2 diet. Larvae reared at 28 °C and 32 °C with Khan’s diet demonstrated low pupation rates (c.80%). We accounted for these phenomena as secondary sex ratio manipulation, because a higher proportion of male adults emerged at 28 °C and 32 °C than that for the IAEA 2 diet. In general, the pupal development time shortened as temperature increased, resulting in higher teneral energy reserves in male mosquitoes. High energy reserves allowed male mosquitoes reared with Khan’s diet to have higher adult longevity (5–6 days longer when sugar-fed and 2–3 days longer when water-fed) and tolerance of heat stress than those fed on the IAEA 2 diet. The IAEA 2 diet produced larger male mosquitoes than Khan’s diet did: mosquitoes fed on Khan’s diet were 1.03–1.05 times smaller than those fed on the IAEA 2 diet at 28 °C and 32 °C. No evidence indicated reduced mating capacity for small mosquitoes fed on Khan’s diet.

Conclusions

Larvae reared at 28 °C and 32 °C with Khan’s diet were characterized by shorter immature development time compared with those fed on the IAEA 2 diet. Adult mosquitoes produced from that larval rearing condition exhibited a significant male bias, long lifespan, and better endurance against extreme temperatures relative to energy reserves. Thus, the larval diet at rearing temperature of 28 °C and 32 °C optimized rearing techniques for the sterile insect programmes. However, mating competitiveness and flight performance of adult males require further investigation.

Black soldier fly (Hermetia illucens) larvae powder as a larval diet ingredient for mass-rearing Aedes mosquitoes

The mass production of mosquitoes is becoming more wide-spread due to the increased application of the sterile insect technique (SIT) and other genetic control programmes. Due to the variable availability and high cost of the bovine liver powder (BLP) constituent of many current larval diets, there is an urgent demand for new ingredients in order to support sustainable and efficient mosquito production while reducing rearing cost, without affecting the quality of the insects produced. Two black soldier fly (BSF) powder-based diet formulations (50% tuna meal, 35% BSF powder, 15% brewer's yeast and 50% tuna meal + 50% BSF powder) were tested for their suitability to support the development of Aedes aegypti and Ae. albopictus mosquitoes in mass-rearing conditions. Overall, the results indicate that the use of the BSF powder did not negatively impact the development and quality of the produced insects in terms of time to pupation, adult production and male flight ability. Furthermore, depending on the species and diet formulations, there were improvements in some parameters such as female body size, egg production, egg hatch rate and male longevity. BSF powder is a valuable ingredient that can effectively replace costly BLP for the mass production of high quality Ae. aegypti and Ae. albopictus mosquitoes. Both diet formulations can be used for Ae. aegypti showing high plasticity to nutrition sources. However, for Ae. albopictus we recommend the combination including brewer's yeast.

Efficacy of Spinosad Granules and Lambda-Cyhalothrin Contrasts with Reduced Performance of Temephos for Control of Aedes spp. in Vehicle Tires in Veracruz, Mexico

The present study examined the efficacy of λ-cyhalothrin, pyriproxyfen and granular formulations of spinosad and temephos for the control of mosquito larvae present in experimental tires in Veracruz State, Mexico in the period 2015-2016. Both λ-cyhalothrin and spinosad granules provided control of larvae and pupae of Aedes aegypti, Ae. albopictus and Culex spp. in used tires in Veracruz State, Mexico, over a 9-12 week period, although numbers of Culex were low. The numbers of Aedes larvae + pupae in pyriproxyfen and temephos-treated tires were slightly less than half of the untreated control tires, probably a result the pupicidal characteristics of pyriproxyfen and possible resistance in the case of temephos. Spinosad was less harmful to predatory Toxorhynchites spp. than λ-cyhalothrin or temephos. The reduced susceptibility to temephos in Aedes populations was confirmed at five other sites in Veracruz. Public health authorities should consider incorporating spinosad as a larvicide in coastal areas at a high risk of dengue, chikungunya and Zika outbreaks in this region.

Optimization of irradiation dose to Aedes aegypti and Ae. albopictus in a sterile insect technique program

The sterile insect technique (SIT) may offer a means to control the transmission of mosquito borne diseases. SIT involves the release of male insects that have been sterilized by exposure to ionizing radiation. We determined the effects of different doses of radiation on the survival and reproductive capacity of local strains of Aedes aegypti and Ae. albopictus in southern Mexico. The survival of irradiated pupae was invariably greater than 90% and did not differ significantly in either sex for either species. Irradiation had no significant adverse effects on the flight ability (capacity to fly out of a test device) of male mosquitoes, which consistently exceeded 91% in Ae. aegypti and 96% in Ae. albopictus. The average number of eggs laid per female was significantly reduced in Ae. aegypti at doses of 15 and 30 Gy and no eggs were laid by females that had been exposed to 50 Gy. Similarly, in Ae. albopictus, egg production was reduced at doses of 15 and 25 Gy and was eliminated at 35 Gy. In Ae. aegypti, fertility in males was eliminated at 70 Gy and was eliminated at 30 Gy in females, whereas in Ae. albopictus, the fertility of males that mated with untreated females was almost zero (0.1%) in the 50 Gy treatment and female fertility was eliminated at 35 Gy. Irradiation treatments resulted in reduced ovary length and fewer follicles in both species. The adult median survival time of both species was reduced by irradiation in a dose-dependent manner. However, sterilizing doses of 35 Gy and 50 Gy resulted in little reduction in survival times of males of Ae. albopictus and Ae. aegypti, respectively, indicating that these doses should be suitable for future evaluations of SIT-based control of these species. The results of the present study will be applied to studies of male sexual competitiveness and to stepwise evaluations of the sterile insect technique for population suppression of these vectors in Mexico.

Both living bacteria and eukaryotes in the mosquito gut promote growth of larvae

We recently reported that larval stage Aedes aegypti and several other species of mosquitoes grow when living bacteria are present in the gut but do not grow when living bacteria are absent. We further reported that living bacteria induce a hypoxia signal in the gut, which activates hypoxia-induced transcription factors and other processes larvae require for growth. In this study we assessed whether other types of organisms induce mosquito larvae to grow and asked if the density of non-living microbes or diet larvae are fed obviate the requirement for living organisms prior results indicated are required for growth. Using culture conditions identical to our own prior studies, we determined that inoculation density of living Escherichia coli positively affected growth rates of Ae. aegypti larvae, whereas non-living E. coli had no effect on growth across the same range of inoculation densities. A living yeast, alga, and insect cell line induced axenic Ae. aegypti first instars to grow, and stimulated similar levels of midgut hypoxia, HIF-α stabilization, and neutral lipid accumulation in the fat body as E. coli. However, the same organisms had no effect on larval growth if heat-killed. In addition, no axenic larvae molted when fed two other diets, when fed diets supplemented with heat-killed microbes or lysed and heat-killed microbes. Experiments conducted with An. gambiae yielded similar findings. Taken together, our results indicate that organisms from different prokaryotic and eukaryotic groups induce mosquito larvae to grow, whereas no conditions were identified that stimulated larvae to grow in the absence of living organisms.

Efficacy of larvicides for the control of dengue, Zika, and chikungunya vectors in an urban cemetery in southern Mexico

Many countries in Latin America have recently experienced outbreaks of Zika and chikungunya fever, in additional to the usual burden imposed by dengue, all of which are transmitted by Aedes aegypti in this region. To identify potential larvicides, we determined the toxicity of eight modern insecticides to A. aegypti larvae from a colony that originated from field-collected insects in southern Mexico. The most toxic compounds were pyriproxyfen (which prevented adult emergence) and λ-cyhalothrin, followed by spinetoram, imidacloprid, thiamethoxam, and acetamiprid, with chlorantraniliprole and spiromesifen the least toxic products. Field trails performed in an urban cemetery during a chikungunya epidemic revealed that insecticide-treated ovitraps were completely protected from the presence of Aedes larvae and pupae for 6 and 7 weeks in spinosad (Natular G30) and λ-cyhalothrin-treated traps in both seasons, respectively, compared to 5-6 weeks for temephos granule-treated ovitraps, but was variable for pyriproxyfen-treated ovitraps with and 1 and 5 weeks of absolute control in the dry and rainy seasons, respectively. Insecticide treatments influenced the mean numbers of Aedes larvae + pupae in each ovitrap, mean numbers of eggs laid, and percentage of egg hatch over time in both trials. The dominant species was A. aegypti in both seasons, although the invasive vector Aedes albopictus was more prevalent in the rainy season (26.7%) compared to the dry season (10.2%). We conclude that the granular formulation of spinosad (Natular G30) and a suspension concentrate formulation of λ-cyhalothrin proved highly effective against Aedes spp. in both the dry and rainy seasons in the cemetery habitat in this region.