Reducing the cost and assessing the performance of a novel adult mass-rearing cage for the dengue, chikungunya, yellow fever and Zika vector, Aedes aegypti (Linnaeus)

Upscaling the production of sterile male mosquitoes with an automated pupa sex sorter

Effective mosquito population suppression has been repeatedly demonstrated in field trials through the release of male mosquitoes to induce sterile mating with wild females using the incompatible insect technique (IIT), the sterile insect technique (SIT), or their combination. However, upscaling these techniques requires a highly efficient and scalable approach for the sex separation of mass-reared mosquitoes to minimize the unintentional release of females, which can lead to either population replacement or biting nuisance, a major bottleneck up to now. Here, we report the successful development of an automated mosquito pupa sex sorter that can effectively separate large numbers of males from females for population suppression of Aedes aegypti, A. albopictus, and Culex quinquefasciatus. The male production capacity of the automated sex sorter was increased by ~17-fold compared with manual sex separation with the Fay-Morlan sorter and enabled one person to separate 16 million males per week. With ~0.5% female contamination, the produced males exhibited high flight ability and mating performance. The field trial demonstrates that the quality of A. albopictus males produced using the automated sex sorter is suitable for inducing population suppression. These results indicate that the automated sex sorter offers the potential to upscale IIT and SIT against mosquito vectors for disease control.

Mating harassment may boost the effectiveness of the sterile insect technique for Aedes mosquitoes

The sterile insect technique is based on the overflooding of a target population with released sterile males inducing sterility in the wild female population. It has proven to be effective against several insect pest species of agricultural and veterinary importance and is under development for Aedes mosquitoes. Here, we show that the release of sterile males at high sterile male to wild female ratios may also impact the target female population through mating harassment. Under laboratory conditions, male to female ratios above 50 to 1 reduce the longevity of female Aedes mosquitoes by reducing their feeding success. Under controlled conditions, blood uptake of females from an artificial host or from a mouse and biting rates on humans are also reduced. Finally, in a field trial conducted in a 1.17 ha area in China, the female biting rate is reduced by 80%, concurrent to a reduction of female mosquito density of 40% due to the swarming of males around humans attempting to mate with the female mosquitoes. This suggests that the sterile insect technique does not only suppress mosquito vector populations through the induction of sterility, but may also reduce disease transmission due to increased female mortality and lower host contact.

From the Lab to the Field: Long-Distance Transport of Sterile Aedes Mosquitoes

Pilot programs of the sterile insect technique (SIT) against Aedes aegypti may rely on importing significant and consistent numbers of high-quality sterile males from a distant mass rearing factory. As such, long-distance mass transport of sterile males may contribute to meet this requirement if their survival and quality are not compromised. This study therefore aimed to develop and assess a novel method for long-distance shipments of sterile male mosquitoes from the laboratory to the field. Different types of mosquito compaction boxes in addition to a simulation of the transport of marked and unmarked sterile males were assessed in terms of survival rates/recovery rates, flight ability and morphological damage to the mosquitoes. The novel mass transport protocol allowed long-distance shipments of sterile male mosquitoes for up to four days with a nonsignificant impact on survival (>90% for 48 h of transport and between 50 and 70% for 96 h depending on the type of mosquito compaction box), flight ability, and damage. In addition, a one-day recovery period for transported mosquitoes post-transport increased the escaping ability of sterile males by more than 20%. This novel system for the long-distance mass transport of mosquitoes may therefore be used to ship sterile males worldwide for journeys of two to four days. This study demonstrated that the protocol can be used for the standard mass transport of marked or unmarked chilled Aedes mosquitoes required for the SIT or other related genetic control programs.

The size of larval rearing container modulates the effects of diet amount and larval density on larval development in Aedes aegypti

Mass-rearing of mosquitoes under laboratory conditions is an important part of several new control techniques that rely on the release of males to control mosquito populations. While previous work has investigated the effect of larval density and diet amount on colony productivity, the role of the size of the container in which larval development takes place has been relatively ignored. We investigated the role of container size in shaping life history and how this varied with density and food availability in Aedes aegypti, an important disease vector and target of mass-rearing operations. For each treatment combination, immature development time and survival and adult body size and fecundity were measured, and then combined into a measure of productivity. We additionally investigated how larval aggregation behaviour varied with container size. Container size had important effects on life history traits and overall productivity. In particular, increasing container size intensified density and diet effects on immature development time. Productivity was also impacted by container size when larvae were reared at high densities (1.4 larva/ml). In these treatments, the productivity metric of large containers was estimated to be significantly lower than medium or small containers. Regardless of container size, larvae were more likely to be observed at the outer edges of containers, even when this led to extremely high localized densities. We discuss how container size and larval aggregation responses may alter the balance of energy input and output to shape development and productivity.

A Mark-Release-Recapture Study to Estimate Field Performance of Imported Radio-Sterilized Male Aedes albopictus in Albania

The pathogen transmitting Aedes albopictus mosquito is spreading rapidly in Europe, putting millions of humans and animals at risk. This species is well-established in Albania since its first detection in 1979. The sterile insect technique (SIT) is increasingly gaining momentum worldwide as a component of area-wide-integrated pest management. However, estimating how the sterile males will perform in the field and the size of target populations is crucial for better decision-making, designing and elaborating appropriate SIT pilot trials, and subsequent large-scale release strategies. A mark-release-recapture (MRR) experiment was carried out in Albania within a highly urbanized area in the city of Tirana. The radio-sterilized adults of Ae. albopictus Albania strain males were transported by plane from Centro Agricoltura Ambiente (CAA) mass-production facility (Bologna, Italy), where they were reared. In Albania, sterile males were sugar-fed, marked with fluorescent powder, and released. The aim of this study was to estimate, under field conditions, their dispersal capacity, probability of daily survival and competitiveness, and the size of the target population. In addition, two adult mosquito collection methods were also evaluated: BG-Sentinel traps baited with BG-Lure and CO_2, (BGS) versus human landing catch (HLC). The overall recapture rates did not differ significantly between the two methods (2.36% and 1.57% of the total male released were recaptured respectively by BGS and HLC), suggesting a similar trapping efficiency under these conditions. Sterile males traveled a mean distance of 93.85 ± 42.58 m and dispersed up to 258 m. Moreover, they were observed living in the field up to 15 days after release with an average life expectancy of 4.26 ± 0.80 days. Whether mosquitoes were marked with green, blue, yellow, or pink, released at 3.00 p.m. or 6.00 p.m., there was no significant difference in the recapture, dispersal, and survival rates in the field. The Fried competitiveness index was estimated at 0.28. This mark-release-recapture study provided important data for better decision-making and planning before moving to pilot SIT trials in Albania. Moreover, it also showed that both BG-traps and HLC were successful in monitoring adult mosquitoes and provided similar estimations of the main entomological parameters needed.

Standardization of the FAO/IAEA Flight Test for Quality Control of Sterile Mosquitoes

Successful implementation of the sterile insect technique (SIT) against Aedes aegypti and Aedes albopictus relies on maintaining a consistent release of high-quality sterile males. Affordable, rapid, practical quality control tools based on the male’s flight ability (ability to escape from a flight device) may contribute to meeting this requirement. Therefore, this study aims to standardize the use of the original FAO/IAEA rapid quality control flight test device (FTD) (version 1.0), while improving handling conditions and reducing the device’s overall cost by assessing factors that could impact the subsequent flight ability of Aedes mosquitoes. The new FTD (version 1.1) is easier to use. The most important factors affecting escape rates were found to be tube color (or “shade”), the combined use of a lure and fan, mosquito species, and mosquito age and density (25; 50; 75; 100 males). Other factors measured but found to be less important were the duration of the test (30, 60, 90, 120 min), fan speed (normal 3000 rpm vs. high 6000 rpm), and mosquito strain origin. In addition, a cheaper version of the FTD (version 2.0) that holds eight individual tubes instead of 40 was designed and successfully validated against the new FTD (version 1.1). It was sensitive enough to distinguish between the effects of cold stress and high irradiation dose. Therefore, the eight-tube FTD may be used to assess Aedes’ flight ability. This study demonstrated that the new designs (versions 1.1 and 2.0) of the FTD could be used for standard routine quality assessments of Aedes mosquitoes required for an SIT and other male release-based programs.

Adult mosquito predation and potential impact on the sterile insect technique

The sterile insect technique is a promising environmentally friendly method for mosquito control. This technique involves releasing laboratory-produced sterile males into a target field site, and its effectiveness may be affected by the extent of adult mosquito predation. Sterile males undergo several treatments. Therefore, it is vital to understand which treatments are essential in minimizing risks to predation once released. The present study investigates the predation propensity of four mantis species (Phyllocrania paradoxa, Hymenopus coronatus, Blepharopsis mendica, Deroplatys desiccata) and two gecko species (Phelsuma standingi, P. laticauda) on adult Aedes aegypti, Ae. albopictus and Anopheles arabiensis mosquitoes in a laboratory setting. First, any inherent predation preferences regarding mosquito species and sex were evaluated. Subsequently, the effects of chilling, marking, and irradiation, on predation rates were assessed. The selected predators effectively preyed on all mosquito species regardless of the treatment. Predation propensity varied over days for the same individuals and between predator individuals. Overall, there was no impact of laboratory treatments of sterile males on the relative risk of predation by the test predators, unless purposely exposed to double the required sterilizing irradiation dose. Further investigations on standardized predation trials may lead to additional quality control tools for irradiated mosquitoes.

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.

Sterile Insect Technique in an Integrated Vector Management Program against Tiger Mosquito Aedes albopictus in the Valencia Region (Spain): Operating Procedures and Quality Control Parameters

Aedes albopictus and Aedes aegypti are the main vectors of arboviral diseases such as dengue, Zika and chikungunya viruses. About a third of the world population is currently at risk of contracting Aedes-borne epidemics. In recent years, A. albopictus has drastically increased its distribution in many countries. In the absence of efficient mosquito vector control methods, the sterile insect technique (SIT) is presented as a very promising and environment-friendly control tool. The Agriculture Department of the Valencian Region is promoting an ongoing pilot project to evaluate the efficacy of an integrated vector management program (IVM) based on the use of the SIT as the main method of control. The laboratory studies for evaluating the entomological efficacy of SIT through the phased conditional testing process recommended by World Health Organization and the International Atomic Energy Agency (WHO-IAEA) are addressed. This study describes the routine operating procedures and quality control parameters for the medium-scale rearing of sterile male A. albopictus. More than 15 million sterile males have been produced and released in an area of 80 ha between 2018 and 2020. Of the initial L1 larvae, we recovered 17.2% of male pupae after sex sorting to be sterilized and released on the field, while the rest of the pupae remained available to maintain the rearing colony. The residual percentage of females after sex sorting was on average 0.17%. The obtained values in terms of production and quality control as well as the proposed rearing methodology can be useful for designing a medium-scale mosquito-rearing pipeline.

Does Tap Water Quality Compromise the Production of Aedes Mosquitoes in Genetic Control Projects?

Simple Summary

Scientists all over the world are continually rearing and producing insects in laboratories for many purposes including pest control programmes. Aedes aegypti and Ae. albopictus are mosquitoes of public health importance due to their ability to vector human and animal pathogens and thus vector control represents an important component of many disease control programmes. Water is a factor of great importance in the larval environment of mosquito species. However, obtaining sufficient water of reliable quality for mosquito rearing is still challenging, especially in developing and least developed countries, where access even to clean drinking water is limited. In prospect of cost-effective methods for improved mass-rearing toward SIT application, we assessed the impact of using tap water on the development and quality of Aedes mosquitoes. Results showed that, tap water with hardness/electrical conductivity beyond certain levels (140 mg/l CaCO₃ or 368 µS/cm) was shown to have a negative impact on the production of Ae. albopictus and Ae. aegypti mosquitoes. These results suggest that the quality of water should be checked when using for rearing mosquitoes for release purposes in order to optimize the production performance of mass-rearing facilities. This may have important implications for the implementation of the sterile insect technique in areas where reverse osmosis water is a scarce or costly resource.

Abstract

A mosquito’s life cycle includes an aquatic phase. Water quality is therefore an important determinant of whether or not the female mosquitoes will lay their eggs and the resulting immature stages will survive and successfully complete their development to the adult stage. In response to variations in laboratory rearing outputs, there is a need to investigate the effect of tap water (TW) (in relation to water hardness and electrical conductivity) on mosquito development, productivity and resulting adult quality. In this study, we compared the respective responses of Aedes aegypti and Ae. albopictus to different water hardness/electrical conductivity. First-instar larvae were reared in either 100% water purified through reverse osmosis (ROW) (low water hardness/electrical conductivity), 100% TW (high water hardness/electrical conductivity) or a 80:20, 50:50, 20:80 mix of ROW and TW. The immature development time, pupation rate, adult emergence, body size, and longevity were determined. Overall, TW (with higher hardness and electrical conductivity) was associated with increased time to pupation, decreased pupal production, female body size in both species and longevity in Ae. albopictus only. However, Ae. albopictus was more sensitive to high water hardness/EC than Ae. aegypti. Moreover, in all water hardness/electrical conductivity levels tested, Ae. aegypti developed faster than Ae. albopictus. Conversely, Ae. albopictus adults survived longer than Ae. aegypti. These results imply that water with hardness of more than 140 mg/l CaCO₃ or electrical conductivity more than 368 µS/cm cannot be recommended for the optimal rearing of Aedes mosquitoes and highlight the need to consider the level of water hardness/electrical conductivity when rearing Aedes mosquitoes for release purposes.

Assessment of a Novel Adult Mass-Rearing Cage for Aedes albopictus (Skuse) and Anopheles arabiensis (Patton)

Successful implementation of the sterile insect technique (SIT) against Aedes albopictus and Anopheles arabiensis relies on a continuous supply of sterile males. To meet this requirement, optimization of the mass-rearing techniques is needed. This study, therefore, aims to assess a new mass-rearing cage (MRC) in terms of egg production efficiency and egg hatch rate (quality). In addition, adult survival was evaluated based on a cage adult-index for Ae. albopictus. Moreover, the cage's suitability for use in mass An. arabiensis egg production was compared to that of the FAO/IAEA Anopheles reference cage. In Ae. albopictus rearing, the new MRC produced 1,112,110 eggs per cage following six blood meals, with minimum loss of eggs in the egging water. Furthermore, the adult index gave a good proxy of daily mortality rates in Ae. albopictus. In An. arabiensis rearing, about 130,000 eggs per egg batch were collected both from the new and the reference MRC. These findings suggest that the new MRC prototype is efficient in terms of egg production and can be used for mass-rearing in SIT programs targeting Ae. albopictus as well as An. arabiensis. The adult index was also positively validated for the detection of unusual mortality rates in Ae. albopictus mass-rearing facilities. Overall, the new MRC has shown several advantages; however, further improvements are necessary to minimize escapes during the egg collection processes.

Aedes aegypti larval development and pupal production in the FAO/IAEA mass-rearing rack and factors influencing sex sorting efficiency

The production of a large number of mosquitoes of high biological qualities and reliable sex sorting before release are key challenges when applying the sterile insect technique as part of an area-wide integrated pest management approach. There is a need to fully evaluate the production capacity of the equipment developed in order to plan and maintain a daily production level for large-scale operational release activities. This study aimed to evaluate the potential use of the FAO/IAEA larval rearing unit for Aedes aegypti and the subsequent female contamination rate after sex sorting with a Fay–Morlan glass separator. Trays from each rack were tilted and their contents sorted either for each individual tray or after mixing the content of all trays from the rack. The pupal production and the female contamination rate were estimated with respect to day of collection, position of the tray, type of pupae collection, and sorting operator. Results showed significant daily variability of pupal production and female contamination rate, with a high male pupal production level achieved on the second day of collection and estimated female contamination of male pupae reached around 1%. Neither tray position nor type of pupae collection affected the pupal production and female contamination rate. However, the operator had a significant effect on the female contamination rate. These results highlight the need to optimize pupal production at early days of collection and to develop a more effective and automated method of sex separation.

Phased Conditional Approach for Mosquito Management Using Sterile Insect Technique

Mosquito-borne diseases represent a major threat to humankind. Recently, the incidence of malaria has stopped decreasing while that of dengue is increasing exponentially. Alternative mosquito-control methods are urgently needed. The sterile insect technique (SIT) has seen significant developments recently and may play an important role. However, testing and implementing SIT for vector control is challenging, and a phased conditional approach (PCA) is recommended, that is, advancement to the next phase depends on completion of activities in the previous one. We herewith present a PCA to test the SIT against mosquitoes within an area-wide-integrated pest-management programme, taking into account the experience gained with plant and livestock pests and the recent developments of the technique against mosquitoes.

Mosquito mass rearing: who's eating the eggs?

For the sterile insect technique, and other related biological control methods where large numbers of the target mosquito are reared artificially, production efficiency is key for the economic viability of the technique. Rearing success begins with high quality eggs. Excess eggs are often stockpiled and stored for longer periods of time. Any pests that prey on these eggs are detrimental to stockpiles and need to be avoided. Psocids of the genus Liposcelis (Psocoptera, Liposcelididae) are common scavengers consuming various types of organic material that are distributed globally and thrive in warm damp environments, making insectaries ideal habitats. In this short report, we investigated the species that has been found scavenging stored mosquito eggs in our insectary and identified it to be Liposcelis bostrychophila Badonnel, 1931. Additional observations were made to determine whether these predators indeed feed on mosquito eggs, and to suggest simple, effective ways of avoiding infestation.

Mass-Rearing of Drosophila suzukii for Sterile Insect Technique Application: Evaluation of Two Oviposition Systems

Drosophila suzukii (Diptera: Drosophilidae) is an invasive pest of a wide range of commercial soft-skinned fruits. To date, most management tactics are based on spraying of conventional and/or organic insecticides, baited traps, and netting exclusion. Interest has been expressed in using the sterile insect technique (SIT) as part of area-wide integrated pest management (AW-IPM) programs to control D. suzukii infestations. Mass-rearing protocols are one of the prerequisites for successful implementation of the SIT. To establish mass-rearing methods for this species, two different egg-collection systems were developed and compared with respect to the number of eggs produced, egg viability, pupa and adult recovery, adult emergence rate, and flight ability. Female flies kept in cages equipped with a wax panel produced significantly more eggs with higher viability and adult emergence rate, as compared to the netted oviposition system. The wax panel system was also more practical and less laborious regarding the collection of eggs. Furthermore, the wax panel oviposition system can be adapted to any size or design of an adult cage. In conclusion, this system bears great promise as an effective system for the mass production of D. suzukii for SIT.

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.